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Transformation behaviour and martensite stabilization in the ferromagnetic Co–Ni–Ga Heusler alloy
Scripta Materialia 50 (2004) 225–229 www.actamat-journals.com
Transformation behaviour and martensite stabilization in the ferromagnetic Co–Ni–Ga Heusler alloy V.A. Chernenko a, J. Pons b
b,*
, E. Cesari b, I.K. Zasimchuk
c
a Institute of Magnetism, Vernadsky str. 36-b, Kiev 03142, Ukraine Universitat de les Illes Balears, Departament de Fısica, E-07122 Palma de Mallorca, Spain c Institute of Metal Physics, Vernadsky str. 36, Kiev 03142, Ukraine
Accepted 4 September 2003
Abstract Some features of the martensitic transformation of a ferromagnetic Co49:0 Ni22:0 Ga29:0 single crystal have been clarified, including an effect of strain-induced stabilization of martensite. Microstructural observations support the origin of such effect, already introduced in non-ferromagnetic alloys, being related to difficulties in the formation of a habit plane between detwinned martensite and austenite. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Martensitic phase transformation; Strain-induced stabilization of martensite; Compression tests; Heusler phases
1. Introduction Recent years have witnessed a steady increase of interest in the ferromagnetic thermoelastic martensites, such as those exhibited by Ni–Mn–Ga Heusler alloys, after it was demonstrated that a large strain can be generated by the magnetic field-induced twin rearrangement (see, e.g. [1,2]). In addition, some other shape memory ferromagnetic systems, namely Fe–Pd [3], Fe3 Pt [4], Co–Ni–Al [5,6], Ni–Fe–Ga [7] and Co–Ni–Ga [5,8,9] were explored as candidates for this new class of magneto-strained materials (MSM). Among Heusler compounds being in martensitic state at room temperature, the Co–Ni–Ga family has special interest in the MSM field in view of the fact that brittleness of this alloy can be reduced by c-phase precipitates [5,10]. Previous few publications were mainly addressed to the influence of the element content on the transformation temperatures in polycrystalline alloys near the stoichiometric compound Co2 NiGa. The martensitic transformation (MT) temperature as a function of electron concentration shows a similar trend to that of Ni–Mn–Ga system [9,11]. Phase equilibria, MT and
Curie temperatures versus Co/Ni ratio in the composition range Co–(10–50)Ni–30Ga (at.%) were examined in [5]. As a result, a narrow concentration region 21– 22at.%Ni in which MT above 300 K is still going in ferromagnetic austenite was found. The crystal structure of martensite was claimed to be tetragonal L10 [5] or, presumably, orthorhombic and/or monoclinic [8], although structural results were not presented. The temperature dependences of damping characteristics [8] suggest reversibility of MT with a temperature hysteresis about 30 K. Other transformation characteristics were not studied yet. The present paper reports some results on the structural, thermal and mechanical characterization of a Co– Ni–Ga single crystal transforming martensitically from ferromagnetic austenite. For the first time in ferromagnetic shape memory alloys, a remarkable strain induced stabilization effect has been found (as already observed in non-magnetic Ni–Ti or Cu–Al–Ni alloys [12]). In order to get new information of this mechanical stabilization effect, it has also been studied in two Ni–Mn–Ga alloys exhibiting different martensitic structures.
2. Experimental procedure *
Corresponding author. Tel.: +34-971-173217; fax: +34-971173426. E-mail address: [email protected] (J. Pons).
