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Martensite ordering and stabilization in copper based shape memory alloys
Mhrials
Pegamon
Research Bulletin, Vol. 30. No. 6, pp. 755-760.1995 CopyriSk 0 1995 Elsevier Science Ltd F’riated in the USA. All ri&ts reserved 0025~5408/95 $9.50 + .OO
MARTRNSITJZORDERING AND STABILIZATION IN COPPER BASED SHAPE MEMORY ALLOYS
0. Adigiizel Department of Physics, Firat University, 23 169 E&g,
Turkey
(Received December 4,1994; Refereed)
ABSTRACT Stabilization behavior of the martensite in shape memory CuZnAl and CuAlMn alloys has been studied by x-ray measurements and electron microscopy. In these alloys infhrenced by both quenching and post-quench heat treatments, the degree of stabilization depends on quenching conditions. The stabilization process is mainly due to structural change and atomic arrangement in the martensitic lattice inherited from the parent phase. From x-ray results, it is suggested that stabilization and loss of memory are directly related to disordering in a martensitic state. In the present study, it was concluded that the spacing di&rences (Ad) between some selected pairs of diffraction planes oftype (h,k,l) and (h&l) satisfying the relation (hr2 - h2*)/3 = &* - k,2)/n, with n = 1 for 9R and n = 4 for 18R martensites reflect the degree of ordering in martensite, in addition to the lattice distortion parameters defined as @ = Isin2%,, - sin29,,,( and @’= ]sin2g,,, - sin20,,,]. MATERIALS INDEX copper, zinc, ahnninium, manganese, shape memory alloys.
Thermal treatments and their effects on the martensitic transformation and stabilization in binary and ternary copper-based shape memory alloys, have recently received considerable attention [l-6]. As pointed out in the recent studies [ 1,5,6], metastable phases of copperbased shape memory alloys are very sensitive to the aging effects, and heat treatments can change the relative stability of both martensite and parent phases. Due to the diffusionless character of martensitic transformations, product martensite inherits the state of order which existed in the original g-phase, and it seems reasonable to expect changes in long or short range order in the martensite during aging which increases the martensite stability [2]. Also 755
0.
756
Vol. 30, No. 6
ADIGI&EL
12%
00 18
M 1210
200
2oit
I
40
42
44 (a)_%2*
48
40
42 Ib) !&j
I
46
FIG. 1. X-ray powder diiEactograms of quenched a) CuZnAl and b) CuAlMn alloys. due to the stabilization of martensite, high-temperature application of the shape memory CuZnAl alloys are limited, and more stable and high transtkmation temperature can be obtained with the CuAlNi alloys [3]. It was reported that the most important observation concerning the stabilization of CuZnAl martensite is that the A,-shift becomes fasterand larger the higher the vacancy concentration in the martensite and the higher the stabilization [4]. The phenomena of the stabilization refxs to the transformation of a system towards its final equilibrium (i.e. a lowering of the free energy of the system) and this can be carried out in different steps [3]. All intermediate steps and heat treatments are regarded as stabilization. In other words, stabilization of martensitic alloys refers to the result of a process in which marten& undergoes no large scale changes in the order parameters and defect structure [3]. The martensite phase in copper based shape memory alloys inherits the atomic ordering and the antiphase domain boundaries. The stabilization process is mainly due to the atom arrangement of the martensitic lattice while the contribution of a pinning mechanism as is suggested for CuZnAl alloy and is small [7,8]. While ordering in the g-phase of these alloys after various heat treatments are ruled out as possible causes for the stabilization after aging in the g-condition, reordering in the martensite can also contribute to the stabilization after aging in the martensitic state [5]. In the present study, the martensite stabilization effect and ordering in the martensitic state in a CuZnAl and CuAlMn alloy given different heat treatments are described, and the derived results are compared and discussed. NTAL A CuZnAl alloy with a nominal composition of 69.9 wt./o Cu, 26.1 wtlo Zn, 4wt/o Al and a CuAlMn alloy with a nominal composition by weight of 83% Copper, 11% Aluminum, 6% Manganese were chosen for investigation. These alloys were supplied by Delta Materials Research Ltd. (Ipswich, England). All specimens obtained from these alloys were solution treated in the g-phase fields: (830 “C for the CuZnAl alloy and 700 “C for the CuAlMn alloy) and then quenched in iced brine to retain the D-phase and given the post-quench heat
Vol. 30, Nmo. 6
SHAPE MEMORY ALLOYS
757
(a) FIG. 2. Electron dif%action patterns taken from a) CuZnAl and b) CuAlMn alloy samples. treatments. In particular for the X-Ray Difhaction (XRD) measurements, the following postquench heat treatments were given on the powder specimens of alloys in the evacuated quartz capsules: CuZnAl:
CuAlMn:
a) Aging at room temperature after quenching from 830°C. b) Aging at room temperature after an intermediate anneal at 100°C for 30 minutes. a) Aging at room temperature after quenching Corn 700°C. b) Aging at 60°C and 100°C after quenching. c) Aging at room temperature after an intermediate anneal at 180 “C for 15 minutes.
