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Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys
Accepted Manuscript Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys Kenji Hirata, Xiao Xu, Toshihiro Omori, Makoto Nagasako, Ryosuke Kainuma PII: DOI: Reference:
S0925-8388(15)01102-0 http://dx.doi.org/10.1016/j.jallcom.2015.03.264 JALCOM 33971
To appear in:
Journal of Alloys and Compounds
Received Date: Accepted Date:
22 February 2015 22 March 2015
Please cite this article as: K. Hirata, X. Xu, T. Omori, M. Nagasako, R. Kainuma, Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.03.264
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys
Kenji Hirata1, Xiao Xu, 1, a) Toshihiro Omori,1 Makoto Nagasako2 and Ryosuke Kainuma1 1
Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-02, Sendai 980-8579, Japan 2
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Abstract
Martensitic transformation behaviors and magnetic properties in off-stoichiometric Co2Cr(AlSi) Heusler alloys were investigated mainly by TEM, DSC, SQUID and mechanical test.
In
Co57Cr 21Al11Si11 alloy, martensitic transformation from Heusler-type L21 to D022 phase was confirmed, where Ms and Af temperatures were 429 and 452 K, respectively, suggesting thermoelastic martensitic transformation because of the small transformation temperature hysteresis.
In polycrystalline
Co57Cr 25Al10Si10 alloy with large grains, superelasticity with a recovery strain of over 2% was detected by compression test at 298 K. Thus, a novel Co-based shape memory alloy was discovered in the Co-Cr-Al-Si system.
Corresponding author’s mail address: [email protected]
1. Introduction A number of shape memory alloys (SMAs) have been reported in Ni-based [1-6], Cu-based [7, 8] and Fe-based [9-11] Heusler alloys with the L21 structure having a chemical composition of X2YZ. Co-based Heusler alloys with half-metallic behaviors, which have large spontaneous magnetizations as well as high Curie temperature, have been intensively studied in the field of spintronics [12]. However, there have been no reports on martensitic transformation in Co-based Heusler alloys except for non-half-metallic Co2NbSn alloys [13, 14], while Co-Ni-Ga ferromagnetic shape memory alloy with the B2 structure has been reported [15-17]. Very recently, an anomalous martensitic transformation has been reported in off-stoichiometric Co2Cr(GaSi) Heusler alloys [18]. In the alloys, after normal martensitic transformation during cooling, the martensite phase transforms back to the parent phase by further cooling, which is called re-entrant transformation.
By the re-entrant
transformation, a cooling-induced shape memory effect and an inverse temperature dependence of superelastic stress has also been reported [18]. Co-based SMAs are attractive because of potential applications such as thermally- or magnetically-driven actuators, half-metallic spintronic materials and corrosion-resistant biomaterials such as Co-Cr-Mo alloys [19-21]. For practical applications of the Co2Cr(GaSi) SMAs, Ga is expensive and a substitution of Ga is desired. One candidate is Al, which is in the same group as Ga in the periodic table. Although unstable in the Co-Cr-Al system [22], the L21 phase is expected to be stabilized by the addition of Si in analogy with Co2Mn(Al1-xSix) [23] and Co2Cr(Ga1-xSix) [24] alloys. In this study, for some off-stoichiometric Co2Cr(AlSi) Heusler alloys, martensitic transformation and magnetic properties were investigated and whether the alloys show superelasticity and re-entrant transformation behaviors as is the case in the Co2Cr(GaSi) SMA was examined.
2. Experimental procedures Co57Cr 21Al11Si11 (denoted as 1M), Co55Cr23Al11Si11 (denoted as 1P) and Co55Cr 25Al10Si10 (denoted as 2P) alloys were prepared by induction melting under an argon atmosphere. Samples of these alloys were solution-treated at 1473 K for 24 h and quenched into water. After removing oxidation layers carefully, small pieces of the samples were cut out using a low-speed cutter. The microstructure was observed using an optical microscope (OM) and a transmission electron microscope (TEM: JEOL, JEM2100) at 200 kV. OM samples were prepared using an etchant composed of 4.8% hydrogen peroxide and 95.2% HCl. TEM samples were prepared by electropolishing with a solution of 72% acetic acid, 12% ethanol, 8% ethyleneglycol, and 8% perchloric acid at room temperature. The crystal structure was determined using X-ray diffraction (XRD) and TEM at room temperature. For XRD measurements, 1P and 1M were prepared as powder and bulk samples, respectively, where Co-K radiation source was used. After grinding the sample of 1P, the obtained powders were annealed at 1473 K for 2 min in order to remove introduced strain. Magnetic properties were investigated using a superconducting quantum interference device (SQUID) magnetometer in the range of 4.2 K to 300 K and using a vibrating sample magnetometer (VSM) in the range of 300 K to 500 K. Thermomagnetization measurements were performed at a heating/cooling rate of 2 K/min under a magnetic field of 500 Oe, and magnetization measurements were performed at 6 K up to 50 kOe. Martensitic transformation temperatures were determined using differential scanning calorimetry (DSC) at a heating and cooling rate of 10 K/min. Compression tests were performed at 298 K on a 2P sample with large grains obtained by prolonged heat treatment at 1473 K for 14 days, the crystal orientations being measured using the electron backscatter scanning diffraction (EBSD) method.