A Co49:0 Ni22:0 Ga29:0 single crystal (denoted as CNG) was grown by Bridgman method using induction
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.09.024
226
V.A. Chernenko et al. / Scripta Materialia 50 (2004) 225–229
Table 1 Values of the forward (TM ) and reverse (TA ) martensitic transformation temperatures, transformation heats (q), Curie temperatures (TC ) and yield stress (ry ) Alloy
TM (K)
TA (K)
q (J/g)
TC ; (K)
ry (MPa)
CNG NMG1 NMG2
316 327 363
346 337 375
3.0 6.6 10.7
381 371 373
70 7 60
melted/cast ingot. Comparative measurements were also performed in two Ni–Mn–Ga single crystals of compositions Ni50:3 Mn30:0 Ga19:7 (NMG1) and Ni53:1 Mn26:6 Ga20:3 (NMG2) taken from the materials set used in our previous studies. The specimens for all measurements were spark cut from single crystals and mechanically and electrolytically polished. Differential scanning calorimetry (TA Instruments 2920 MDSC) measurements were performed at a heating–cooling rate of 10 K/min. The MT temperatures TM and TA were taken as the DSC peak temperatures for the forward (M) and reverse (A) transformations, respectively. The transformation temperatures and latent heat values shown in Table 1 were averaged over several DSC runs. The structural and phase characterization of martensites was made by optical microscopy equipped with a simple compression device, as well as SEM (Hitachi S-530 with Link ISIS EDS microanalysis) and TEM (Hitachi H-600 100 kV and Jeol 2011 200 kV). Compression tests of prismatic shaped specimens with 2 · 2 mm2 cross-section and about 4 mm gauge length were carried out using a ZWICK-100 testing machine equipped with a temperature chamber. The details of stress–strain measurements can be found in Ref. [13]. The NMG1 prism was {1 0 0} faced while NMG2 and CNG prisms were non-oriented. It is important to specify that the different experiments (except for the mechanical failure test) which require a bulk test sample have been carried out using the same aforementioned prismatic specimen in each alloy.
consider them as c-phase. This phase may play a positive role in improving the ductility at room temperature, which still remains a problem for Ni–Mn–Ga alloys. In fact, the re-oriented martensitic phase of CNG alloy under compression can still be plastically deformed by 4% before failure. The precipitates are probably also responsible of the relatively high temperature hysteresis of MT in CNG alloy (30 K), which is nearly three times larger than in the Ni–Mn–Ga alloys (Table 1). The TEM specimens were indeed useful to characterize the martensitic phase of this alloy. Several selected-area electron diffraction patterns (SAEDP) taken from martensite at room temperature are collected in Fig. 1. They can be consistently indexed according to a L10 fct structure, or a non-modulated bct with c=a > 1 in the axes directly derived from cubic austenite. The lattice parameters that can be estimated from the SAEDP are: aL10 ¼ 0:38 nm, cL10 ¼ 0:32 nm or abct ¼ 0.27 nm, cbct ¼ 0:32 nm. The patterns reveal an atomic order derived from B2-type in austenite; the reflections from L21 order of parent phase being absent (this is clear in Fig. 1c, corresponding to the [0 1 0]L10 or [1 1 0]bct zone axis). Probably, the L21 -type reflections are not seen due to closeness of the atomic scattering factors, as it happens in some Ni–Mn–Ga alloys. It is also worth to mention that thin ribbons elaborated by rapid solidification (planar flow melt spinning technique) were partially in austenite at room temperature. This allowed to determine the lattice parameter of cubic austenite by means of X-ray diffraction (powder method), being ac ¼ 0:286 nm. Concerning the Ni–Mn–Ga alloys studied for comparison, the NMG1 exhibits at room temperature the five-layered tetragonal phase with c=a < 1 (or 10 M microtwinned structure with (3 2)2
3. Results and discussion The transformation temperatures listed in Table 1 indicate that all alloys are in martensitic state at ambient conditions. Optical microscopy observations of CNG alloy show dispersed colonies of particles of a second phase within the martensite plates. SEM observations linked with EDS microanalysis revealed that these particles have an enriched Co content, higher than 70 at.%, close to the Co-rich fcc c-phase. Several TEM specimens of CNG alloy were prepared by double jet electropolishing, the thin foil being formed in all cases in regions not containing such particles, which restricted their further study. However, the SEM-EDS microanalysis results together with other published works [5] allow to
Fig. 1. SAED patterns showing L10 martensitic phase in Co–Ni–Ga.
