Title of Invention

METHODS OF PROCESSING NICKEL-TITANIUM SHAPE MEMORY ALLOYS

Abstract The invention discloses a method of processing a nickel-titanium alloy comprising 50 to 55 atomic percent nickel to provide a pre-selected austenite transformation temperature of -100°C to 100°C, the method comprising: selecting the pre-selected austenite transformation temperature; and thermally processing the nickel-titanium alloy by isothermally aging the nickel-titanium alloy at a temperature of 500°C to 800°C for at least 2 hours to adjust an amount of nickel in solid solution in a TiNi phase of the alloy such that a stable austenite transformation temperature is reached during thermally processing the nickel-titanium alloy, wherein the stable austenite transformation temperature is essentially equal to the pre-selected austenite transformation temperature, wherein the nickel-titanium alloy comprises a nickel concentration that reaches a solid solubility limit during thermally processing the nickel-titanium alloy.
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BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The various embodiments of the present invention generally relate to
methods of processing nickel-titanium alloys. More particularly, certain
embodiments of the present invention relate to thermally processing nickel-titanium
, alloys to predictably adjust the austenite transformation temperature and/or
transformation temperature range of the alloy.
DESCRIPTION OF RELATED ART
Equiatomic and near-equiatomic nickel-titanium alloys are known to
possess both "shape memory" and "superelastic" properties. More specifically,
these alloys, which are commonly referred to as "Nitinol" alloys, are known to
undergo a martensitic transformation from a parent phase (commonly referred to as
the austenite phase) to at least one martensite phase on cooling to a temperature
below the martensite start (or "Ms") temperature of the alloy. This transformation is

complete on cooling to the martensite finish (or "Mf") temperature of the alloy.
Further, the transformation is reversible when the material is heated to a temperature
above its austenite finish (or "Af") temperature. This reversible martensitic
transformation gives rise to the shape memory properties of the alloy. For example,
a nickel-titanium alloy can be formed into a first shape while in the austenite phase
(i.e., above the austenite finish temperature, or Af, of the alloy), and subsequently
cooled 1o a temperature below the Mf and formed into a second shape. As long as
the material remains below the As (i.e., the temperature at which the transition to
austenite begins or the austenite start temperature) of the alloy, the alloy will retain
the second shape. However, if the alloy is heated to a temperature above the Af the
alloy will revert back to the first shape.
The transformation between the austenite and martensite phases also
gives rise to the "superelastic" properties of nickel-titanium alloys. When a nickel-
titanium alloy is strained at a temperature above Ms, the alloy can undergo a strain-
induced transformation from the austenite phase to the martensite phase. This
transformation, combined with the ability of the martensite phase to deform by
movement of twinned boundaries without the generation of dislocations, permits the
nickel-titanium alloy to absorb a large amount of strain energy by elastic deformation
without plastically (i.e., permanently) deforming. When the strain is removed, the
alloy is able to almost fully revert back to its unstrained condition.
The ability to make commercial use of the unique properties of nickel-
titanium alloys, and other shape memory alloys, is to a great extent dependent upon
the temperatures at which these transformations occur, i.e, the As and Af, and Ms
and Mf of the alloy, as well as the range of temperatures over which these
transformations occur. However, in binary nickel-titanium alloy systems, it has been
observed that the transformation temperatures of the alloy are highly dependent on
composition. That is, for example, it has been observed that the Ms temperature of a
nickel-titanium alloy can change more than 100K for a 1 atomic percent change in
composition of the alloy. See K. Otsuka and T. Kakeshia, "Science and Technology
of Shape-Memory Alloys: New Developments," MRS Bulletin. Feb. 2002, at pages
91-100.

Further, as will be appreciated by those skilled in the art, the tight
compositional control of nickel-titanium alloys necessary to achieve predictable
transformation temperatures is extremely difficult to achieve. For example, in order
to achieve a desired transformation temperature in a typical nickel-titanium process,
after a nickel-titanium ingot or billet is cast, the transformation temperature of the
ingot must be measured. If the transformation temperature is not the desired
transformation temperature, the composition of the ingot must be adjusted by
remelting and alloying the ingot. Further, if the ingot is compositionally segregated,
which may occur for example during solidification, the transformation temperature of
several regions across the ingot must be measured and the transformation
temperature in each region must be adjusted. This process must be repeated until
the desired transformation temperature is achieved. As will be appreciated by those
skilled in the art, such methods of controlling transformation temperature by
controlling composition are both time consuming and expensive. As used herein, the
term "transformation temperature(s)" refers generally to any of the transformation
temperatures discussed above; whereas the term "austenite transformation
temperature(s)" refers to at least one of the austenite start (As) or austenite finish (Af)
temperatures of the alloy, unless specifically noted.
Methods of generally increasing or decreasing the transformation
temperatures of nickel-titanium alloys using thermal processes are known in the art.
For example, U.S. Patent No. 5,882,444 to Flomenblit et al. discloses a memorizing
treatment for a two-way shape memory alloy, which involves forming a nickel-
titanium alloy into a shape to be assumed in the austenitic phase, and then
polygonizing the alloy by heating at 450°C to 550°C for 0.5 to 2.0 hours, solution
treating the alloy at 600°C to 800°C for 2 to 50 minutes, and finally aging at about
350°C to 500°C for about 0 to 2.5 hours. According to Flomenblit et al., after this
treatment, the alloy should have an Af ranging from 10°C-60oC and a transformation
temperature range (i.e., Af-As) of 1°C to 5°C. Thereafter, the Af of the alloy may be
increased by aging the alloy at a temperature of about 350°C to 500°C.
Alternatively, the alloy may be solution treated at a temperature of about 510°C to
800°C to decrease the Af of the alloy. See Flomenblit et al. at col. 3, lines 47-53.

U.S. Patent No. 5,843,244 to Pelton et al. discloses a method of
treating a component formed from a nickel-titanium alloy to decrease the Af of the
alloy by exposing the component to a temperature greater than a temperature to
which it is exposed to shape-set the alloy and less than the solvus temperature of
the alloy for not morelhan 10 minutes to reduce the Afof the alloy.
However, there remains a need for an efficient method of predictably
controlling the austenite transformation temperatures and/or austenite transformation
temperature range of nickel-titanium alloys to achieve a desired austenite
transformation temperature and/or austenite transformation temperature range.
Further, there remains a need for a method of predictably controlling the austenite
transformation temperatures and austenite transformation temperature range of
nickel-titanium alloys having varying nickel contents.
BRIEF SUMMARY OF THE INVENTION
Embodiments of the present invention provide methods of processing
nickel-titanium alloys to achieve a desired austenite transformation temperature. For
example, one non-limiting method of processing a nickel-titanium alloy comprising
from greater than 50 up to 55 atomic percent nickel to provide a desired austenite
transformation temperature comprises selecting the desired austenite transformation
temperature, and thermally processing the nickel-titanium alloy to adjust an amount
of nickel in solid solution in a TiNi phase of the alloy such that a stable austenite
transformation temperature is reached during thermally processing the nickel-
titanium alloy, wherein the stable austenite transformation temperature is essentially
equal to the desired austenite transformation temperature.
Another non-limiting method of processing a nickel-titanium alloy to
provide a desired austenite transformation temperature comprises selecting a nickel-
titanium alloy comprising from greater than 50 up to 55 atomic percent nickel,
selecting the desired austenite transformation temperature, and thermally processing
the selected nickel-titanium alloy to adjust an amount of nickel in solid solution in a

TiNi phase of the alloy such that a stable austenite transformation temperature is
reached during thermally processing the selected nickel-titanium alloy, the stable
austenite transformation temperature being essentially equal to the desired austenite
transformation temperature, wherein the selected nickel-titanium alloy comprises
sufficient nickel to reach a solid solubility limit during thermally processing the
selected nickel-titanium alloy.
Still another non-limiting method of processing "two or more nickel-
titanium alloys having varying compositions comprising from greater than 50 up to 55
atomic percent nickel to achieve a desired austenite transformation temperature
comprises selecting the desired austenite transformation temperature, and
subjecting the nickel-titanium alloys to similar thermal processing such that after
thermal processing, the nickel-titanium alloys have stable austenite transformation
temperatures, the stable austenite transformation temperatures being essentially
equal to the desired austenite transformation temperature.
Another non-limiting method of processing a nickel-titanium alloy
including regions of varying composition comprising from greater than 50 up to 55
atomic percent nickel such that each region has a desired austenite transformation
temperature comprises thermally processing the nickel-titanium alloy to adjust an
amount of nickel in solid solution in a TiNi phase of the alloy in each region of the
nickel-titanium alloy, wherein after thermally processing the nickel-titanium alloy,
each of the regions of the nickel-titanium alloy has a stable austenite transformation
temperature that is essentially equal to the desired austenite transformation
temperature.
Embodiments of the present invention also provide methods of
processing nickel-titanium alloys to achieve a desired austenite transformation
temperature range. For example, one non-limiting method of processing a nickel-
titanium alloy comprising from greater than 50 up to 55 atomic percent nickel to
achieve a desired austenite transition temperature range comprises isothermally
aging the nickel-titanium alloy in a furnace at a temperature ranging from 500°C to