X-ray powder diftractograms were taken from the above heat treated specimens at intervals during martensite aging using a nickel filtered Cu-Ku radiation at a scanning rate of 2”, 20hninute. Electron diffraction pattern were also obtained from the alloy samples mentioned above and the specimens were examined in a Jeol2OOCX electron microscope operated at 160kV aud 200kV. Specimen preparation are described elsewhere [9]. &lXILT!3
AND DISCUSSION
Two x-ray dif&action profiles taken fi-om the quenched CuZnAl and CuZnMn alloys, and are shown in figures 1 a and b, respectively. Two electron diffraction patterns taken from the alloys samples above mentioned are also shown in figure 2. In the x-ray diB?raction profiles, the limited 2 8 intervals of 40” to 48” are shown. The wide range profiles of 2 8 including also 040 and 320 peaks were given elsewhere [lO,l 11.Both x-ray dilfractograms and electron
758
0. ADIG~ZEL
Vol. 30, No. 6
24 J 15 -
23
1 131
aging at R.T. after step anneal a aging at 100°C after quench l
4’ eaging at R.T. after quench . aging at R.T. after step anneal
1
6.5'] 0
+
2
4
6
Aging time (x10* hours) (a)
4.2‘1 8 0
.
I
,
*
1
h . I
+
A&g
time4(x10’ (b)
hok)
FIG. 3. Variation of lattice distortion parameters, 4 and #, of a) CuZnAl and b) CuAlMn alloy samples versus the holding time at different aging temperatures.
diffraction patterns reveal that these alloys have the ordered structure in the martensitic condition inherited from the parent phase. Both alloys are in the martensitic state at the room temperature, and exhibit the order of parent the state. The lines observed in the x-ray powder difiractograms were identified as Ml 8R martensite superlattice reflections and indexed on the monoclinic base. Figure 3 also shows the lattice distortion parameters versus the holding time at the different anneal temperatures for the used alloys given different heat treatments defined as @= Isin* 9202- sin*B,,,] and $’ = ]sin* 9,,, - sin* t&J. As seen from these figures, + and #’ show a gradual decrease with holding time. $ and 4’ parameters are closely related to the spacing differences (Ad) between the selected pairs of diffraction planes types. These are described in the abstract section. The variation of Ad spacing differences against the holding time at room temperature for two quenched alloys is shown in figure 4. They also show a trend to decrease with holding time. This trend is closely related to the stabilization. The martensitic phase in the copper-based g-phase alloys is based on one of the (110) plane of parent phase. A (110) plane in the B-phase of DO, type ordered is rectangular, as shown in figure 5a, and it transforms to a hexagon with hexagonal distortion during the martensitic transformation. As shown in figure 5a, the order in the long range ordered g-matrix, can be described with reference to sublattices I, III and IV. Sublattice I is occupied mainly by Cu atoms, while sublattice III is occupied by Zn atoms and the remaining Cu atoms, and sublattice IV is also occupied with the most of Al atoms in CuZnAl alloys [ 1214]. CuAlMn alloys of g-phase also show the similar configuration and sublattices III and IV are occupied by Al and Mn atoms, respectively, and the remaining Cu atoms, while Cu atoms are dominant at sublattice I. Although parent B in copper based alloys is a stable phase, the martensite is not stable and can reduce its free energy if atoms are able to jump with sufEcient frequency [ 121. Iu the ordered case, the next nearest Cu-Al bonds are obtained by putting all Al atoms on sublattice IV, and a Cu atom on sublattice I is interchanged with a Zn atom on a neighboring sublattice III, in the case of Al atom on a neighboring IV side, after the martensitic transformation [ 121. Similar bonds are also obtained between Cu-Zn atoms in CuZnAl alloys and between Cu-Mn atoms in CuAlMn alloys. Stabilization also leads to this
Vol. 30, No. 6
759
SHAPEMEMORYALLOYS
(040-320)
3-
t5 (040-320) =
0
2
+
I Timc(xqlo2hoursp (a)
.
