Method of manufacturing high-temperature shape memory alloys

A method of manufacturing a high-temperature shape memory alloy includes the steps of cold-working a high-temperature shape memory alloy, in which a reverse martensite transformation start temperature (As) in a first heating after cold working reaches 350.degree. C. or above. Thereafter, the cold-worked alloy undergoes a first heat treatment for a period of time within the incubation time required for recrystallization or less, and at a temperature higher than a reverse martensite transformation finish temperature (Af). Finally, the resultant alloy is annealed with a second heat treatment, at a temperature which is not less than the plastic strain recovery temperature and not more than the recrystallization temperature. Specifically, the first heat treatment is performed for a period of three minutes or less at a temperature which exceeds 500.degree. C. and which is lower than the melting point of the alloy. The composition of the high-temperature shape memory alloy is Ti.sub.50 Ni.sub.50-x Pd.sub.x (x being 35 to 50 at %), Ti.sub.50-x Ni.sub.50 Zr.sub.x (x being 22 to 30 at %), Ti.sub.50-x Ni.sub.50 Hf.sub.x (x being 20 to 30 at %) or the like.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of manufacturing high-temperature shape 
memory alloys, and more particularly, to a manufacturing method for 
substantially improving shape recovery characteristics of high-temperature 
shape memory alloys such as Ti--Pd--Ni, Ti--Ni--Zr and Ti--Ni--Hf alloys. 
2. Description of the Prior Art 
Ti--Ni alloys are well known as shape memory alloys and superelastic 
alloys. Shape recovery temperature (i.e., reverse martensite 
transformation finish temperature, which will hereafter be referred to as 
"Af temperature") can be varied in the range of approximately -100.degree. 
to +100.degree. C., depending on the ratio of Ti to Ni, by addition of a 
third element and by varying conditions of thermo-mechanical treatment or 
the like. 
In the shape memory treatment, these shape memory alloys are cold-worked 
and thereafter annealed at a temperature (approximately 400.degree. C. in 
general) which is not less than a plastic strain recovery temperature. The 
plastic strain recovery temperature corresponds to a temperature at which 
dislocations induced by cold working are rearranged. Since the plastic 
strain recovery temperature is higher than the Af temperature, the shape 
memory alloys are heated up to the Af temperature or above simultaneously 
with annealing for the shape memory treatment and then transformed to a 
parent phase state once to permit the memory of shape. 
It is important for the shape memory treatment to satisfy the following 
three conditions for obtaining satisfactory shape memory characteristics. 
1) Saturation of reorientation of martensite variants due to cold working 
should be settled. 2) Dislocations induced by cold working should be 
rearranged. 3) No recrystallization should be caused. 
The Af temperature (shape recovery temperature) of Ti--Ni shape memory 
alloys slightly exceeds 100.degree. C. at most. Thus, in order to obtain 
shape memory alloys having an Af temperature higher than 100.degree. C., 
i.e., high-temperature shape memory alloys, it is necessary to substitute 
different kinds of alloys such as Ti--Ni--Pd and Ti--Ni--Zr alloys for 
Ti--Ni alloys. 
The high-temperature shape memory alloys can be used for components 
operated by detection of the boiling of water, the overheating of oil and 
the melting of a polymer or the like, or for safety valves for cooling 
water in nuclear reactors. 
A large number of alloys such as Ti--Pd--X, Ti--Au--X (X.dbd.Ni, Cu, W, Ta, 
Co, Cr, Fe) and Ti--Ni--X (X.dbd.Zr, Hf) alloys are well known as 
high-temperature shape memory alloys, in which the Af temperature greatly 
exceeds 100.degree. C. These alloys can vary in reverse martensite 
transformation start temperature (hereafter referred to as "As 
temperature") or in Af temperature, depending on the kind of substituent 
element and the composition range thereof. The As or Af temperature may 
reach 500.degree. C. or above depending on the composition. 
In general, a difference between the As temperature and the Af temperature 
in an annealing state is not more than several multiples of ten degrees. 
However, when these alloys are cold-worked, the Af temperature in the 
first heating after cold working further rises by approximately 
150.degree. C. due to induction of strain or deformation and, therefore, 
the difference between the As temperature and the Af temperature widens. 