800°C for at least 2 hours, wherein after aging the nickel-titanium alloy has an
austenite transformation temperature range no greater than 15°C.
Another non-limiting method of processing a nickel-titanium alloy
including regions of varying composition comprising from greater than 50 up to 55
atomic percent nickel such that each region has a desired austenite transformation
temperature range comprises isothermally aging the nickel-titanium alloy to adjust an
amount of nickel in solid solution in a TiNi phase of the alloy in each region of the
nickel-titanium alloy, wherein after isothermally aging the nickel-titanium alloy, each
of the regions of the nickel-titanium alloy has an austenite transformation
temperature range of no greater than 15°C.
Still another non-limiting method of processing a nickel-titanium alloy
comprising from greater than 50 up to 55 atomic percent nickel to achieve a desired
austenite transformation temperature range comprises isothermally aging the nickel-
titanium alloy in a furnace at a first aging temperature to achieve a stable austenite
transformation temperature, and isothermally aging the nickel-titanium alloy at a
second aging temperature that is different than the first aging temperature, wherein
after aging at the second aging temperature, the nickel-titanium alloy has an
austenite transformation temperature range that is essentially equal to the desired
transformation temperature range.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
The various embodiments of the present invention will be better
understood when read in conjunction with the drawings, in which:
Fig. 1 is a schematic graph of the austenite transformation
temperatures versus aging time at 675°C for two different nickel-
titanium alloys.
Fig. 2 is a schematic graph of the stable austenite transformation
temperature versus aging temperature for two different nickel-titanium
alloys.

Fig. 3 is a schematic graph of the austenite transformation
temperatures versus aging time at 566°C for two different nickel-
titanium alloys.
Fig. 4 is a schematic differential scanning calorimeter ("DSC") plot of a
nickel-titanium alloy after 2 hours aging at 650°C.
Fig. 5 is a schematic DSC plot of a nickel-titanium alloy after 24 hours
aging at 650°C.
Fig. 6 is a schematic DSC plot of a nickel-titanium alloy after 216 hours
aging at 650°C.
DETAILED DESCRIPTION OF THE INVENTION
As previously discussed, typically, the austenite transformation
temperatures of bulk nickel-titanium alloys are adjusted by adjusting the composition
of the alloy. However, because the austenite transformation temperatures of nickel-
titanium alloys are sensitive to minor compositional variations, attempts to control the
austenite transformation temperatures through composition have proven to be both
time consuming and expensive. Moreover, where the bulk alloy is compositionally
segregated, which can occur, for example, during solidification, adjusting the
austenite transformation temperatures of the alloy can require numerous
compositional adjustments. In contrast, the methods of processing nickel-titanium
alloys according to various embodiments of the present invention can be
advantageous in providing efficient methods of predictably controlling the austenite
transformation temperatures and/or austenite transformation temperature range of
nickel-titanium alloys to achieve a desired austenite transformation temperature
and/or austenite transformation temperature range, without the need for
compositional adjustments. Further, the methods according to various embodiments
of the present invention can be advantageous in providing efficient methods of
predictably controlling the austenite transformation temperatures and/or austenite
transformation temperature range for nickel-titanium alloys having varying nickel
contents, for example, when the bulk alloy is compositionally segregated or where

different alloys are processed simultaneously. Other advantages of the methods of
processing nickel-titanium alloys according to certain embodiments of the present
invention can include increased tensile strength and hardness of the alloys.
It will be appreciated by those skilled in the art that the As and Af of
nickel-titanium alloys can be generally adjusted by exposing the nickel-titanium alloy
to an elevated temperature for relatively short periods of time. For example, if the
alloy is exposed to a temperature sufficient to cause the formation of nickel-rich
precipitates, the transformation temperatures of the alloy will generally increase. In
contrast, if the alloy is exposed to a temperature sufficient to cause nickel-rich
precipitates to dissolve, (i.e., the nickel goes into solid solution in the TiNi phase), the
transformation temperature of the alloy will generally decrease.
However, it has been observed by the inventor that the extent of the
increase or decrease in the austenite transformation temperatures during thermal
processing will depend on several factors, including, but not limited to the initial As
and AfOf the alloy, the overall composition of the alloy, and the time and temperature
to which it is exposed. For example, referring now to Fig. 1, there is shown a plot of
austenite transformation temperature (As and Af) versus aging time at 675°C for two
nickel-titanium alloys, one containing 55 atomic percent nickel (represented by solid
circles and squares), and the other containing 52 atomic percent nickel (represented
by open circles and squares). As can be seen from the plot of Fig. 1, when these
alloys are aged for 2 hours, the As and Af for both alloys change substantially with
increased aging time. However, after about 24 hours of aging, the changes in the As
(represented in Fig. 1 by squares) and Af (represented in Fig. 1 by circles) for both
alloys with increased aging time are relatively small. For example, after 216 hours of
aging, the austenite transformation temperatures fluctuate only slightly from the
austenite transformation temperatures observed after 24 hours of aging. In. other
words, it appears that after aging these alloys at 675°C for about 24 hours, stable
austenite transformation temperatures (both Asand Af) are achieved. As used herein
the term "stable austenite transformation temperature" means the at least one of the
austenite start (As) or austenite finish (Af) temperatures of the nickel-titanium alloy

achieved after thermal processing deviates no more than 10°C upon thermally
processing the nickel-titanium alloy under the same conditions for an additional 8
hours.
For example, although not limiting herein, after aging the 55 atomic
percent nickel alloy ("55 at.% Ni") at 675°Cfor 24 hours, the nickel-titanium alloy has
an As of about -12°C, and the 52 atomic percent nickel alloy ("52 at.% Ni") has an As
of about -1 S°C. After aging the 55 at.% Ni alloy at 675°C for 24 hours, the nickel-
titanium alloy has an Af of about -9°C, and the 52 at.% Ni alloy has an Af of about
-14°C. When these alloys are aged for 216 hours at 675°C, neither the As nor the Af
of the individual alloys deviates more than 10°C from the As or Af of the alloys
observed after 24 hours aging. In this particular non-limiting example, the As and Af
of the individual alloys after aging for 216 hours at 675°C deviate less than about
5°C from the As and Af of the alloys observed after 24 hours aging at 675°C.
As discussed in more detail below, and while not intending to be bound
by any particular theory, it is believed by the inventor that variability in the As and Af
of the alloys after aging for 2 hours can be largely attributed to the inability to achieve
compositional equilibrium or near-equilibrium conditions within these alloys during
this relatively short duration thermal process. Thus, as can be seen from the plot of
Fig. 1, while non-equilibrium thermal processes can be used to generally increase
(or decrease) the austenite transformation temperature of an alloy, they are not
particularly useful in making predictable adjustments to the austenite transformation
temperature of an alloy in order to achieve a desired austenite transformation
temperature.
Referring again to Fig. 1, it can be seen that the austenite
transformation temperatures of the alloys are dependent upon composition when the
alloys are aged for less than about 24 hours. For example, after 2 hours aging at
675°C, the As of the 55 at.% Ni alloy is about 27°C higher than the As of the 52 at.%
Ni alloy; and the Af of the 55 at.% Ni alloy Is about 30°C higher than the Af of the 52
at.% Ni alloy. Even after 6 hours of aging at 675°C, the As of the 55 at.% Ni alloy is

about 19°C higher than the Asof the 52 at.% Ni alloy; while the Af of the 55 at.% Ni
alloy is about 21 °C higher than the Af of the 52 at.% Ni alloy. However, after about
24 hours of aging at 675°C, the difference between the As of the 55 at.% Ni alloy and
that of the 52 at.% Ni alloy decreases dramatically, as does the difference between
the Af for both the alloys. Although not limiting herein, in this particular example after
24 hours aging at 675°C, the difference between austenite start temperatures
between the two alloys is only about 6°C, whereas the difference between the
austenite finish temperatures between the two alloys is about 5°C.
Thus, it appears that the austenite transformation temperatajres_achieved
after aging these two alloys for about 24 hours at 675°C are independent of overall
composition of the alloys. As used herein, the term "independent of overall
composition" means at least one of the austenite start (As) or austenite finish (Af)
temperatures of a nickel-titanium alloy after thermal processing is within 10°C of any
other nickel-titanium alloy similarly processed and having sufficient nickel to reach
the solid solubility limit during thermal processing, as discussed below in more detail.
Consequently, as can be seen from the plot of Fig. 1, although
relatively short duration thermal processes can be used to make general shifts in the
austenite transformation temperatures of nickel-titanium alloys (i.e., generally
increase or decrease the austenite transformation temperatures), they are not
particularly useful in making predictable adjustments to the austenite transformation
temperatures of nickel-titanium alloys in order to achieve a desired austenite
transformation temperature that is independent of overall composition of the alloy.
As previously discussed, it is believed by the inventor that variability
associated with relatively short duration thermal processes can be largely attributed
to the non-equilibrium conditions achieved within the alloy during thermal processing.
However, the inventor has observed that predictable and stable transformation
temperatures, and in particular austenite transformation temperatures, can be
achieved by thermally processing nickel-titanium alloys to achieve a compositional
equilibrium or near-equilibrium condition within the alloy. More particularly, it has