V.A. Chernenko et al. / Scripta Materialia 50 (2004) 225–229 600
227
400
Stress (MPa)
500 400 1
T = 400 K
200
0 300
300
350
400
450
Stress (MPa)
σ (MPa)
400
500 T (K)
2
300
3 4
200
200
100
5
100 0 T = 445 K
0 2 3 Strain (%)
4
300
5
Fig. 2. Stress–strain curves taken from Co–Ni–Ga austenite at different temperatures: (1) 494 K, (2) 456 K, (3) 425 K, (4) 401 K and (5) 371 K. In the cases of unclosed loops the specimen length was reset by heating above 420 K. Inset: temperature dependence of the critical stress needed to induce MT. Slope 2.2 MPa/K.
stacking sequence), together with a minor presence of seven-layered orthorhombic phase (14 M with (52)2 stacking sequence). The NMG2 has the non-modulated tetragonal cell with c=a > 1 (or a ‘‘double’’ L10 , as derived from a L21 austenite). Thus, with the uncertainty of the atomic order as being derived from B2 or L21 austenite, the crystal structure of the thermally induced martensite in the present CNG alloy can be considered to be equivalent to that exhibited at room temperature by NMG2 martensite, belonging to Group III [11,14]. The stress–strain behaviour of the ferromagnetic austenite of CNG alloy is collected in Fig. 2. The typical linear dependence of the critical stress to induce the transformation as a function of temperature is obtained. Indeed, superelasticity is found for this compound, but the strain recovery is accompanied by a very large stress hysteresis of more than 200 MPa and takes place at temperatures above 420 K, much higher than the temperature for the finish of the reverse MT (TAf ¼ 360 K). What is more interesting is that the degree of strain recovery and stress hysteresis width are dependent on the deformation value achieved in partial stress–strain cycles (Fig. 3). This behaviour is different from Ni–Mn–Ga alloys. The r–e curves of NMG2 were studied in Ref. [13], while NMG1 alloy has been studied in the present work, giving equivalent results to those published in Ref. [15] for an alloy with similar composition. For each NMG specimen, the stress induced MT is characterised by a relatively low stress hysteresis (typically, a few tens of MPa) and full strain recovery is achieved at temperatures a few degrees above TAf . In addition, the stress hysteresis is practically independent on the amount of strain, as can be observed in Fig. 4. A similar behaviour as that depicted in Fig. 3 was also observed in the c0 martensite of Cu–Al–Ni alloys,
Stress (MPa)
1
200
100
0 0
1
2
3 Strain (%)
4
5
Fig. 3. Stress–strain curves of Co–Ni–Ga up to different strain levels recorded at two temperatures. In case of unclosed loops, the specimen length was reset by heating above 420 K.
80
60 Stress (MPa)
0
40
20
0 0
1
2
3
4
Strain (%)
Fig. 4. Superelastic partial and full loops for NMG1 crystal compressed along [100] axis at T ¼ 344 K.
which exhibits as well an effect of martensite stabilization after compression, similarly to Ni–Ti alloys (see Ref. [12] and references therein). Compressing the internally twinned c0 martensite of Cu–Al–Ni causes its detwinning. If this martensite is stress-induced from parent phase, a non-twinned c0 phase is obtained as well. The martensite stabilization, i.e. the shift of the reverse transformation to higher temperatures/lower stresses is attributed to the difficulties in the formation of a habit plane between austenite and the detwinned martensite,
2nd heating, ε = 0 1st heating ε = 1.25% ε = 2.20%
Alloy CNG
ε = 2.80%
5 mW
ε = 3.50%
Cooling, ε = 0
Alloy NMG1 5 mW
2nd heating, ε = 0 1st heating, ε = 6% Tc Cooling, ε = 0 2nd heating, ε = 0
Alloy NMG2
1st heating, ε = 6.5% 20 mW
as it is well known that the internal twinning of c0 martensite is essential to achieve an undistorted habit plane [12]. Such an effect was also studied in the different martensites of the present ferromagnetic alloys. Different strain values were set at room temperature, then the specimen was removed and placed into the DSC, where its transformation characteristics were measured during the first heating run and subsequent cooling–heating cycle. The results are compiled in Fig. 5, where it is clear that in the CNG alloy, the first reverse transformation is shifted to higher temperatures by an extent which increases with the applied strain, reaching a value of 80 K after 4% strain. On the other hand, for the crystallographically similar martensite in NMG2 alloy, this shift reaches only 10 K after 6.5% strain, while NMG1 does not show any stabilization, even though the applied strain was extended up to 6%. In all cases, it was verified by specimen length control that, after first heating, the full strain was recovered due to the shape memory effect, and the subsequent temperature cycling reflects the standard behaviour of the strain-free specimens. In order to understand the different behaviour exhibited by these alloys, non-oriented pieces of CNG and NMG1 were compressed by 4% and sliced to prepare a thin foil for TEM. TEM micrographs of deformed and strain-free martensites are presented in Fig. 6. For CNG alloy, the non-deformed microstructure consists of martensite plates with rather thin {1 1 1}L10 transformation twins, the two lamellae having nearly the same thickness. In the deformed specimen, one of the twin lamella is several times thicker than the other. Thus, the deformation in this Co–Ni–Ga alloy causes detwinning of the internally {1 1 1} twinned L10 martensite. On its turn, for the NMG1 alloy, the deformed specimens ex-
Exo
V.A. Chernenko et al. / Scripta Materialia 50 (2004) 225–229
Heat Flow
228
Cooling, ε = 0
250
300
350
400
450
Temperature (K)
Fig. 5. First heating DSC curves after deformation of martensite and second complete thermal cycles recorded in the different alloys.