Thus, in case of alloys in which the As temperature is not less than 
350.degree. C., the Af temperature in the first heating after cold working 
reaches 500.degree. C. or above, exceeding recrystallization temperature. 
For instance, where the composition of a Ti--Ni--Pd alloy is Ti.sub.50 
Ni.sub.50-x Pd.sub.x (a numerical value represents at %, and the same 
shall apply hereafter), when x is 43 or more, the Af temperature in the 
annealing state reaches 500.degree. C. or more. Further, when x is 35 or 
more, the As temperature is not less than 350.degree. C., and the Af 
temperature in the first heating after cold working reaches 500.degree. C. 
or above. 
In case where the Ti--Ni--Zr alloy has a composition expressed as 
Ti.sub.50-x Ni.sub.50 Zr.sub.x, when x is 29 or more, the Af temperature 
in the annealing state reaches 500.degree. C. or above. 
When x is 22 or more, the As temperature is not less than 350.degree. C., 
and the Af temperature in the first heating after cold working reaches 
500.degree. C. or above. 
Further, in case where the Ti--Ni--Hf alloy has a composition expressed as 
Ti.sub.50-x Ni.sub.50 Hf.sub.x, when x is 27 or more, the Af temperature 
in the annealing state reaches 500.degree. C. or above. Further, when x is 
20 or more, the As temperature is not less than 350.degree. C., and the Af 
temperature in the first heating after cold working reaches 500.degree. C. 
or above. 
As described above, in case of the alloys in which the As temperature is 
not less than 350.degree. C., the Af temperature in the first heating 
after cold working reaches 500.degree. C. or above, exceeding 
recrystallization temperature. As a matter of course, in case of alloys in 
which the As temperature is not less than 500.degree. C. from the 
beginning, the Af temperature in the first heating after cold working is 
also not less than 500.degree. C. 
However, even if such alloys described above are cold-worked and thereafter 
annealed at 400.degree. C. for an hour, similar to the conventional Ti--Ni 
shape memory alloys, it is not possible to cause the memory of shape. 
On the other hand, when the above alloys are annealed at a temperature 
higher than the Af temperature in the first heating after cold working, it 
is possible to produce shape memory. However, since the recrystallization 
starts for the above alloys at such a high temperature, the shape recovery 
rate is reduced. 
For the reasons described above, the high-temperature shape memory alloys, 
in which the Af temperature in the first heating after cold working 
reaches a recrystallization temperature or above, have presented a problem 
in that a satisfactory shape recovery cannot be obtained. 
As a result of various studies of the above problems, the present inventors 
have developed a manufacturing method in which a high-temperature shape 
memory alloy exhibits an As temperature in the first heating after cold 
working of not less than 350.degree. C., and is imparted with shape memory 
and a satisfactory shape recovery rate. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a method of 
manufacturing a high-temperature shape memory alloy, comprising the steps 
of cold-working a high-temperature shape memory alloy, in which a reverse 
martensite transformation start temperature (As) in the first heating 
after cold working reaches 350.degree. C. or above, thereafter heating the 
cold-worked alloy in a first heat treatment for a period of time not 
exceeding the incubation time required for recrystallization and at a 
temperature higher than a reverse martensite transformation finish 
temperature (Af), and finally annealing the resultant alloy in a second 
heat treatment at a temperature which is not less than the plastic strain 
recovery temperature and not more than the recrystallization temperature. 
In a preferred embodiment of the present invention the first heat treatment 
is performed for a period of three minutes or less at a temperature which 
exceeds 500.degree. C. and which is less than the melting point of the 
alloy. 
In another preferred aspect of the present invention the composition of the 
high-temperature shape memory alloy is Ti.sub.50 Ni.sub.50-x Pd.sub.x, in 
which x is in the range of 35 to 50 at %, Ti.sub.50-x Ni.sub.50 Zr.sub.x, 
in which x is in the range of 22 to 30 at %, or Ti.sub.50-x Ni.sub.50 
Hf.sub.x, in which x is in the range of 20 to 30 at %. 
Hereafter will be described the present invention in detail. First of all, 
a general principle of shape memory treatment of shape memory alloys will 
be given as follows. 