been observed by the inventor that nickel-titanium alloys can be thermally processed
to achieve a stable austenite transformation temperature that is characteristic of the
temperature at which the material is thermally processed, provided the nickel-
titanium alioy has sufficient nickel to reach the solid solubility limit (discussed below)
of nickel in the TiNi phase at the thermal processing temperature. Although not
meaning to be bound by any particular theory or limit the present invention, it is
believed that the stable austenite transformation temperatures observed after
thermally processing the nickel-titanium alloys at a given temperature are
characteristic of an equilibrium or near-equilibrium amount of nickel in solid solution
in the TiNi phase at the thermal processing temperature.
Although not limiting herein, one skilled in the art will recognize that in
binary nickel-titanium alloys, the maximum amount of nickel that can exist in a stable
solid solution in the TiNi phase varies with temperature. In other words, the solid
solubility limit of nickel in the TiNi phase varies with temperature. As used herein,
the term "solid solubility limit" means the maximum amount of nickel that is retained
in the TiNi phase at a given temperature. In other words, the solid solubility limit is
the equilibrium amount of nickel that can exist in solid solution in the TiNi phase at a
given temperature. For example, although not limiting herein, as will be understood
by those skilled in the art, generally, the solid solubility limit of nickel in the TiNi
phase is given by the solvus line separating the TiNi and TiNi + TiNi3 phase fields in
a Ti-Ni equilibrium phase diagram. See ASM Materials Engineering Dictionary. J.R.
Davis, ed. ASM International, 1992 at page 432, which is hereby specifically
incorporated by reference. A non-limiting example of one Ti-Ni phase diagram is
shown in K. Otsuka and T. Kakeshia at page 96. However, alternative methods of
determining the solid solubility limit of nickel in the TiNi phase will be apparent to
those skilled in the art.
It will also be appreciated by those skilled in the art that if the amount
of nickel in the TiNi phase exceeds the solid solubility limit of nickel in the TiNi phase
(i.e., the TiNi phase is supersaturated with nickel) at a given temperature, nickel will
tend to precipitate out of solution to form one or more nickel-rich precipitates, thereby

relieving the supersaturation. However, because the diffusion rates in the Ti-Ni
system can be slow, the supersaturation is not instantaneously relieved. Instead, it
can take a substantial amount of time for equilibrium conditions in the alloy to be
reached. Conversely, if the amount of nickel in the TiNi phase is less than the solid
solubility limit at a given temperature, nickel will diffuse into the TiNi phase until the
solid solubility limit is reached. Again, it can take a substantial amount of time for
equilibrium conditions in the alloy to be reached.
Further, when nickel precipitates out of the TiNi phase to form nickel-
rich precipitates, both the hardness and the ultimate tensile strength of the alloy can
be increased due to the presence of the nickel-precipitates distributed throughout the
alloy. This increase in strength is commonly referred to as "age hardening" or
"precipitation hardening." See ASM Materials Engineering Dictionary at page 339.
As previously discussed, the transformation temperatures of a nickel-
titanium alloy are strongly influenced by the composition of the alloy. In particular, it
has been observed that the amount of nickel in solution in the TiNi phase of a nickel-
titanium alloy will strongly influence the transformation temperatures of the alloy. For
example, it has been observed that the Ms of a nickel-titanium alloy will generally
decrease with increasing amounts of nickel in solid solution in the TiNi phase of the
alloy; whereas the Ms of a nickel-titanium alloy will generally increase with
decreasing amounts of nickel in solid solution in the TiNi phase of the alloy. See
R.J. Wasilewski et al., "Homogenity Range and the Martensitic Transformation in
TiNi," Metallurgical Transactions. Vol. 2, January 1971 at pages 229-238.
However, although not meant to be bound by any particular theory, it is
believed by the inventor that when an equilibrium or near-equilibrium amount of
nickel exists in solid solution in the TiNi phase of a nickel-titanium alloy at a given
temperature, the alloy will have a stable austenite transformation temperature that is
characteristic of the given temperature, regardless of the overall composition of the
alloy. In other words, so long as sufficient nickel is present in the nickel-titanium
alloy to reach the solid solubility limit of nickel in the TiNi phase of the alloy at a given

thermal processing temperature, all nickel-titanium alloys should have essentially the
same austenite transformation temperature after thermally processing the alloys at a
particular thermal processing temperature to achieve a equilibrium or near-
equilibrium amount of nickel in solid solution in theTiNi phase of the alloys at the
thermal processing temperature. Therefore, the stable austenite transformation
temperature reached after thermally processing a nickel-titanium alloy is
characteristic of an equilibrium or near-equilibrium amount of nickel in solid solution
in the TiNi phase of the alloy at the particular thermal processing temperature.
Consequently, although not limiting herein, as the amount of nickel in
solid solution in the TiNi phase of a nickel-titanium alloy approaches the equilibrium
amount (i.e. the solid solubility limit) at a given temperature, the less the austenite
transformation temperature of the alloy should fluctuate with additional thermal
processing at that temperature. In other words, a stable austenite transformation
temperature that is characteristic of a compositional equilibrium or near-equilibrium
condition within the alloy will be observed.
It will also be appreciated by those skilled in the art that if, after thermal
processing, the alloy is cooled too slowly to room temperature, the equilibrium or
near-equilibrium conditions achieved during thermal processing can be lost.
Accordingly, it is generally desirable to cool the nickel-titanium alloys after thermal
processing sufficiently quickly retain the equilibrium or near-equilibrium conditions
achieved during thermal processing. For example, after thermal processing the
alloy, the alloy can be air cooled, liquid quenched, or air quenched.
Referring now to Fig. 2, there is shown a plot of stable austenite
transformation temperature versus aging temperature for two nickel-titanium alloys
containing varying amounts of nickel. The two nickel-titanium alloys were
isothermally aged at the indicated temperatures for about 24 hours in order to
achieve stable austenite transformation temperatures. As discussed above, the
stable transformation temperatures are characteristic of an equilibrium or near-

equilibrium amount of nickel in solid solution in the TiNi phase of the alloys at the
thermal processing temperature.
Further, as can be seen from the plot of Fig. 2, it is possible to
thermally process a nickel-titanium alloy to achieve a desired austenite
transformation temperature by selecting athermal processing temperature having
associated with it a stable austenite transformation temperature essentially equal to
the desired austenite transformation temperature, and then thermally processing the
nickel-titanium alloy atthattemperature to achieve the stable austenite
transformation temperature. Since the stable austenite transformation temperature
for a given thermal processing temperature can be readily determined (for example
by isothermal aging studies), it is possible to predictably adjust the As and Af of
nickel-titanium alloys by thermally processing the nickel-titanium alloys to achieve
compositional equilibrium or near-equilibrium conditions within the alloy.
Additionally, as long as the nickel content of the alloy is sufficient to reach the solid
solubility limit at the thermal processing temperature selected, the stable austenite
transformation temperature achieved will be independent of overall composition of
the alloy. As used herein with respect to transformation temperatures, the term
"essentially equal" means that the transformation temperatures are within 10°C or
less of each other. Therefore, although not required, transformation temperatures
that are essentially equal to each other can be equal to each other.
Various non-limiting embodiments of the present invention will now be
described. It will be understood by those skilled in the art that the methods
according to certain embodiments of the present invention can be utilized in
conjunction with a variety of nickel-titanium alloy systems, as well as other alloy
systems having properties sensitive to minor compositional variations; however, for
clarity, aspects of the present invention have been described with reference to binary
nickel-titanium alloy systems. Although not limiting herein, the methods according to
certain embodiments of the present invention are believed to be useful in processing
binary, ternary, and quaternary alloy systems comprising nickel and titanium in
conjunction with at least one other alloying element. For example, ternary nickel-

titanium alloy systems believed to be useful in various embodiments of the present
invention include, but are not limited to: nickel-titanium-hafnium; nickel-titanium-
copper; and nickel-titanium-iron alloy systems.
In one non-limiting embodiment of the present invention, a nickel-
titanium alloy comprising from greater than 50 up to 55 atomic percent nickel is
thermally processed to provide a desired austenite transformation temperature.
More particularly, according to this embodiment of the present invention, the method
comprises selecting a desired austenite transformation temperature, and thermally
processing the nickel-titanium alloy tp.adjust.an amount of nickel in solid solution in a
TiNi phase of the alloy such that a stable austenite transformation temperature,
which is essentially equal to the desired austenite transformation temperature, is
reached during thermal processing. Further, as discussed above, as long as the
amount of nickel present in the nickel-titanium alloy is sufficient to reach the solid
solubility limit at the thermal processing temperature, the austenite transformation
temperature achieved can be independent of overall composition of the alloy.
Additionally, although not required, according to this non-limiting embodiment, the
desired austenite transformation temperature can range from about -100°C to about
100°C.
Although not meant to be limiting herein, the effect of thermal
processing on the austenite transformation temperature of nickel-titanium alloys
comprising 50 atomic percent or less nickel is believed to be too small to be
commercially useful; whereas nickel-titanium alloys having greater than 55 atomic
percent nickel are believed to be too brittle for commercial processing. However,
those skilled in the art may recognize certain applications for which nickel-titanium
alloys comprising greater than 55 atomic percent nickel are desirable. In such
cases, alloys comprising greater than 55 atomic percent nickel may be utilized in
conjunction with the various embodiments of the present invention. Theoretically,
alloys comprising up to about 75 atomic percent nickel (i.e., within the TiNi + TiNi3
phase field) should be capable of processing according to the various embodiments
of the present invention; however, the time required to thermally process such high