hibited internally twinned non-modulated martensite (‘‘double’’ L10 , derived from L21 ordered austenite), with irregular but similar thickness of each twin lamella (Fig. 6c). In this case, the compression destroys the
Fig. 6. Microstructure of CNG martensite before (a) and after (b) 4% deformation. (c) Microstructure of NMG1 deformed by 4%.
V.A. Chernenko et al. / Scripta Materialia 50 (2004) 225–229
nano-twinning characteristic of the five- and seven-layered structures, leading to a ‘‘nearly standard’’ internally twinned non-modulated martensite. In case of CNG alloy, the same ingredients as in the c0 martensite of Cu–Al–Ni are on the table: the thermal martensite (L10 ) has internal twinning as lattice invariant shear and becomes detwinned after compression. So, the origin of the mechanical stabilization effect outlined above and more deeply explained in Ref. [12] seems to work in this case as well. The absence of stabilization effect in NMG1 can be attributed to the fact that the martensite obtained after compression is still internally twinned; so, it has not much problems to form a habit plane with austenite, and, then, no mechanical stabilization effect is present. This latter result gives an additional strong support to the proposed mechanism. The relatively low amount of mechanical stabilization obtained in NMG2 alloy compared to CNG (and Cu–Al– Ni [12]) remains still unclear. The precipitates present in CNG may play some role, but further studies are needed to clarify this point.
4. Conclusions 1. The thermally induced martensitic phase in single crystalline ferromagnetic Co49:0 Ni22:0 Ga29:0 alloy has a L10 structure or non-modulated bct cell in other axes. No reflections coming from the L21 type of atomic order are visible. Besides the atomic order, the structure is equivalent to that exhibited by Ni– Mn–Ga alloys of Group III [11,14] or by other Heusler alloy systems [5,6]. 2. Stress-induced MT with large stress hysteresis and related superelasticity effect has been found in Co–Ni– Ga. 3. A strong strain-induced stabilization effect of ferromagnetic martensitic phase is observed in Co–Ni– Ga, the effect being weaker in a Ni–Mn–Ga alloy of group III having a similar thermal induced martensite. Another Ni–Mn–Ga alloy showing five- and
229
seven-layered martensite does not exhibit such stabilization effect. These results together with some microstructural observations strongly support the origin of the mechanical stabilization effect, already observed and discussed in non ferromagnetic alloys.
Acknowledgements V.A.C. is grateful to the UIB (Palma, Spain) for support of his stay at Departament de Fısica. Authors thank to Dr. F. Hierro, Dr. J. Cifre and Dr. A.E. Perekos for their assistance with experiments. Support from DGI (project MAT2002-00319) is acknowledged.