Crystal dislocations are induced at high density by cold working. The 
resultant cold-worked alloy is then annealed for a proper period of time 
and at a proper temperature, higher than a plastic strain recovery 
temperature, to cause rearrangement of the dislocations. Since the 
rearranged dislocations offer resistance to slip, the critical stress for 
the slip is increased more than the critical stress for the rearrangement 
of martensite or for the appearance of stress-induced martensite. Thus, 
the martensite is rearranged or the stress-induced martensite appears 
without causing any slip at the time of deformation to produce 
satisfactory shape memory characteristics. 
On the other hand, when the annealing temperature is at the 
recrystallization temperature or above, not only are the dislocations 
rearranged, but also recrystallization is caused. Since a recrystallized 
portion has an extremely reduced density of dislocations, the resistance 
to the slip is reduced. Therefore, the critical stress for the slip is 
reduced more than the critical stress for the rearrangement of martensite, 
and the slip is easily caused, resulting in degradation of shape memory 
characteristics. 
In case of the conventional Ti--Ni shape memory alloys, since the Af 
temperature (-100.degree. to 100.degree. C.) is not more than the plastic 
strain recovery temperature (approximately 400.degree. C.), the 
transformation to a parent phase state occurs due to heating up to the 
plastic strain recovery temperature or above. Accordingly, the 
rearrangement of dislocations caused by cold working is attained. 
Therefore, the conventional Ti--Ni shape memory alloys permit the memory 
of shape, and have no problem. 
However, in case of Ti--Pd--X, Ti--Au--X, Ti--Ni--X or like shape memory 
alloys, in which the Af temperature is higher than the recrystallization 
temperature, when the annealing is performed at a temperature exceeding 
the Af temperature, recrystallization is caused to degrade the shape 
recovery characteristics. On the other hand, when the annealing is 
performed at a temperature less than the Af temperature, the above shape 
memory alloys retain the dislocations of martensite structure caused by 
cold working even after the heat treatment, and therefore, shape memory 
cannot be attained. 
According to the present invention, a high-temperature shape memory alloy, 
in which As temperature in the first heating after cold working reaches 
350.degree. C. or above, i.e., Ti--Pd--X, Ti--Au--X, Ti--Ni--X or like 
alloy described above, is cold-worked and thereafter heated as the first 
heat treatment for a period of time equal to the incubation time for 
recrystallization or less, at a temperature higher than the Af 
temperature. 
The crystal structure of the alloy is transformed to the parent phase by 
the first heat treatment. 
Once the crystal structure of the alloy is transformed to the parent phase, 
the dislocations in the martensite caused by cold working can be 
reoriented. 
The temperature in the heat treatment described above is set to be not less 
than the recrystallization temperature of the alloy. However, since the 
transformation to the parent phase is finished within the incubation time 
for recrystallization, the heat treatment for a short period of time is 
sufficient to heat to the Af temperature or above, and the start of 
recrystallization can be avoided. 
In other words, the first heat treatment of the present invention is 
performed at a temperature higher than both the Af temperature and the 
recrystallization temperature. However, since the heating time in the 
first heat treatment is as extremely short, i.e. equal to the incubation 
time for recrystallization or less, a shape memory alloy having a high 
shape recovery rate can be obtained without causing recrystallization. 
The temperature in the first heat treatment preferably exceeds 500.degree. 
C. and is less than the melting point of the alloy. When the temperature 
is less than 500.degree. C., the shape recovery rate is reduced. On the 
other hand, when the temperature exceeds the melting point, the alloy is 
melted. A temperature in the range of 500.degree. to 1000.degree. C. is 
preferable for practical use. 
The melting point of Ti--Au--Ni alloy is approximately in the range of 
1310.degree. to 1495.degree. C., the melting point of Ti--Ni--Pd alloy is 
approximately in the range of 1310.degree. to 400.degree. C., the melting 
point of Ti--Ni--Zr alloy is approximately in the range of 1260.degree. to 
1310.degree. C., and the melting point of Ti--Ni--Hf alloy is 
approximately in the range of 1310.degree. to 1530.degree. C. 
The recrystallization temperature of each of the above alloys is not less 
than 500.degree. C. 
The heating time in the first heat treatment is preferably set to be three 
minutes or less. When the heating time exceeds three minutes, 
recrystallization degrades the shape recovery characteristics. More 
preferably, the heating time is one minute or less. 
After the first heat treatment, the annealing is performed as the second 
heat treatment at a temperature which is not less than the plastic strain 
recovery temperature of the alloy and not more than the recrystallization 
temperature. The second heat treatment causes only the rearrangement of 
dislocations without recrystallization. Therefore, satisfactory shape 
memory effects can be obtained by the second heat treatment. 