nickel alloys, as well as the brittle nature of these high nickel alloys, renders them
not well suited for most commercial applications.
Another non-limiting embodiment of a method of processing a nickel-
titanium alloy to provide a desired austenite transformation temperature according to
the present invention comprises, selecting a nickel-titanium alloy comprising from
greaterthan 50 up to 55 atomic percent nickel, selecting a desired austenite
transformation temperature, and thermally processing the selected nickel-titanium
alloy to adjust an amount of nickel in solid solution in a TiNi phase of the alloy, such
that a stable austenite transformation temperature is reached during thermal
processing, the stable austenite transformation temperature being essentially equal
to the desired austenite transformation temperature. According to this non-limiting
embodiment, the selected nickel-titanium alloy comprises sufficient nickel to reach a
solid solubility limit during thermal processing. Further, according to this non-limiting
embodiment, the stable austenite transformation temperature can be independent of
overall composition of the alloy. Additionally, although not required, the desired
austenite transformation temperature according to this non-limiting embodiment can
range from about -100°C to about 100°C.
In another non-limiting embodiment of the present invention, two or
more nickel-titanium alloys having varying compositions and comprising from greater
than 50 up to 55 atomic percent nickel are processed such that the alloys have a
desired austenite transformation temperature. According to this non-limiting
embodiment, the method comprises selecting a desired austenite transformation
temperature, and subjecting the nickel-titanium alloys to similar thermal processing
such that after thermal processing, the nickel-titanium alloys have stable austenite
transformation temperatures that are essentially equal to the desired austenite
transformation temperature. As previously discussed, as long as the nickel-titanium
alloys have sufficient nickel to reach a solid solubility limit during thermal processing,
the stable austenite transformation temperature of the alloys will be independent of
overall composition of the alloys. Further, although not required, according to this
non-limiting embodiment, the desired austenite transformation temperature can

possess increased tensile strength and/or increased hardness as compared to the
alloys prior to thermal processing.
Suitable, non-limiting methods of thermally processing nickel-titanium
alloys according to the foregoing, non-limiting embodiments of the present invention
will now be discussed. Methods of thermally processing nickel-titanium alloys
according to the various embodiments of the present invention include, but are not
limited to, isothermal aging treatments, staged or stepped aging treatments, and
controlled cooling treatments. As used herein, the term "isothermal aging" means
holding the alloy in a furnace at a constant furnace temperature for a period of time.
However, it will be appreciated by those skilled in the art that, due to the equipment
limitations, minor fluctuations in furnace temperature can occur during isothermal
aging treatments.
For example, in certain embodiment of the present invention, thermally
processing the nickel-titanium alloy includes isothermally aging the nickel-titanium
alloy. As previously discussed, the temperature at which the nickel-titanium alloy is
thermally processed will depend upon the desired austenite transformation
temperature. Thus, for example, in certain non-limiting embodiments of the present
invention, wherein thermally processing the nickel-titanium alloy includes
isothermally aging the nickel-titanium alloy, the isothermal aging temperature can
range from 500°C to 800°C.
Although not limiting herein, it is believed that although isothermal
aging at temperatures below about 500°C can be utilized in accordance with various
embodiments of the present invention, the time required to achieve equilibrium or
near-equilibrium conditions at aging temperature below about 500°C is generally too
long to be useful for many commercial applications. Further, isothermal aging at
temperatures above about 800°C can be utilized in accordance with various
embodiments of the present invention; however, nickel-rich alloys aged at
temperatures above about 800°C tend to be too brittle to be useful in many
commercial applications. However, those skilled in the art may recognize

applications for which aging temperatures below about 500°C or above about 800°C
can be useful. Accordingly, embodiments of the present Invention contemplate
thermally processing nickel-titanium alloys at temperatures below about 500°C or
above about 800°C.
It will be appreciated by those skilled in the art that the duration of the
isothermal aging treatment required to achieve a stable austenite transformation
temperature will vary depending, in part, on the configuration (or cross-sectional
area) of the alloy (i.e, bars, wire, slabs, etc.), the aging temperature, as well as the
overall nickel content of the alloy. For example, although not limiting herein, where
super-fine nickel-titanium wire (i.e., wire with a diameter of less than about 0.03
inches) or nickel-titanium foil is thermally processed, isothermal aging times of at
least 2 hours can be utilized in accordance with embodiments of the present
invention. Where alloys with larger cross-sections are isothermally aged, aging time
can be greater than 2 hours, and may be least 24 hours or more. Similarly, if alloys
having smaller cross-sections are thermally processed, the isothermal aging time
can be less than 2 hours.
Further, where the overall composition of the nickel-titanium alloy is
very nickel-rich as compared to the solid solubility limit at the thermal processing
temperature and/or relatively low thermal processing temperature is employed to
achieve a desired austenite transformation temperature, the time required to achieve
a stable austenite transformation temperature can be longer than desired for some
commercial applications. However, it has been found by the inventors that the time
required to achieve a stable austenite transformation temperature in very nickel-rich
alloys and/or at low thermal processing temperatures can be reduced by employing
a staged thermal process as described below.
More specifically, according to certain embodiments of the present
invention, thermally processing the nickel-titanium alloy to achieve a stable austenite
transformation temperature that is essentially equal to the desired austenite
transformation temperature includes aging the nickel-titanium alloy at a first aging

temperature and subsequently aging the nickel-titanium alloy at a second aging
temperature, wherein the first aging temperature is higher than the second aging
temperature. According to this embodiment, the second aging temperature is
chosen so as to achieve the desired austenite transformation temperature as
described in detail above. That is, after aging at the second aging temperature, the
alloy will have a stable austenite transformation temperature that is essentially equal
to the desired transformation temperature, and characteristic of a compositional
equilibrium or near-equilibrium condition within the alloy at the second aging
temperature.
While not intending to be bound by any particular theory, a first aging
temperature that is higher than the second aging temperature, but below the solvus
temperature of the alloy, is selected to increase the initial diffusion rate of nickel
within the alloy. Thereafter, the desired austenite transformation temperature is
achieved by aging the nickel-titanium alloy at a second aging temperature having a
stable austenite transformation temperature essentially equal to the desired
transformation temperature. Although not required, after aging at the second aging
temperature, the nickel-titanium alloy can have an equilibrium amount of nickel in
solid solution in the TiNi phase.
Referring now to Fig. 3, there is shown a plot of austenite
transformation temperature versus aging time for two nickel-titanium alloys that were
aged using a two-stage aging process. Although not indicated on the plot, prior to
aging at 566°C, both alloys were aged for about 24 hours at 675°C to increase the
initial diffusion rate of nickel in the alloy. Thereafter, both alloys were aged at 566°C
as indicated by the plot of Fig. 3. As can be seen from the plot of Fig. 3, after about
72 hours, stable As and Af temperatures, which are also independent of overall
composition of the alloy, are achieved. In contrast, had the alloys been isothermally
aged in one-stage aging process (i.e., at 566°C only), aging times in excess of 72
hours would have been required to achieve stable transformation temperatures due
to the relatively low nickel diffusion at this temperature and relatively high nickel
content.