References [1] Ullakko K, Huang JK, Kanter C, Kokorin VV, O’Handley RC. Appl Phys Lett 1996;69:1966. [2] M€ ullner P, Chernenko VA, Wollgarten M, Kostorz G. J Appl Phys 2002;92:6708. [3] James RD, Wuttig M. Philos Mag 1998;A77:1273. [4] Kakeshita T, Takeuchi T, Fukuda T, Saburi T, Oshima R, Muto S, et al. Mater Trans JIM 2000;41:882. [5] Oikawa K, Ota T, Gejima F, Ohmori T, Kainuma R, Ishida K, et al. Mater Trans JIM 2001;42:2472. [6] Oikawa K, Wulff L, Iijima T, Gejima F, Ohmori T, Fujita A, Fukamichi K, Kainuma R, Ishida K, et al. Appl Phys Lett 2001;79:3290. [7] Oikawa K, Ota T, Ohmori T, Tanaka Y, Morito H, Fujita A, et al. Appl Phys Lett 2002;81:5201. [8] Wuttig M, Li J, Craciunescu C. Scripta Mater 2001;44:2393. [9] Craciunescu C, Kishi Y, Lograsso TA, Wuttig M. Scripta Mater 2002;47:285. [10] Booth JG, Cywinski R, Prince JG. J Magn Magn Mater 1978;7:127. [11] Chernenko VA. Scripta Mater 1999;40:523. [12] Picornell C, Pons J, Cesari E. Acta Mater 2001;49:4221. [13] Chernenko VA, L’vov VA, Pons J, Cesari E. J Appl Phys 2003;93:2394. [14] Pons J, Chernenko VA, Santamarta R, Cesari E. Acta Mater 2000;48:3027. [15] Pons J, Chernenko VA, Cesari E, L’vov VA. J de Phys IV, Stress– strain behaviour of Ni–Mn–Ga alloys: experiment and modelling. Journal de Physique, in press.
Transformation behaviour and martensite stabilization in the ferromagnetic Co–Ni–Ga Heusler alloy V.A. Chernenko a, J. Pons b
b,*
, E. Cesari b, I.K. Zasimchuk
c
a Institute of Magnetism, Vernadsky str. 36-b, Kiev 03142, Ukraine Universitat de les Illes Balears, Departament de Fısica, E-07122 Palma de Mallorca, Spain c Institute of Metal Physics, Vernadsky str. 36, Kiev 03142, Ukraine
Accepted 4 September 2003
Abstract Some features of the martensitic transformation of a ferromagnetic Co49:0 Ni22:0 Ga29:0 single crystal have been clarified, including an effect of strain-induced stabilization of martensite. Microstructural observations support the origin of such effect, already introduced in non-ferromagnetic alloys, being related to difficulties in the formation of a habit plane between detwinned martensite and austenite. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Martensitic phase transformation; Strain-induced stabilization of martensite; Compression tests; Heusler phases
1. Introduction Recent years have witnessed a steady increase of interest in the ferromagnetic thermoelastic martensites, such as those exhibited by Ni–Mn–Ga Heusler alloys, after it was demonstrated that a large strain can be generated by the magnetic field-induced twin rearrangement (see, e.g. [1,2]). In addition, some other shape memory ferromagnetic systems, namely Fe–Pd [3], Fe3 Pt [4], Co–Ni–Al [5,6], Ni–Fe–Ga [7] and Co–Ni–Ga [5,8,9] were explored as candidates for this new class of magneto-strained materials (MSM). Among Heusler compounds being in martensitic state at room temperature, the Co–Ni–Ga family has special interest in the MSM field in view of the fact that brittleness of this alloy can be reduced by c-phase precipitates [5,10]. Previous few publications were mainly addressed to the influence of the element content on the transformation temperatures in polycrystalline alloys near the stoichiometric compound Co2 NiGa. The martensitic transformation (MT) temperature as a function of electron concentration shows a similar trend to that of Ni–Mn–Ga system [9,11]. Phase equilibria, MT and
Curie temperatures versus Co/Ni ratio in the composition range Co–(10–50)Ni–30Ga (at.%) were examined in [5]. As a result, a narrow concentration region 21– 22at.%Ni in which MT above 300 K is still going in ferromagnetic austenite was found. The crystal structure of martensite was claimed to be tetragonal L10 [5] or, presumably, orthorhombic and/or monoclinic [8], although structural results were not presented. The temperature dependences of damping characteristics [8] suggest reversibility of MT with a temperature hysteresis about 30 K. Other transformation characteristics were not studied yet. The present paper reports some results on the structural, thermal and mechanical characterization of a Co– Ni–Ga single crystal transforming martensitically from ferromagnetic austenite. For the first time in ferromagnetic shape memory alloys, a remarkable strain induced stabilization effect has been found (as already observed in non-magnetic Ni–Ti or Cu–Al–Ni alloys [12]). In order to get new information of this mechanical stabilization effect, it has also been studied in two Ni–Mn–Ga alloys exhibiting different martensitic structures.