The second heat treatment is preferably performed at a temperature of 
300.degree. to 500.degree. C. for 30 minutes to 2 hours. When the 
temperature is less than 300.degree. C., it is not possible to 
satisfactorily produce shape memory. On the other hand, when the 
temperature is not less than 500.degree. C., recrystallization is liable 
to occur. 
The high-temperature shape memory alloy to be manufactured according to the 
present invention corresponds to an alloy in which the As temperature in 
the first heating after cold working reaches 350.degree. C. or above, 
i.e., a shape memory alloy recovering at a temperature as high as 
350.degree. C. or above. At present, the Ti--Pd--X, Ti--Au--X (X.dbd.Ni, 
Cu, W, Ta, Co, Cr, Fe), and Ti--Ni--X (X.dbd.Zr, Hf) alloys described 
above are representative of such high-temperature shape memory alloys. In 
particular, the Ti--Pd--X and Ti--Ni--X alloys are of practical use. From 
the viewpoint of composition, alloys having the compositions respectively 
expressed as Ti50Ni50 XPdx, in which x is in the range of 35 to 50 at %, 
Ti50 Ni50Zrx, in which x is in the range of 22 to 30 at %, and Ti50 
XNi50Hfx, in which x is in the range of 20 to 30 at %, show satisfactory 
characteristics and are preferable for practical use. 
These high-temperature shape memory alloys can be manufactured according to 
a conventional method. For instance, a billet is manufactured by means of 
high frequency induction melting, plasma melting, powder metallurgy or the 
like. Subsequently, the billet thus manufactured is hot-worked by means of 
hot rolling, hot extrusion or the like, and then cold-worked by means of 
cold rolling, drawing or the like and thereby formed into a sheet, strip, 
rod, wire or like product. 
An ordinary heating furnace may be used in the heat treatment. High 
frequency heating, annealing by direct current or the like can be applied 
for the heat treatment. Also, air cooling, water quenching or the like can 
be properly used for cooling after annealing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
(Embodiment 1) 
An alloy having a composition expressed as Ti.sub.50 Ni.sub.50-x Pd.sub.x 
was used to prepare three samples varying in concentration of Pd such that 
x was 35, 40 and 50 at %, respectively. 30 g of each sample was melted by 
means of plasma melting and worked into a sheet 1.0 mm in thickness 
through hot rolling and cold rolling (cold-rolling work rate: 
approximately 25%). A tension test piece (of 16 mm in gauge length) was 
cut off from the sheet by means of electric discharge machining. The 
surface of each test piece was polished and, thereafter, each test piece 
was heat-treated at the various temperatures shown in Table 1. 
A test for shape recovery characteristics was given to each test piece. The 
results are shown in Table 1. 
With respect to test pieces retaining approximately 3% of apparent plastic 
strain resulting from the removal of stress after 4% of tensile strain has 
been applied to the test pieces at room temperature, the evaluation was 
made as follows. The above test pieces were heated up to the shape 
recovery test temperature shown in Table 1 to cause reverse 
transformation. The test pieces which showed an almost 100% shape recovery 
are represented by .largecircle. (i.e., the shape recovery rate was not 
less than 95%), the test pieces which showed hardly any recovery of shape 
are represented by X (i.e., the shape recovery was not more than 20%), and 
the test pieces intermediate between the test pieces represented by 
.largecircle. and X are represented by .DELTA.. 
In Table 1, the As temperature in the first heating represents a reverse 
martensite transformation start temperature after cold working. In this 
case, the As temperature was determined by thermal analysis. 
In the heat treatment temperatures, Tf represents the temperature in the 
first heat treatment, and the time the test pieces were held at Tf was one 
minute, while Ta represents the temperature in the second heat treatment, 
and the time the test pieces were held at Ta was one hour. 
TABLE 1 
__________________________________________________________________________ 
REVERSE 
TRANSFORMATION SHAPE RECOVERY 
Pd START HEAT CHARACTERISTICS 
CONCENTRATION TEMPERATURE IN 
TREATMENT SHAPE 
X FIRST HEATING 
TEMPERATURE 
RECOVERY RECOVERY 
NO. 