In one non-limiting example of a two-stage aging process according to
certain embodiments of the present invention, a nickel-titanium alloy is isothermally
aged at a first aging temperature ranging from 600°C to 800°C, and subsequently
aged at a lower second aging temperature ranging from 500°C to 600°C. Further,
although not required, the nickel-titanium alloy can be aged at the first aging
temperature for at least 2 hours and at the second aging temperature for at least 2
hours. As previously discussed, according to this embodiment, the stable austenite
transformation temperature is achieved during aging at the second aging
temperature.
It will also be appreciated by those skilled in the art that, as the excess
nickel content of the nickel-titanium alloy diminishes, the driving force for nucleation
of nickel-rich precipitates also diminishes. Further, if in order to achieve the desired
austenite transformation temperature, the alloy is to be thermally processed at a
temperature near the solvus temperature of the alloy, the driving force for and rate of
nucleation of the nickel-rich precipitates will be quite low during thermal processing.
Accordingly, the time required to achieve a stable austenite transformation
temperature that is essentially equal to the desired austenite transformation
temperature can be longer than desired for some commercial applications.
However, it has been found by the inventor that by employing a two-stage thermal
process, the time required to achieve the stable austenite transformation
temperature can be reduced. More specifically, according to certain embodiments of
the present invention, thermally processing the nickel-titanium alloy to achieve a
stable austenite transformation temperature essentially equal to the desired
austenite transformation temperature includes aging the nickel-titanium alloy at a first
aging temperature and subsequently aging the nickel-titanium alloy at a second
aging temperature, wherein the first aging temperature is lower than the second
aging temperature.
While not intending to be bound by any particular theory, one skilled in
the art will appreciate that the driving force for homogenous nucleation of nickel-rich
precipitates from a supersaturated TiNi phase can be increased by decreasing the

temperature of the alloy below the solvus temperature of the alloy, i.e, undercooling
below the solvus temperature of the alloy. Thus, by utilizing a first aging temperature
that is lower than the aging temperature needed to achieve the desired
transformation temperature, the rate of nucleation of the nickel-rich precipitates can
be increased. However, once the nuclei are generated at the first aging
temperature, growth oflhe precipitates by diffusion of the nickel will occur more
rapidly if the aging temperature is increased. Accordingly, after aging the nickel-
titanium alloy at the first aging temperature, the nickel-titanium alloy is aged at a
second aging temperature that is higher than the first aging temperature. More
particularly, the second aging temperature is chosen such that the stable austenite
transformation temperature reached during aging at the second aging temperature is
essentially equal to the desired austenite transformation temperature.
By employing a two-stage aging process using a first aging
temperature that is lower than the second aging temperature, it has been observed
that the total aging time required to achieve a stable austenite transformation
temperature essentially equal to a desired austenite transformation temperature can
be reduced. In one specific non-limiting example of a two-stage aging process
according to this embodiment of the present invention, a nickel-titanium alloy is
isothermally aged at a first aging temperature ranging from 500°C to 600CC, and
subsequently aged at a second aging temperature ranging from 6008C to 800°C.
Further, although not required, the nickel-titanium alloy can be aged at the first aging
temperature for at least 2 hours and at the second aging temperature for at least 2
hours. As previously discussed, according to this embodiment, the stable austenite
transformation temperature is achieved during aging at the second aging
temperature.
Methods of processing nickel-titanium alloys to achieve a desired
transformation temperature range will now be discussed. As previously discussed,
the utility of shape memory alloys depends upon the transformation temperatures of
the alloy, as well as the transformation temperature range. As used herein, the term
"transformation temperature range" means the difference between the start and

finish temperatures for a given phase transformation for a given alloy (i.e., Af- As or
Ms-Mf). As used herein, the term "austenite transformation temperature range"
means the difference between the As and Af temperature for a given alloy (i.e., Af-
As). Further, as used herein with respect to transformation temperature ranges, the
term "essentially equal" means that the transformation temperature ranges are within
10°C or less of each other. Therefore, although not required, transformation
temperature ranges that are essentially equal to each other can be equal to each
other.
Although not limiting herein, in some applications, a narrow austenite
transformation temperature range is desired. Generally a narrow austenite
transformation temperature range is desirable in applications that utilize the
superelastic properties of the nickel-titanium alloys, for example, but not limited to,
antenna wire and eyeglass frames. While in other applications, a broad austenite
transformation temperature range is desired. Generally a broad austenite
transformation temperature range is desirable in applications requiring different
degrees of transformation at different temperatures, for example, but not limited to,
temperature actuators.
Referring again to Fig. 1, as can be seen from the plot in this figures,
as the aging time increases, the austenite transformation temperature range for both
the 55 at.% Ni alloy and the 52 at.% Ni alloy decreases. For example, after aging
the 52 at.% Ni alloy for 2 hours at 675°C, the alloy has an austenite transformation
temperature range of about 18°C, and after 6 hours of aging, the austenite
transformation temperature range is about 11 °C. However, after 24 hours aging at
675°C, the 52 at.% Ni alloy has an austenite transformation temperature range of
less than about 5°C. Further, as aging time increases beyond 24 hours, this
austenite transformation temperature range does not change appreciably. Similarly,
after aging the 55 at.% Ni alloy for 2 hours at 675°C, the alloy has an austenite
transformation temperature range of about 21°C, and after 6 hours of aging, the
austenite transformation temperature range is about 13°C. However, after 24 hours
aging at 675°C, the 52 at.% Ni alloy has an austenite transformation temperature

range of less than about 5°C. Further, as aging time increases beyond 24 hours,
this austenite transformation temperature range does not change appreciably.
Referring now to Figs. 4-6, there are shown three, schematic
differential scanning calorimeter ("DSC") plots obtained for a nickel-titanium alloy
comprising 55 atomic percent nickel. The DSC plot in Fig. 4 was obtained from a 55
atomic percent nickel alloy that was isothermally aged at 6508Cfor 2 hours. The
DSC plot in Fig. 5 was obtained after isothermally aging the 55 atomic percent nickel
alloy at 650°C for 24 hours, and the DSC plot in Fig. 6 was obtained after
isothermally aging the 55 atomic percent nickel alloy at 650°C for 216 hours.
Referring to Fig. 4, the upper peak, generally indicated as 40,
represents the temperature range over which the martensitic transformation occurs
on cooling the alloy. For example, as generally indicated in Fig. 4, the martensitic
transformation starts at the Ms temperature, generally indicated as 42, and is
complete at the Mf temperature, generally indicated as 44, of the alloy. The lower
peak, generally indicated as 45, represents the temperature range over which the
austenitic transformation occurs on heating the alloy. For example, as indicated in
Fig. 4, the austenite transformation starts at the As temperature, generally indicated
as 47, and is complete at the Af temperature, generally indicated as 49, of the alloy.
As can been seen from the DSC plots in Figs. 4-6, both the martensite
and austenite transformation temperature ranges narrow with increasing aging time
at 650°C. Thus, for example, upper peak 50 (in Fig. 5) is sharper and more narrow
then upper peak 40 (in Fig. 4); and upper peak 60 (in Fig. 6) is sharper and more
narrow than both upper peak 40 and upper peak 50. Similarly, lower peak 55 (in Fig.
5) is sharper and more narrow then lower peak 45 (in Fig. 4); and lower peak 65 (in
Fig. 6) is sharper and more narrow than both lower peak 45 and lower peak 55.
As discussed above, along with the austenite transformation
temperature, controlling the austenite transformation temperature range to a narrow
interval is desirable in certain applications. Therefore, certain embodiments of the

present invention provide methods of processing' a nickel-titanium alloy comprising
from greater than 50 up to 55 atomic percent nickel to achieve a desired austenite
transformation temperature range. More specifically, the methods comprise
isothermally aging the nickel-titanium alloy in a furnace at a temperature ranging
from 500°C to 800°C for at least 2 hours, wherein after isothermally aging, the nickel-
titanium alloy has an austenite transformation temperature range no greater than
15°C. Although not required, according to this non-limiting embodiment, the aging
time can be at least 3 hours, at least 6 hours, and can be at least 24 hours
depending upon, among other things, the desired austenite transformation
temperature range. Further, according to this non-limiting embodiment, the austenite
transformation temperature range achieved after isothermal aging can be no greater
than 10°C, and can be no greater than 6"C, depending, in part, on the isothermal
aging conditions.
Further, as previously discussed, nickel-titanium alloys can become
compositionally segregated during solidification. Therefore, various embodiments of
the present invention also contemplate methods of processing nickel-titanium alloys
including regions of varying composition.comprising from greater than 50 up to 55
atomic percent nickel, such that each region has a desired austenite transformation
temperature range. According to these embodiments, the method comprises
isothermally aging the nickel-titanium alloy to adjust an amount of nickel in solid
solution in a TiNi phase in each region of the nickel-titanium alloy, wherein after
isothermally aging the nickel-titanium alloy, each of the regions of the nickel-titanium
alloy has an austenite transformation temperature range of no greater than 15°C.
Although not required, according to this non-limiting embodiment, the aging time can
be at least 2 hours, at least 3 hours, at least 6 hours, and at least 24 hours
depending upon, among other things, the desired austenite transformation
temperature range. Further, according to this non-limiting embodiment, the austenite
transformation temperature range achieved after isothermal aging can be no greater
than 10°C, and can be no greater than 6°C, depending, in part, on the isothermal
aging conditions.