2. Experimental procedure *
Corresponding author. Tel.: +34-971-173217; fax: +34-971173426. E-mail address: [email protected] (J. Pons).
A Co49:0 Ni22:0 Ga29:0 single crystal (denoted as CNG) was grown by Bridgman method using induction
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.09.024
226
V.A. Chernenko et al. / Scripta Materialia 50 (2004) 225–229
Table 1 Values of the forward (TM ) and reverse (TA ) martensitic transformation temperatures, transformation heats (q), Curie temperatures (TC ) and yield stress (ry ) Alloy
TM (K)
TA (K)
q (J/g)
TC ; (K)
ry (MPa)
CNG NMG1 NMG2
316 327 363
346 337 375
3.0 6.6 10.7
381 371 373
70 7 60
melted/cast ingot. Comparative measurements were also performed in two Ni–Mn–Ga single crystals of compositions Ni50:3 Mn30:0 Ga19:7 (NMG1) and Ni53:1 Mn26:6 Ga20:3 (NMG2) taken from the materials set used in our previous studies. The specimens for all measurements were spark cut from single crystals and mechanically and electrolytically polished. Differential scanning calorimetry (TA Instruments 2920 MDSC) measurements were performed at a heating–cooling rate of 10 K/min. The MT temperatures TM and TA were taken as the DSC peak temperatures for the forward (M) and reverse (A) transformations, respectively. The transformation temperatures and latent heat values shown in Table 1 were averaged over several DSC runs. The structural and phase characterization of martensites was made by optical microscopy equipped with a simple compression device, as well as SEM (Hitachi S-530 with Link ISIS EDS microanalysis) and TEM (Hitachi H-600 100 kV and Jeol 2011 200 kV). Compression tests of prismatic shaped specimens with 2 · 2 mm2 cross-section and about 4 mm gauge length were carried out using a ZWICK-100 testing machine equipped with a temperature chamber. The details of stress–strain measurements can be found in Ref. [13]. The NMG1 prism was {1 0 0} faced while NMG2 and CNG prisms were non-oriented. It is important to specify that the different experiments (except for the mechanical failure test) which require a bulk test sample have been carried out using the same aforementioned prismatic specimen in each alloy.
consider them as c-phase. This phase may play a positive role in improving the ductility at room temperature, which still remains a problem for Ni–Mn–Ga alloys. In fact, the re-oriented martensitic phase of CNG alloy under compression can still be plastically deformed by 4% before failure. The precipitates are probably also responsible of the relatively high temperature hysteresis of MT in CNG alloy (30 K), which is nearly three times larger than in the Ni–Mn–Ga alloys (Table 1). The TEM specimens were indeed useful to characterize the martensitic phase of this alloy. Several selected-area electron diffraction patterns (SAEDP) taken from martensite at room temperature are collected in Fig. 1. They can be consistently indexed according to a L10 fct structure, or a non-modulated bct with c=a > 1 in the axes directly derived from cubic austenite. The lattice parameters that can be estimated from the SAEDP are: aL10 ¼ 0:38 nm, cL10 ¼ 0:32 nm or abct ¼ 0.27 nm, cbct ¼ 0:32 nm. The patterns reveal an atomic order derived from B2-type in austenite; the reflections from L21 order of parent phase being absent (this is clear in Fig. 1c, corresponding to the [0 1 0]L10 or [1 1 0]bct zone axis). Probably, the L21 -type reflections are not seen due to closeness of the atomic scattering factors, as it happens in some Ni–Mn–Ga alloys. It is also worth to mention that thin ribbons elaborated by rapid solidification (planar flow melt spinning technique) were partially in austenite at room temperature. This allowed to determine the lattice parameter of cubic austenite by means of X-ray diffraction (powder method), being ac ¼ 0:286 nm. Concerning the Ni–Mn–Ga alloys studied for comparison, the NMG1 exhibits at room temperature the five-layered tetragonal phase with c=a < 1 (or 10 M microtwinned structure with (3 2)2
3. Results and discussion The transformation temperatures listed in Table 1 indicate that all alloys are in martensitic state at ambient conditions. Optical microscopy observations of CNG alloy show dispersed colonies of particles of a second phase within the martensite plates. SEM observations linked with EDS microanalysis revealed that these particles have an enriched Co content, higher than 70 at.%, close to the Co-rich fcc c-phase. Several TEM specimens of CNG alloy were prepared by double jet electropolishing, the thin foil being formed in all cases in regions not containing such particles, which restricted their further study. However, the SEM-EDS microanalysis results together with other published works [5] allow to
Fig. 1. SAED patterns showing L10 martensitic phase in Co–Ni–Ga.