(at %) As (.degree.C.) 
Tf (.degree.C.) 
Ta (.degree.C.) 
TEST TEMP. (.degree.C.) 
RATE REMARKS 
__________________________________________________________________________ 
1 35 APPROX. 350 500 400 380 .largecircle. 
PRESENT 
INVENTION 
2 " " -- 400 " X COMATIVE 
EXAMPLE 
3 " " -- 500 " .DELTA. 
COMATIVE 
EXAMPLE 
4 " " -- 900 " .DELTA. 
COMATIVE 
EXAMPLE 
5 " " 600 400 " .largecircle. 
PRESENT 
INVENTION 
6 40 APPROX. 520 570 400 460 .largecircle. 
PRESENT 
INVENTION 
7 " " -- 400 " X COMATIVE 
EXAMPLE 
8 " " -- 900 " .DELTA. 
COMATIVE 
EXAMPLE 
9 " " 600 400 " .largecircle. 
PRESENT 
INVENTION 
10 50 APPROX. 670 730 400 620 .largecircle. 
PRESENT 
INVENTION 
11 " " -- 400 " X COMATIVE 
EXAMPLE 
12 " " -- 900 " .DELTA. 
COMATIVE 
EXAMPLE 
__________________________________________________________________________ 
As is apparent from Table 1, it was found that each of the test pieces Nos. 
1, 5, 6, 9 and 10 showed not less than 350.degree. C. in As temperature in 
the first heating after cold working and showed an almost 100% shape 
recovery. 
On the other hand, it was found that each of the test pieces Nos. 2, 3, 4, 
7, 8, 11 and 12 of the comparative examples hardly showed any recovery of 
shape, or was inferior in shape recovery, because the first heat treatment 
(Tf) was omitted. 
(Embodiment 2) 
With respect to the samples of 35 and 40, the at % in concentration of Pd, 
the temperatures (Tf, Ta) and time of heat treatment were varied as shown 
in Table 2 to prepare different samples. The shape recovery 
characteristics were examined as in embodiment 1, and the results are 
shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
SHAPE RECOVERY 
Pd HEAT CHARACTERISTICS 
CONCEN- TREATMENT HOLDING 
PRESENCE OF 
SHAPE 
TRATION TEMPERATURE 
TIME (min.) 
RECRYSTALLI- 
RECOVERY RECOVERY 
NO. 
X (at %) 
Tf (.degree.C.) 
Ta (.degree.C.) 
Tf Ta 
ZATION TEST TEMP. (.degree.C.) 
RATE REMARKS 
__________________________________________________________________________ 
1 35 500 400 1 60 
ABSENCE 380 .largecircle. 
PRESENT 
INVENTION 
2 " 600 400 2 60 
ABSENCE " .largecircle. 
PRESENT 
INVENTION 
3 " 600 400 10 60 
PRESENCE " .DELTA. 
COMATIVE 
EXAMPLE 
4 40 570 400 1 60 
ABSENCE 460 .largecircle. 
PRESENT 
INVENTION 
5 " 600 400 30 (sec.) 
60 
ABSENCE " .largecircle. 
PRESENT 
INVENTION 
6 " 600 400 10 60 
PRESENCE " .DELTA. 
COMATIVE 
EXAMPLE 
__________________________________________________________________________ 
As is apparent from Table 2, each of the test pieces Nos. 1, 2, 4 and 5 of 
the present invention shows satisfactory shape recovery characteristics 
without recrystallization. In this case, as long as the time the test 
pieces are held at Tf is within 2 minutes, the first heat treatment can be 
performed within the incubation time of recrystallization, even if Tf 
exceeds the recrystallization temperature. 
On the other hand, each of the test pieces Nos. 3 and 6 of the comparative 
examples underwent recrystallization and was inferior in shape recovery 
characteristics, because these test pieces were held at Tf for a longer 
period of time. 
(Embodiment 3) 
An alloy having a composition expressed as Ti.sub.50-x Ni.sub.50 Zr.sub.X 
was used to prepare two kinds of samples varying in concentration of Zr, 
with x being 22 and 30 at %, respectively. 3 Kg of each sample was melted 
by means of high frequency induction melting, and then subjected to 
casting, hot-extrusion and hot-rolling with a grooved roll. Subsequently, 
the resultant samples were repeatedly drawn with a die, annealed and 
worked into a wire of 1.0 mm in diameter (final cold working rate: 
approximately 30%). 140 mm of the rod was cut off, then linearly fixed in 
position and heat-treated at the various temperatures shown in Table 3. 