As also discussed above, along with the austenite transformation
temperatures, controlling the austenite transformation temperature range to a broad
interval is desirable in certain applications. Accordingly, certain embodiments of the
present invention provide methods of processing a nickel-titanium alloy comprising
from greater than 50 up to 55 atomic percent nickel to achieve a desired austenite
transformation temperature and a desired transformation temperature range. More
specifically, the method comprises aging the nickel-titanium alloy in a furnace at a
first aging temperature to achieve a stable austenite transformation temperature, and
subsequently aging the nickel-titanium alloy at a second aging temperature that is
lower than the first aging temperature, wherein after aging the nickel-titanium alloy at
the second aging temperature, the nickel-titanium alloy has an austenite
transformation temperature range that is essentially equal to the desired austenite
transformation temperature range. Further, according to this non-limiting
embodiment, the transformation temperature range achieved on aging at the second
aging temperature is greater than an austenite transformation temperature achieved
on aging nickel-titanium alloy at a first aging temperature.
In another non-limiting embodiment of the present invention, the
method of processing the nickel-titanium alloy comprising from greater than 50 up to
55 atomic percent nickel to achieve a desired transformation temperature range
comprises aging the nickel-titanium alloy in a furnace at a first aging temperature to
achieve a stable austenite transformation temperature, and subsequently aging the
nickel-titanium alloy at a second aging temperature that is higher than the first aging
temperature, wherein after aging at the second aging temperature, the nickel-
titanium alloy has an austenite transformation temperature range that is essentially
equal to the desired austenite transformation temperature range. Further, according
to this non-limiting embodiment, the transformation temperature range achieved on
aging at the second aging temperature is greater than an austenite transformation
temperature achieved on aging nickel-titanium alloy at a first aging temperature.
Various embodiments of the present invention will now be illustrated by
the following, non-limiting examples.

EXAMPLES
Example 1
Two nickel-titanium alloys, one containing approximately 52 atomic
percent nickel and one containing approximately 55 atomic percent nickel, were
prepared as follows. The pure nickel and titanium alloying additions necessary for
each alloy were weighed and transferred to a vacuum arc remelting furnace. The
alloys were then melted and subsequently cast into a rectangular slab. After casting,
each nickel-titanium alloy was then hot worked to refine the grain structure.
Attempts were then made to measure the austenite transformation temperatures
(both As and Af) of the alloys prior to any aging treatments. However, because the
alloys were compositionally segregated, the austenite transformation temperatures
could not be determined. Thereafter, samples of each alloy were isothermally aged
in a furnace for the times and temperatures shown in Table 1.
After each aging time interval, the austenite transformation
temperatures for each alloy were determined using a bend free recovery test, which
was conducted as follows. An initially flat specimen to be tested was cooled to a
temperature approximately -196°C (i.e., below Ms of the alloy) by immersing the
specimen in liquid nitrogen. Thereafter, the specimen was deformed in to an
inverted "U" shape using a mandrel, which was also cooled by immersion in liquid
nitrogen. The diameter of the mandrel was selected according to the following
equation:

Where Dm is the mandrel diameter, T is the thickness of the specimen, and s is the
percent strain desired, here, three percent. Thereafter, the specimen having the
inverted "U" shape was placed directly under a linear variable differential transformer
("LVDT") probe in a bath of methanol and liquid nitrogen having a temperature
approximately 10°C below the suspected As of the alloy. The bath containing the
specimen and the LVDT probe were then and heated using a hot plate. As the
specimen warmed in the bath, it began to revert back to it original shape (i.e., flat)
once the temperature of the specimen reached the As temperature of the alloy. The
reversion to the initially flat shape was complete at the Af temperature of the alloy.

Data corresponding to relative displacement of the specimen was collected using the
LVDT probe as the specimen was warmed and the data was stored in a computer.
A graph of displacement versus temperature was then plotted and the As and Af
temperatures determined based on an approximation of the inflection points of the
curve. In particular, the intersection points of three linear regression-fit lines
corresponding to the three regions of the graph- i.e., the low temperature and high
temperature regions where the graph of displacement versus temperature has
relatively small slope, and the intermediate region where graph has a relatively large
slope- were used to approximate the As and Af temperatures of the specimen.

As can be seen from Table 1, by aging either of the alloys for 24 hours
stable austenite transformation temperatures (both As and Af) can be achieved, (i.e.
the As and Af of each of the alloys after 24 hours aging at 675°C does not deviate
more than 10°C upon thermally processing the nickel-titanium alloy under the same
conditions for an additional 8 hours.) Further, the stable austenite transformation
temperatures achieved after 24 hours aging at 675°C are also independent of

overall composition of the nickel-titanium alloy. That is, the As of the 55 at.% Ni alloy
is within 10°C of the As of the 52 at.% Ni alloy after thermally processing the alloys at
675°C for 24 hours; and the Af of the 55 at.% Ni alloy is within 10°C of the Af of the
52 at.% Ni alloy after thermally processing the alloys at 675°C for 24 hours. It is
believed that the decrease in As and Af observed after 72 hours aging at 675°C is not
representative and can be attributed to fluctuations in the furnace temperature during
aging.
In comparison, although it appears after aging the alloys for 6 hours at
675°C, the As and Af of the 52 at.% Ni alloy and the As of the 55 at.% Ni alloy are
stable, the austenite transformation temperatures are not independent of overall
composition. Further, after 2 hours aging at 675°C, the austenite transformation
temperature for both alloys are neither stable nor independent of overall
composition.
Stable austenite transformation temperatures (both As and Af) can also
be achieved for both alloys by aging the alloys for 24 hours at 650°C, (i.e. the As and
Af of each of the alloys after about 24 hours aging at 650°C does not deviate more
than 10°C upon thermally processing the nickel-titanium alloy under the same
conditions for an additional 8 hours.) Further, the stable austenite transformation
temperatures achieved after 24 hours aging at 650°C are also independent of overall
composition of the nickel-titanium alloy. That is, the As of the 55 at.% Ni alloy is
within 10°C of the As of the 52 at.% Ni alloy after thermally processing the alloys at
650°C for 24 hours; and the Af of the 55 at.% Ni alloy is within 10°C of the Af of the
52 at.% Ni alloy after thermally processing the alloys at 650°C for 24 hours.
In comparison, although it appears after aging the alloys for about 6
hours at 650°C that the Af of the 52 at.% Ni alloy and the As and Af of the 55 at.% Ni
alloy are stable, the austenite start temperatures are not independent of overall
composition. Further, after about 2 hours aging at 650°C, only the Af of the 55 at.%
Ni alloy appears to be stable, but neither the As nor the Af of the alloys is
independent of overall composition of the alloys.

Although not limiting herein, it is believed that the initial amount of
nickel in solid solution in the TiNi phase in the 55 at.% Ni alloy before aging was
closer to the solid solubility limit of nickel in the TiNi phase at 650°C than for the 52
at.% Ni alloy. Therefore, the aging time at 650°C required to achieve stable
austenite transformation temperatures for the 55 at.% nickel alloy was less than for
the 52 at.% Ni alloy. However, as indicated by Table 1, austenite transformation
temperatures that are both stable and independent of overall composition can be
achieved by aging the alloys for 24 hours at 650°C. Therefore, the same thermal
processing can be used for both alloys without regard to the initial condition of the
alloy.
Further, as indicated in Table 1, the stable austenite transformation
temperatures (As and Af) achieved after aging the nickel-titanium alloys for 24 hours
at 675°C are lower than the stable transformation temperatures achieved after aging
the nickel-titanium alloys for 24 hours at 650°C. Although not meant to be bound by
any particular theory, as previously discussed, this is believed to be attributable to
the different solid solubility limit for nickel in the TiNi phase at 675°C than at 650°C.
In other words, the characteristic austenite transformation temperatures for nickel-
titanium alloys having an equilibrium amount of nickel in solid solution in the TiNi
phase at 675°C are lower than the characteristic austenite transformation
temperatures for nickel-titanium alloys having an equilibrium amount of nickel in solid
solution in the TiNi phase at 650°C.
Moreover, as indicated in Table 1, the austenite transformation
temperature range generally tends to narrow with increasing aging time at a given
aging temperature for both alloys.
Example 2
Additional samples of the two alloys prepared according to Example 1
above were aged using the following two-stage aging process. The alloys were
aged at a first aging temperature of about 675°C for 24 hours and subsequently
aged at a second aging temperature as indicated below in Table 2. After each aging

time interval, the austenite transformation temperatures for each alloy were
determined using the bend free recover test described above in Example 1.

at a second aging temperature of 600°C, stable austenite transformation
temperatures (both As and Af) can be achieved, (i.e. the As and Af of each of the
alloys after 24 hours aging at 600°C does not deviate more than 10°C upon
thermally processing the nickel-titanium alloy under the same conditions for an
additional 8 hours.) Further, the stable austenite transformation temperatures
achieved after 24 hours aging at the second aging temperature of 600°C are also
independent of overall composition of the nickel-titanium alloy. That is, the As of the
55 at.% Ni alloy is within 10°C of the As of the 52 at.% Ni alloy after thermally
processing the alloys at a second aging temperature of 600°C for 24 hours; and the
Af of the 55 at.% Ni alloy is within 10°C of the Af of the 52 at.% Ni alloy after
thermally processing the alloys at a second aging temperature of 600°C for 24 hours.