V.A. Chernenko et al. / Scripta Materialia 50 (2004) 225–229 600
227
400
Stress (MPa)
500 400 1
T = 400 K
200
0 300
300
350
400
450
Stress (MPa)
σ (MPa)
400
500 T (K)
2
300
3 4
200
200
100
5
100 0 T = 445 K
0 2 3 Strain (%)
4
300
5
Fig. 2. Stress–strain curves taken from Co–Ni–Ga austenite at different temperatures: (1) 494 K, (2) 456 K, (3) 425 K, (4) 401 K and (5) 371 K. In the cases of unclosed loops the specimen length was reset by heating above 420 K. Inset: temperature dependence of the critical stress needed to induce MT. Slope 2.2 MPa/K.
stacking sequence), together with a minor presence of seven-layered orthorhombic phase (14 M with (52)2 stacking sequence). The NMG2 has the non-modulated tetragonal cell with c=a > 1 (or a ‘‘double’’ L10 , as derived from a L21 austenite). Thus, with the uncertainty of the atomic order as being derived from B2 or L21 austenite, the crystal structure of the thermally induced martensite in the present CNG alloy can be considered to be equivalent to that exhibited at room temperature by NMG2 martensite, belonging to Group III [11,14]. The stress–strain behaviour of the ferromagnetic austenite of CNG alloy is collected in Fig. 2. The typical linear dependence of the critical stress to induce the transformation as a function of temperature is obtained. Indeed, superelasticity is found for this compound, but the strain recovery is accompanied by a very large stress hysteresis of more than 200 MPa and takes place at temperatures above 420 K, much higher than the temperature for the finish of the reverse MT (TAf ¼ 360 K). What is more interesting is that the degree of strain recovery and stress hysteresis width are dependent on the deformation value achieved in partial stress–strain cycles (Fig. 3). This behaviour is different from Ni–Mn–Ga alloys. The r–e curves of NMG2 were studied in Ref. [13], while NMG1 alloy has been studied in the present work, giving equivalent results to those published in Ref. [15] for an alloy with similar composition. For each NMG specimen, the stress induced MT is characterised by a relatively low stress hysteresis (typically, a few tens of MPa) and full strain recovery is achieved at temperatures a few degrees above TAf . In addition, the stress hysteresis is practically independent on the amount of strain, as can be observed in Fig. 4. A similar behaviour as that depicted in Fig. 3 was also observed in the c0 martensite of Cu–Al–Ni alloys,
Stress (MPa)
1
200
100
0 0
1
2
3 Strain (%)
4
5
Fig. 3. Stress–strain curves of Co–Ni–Ga up to different strain levels recorded at two temperatures. In case of unclosed loops, the specimen length was reset by heating above 420 K.
80
60 Stress (MPa)
0
40
20
0 0
1
2
3
4
Strain (%)
Fig. 4. Superelastic partial and full loops for NMG1 crystal compressed along [100] axis at T ¼ 344 K.