A test for shape recovery characteristics was given to each test piece. The 
results are shown in Table 3. 
A strain gauge of 50 mm in length between gauges was used for applying 
tensile strain. The evaluation method, the heat-treatment method and the 
symbols in Table 3 are similar to those in embodiment 1. 
TABLE 3 
__________________________________________________________________________ 
REVERSE 
TRANSFORMATION SHAPE RECOVERY 
Zr START HEAT CHARACTERISTICS 
CONCENTRATION TEMPERATURE IN 
TREATMENT SHAPE 
X FIRST HEATING 
TEMPERATURE 
RECOVERY RECOVERY 
NO. 
(at %) As (.degree.C.) 
Tf (.degree.C.) 
Ta (.degree.C.) 
TEST TEMP. (.degree.C.) 
RATE REMARKS 
__________________________________________________________________________ 
1 22 APPROX. 350 600 450 380 .largecircle. 
PRESENT 
INVENTION 
2 " " -- 400 " X COMATIVE 
EXAMPLE 
3 " " -- 600 " .DELTA. 
COMATIVE 
EXAMPLE 
4 30 APPROX. 500 700 400 530 .largecircle. 
PRESENT 
INVENTION 
5 " " -- 400 " X COMATIVE 
EXAMPLE 
6 " " -- 700 " .DELTA. 
COMATIVE 
EXAMPLE 
__________________________________________________________________________ 
As is apparent from Table 3, each of the test pieces Nos. 1 and 4 of the 
present invention showed not less than 350.degree. C. in As temperature in 
the first heating, and almost 100% shape recovery. On the other hand, each 
of the test pieces Nos. 2, 3, 5 and 6 of the comparative examples hardly 
showed any recovery of shape or was inferior in shape recovery, because 
the first heat treatment (Tf) was omitted. 
(Embodiment 4) 
With respect to the samples of 22 and 30, the at % in concentration of Zr, 
the temperatures (Tf, Ta) and time of heat treatment were varied as shown 
in Table 4 to prepare different samples. Then, the shape recovery 
characteristics were examined as in embodiment 3. The results are shown in 
Table 4. 
TABLE 4 
__________________________________________________________________________ 
SHAPE RECOVERY 
Zr HEAT CHARACTERISTICS 
CONCEN- TREATMENT HOLDING 
PRESENCE OF 
SHAPE 
TRATION TEMPERATURE 
TIME (min.) 
RECRYSTALLI- 
RECOVERY RECOVERY 
NO. 
X (at %) 
Tf (.degree.C.) 
Ta (.degree.C.) 
Tf Ta ZATION TEST TEMP. (.degree.C.) 
RATE REMARKS 
__________________________________________________________________________ 
1 22 600 400 1 60 ABSENCE 380 .largecircle. 
PRESENT 
INVENTION 
2 " 600 400 10 60 PRESENCE " .DELTA. 
COMATIVE 
EXAMPLE 
3 30 700 400 1 60 ABSENCE 530 .largecircle. 
PRESENT 
INVENTION 
4 " 700 400 10 60 PRESENCE " .DELTA. 
COMATIVE 
EXAMPLE 
__________________________________________________________________________ 
As is apparent from Table 4, each of the test pieces Nos. 1 and 3 of the 
present invention showed satisfactory shape recovery characteristics 
without recrystallization. In this case, as long as the test pieces were 
held at Af within one minute, the first heat treatment can be performed 
within the incubation time of recrystallization, even if Tf exceeds the 
recrystallization temperature. 
On the other hand, each of the test pieces Nos. 2 and 4 of the comparative 
examples underwent recrystallization and were inferior in shape recovery 
characteristics, because the test pieces were held at Tf for a longer 
period of time. 