1 8
t 0
.
, . I 2 -+Ti&102hDUS~
,
.
8
0)
FIG. 4. The variation of Ad values of a) CuZnAl and b) uAlMn alloy samples against the holding time at room temperature [9]. interchange between the interrelated atoms. The g-phase orders at rather high temperatures depending on the alloy composition and transforms to the close packed structures with martensitic transformation. As stated in reference 13, the g-phase is not in the thermal equilibrium due to the quenching treatment prior to the martensitic transformation, and this verifies different vacancy concentration and existence of short range order in the long range ordered matrix [4]. Stabilization in the copper based shape memory alloys is closely related to the disordering in martensitic state, and Ad, spacing di%rences of some particular dfiaction plane pairs and lattice distortion parameters, # and 4’. reflect the degree of ordering and stabilization in martensite. As seen from figures 3 and 4, Ad and I#, v parameters gradually decrease with holding time at the aging temperatures. This decrease in these parameters leads to disordering in martensite. The martensite in the used alloys is progressively stabilized the longer it is
FIG. 5. Atomic con&uration on the (110) basal plane of DO, type ordered g-matrix. a) before and b) after hexagonal distortion, c) e%ct of atomic sizes.
760
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retained. The stabilization of the product phase is aided by quenching-in vacancies. This also requires that a structural change occurs during the aging in the martensitic condition and this gives rise to a change in the configurational order. The basal plane is not close packed in the ordered case due to the atomic size differences of the contributed elements both before and after hexagonal distortion with martensitic transformation as shown in figure 5c and disordering in martensite makes the basal plane nearly close packed taking the atomic sizes approximately equal. One can say that the aging of martensite causes the changes in the unit cell, too. CONCLUSIt can be concluded with the above results that the aging the alloys at the appropriate temperatures leads to the martensite stabilization. Stabilization and disordering in martensite increase with holding time, and Ad, 4 and 4’ parameters do not reach zero and specimens do not transform to fully disordered state with stabilization.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Arab, A.A. and M. Ahlers, Joum. de Phys., 1982, fi C4-709. Arab, A.A., M. Chandrasekaran, and M. Ahlers, Scripta Metal1.,1984, .& 709. J.V. Humbeeck, J. Janssen, N. Mwamba and L. Delaey, Scripta Metall., 1984, fi 893. A. Rapacioli and M. Ahlers, Acta Metall., 1979a 777. J. Janssen, J.V. Humbeeck, M. Chandrasekaran, N. Mwamba and L. Delaey, Joum. de Phys., 1982,43. c4-715. M.D. Graef, J.V.Humbeeck, MAndrade and L.Delaey, Scripta Metall., 1985, a 643. T.Y. Hsu, Proc. Int. Symposium on Shape Memory Alloys, 1986, Guilin, China, p.207. J.V. Humbeeck, L. Delaey and D. Roedolf, Proc. Int. Conf. on Martensitic Transf., 1986, Japan Inst. Metals, p. 862. 0. Adiguzel, L. Chandrasekaran and A.P. Miodownik, The Martensitic Transformation in Science and Tech. (E. Hombogen, N. Jest, ed.), Informationsgesellschaft Verlag, 1989, p. 109. 0. Adiguzel, Turkish Journal of Physics, 1989, & 17 1. 0. Adiguzel, Commun. Fat. Sci. Univ. Ank. Series A,, A,, 1987, & 47. AA. Arab, M. Chandrasekaran and M. Ahlers, Scripta Metall., 1984,18,1125. A.A. Arab and M. Ahlers, Acta Metall., 1988, & 2627. R. Rapacioli and M. Ahlers, ScriptaMetall., 1977,L 1147. W. Mingpu and C. Mingsheng, Proo. Int. Symposium on Shape Memory Alloys, 1986, Guilin, China, p. 255. F.E. Fujita, Proc.Int.Conf. on Martensitic Transf., 1979, Cambridge, USA, p. 106.