In comparison, although it appears after aging the alloys for 6 hours at
a second aging temperature of 600°C, the Af of the 52 at.% Ni alloy and the As and Af
of the 55 at.% Ni alloy are stable, the austenite start temperatures are not
independent of overall composition. Further, after 2 hours aging at the second aging
temperature of 600°C, neither the As nor Af of the 52 at.% Ni alloy is stable and the
austenite start temperatures are not independent of overall composition.
Although not limiting herein, it is believed that the amount of nickel in
solid solution in the TiNi phase in the 55 at.% Ni alloy before aging at the second
aging temperature was closer to the solid solubility limit of nickel in the TiNi phase at
600°C than for the 52 at.% Ni alloy. Therefore, the aging time at 600°C required to
achieve stable austenite transformation temperatures for the 55 at.% nickel alloy was
less than for the 52 at.% Ni alloy. However, as indicated by Table 2, austenite
transformation temperatures that are both stable and independent of overall
composition can be achieved by aging the alloys for 24 hours at 600°C. Therefore,
the same thermal processing can be used for both alloys without regard to the initial
condition of the alloy.
As can be seen from Table 2, by aging either of the alloys for 72 hours
at a second aging temperature of 566°C, stable austenite transformation
temperatures (both As and Af) can be achieved, (i.e. the As and Af of each of the
alloys after 72 hours aging at 566°C does not deviate more than 10°C upon
thermally processing the nickel-titanium alloy under the same conditions for an
additional 8 hours.) Further, the stable austenite transformation temperatures
achieved after 72 hours aging at the second aging temperature 566°C are also
independent of overall composition of the nickel-titanium alloy. That is, the As of the
55 at.% Ni alloy is within 10°C of the As of the 52 at.% Ni alloy after thermally
processing the alloys at a second aging temperature of 566°C for 72 hours; and the
Af of the 55 at.% Ni alloy is within 10°C of the Af of the 52 at.% Ni alloy after
thermally processing the alloys at a second aging temperature of 566°C for 72 hours.

In comparison, although it appears after aging the alloys for 24 hours at
a second aging temperature of 566°C, the Af of the 52 at.% Ni alloy and the As and Af
of the 55 at.% Ni alloy are stable, the austenite start temperatures are not
independent of overall composition. Further, from 2 to 6 hours aging at the second
aging temperature of 566°C, the austenite transformation temperatures are neither
stable nor independent of overall composition.
Further, as indicated in Table 2, the stable austenite transformation
temperatures (As and Af) achieved after aging the nickel-titanium alloys for 24 hours
at 600°C are lower than the stable transformation temperatures achieved after aging
the nickel-titanium alloys for 24 hours at 566°C. Although not meant to be bound by
any particular theory, as previously discussed, this is believed to be attributable to
the different solid solubility limit for nickel in the TiNi phase at 600°C than at 566°C.
In other words, the characteristic austenite transformation temperatures for nickel-
titanium alloys having an equilibrium amount of nickel in solid solution in the TiNi
phase at 600°C are lower than the characteristic austenite transformation
temperatures for nickel-titanium alloys having an equilibrium amount of nickel in solid
solution in the TiNi phase at 566°C.
Moreover, as indicated in Table 2, the austenite transformation
temperature range generally tends to narrow with increasing aging time at a given
aging temperature for both alloys. As previously discussed with respect to austenite
transformation temperatures, the relatively small fluctuations in the austenite
transformation temperature range for the 55 at.% Ni alloy aged at 600°C is believed
to be attributable to the alloy having an amount of nickel in solid solution in the TiNi
phase that is close to the solid solubility limit before aging at 600°C.
It is to be understood that the present description illustrates aspects of
the invention relevant to a clear understanding of the invention. Certain aspects of
the invention that would be apparent to those of ordinary skill in the art and that,
therefore, would not facilitate a better understanding of the invention have not been
presented in order to simplify the present description. Although the present invention

has been described in connection with certain embodiments, those of ordinary skill
the art will, upon considering the foregoing description, recognize that many
modifications and variations of the invention may be employed. All such variations
and modifications of the invention are intended to be covered by the foregoing
description and the following claims.

WE CLAIM:
1. A method of processing a nickel-titanium alloy comprising 50 to 55 atomic
percent nickel to provide a pre-selected austenite transformation temperature of
-100°C to 100°C, the method comprising:
selecting the pre-selected austenite transformation temperature; and
thermally processing the nickel-titanium alloy by isothermally aging the nickel-
titanium alloy at a temperature of 500°C to 800°C for at least 2 hours to
adjust an amount of nickel in solid solution in a TiNi phase of the alloy
such that a stable austenite transformation temperature is reached during
thermally processing the nickel-titanium alloy,
wherein the stable austenite transformation temperature is essentially equal
to the pre-selected austenite transformation temperature,
wherein the nickel-titanium alloy comprises a nickel concentration that
reaches a solid solubility limit during thermally processing the nickel-titanium alloy.
2. The method of claim 1, wherein after thermally processing the nickel-titanium alloy,
the stable austenite transformation temperature of the nickel-titanium alloy is
independent of overall composition of the nickel-titanium alloy.
3. The method of claim 1, wherein thermally processing the nickel-titanium alloy
includes isothermally aging the nickel-titanium alloy for at least 24 hours.
4. The method of claim 1, wherein thermally processing the nickel-titanium alloy
includes aging the nickel-titanium alloy at a first aging temperature ranging from
600°C to 800°C and subsequently aging the nickel-titanium alloy at a second aging
y
temperature ranging from 500°C to 600oC, the first aging temperature being
greater than the second aging temperature.

5. The method of claim 4, wherein the nickel-titanium alloy reaches the stable
austenite transformation temperature during aging at the second aging
temperature.
6. The method of claim 1, wherein thermally processing the nickel-titanium alloy
includes aging the nickel-titanium alloy at a first aging temperature ranging from
500°C to 600°C and subsequently aging the nickel-titanium alloy at a second aging
temperature ranging from 600°C to 800°C, the first aging temperature being less
than the second aging temperature.
7. The method of claim 6, wherein the nickel-titanium alloy reaches the stable
austenite transformation temperature during aging at the second aging
temperature.
8. The method of claim 1, wherein the nickel-titanium is a binary nickel-titanium alloy.
9. The method of claim 1, wherein the nickel-titanium alloy optionally comprises at
least one additional alloying element.
10. The method of claim 1, wherein the nickel-titanium alloy optionally comprises at
least one additional alloying element selected from the group consisting of copper,
iron, and hafnium.
11. A method of processing a nickel-titanium alloy to provide a
pre-selected austenite transformation temperature of-100°C to 100°C, the method
comprising:
selecting a nickel-titanium alloy comprising 50 to 55 atomic percent nickel;
selecting the pre-selected austenite transformation temperature; and
thermally processing the selected nickel-titanium alloy by isothermally aging
the nickel-titanium alloy at a temperature of 500°C to 800°C for at least 2

hours to adjust an amount of nickel in solid solution in a TiNi phase of the
alloy such that a stable austenite transformation temperature is reached
during thermally processing the selected nickel-titanium alloy, the stable
austenite transformation temperature being essentially equal to the pre-
selected austenite transformation temperature; and
wherein the selected nickel-titanium alloy comprises a nickel concentration that
reaches a solid solubility limit during thermally processing the selected nickel-
titanium alloy.
12. The method of claim 11, wherein after thermally processing the nickel-titanium
alloy, the stable austenite transformation temperature of the nickel-titanium alloy is
independent of overall composition of the nickel-titanium alloy.
13. A method of processing two or more nickel-titanium alloys having varying
compositions comprising 50 to 55 atomic percent nickel to achieve a pre-selected
austenite transformation temperature of-100°C to 100°C, the method comprising:
selecting the pre-selected austenite transformation temperature; and
subjecting the nickel-titanium alloys to similar thermal processing such that
after thermal processing, the nickel-titanium alloys have stable austenite
transformation temperatures, the stable austenite transformation
temperatures being essentially equal to the pre-selected austenite
transformation temperature.
14. The method of claim 13 wherein the two or more nickel-titanium alloys comprise a
nickel concentration that reaches a solid solubility limit during thermal processing.
15. The method of claim 13, wherein thermal processing the two or more nickel-
titanium alloys includes isothermally aging the two or more nickel-titanium alloys.