(Embodiment 5) 
An alloy having a composition expressed as Ti.sub.50-x N.sub.50 Hf.sub.x 
was used to prepare two samples varying in concentration of Hf, with x at 
20 and 30 at %, respectively. 1 Kg of each sample was formed into a billet 
by means of powder metallurgy. Subsequently, the billet was subjected to 
hot isostatic pressing treatment, hot-extrusion and hot-rolling with a 
grooved roll. Thereafter, the rolled product was repeatedly drawn with a 
die, annealed and worked into a wire of 1.0 mm in diameter (final cold 
working rate: approximately 30%). 140 mm of the rod was cut off, then 
linearly fixed in position and heat-treated at the various temperatures 
shown in Table 5. A test for shape recovery characteristics was given to 
each test piece. The results are shown in Table 5. 
The testing method, the evaluation method, the heat-treatment method and 
the symbols in Table 5 are similar to those in embodiment 3. 
TABLE 5 
__________________________________________________________________________ 
REVERSE 
TRANSFORMATION SHAPE RECOVERY 
Hf START HEAT CHARACTERISTICS 
CONCENTRATION TEMPERATURE IN 
TREATMENT SHAPE 
X FIRST HEATING 
TEMPERATURE 
RECOVERY RECOVERY 
NO. 
(at %) As (.degree.C.) 
Tf (.degree.C.) 
Ta (.degree.C.) 
TEST TEMP. (.degree.C.) 
RATE REMARKS 
__________________________________________________________________________ 
1 20 APPROX. 350 600 400 390 .largecircle. 
PRESENT 
INVENTION 
2 " " -- 400 " X COMATIVE 
EXAMPLE 
3 " " -- 600 " .DELTA. 
COMATIVE 
EXAMPLE 
4 30 APPROX. 600 800 400 640 .largecircle. 
PRESENT 
INVENTION 
5 " " -- 400 " X COMATIVE 
EXAMPLE 
6 " " -- 800 " .DELTA. 
COMATIVE 
EXAMPLE 
__________________________________________________________________________ 
As is apparent from Table 5, each of the test pieces Nos. 1 and 4 of the 
present invention showed not less than 350.degree. C. in As temperature in 
the first heating, and showed almost 100% shape recovery. On the other 
hand, each of the test pieces Nos. 2, 3, 5 and 6 of the comparative 
examples hardly showed any recovery of shape or was inferior in shape 
recovery, because the first heat treatment (Tf) was omitted. 
(Embodiment 6) 
With respect to the samples of 20 and 30, the at % in Hf, the temperatures 
(Tf, Ta) and time of the heat treatment were varied as shown in Table 6 to 
prepare different samples. Then, the shape recovery characteristics were 
examined as in embodiment 5. The results are shown in Table 6. 
TABLE 6 
__________________________________________________________________________ 
SHAPE RECOVERY 
Hf HEAT CHARACTERISTICS 
CONCEN- TREATMENT HOLDING 
PRESENCE OF 
SHAPE 
TRATION TEMPERATURE 
TIME (min.) 
RECRYSTALLI- 
RECOVERY RECOVERY 
NO. 
X (at %) 
Tf (.degree.C.) 
Ta (.degree.C.) 
Tf Ta ZATION TEST TEMP. (.degree.C.) 
RATE REMARKS 
__________________________________________________________________________ 
1 20 600 400 1 60 ABSENCE 390 .largecircle. 
PRESENT 
INVENTION 
2 " 600 400 10 60 PRESENCE " .DELTA. 
COMATIVE 
EXAMPLE 
3 30 800 400 1 60 ABSENCE 640 .largecircle. 
PRESENT 
INVENTION 
4 " 800 400 10 60 PRESENCE " .DELTA. 
COMATIVE 
EXAMPLE 
__________________________________________________________________________ 
As is apparent from Table 6, each of the test pieces Nos. 1 and 3 of the 
present invention showed satisfactory shape recovery characteristics 
without recrystallization. In this case, as long as the time the test 
pieces were held at Tf was within one minute, the first heat treatment was 
performed within the incubation time of recrystallization, even where Tf 
exceeded the recrystallization temperature. 
On the other hand, each of the test pieces Nos. 2 and 4 of the comparative 
examples underwent recrystallization and was inferior in shape recovery 
characteristics, because the test pieces were held at Tf for a longer 
period of time. 
According to the present invention, it is possible to obtain a 
high-temperature shape memory alloy which is excellent in shape recovery 
characteristics. Thus, the high-temperature shape memory alloy of the 
present invention can be expected to be useful for components operating by 
detecting the boiling of water, the overheating of oil, and the melting of 
polymer or the like, or as safety valves for cooling water in nuclear 
reactors.