Pegamon
Research Bulletin, Vol. 30. No. 6, pp. 755-760.1995 CopyriSk 0 1995 Elsevier Science Ltd F’riated in the USA. All ri&ts reserved 0025~5408/95 $9.50 + .OO
MARTRNSITJZORDERING AND STABILIZATION IN COPPER BASED SHAPE MEMORY ALLOYS
0. Adigiizel Department of Physics, Firat University, 23 169 E&g,
Turkey
(Received December 4,1994; Refereed)
ABSTRACT Stabilization behavior of the martensite in shape memory CuZnAl and CuAlMn alloys has been studied by x-ray measurements and electron microscopy. In these alloys infhrenced by both quenching and post-quench heat treatments, the degree of stabilization depends on quenching conditions. The stabilization process is mainly due to structural change and atomic arrangement in the martensitic lattice inherited from the parent phase. From x-ray results, it is suggested that stabilization and loss of memory are directly related to disordering in a martensitic state. In the present study, it was concluded that the spacing di&rences (Ad) between some selected pairs of diffraction planes oftype (h,k,l) and (h&l) satisfying the relation (hr2 - h2*)/3 = &* - k,2)/n, with n = 1 for 9R and n = 4 for 18R martensites reflect the degree of ordering in martensite, in addition to the lattice distortion parameters defined as @ = Isin2%,, - sin29,,,( and @’= ]sin2g,,, - sin20,,,]. MATERIALS INDEX copper, zinc, ahnninium, manganese, shape memory alloys.
Thermal treatments and their effects on the martensitic transformation and stabilization in binary and ternary copper-based shape memory alloys, have recently received considerable attention [l-6]. As pointed out in the recent studies [ 1,5,6], metastable phases of copperbased shape memory alloys are very sensitive to the aging effects, and heat treatments can change the relative stability of both martensite and parent phases. Due to the diffusionless character of martensitic transformations, product martensite inherits the state of order which existed in the original g-phase, and it seems reasonable to expect changes in long or short range order in the martensite during aging which increases the martensite stability [2]. Also 755
0.
756
Vol. 30, No. 6
ADIGI&EL
12%
00 18
M 1210
200
2oit
I
40
42
44 (a)_%2*
48
40
42 Ib) !&j
I
46
FIG. 1. X-ray powder diiEactograms of quenched a) CuZnAl and b) CuAlMn alloys. due to the stabilization of martensite, high-temperature application of the shape memory CuZnAl alloys are limited, and more stable and high transtkmation temperature can be obtained with the CuAlNi alloys [3]. It was reported that the most important observation concerning the stabilization of CuZnAl martensite is that the A,-shift becomes fasterand larger the higher the vacancy concentration in the martensite and the higher the stabilization [4]. The phenomena of the stabilization refxs to the transformation of a system towards its final equilibrium (i.e. a lowering of the free energy of the system) and this can be carried out in different steps [3]. All intermediate steps and heat treatments are regarded as stabilization. In other words, stabilization of martensitic alloys refers to the result of a process in which marten& undergoes no large scale changes in the order parameters and defect structure [3]. The martensite phase in copper based shape memory alloys inherits the atomic ordering and the antiphase domain boundaries. The stabilization process is mainly due to the atom arrangement of the martensitic lattice while the contribution of a pinning mechanism as is suggested for CuZnAl alloy and is small [7,8]. While ordering in the g-phase of these alloys after various heat treatments are ruled out as possible causes for the stabilization after aging in the g-condition, reordering in the martensite can also contribute to the stabilization after aging in the martensitic state [5]. In the present study, the martensite stabilization effect and ordering in the martensitic state in a CuZnAl and CuAlMn alloy given different heat treatments are described, and the derived results are compared and discussed. NTAL A CuZnAl alloy with a nominal composition of 69.9 wt./o Cu, 26.1 wtlo Zn, 4wt/o Al and a CuAlMn alloy with a nominal composition by weight of 83% Copper, 11% Aluminum, 6% Manganese were chosen for investigation. These alloys were supplied by Delta Materials Research Ltd. (Ipswich, England). All specimens obtained from these alloys were solution treated in the g-phase fields: (830 “C for the CuZnAl alloy and 700 “C for the CuAlMn alloy) and then quenched in iced brine to retain the D-phase and given the post-quench heat
Vol. 30, Nmo. 6
SHAPE MEMORY ALLOYS
757
(a) FIG. 2. Electron dif%action patterns taken from a) CuZnAl and b) CuAlMn alloy samples. treatments. In particular for the X-Ray Difhaction (XRD) measurements, the following postquench heat treatments were given on the powder specimens of alloys in the evacuated quartz capsules: CuZnAl:
CuAlMn:
a) Aging at room temperature after quenching from 830°C. b) Aging at room temperature after an intermediate anneal at 100°C for 30 minutes. a) Aging at room temperature after quenching Corn 700°C. b) Aging at 60°C and 100°C after quenching. c) Aging at room temperature after an intermediate anneal at 180 “C for 15 minutes.