16. The method of claim 13, wherein thermally processing the two or more nickel-
titanium alloys includes aging the two or more nickel-titanium alloys at a first aging
temperature ranging from 600°C to 800°C and subsequently aging the two or more
nickel-titanium alloys at a second aging temperature ranging from 500oC to 600°C,
the first aging temperature being greater than the second aging temperature.
17. The method of claim 16, wherein the two or more nickel-titanium alloys reach the
stable austenite transformation temperature during aging at the second aging
temperature.
18. The method of claim 13, wherein thermally processing the two or more nickel-
titanium alloys includes aging the two or more nickel-titanium alloys at a first aging
temperature ranging from 500°C to 600°C and subsequently aging the two or more
nickel-titanium alloys at a second aging temperature ranging from 600°C to 800°C,
the first aging temperature being less than the second aging temperature.
19. The method of claim 18, wherein the two or more nickel-titanium alloys reach the
stable austenite transformation temperature during aging at the second aging
temperature.
20. A method of processing a nickel-titanium alloy including regions of varying
composition comprising 50 to 55 atomic percent nickel such that each region has a
pre-selected austenite transformation temperature of
-100°C to 100°C, the method comprising:
thermally processing the nickel-titanium alloy by heating the alloy to adjust an
amount of nickel in solid solution in a TiNi phase in each region of the
nickel-titanium alloy,
wherein after thermally processing the nickel-titanium alloy, each of the regions of
the nickel-titanium alloy has a stable austenite transformation temperature that is

essentially equal to the pre-selected austenite transformation temperature, wherein
an austenite transformation temperature of the nickel-titanium alloy is a stable
austenite transformation temperature if at least one of the austenite start (As) or
austenite finish (Af) temperatures of the nickel-titanium alloy achieved after
thermally processing the nickel-titanium alloy deviates no more than 10°C upon
thermally processing the nickel-titanium alloy under the same conditions for an
additional 8 hours.
21. The method of claim 20, wherein thermally processing the nickel-titanium alloy
includes isothermally aging the nickel-titanium alloy.
22. The method of claim 20, wherein thermally processing the nickel-titanium alloy
includes aging the nickel-titanium alloy at a first aging temperature ranging from
600°C to 800°C and subsequently aging the nickel-titanium alloy at a second aging
temperature ranging from 500°C to 600°C, the first aging temperature being
greater than the second aging temperature.
23. The method of claim 22, wherein the nickel-titanium alloy reaches the stable
austenite transformation temperature during aging at the second aging
temperature.
24. The method of claim 20, wherein thermally processing the nickel-titanium alloy
includes aging the nickel-titanium alloy at a first aging temperature ranging from
500°C to 600°C and subsequently aging the nickel-titanium alloy at a second aging
temperature ranging from 600°C to 800°C, the first aging temperature being less
than the second aging temperature.

25. The method of claim 24, wherein the nickel-titanium alloy reaches the stable
austenite transformation temperature during aging at the second aging
temperature.
26. A method of processing a nickel-titanium alloy comprising 50 to 55 atomic percent
nickel to achieve a pre-selected austenite transformation temperature range of
-100°C to 100°C, the method comprising isothermally aging the nickel-titanium
alloy in a furnace at a temperature ranging from 500°C to 800°C for at least 2
hours, wherein after aging the nickel-titanium alloy has an austenite transformation
temperature of 6°C to 15°C.
27. The method of claim 26, wherein the nickel-titanium alloy is a binary nickel-titanium
alloy.
28. The method of claim 26, wherein the nickel-titanium alloy optionally comprises at
least one additional alloying element.
29. The method of claim 26, wherein the nickel-titanium alloy optionally comprises at
least one additional alloying element selected from the group consisting of copper,
iron, and hafnium.
30. A method of processing a nickel-titanium alloy including regions of varying
composition comprising 50 to 55 atomic percent nickel such that each region has a
pre-selected austenite transformation temperature range of-100°C to 100°C, the
method comprising:
isothermally aging the nickel-titanium alloy by heating the alloy to adjust an
amount of nickel in solid solution in a TiNi phase in each region of the
nickel-titanium alloy,

wherein after isothermally aging the nickel-titanium alloy, each of the regions of the
nickel-titanium alloy has an austenite transformation temperature of 6°C to 15°C.
31. A method of processing a nickel-titanium alloy comprising from 50 to 55 atomic
percent nickel to achieve a pre-selected austenite transformation temperature
range of-100°C to 100°C, the method comprising:
aging the nickel-titanium alloy in a furnace at a first aging temperature to
achieve a stable austenite transformation temperature; and
aging the nickel-titanium alloy at a second aging temperature that is different
than the first aging temperature, wherein after aging at the second aging
temperature, the nickel-titanium alloy has an austenite transformation
temperature range that is essentially equal to the pre-selected
transformation temperature range.
32. The method of claim 31, wherein the second aging temperature is less than the
first aging temperature.
33. The method of claim 31, wherein the second aging temperature is greater than the
first aging temperature.
34. The method of claim 31, wherein the austenite transformation temperature range
achieved after aging the nickel-titanium alloy at the second aging temperature is
greater than an austenite transformation temperature range achieved after aging
the nickel-titanium alloy at the first aging temperature.


The invention discloses a method of processing a nickel-titanium alloy comprising 50 to
55 atomic percent nickel to provide a pre-selected austenite transformation temperature
of -100°C to 100°C, the method comprising: selecting the pre-selected austenite
transformation temperature; and thermally processing the nickel-titanium alloy by
isothermally aging the nickel-titanium alloy at a temperature of 500°C to 800°C for at
least 2 hours to adjust an amount of nickel in solid solution in a TiNi phase of the alloy
such that a stable austenite transformation temperature is reached during thermally
processing the nickel-titanium alloy, wherein the stable austenite transformation
temperature is essentially equal to the pre-selected austenite transformation
temperature, wherein the nickel-titanium alloy comprises a nickel concentration that
reaches a solid solubility limit during thermally processing the nickel-titanium alloy.

Documents:

02031-kolnp-2005-abstract.pdf

02031-kolnp-2005-claims.pdf

02031-kolnp-2005-description complete.pdf

02031-kolnp-2005-drawings.pdf

02031-kolnp-2005-form 1.pdf

02031-kolnp-2005-form 3.pdf

02031-kolnp-2005-form 5.pdf

02031-kolnp-2005-international publication.pdf

2031-KOLNP-2005-(30-08-2011)-CORRESPONDENCE.pdf

2031-KOLNP-2005-(30-08-2011)-OTHERS.pdf

2031-KOLNP-2005-ABSTRACT 1.1.pdf

2031-kolnp-2005-abstract 1.2.pdf

2031-KOLNP-2005-AMANDED CLAIMS 1.1.pdf

2031-kolnp-2005-amanded pages of specification 1.2.pdf

2031-kolnp-2005-assignment.pdf

2031-kolnp-2005-assignment1.1.pdf

2031-kolnp-2005-claims 1.2.pdf

2031-kolnp-2005-correspondence 1.2.pdf

2031-kolnp-2005-correspondence.pdf

2031-kolnp-2005-correspondence1.3.pdf

2031-KOLNP-2005-DESCRIPTION (COMPLETE) 1.1.pdf

2031-kolnp-2005-description (complete) 1.2.pdf

2031-KOLNP-2005-DRAWINGS 1.1.pdf

2031-kolnp-2005-drawings 1.2.pdf

2031-kolnp-2005-examination report.pdf

2031-KOLNP-2005-FORM 1 1.1.pdf

2031-kolnp-2005-form 1 1.2.pdf

2031-kolnp-2005-form 18.1.pdf

2031-kolnp-2005-form 18.pdf

2031-kolnp-2005-form 2 1.2.pdf

2031-KOLNP-2005-FORM 2.pdf

2031-KOLNP-2005-FORM 3 1.1.pdf

2031-kolnp-2005-form 3 1.2.pdf

2031-kolnp-2005-form 3.pdf

2031-kolnp-2005-form 5.pdf

2031-KOLNP-2005-FORM-27.pdf

2031-kolnp-2005-gpa.pdf

2031-kolnp-2005-granted form 2.pdf

2031-kolnp-2005-granted-abstract.pdf

2031-kolnp-2005-granted-claims.pdf

2031-kolnp-2005-granted-description (complete).pdf

2031-kolnp-2005-granted-drawings.pdf

2031-kolnp-2005-granted-form 1.pdf

2031-kolnp-2005-granted-specification.pdf

2031-kolnp-2005-intenational publication.pdf

2031-kolnp-2005-international search report.pdf

2031-KOLNP-2005-OTHERS 1.1.pdf

2031-kolnp-2005-others 1.2.pdf

2031-kolnp-2005-others.pdf

2031-kolnp-2005-others1.3.pdf

2031-kolnp-2005-pct priority document notification.pdf

2031-kolnp-2005-pct request form.pdf

2031-kolnp-2005-petition under rule 137 1.2.pdf

2031-KOLNP-2005-PETITION UNDER RULE 137.pdf

2031-KOLNP-2005-REPLY TO EXAMINATION REPORT.pdf

2031-kolnp-2005-reply to examination report1.1.pdf

abstract-02031-kolnp-2005.jpg


Patent Number 248140
Indian Patent Application Number 2031/KOLNP/2005
PG Journal Number 25/2011
Publication Date 24-Jun-2011
Grant Date 21-Jun-2011
Date of Filing 17-Oct-2005
Name of Patentee ATI PROPERTIES, INC.
Applicant Address 1600 NE OLD SALEM ROAD, POST OFFICE BOX: 460, ALBANY, OR
Inventors:
# Inventor's Name Inventor's Address
1 WOJCIK, CRAIG 1124 35TH AVENUE N.W., SALEM, OR 97304
PCT International Classification Number C22C 14/00
PCT International Application Number PCT/US2004/010758
PCT International Filing date 2004-04-07
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 10/427,783 2003-05-01 U.S.A.