which exhibits as well an effect of martensite stabilization after compression, similarly to Ni–Ti alloys (see Ref. [12] and references therein). Compressing the internally twinned c0 martensite of Cu–Al–Ni causes its detwinning. If this martensite is stress-induced from parent phase, a non-twinned c0 phase is obtained as well. The martensite stabilization, i.e. the shift of the reverse transformation to higher temperatures/lower stresses is attributed to the difficulties in the formation of a habit plane between austenite and the detwinned martensite,
2nd heating, ε = 0 1st heating ε = 1.25% ε = 2.20%
Alloy CNG
ε = 2.80%
5 mW
ε = 3.50%
Cooling, ε = 0
Alloy NMG1 5 mW
2nd heating, ε = 0 1st heating, ε = 6% Tc Cooling, ε = 0 2nd heating, ε = 0
Alloy NMG2
1st heating, ε = 6.5% 20 mW
as it is well known that the internal twinning of c0 martensite is essential to achieve an undistorted habit plane [12]. Such an effect was also studied in the different martensites of the present ferromagnetic alloys. Different strain values were set at room temperature, then the specimen was removed and placed into the DSC, where its transformation characteristics were measured during the first heating run and subsequent cooling–heating cycle. The results are compiled in Fig. 5, where it is clear that in the CNG alloy, the first reverse transformation is shifted to higher temperatures by an extent which increases with the applied strain, reaching a value of 80 K after 4% strain. On the other hand, for the crystallographically similar martensite in NMG2 alloy, this shift reaches only 10 K after 6.5% strain, while NMG1 does not show any stabilization, even though the applied strain was extended up to 6%. In all cases, it was verified by specimen length control that, after first heating, the full strain was recovered due to the shape memory effect, and the subsequent temperature cycling reflects the standard behaviour of the strain-free specimens. In order to understand the different behaviour exhibited by these alloys, non-oriented pieces of CNG and NMG1 were compressed by 4% and sliced to prepare a thin foil for TEM. TEM micrographs of deformed and strain-free martensites are presented in Fig. 6. For CNG alloy, the non-deformed microstructure consists of martensite plates with rather thin {1 1 1}L10 transformation twins, the two lamellae having nearly the same thickness. In the deformed specimen, one of the twin lamella is several times thicker than the other. Thus, the deformation in this Co–Ni–Ga alloy causes detwinning of the internally {1 1 1} twinned L10 martensite. On its turn, for the NMG1 alloy, the deformed specimens ex-
Exo
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Heat Flow
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Cooling, ε = 0
250
300
350
400
450
Temperature (K)
Fig. 5. First heating DSC curves after deformation of martensite and second complete thermal cycles recorded in the different alloys.
hibited internally twinned non-modulated martensite (‘‘double’’ L10 , derived from L21 ordered austenite), with irregular but similar thickness of each twin lamella (Fig. 6c). In this case, the compression destroys the
Fig. 6. Microstructure of CNG martensite before (a) and after (b) 4% deformation. (c) Microstructure of NMG1 deformed by 4%.
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nano-twinning characteristic of the five- and seven-layered structures, leading to a ‘‘nearly standard’’ internally twinned non-modulated martensite. In case of CNG alloy, the same ingredients as in the c0 martensite of Cu–Al–Ni are on the table: the thermal martensite (L10 ) has internal twinning as lattice invariant shear and becomes detwinned after compression. So, the origin of the mechanical stabilization effect outlined above and more deeply explained in Ref. [12] seems to work in this case as well. The absence of stabilization effect in NMG1 can be attributed to the fact that the martensite obtained after compression is still internally twinned; so, it has not much problems to form a habit plane with austenite, and, then, no mechanical stabilization effect is present. This latter result gives an additional strong support to the proposed mechanism. The relatively low amount of mechanical stabilization obtained in NMG2 alloy compared to CNG (and Cu–Al– Ni [12]) remains still unclear. The precipitates present in CNG may play some role, but further studies are needed to clarify this point.
4. Conclusions 1. The thermally induced martensitic phase in single crystalline ferromagnetic Co49:0 Ni22:0 Ga29:0 alloy has a L10 structure or non-modulated bct cell in other axes. No reflections coming from the L21 type of atomic order are visible. Besides the atomic order, the structure is equivalent to that exhibited by Ni– Mn–Ga alloys of Group III [11,14] or by other Heusler alloy systems [5,6]. 2. Stress-induced MT with large stress hysteresis and related superelasticity effect has been found in Co–Ni– Ga. 3. A strong strain-induced stabilization effect of ferromagnetic martensitic phase is observed in Co–Ni– Ga, the effect being weaker in a Ni–Mn–Ga alloy of group III having a similar thermal induced martensite. Another Ni–Mn–Ga alloy showing five- and
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seven-layered martensite does not exhibit such stabilization effect. These results together with some microstructural observations strongly support the origin of the mechanical stabilization effect, already observed and discussed in non ferromagnetic alloys.
Acknowledgements V.A.C. is grateful to the UIB (Palma, Spain) for support of his stay at Departament de Fısica. Authors thank to Dr. F. Hierro, Dr. J. Cifre and Dr. A.E. Perekos for their assistance with experiments. Support from DGI (project MAT2002-00319) is acknowledged.
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