X-ray powder diftractograms were taken from the above heat treated specimens at intervals during martensite aging using a nickel filtered Cu-Ku radiation at a scanning rate of 2”, 20hninute. Electron diffraction pattern were also obtained from the alloy samples mentioned above and the specimens were examined in a Jeol2OOCX electron microscope operated at 160kV aud 200kV. Specimen preparation are described elsewhere [9]. &lXILT!3
AND DISCUSSION
Two x-ray dif&action profiles taken fi-om the quenched CuZnAl and CuZnMn alloys, and are shown in figures 1 a and b, respectively. Two electron diffraction patterns taken from the alloys samples above mentioned are also shown in figure 2. In the x-ray diB?raction profiles, the limited 2 8 intervals of 40” to 48” are shown. The wide range profiles of 2 8 including also 040 and 320 peaks were given elsewhere [lO,l 11.Both x-ray dilfractograms and electron
758
0. ADIG~ZEL
Vol. 30, No. 6
24 J 15 -
23
1 131
aging at R.T. after step anneal a aging at 100°C after quench l
4’ eaging at R.T. after quench . aging at R.T. after step anneal
1
6.5'] 0
+
2
4
6
Aging time (x10* hours) (a)
4.2‘1 8 0
.
I
,
*
1
h . I
+
A&g
time4(x10’ (b)
hok)
FIG. 3. Variation of lattice distortion parameters, 4 and #, of a) CuZnAl and b) CuAlMn alloy samples versus the holding time at different aging temperatures.
diffraction patterns reveal that these alloys have the ordered structure in the martensitic condition inherited from the parent phase. Both alloys are in the martensitic state at the room temperature, and exhibit the order of parent the state. The lines observed in the x-ray powder difiractograms were identified as Ml 8R martensite superlattice reflections and indexed on the monoclinic base. Figure 3 also shows the lattice distortion parameters versus the holding time at the different anneal temperatures for the used alloys given different heat treatments defined as @= Isin* 9202- sin*B,,,] and $’ = ]sin* 9,,, - sin* t&J. As seen from these figures, + and #’ show a gradual decrease with holding time. $ and 4’ parameters are closely related to the spacing differences (Ad) between the selected pairs of diffraction planes types. These are described in the abstract section. The variation of Ad spacing differences against the holding time at room temperature for two quenched alloys is shown in figure 4. They also show a trend to decrease with holding time. This trend is closely related to the stabilization. The martensitic phase in the copper-based g-phase alloys is based on one of the (110) plane of parent phase. A (110) plane in the B-phase of DO, type ordered is rectangular, as shown in figure 5a, and it transforms to a hexagon with hexagonal distortion during the martensitic transformation. As shown in figure 5a, the order in the long range ordered g-matrix, can be described with reference to sublattices I, III and IV. Sublattice I is occupied mainly by Cu atoms, while sublattice III is occupied by Zn atoms and the remaining Cu atoms, and sublattice IV is also occupied with the most of Al atoms in CuZnAl alloys [ 1214]. CuAlMn alloys of g-phase also show the similar configuration and sublattices III and IV are occupied by Al and Mn atoms, respectively, and the remaining Cu atoms, while Cu atoms are dominant at sublattice I. Although parent B in copper based alloys is a stable phase, the martensite is not stable and can reduce its free energy if atoms are able to jump with sufEcient frequency [ 121. Iu the ordered case, the next nearest Cu-Al bonds are obtained by putting all Al atoms on sublattice IV, and a Cu atom on sublattice I is interchanged with a Zn atom on a neighboring sublattice III, in the case of Al atom on a neighboring IV side, after the martensitic transformation [ 121. Similar bonds are also obtained between Cu-Zn atoms in CuZnAl alloys and between Cu-Mn atoms in CuAlMn alloys. Stabilization also leads to this
Vol. 30, No. 6
759
SHAPEMEMORYALLOYS
(040-320)
3-
t5 (040-320) =
0
2
+
I Timc(xqlo2hoursp (a)
.
1 8
t 0
.
, . I 2 -+Ti&102hDUS~
,
.
8
0)
FIG. 4. The variation of Ad values of a) CuZnAl and b) uAlMn alloy samples against the holding time at room temperature [9]. interchange between the interrelated atoms. The g-phase orders at rather high temperatures depending on the alloy composition and transforms to the close packed structures with martensitic transformation. As stated in reference 13, the g-phase is not in the thermal equilibrium due to the quenching treatment prior to the martensitic transformation, and this verifies different vacancy concentration and existence of short range order in the long range ordered matrix [4]. Stabilization in the copper based shape memory alloys is closely related to the disordering in martensitic state, and Ad, spacing di%rences of some particular dfiaction plane pairs and lattice distortion parameters, # and 4’. reflect the degree of ordering and stabilization in martensite. As seen from figures 3 and 4, Ad and I#, v parameters gradually decrease with holding time at the aging temperatures. This decrease in these parameters leads to disordering in martensite. The martensite in the used alloys is progressively stabilized the longer it is
FIG. 5. Atomic con&uration on the (110) basal plane of DO, type ordered g-matrix. a) before and b) after hexagonal distortion, c) e%ct of atomic sizes.
760
0.
ADIGUZEL
Vol. 30, No. 6
retained. The stabilization of the product phase is aided by quenching-in vacancies. This also requires that a structural change occurs during the aging in the martensitic condition and this gives rise to a change in the configurational order. The basal plane is not close packed in the ordered case due to the atomic size differences of the contributed elements both before and after hexagonal distortion with martensitic transformation as shown in figure 5c and disordering in martensite makes the basal plane nearly close packed taking the atomic sizes approximately equal. One can say that the aging of martensite causes the changes in the unit cell, too. CONCLUSIt can be concluded with the above results that the aging the alloys at the appropriate temperatures leads to the martensite stabilization. Stabilization and disordering in martensite increase with holding time, and Ad, 4 and 4’ parameters do not reach zero and specimens do not transform to fully disordered state with stabilization.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Arab, A.A. and M. Ahlers, Joum. de Phys., 1982, fi C4-709. Arab, A.A., M. Chandrasekaran, and M. Ahlers, Scripta Metal1.,1984, .& 709. J.V. Humbeeck, J. Janssen, N. Mwamba and L. Delaey, Scripta Metall., 1984, fi 893. A. Rapacioli and M. Ahlers, Acta Metall., 1979a 777. J. Janssen, J.V. Humbeeck, M. Chandrasekaran, N. Mwamba and L. Delaey, Joum. de Phys., 1982,43. c4-715. M.D. Graef, J.V.Humbeeck, MAndrade and L.Delaey, Scripta Metall., 1985, a 643. T.Y. Hsu, Proc. Int. Symposium on Shape Memory Alloys, 1986, Guilin, China, p.207. J.V. Humbeeck, L. Delaey and D. Roedolf, Proc. Int. Conf. on Martensitic Transf., 1986, Japan Inst. Metals, p. 862. 0. Adiguzel, L. Chandrasekaran and A.P. Miodownik, The Martensitic Transformation in Science and Tech. (E. Hombogen, N. Jest, ed.), Informationsgesellschaft Verlag, 1989, p. 109. 0. Adiguzel, Turkish Journal of Physics, 1989, & 17 1. 0. Adiguzel, Commun. Fat. Sci. Univ. Ank. Series A,, A,, 1987, & 47. AA. Arab, M. Chandrasekaran and M. Ahlers, Scripta Metall., 1984,18,1125. A.A. Arab and M. Ahlers, Acta Metall., 1988, & 2627. R. Rapacioli and M. Ahlers, ScriptaMetall., 1977,L 1147. W. Mingpu and C. Mingsheng, Proo. Int. Symposium on Shape Memory Alloys, 1986, Guilin, China, p. 255. F.E. Fujita, Proc.Int.Conf. on Martensitic Transf., 1979, Cambridge, USA, p. 106.