3. Results and discussion
3-1. Microstructures and crystal structures Typical microstructures of the 1P and 1M alloys are shown in Fig. 1. The 1P alloy has a parent phase with grain size up to 500 μm, and cracks along the grain boundaries can be found in Fig. 1(a), suggesting the brittleness of the present alloy. The 1M alloy has a martensite microstructure with thin plates. The 1P and 1M samples were subjected to TEM observations. Figure 2 (a) shows the TEM bright-field image of the 1P alloy tilted to the
matrix zone axis. The crystal structure was
determined to be the L21 structure in the selected area diffraction (SAD) pattern with a superlattice diffraction spot of (111). Figure 2(b) shows the TEM bright-field image of the 1M alloy with the SAD pattern shown in the inset, the incident direction being
. The corresponding diffraction
spots can be indexed as the D022 structure with the twin relation. Since 1P has a composition very close to 1M, the parent phase of 1M is considered to have the L21 structure. Therefore, 1M is concluded to show L21→D022 martensitic transformation, as reported in the Co-Cr-Ga-Si system [18]. Note that the tweed microstructure in the bright-field images and the streak in the SAD patterns are observed in Fig. 2, which may be related to spinodal phase decomposition, because such decomposition has been reported in stoichiometric Co2CrAl ternary alloy quenched from elevated temperatures [22]. Figure 3 shows the results of XRD measurements for 1P and 1M. The diffraction peaks were confirmed to be consistent with the L21 and D022 structures for the parent phase in 1P and the martensite phase in 1M, respectively. The lattice parameters were determined to be and
=0.378 nm and
=0.569 nm
=0.644 nm, respectively. A small amount of the fcc phase was
observed in the 1P powder sample, which is considered to have been formed during the cooling process. The weak peak of the parent phase, with lattice parameter of
=0.568 nm, was observed in
1M. From this XRD pattern, it was found that the martensitic transformation from the L21 structure
to the D022 structure was accompanied by a negative volume change (ΔV=－0.137%), which has the same tendency as in the Co-Cr-Ga-Si system [18].
3-2. Martensitic transformation and magnetic properties Figure 4 shows the results of DSC measurement for 1M alloy. Reverse martensitic transformation during heating and forward martensitic transformation during cooling were observed in the 1st cycle. The martensitic transformation temperatures were determined to be Ms = 429 K, Mf = 358 K, As = 398 K and Af = 452 K (Ms, martensitic transformation starting temperature; Mf, martensitic transformation finishing temperature; As, reverse transformation starting temperature; Af, reverse transformation finishing temperature), and the thermal hysteresis defined as Af - Ms was 23 K. This small thermal hysteresis is typical in thermoelastic martensitic transformation.
No re-entrant martensitic
transformation was observed in the present alloy. After the 1st cycle, martensitic transformation was hardly detected in the 2nd heating/cooling cycle, which may be attributed to the aging effect. Further studies on this phenomenon are required. In order to investigate the magnetic properties of the parent and martensite phases, magnetization and thermomagnetization measurements were conducted for 1P and 1M. Figure 5(a) shows the magnetization curves at 6 K. The spontaneous magnetization of the parent phase in the 1P sample at 6 K was determined by the Arrott plot [25] to be 68 emu/g. Figure 5(b) shows the thermomagnetization curves in a heating process under a magnetic field of 500 Oe, where the Curie temperature of the parent phase in 1P is 429 K. The spontaneous magnetization of the ferromagnetic parent phase is comparable to 70~80 emu/g of the Co-Cr-Ga-Si system, but the Curie temperature is lower than that (about 550 K) of Co-Cr-Ga-Si [18, 24]. On the other hand, in 1M alloy, a weak ferromagnetic behavior was observed in Fig. 5(a). The residual parent phase was detected by XRD
measurement (Fig. 3) and the volume fraction of the parent phase can be roughly estimated as 9.2% using the peak ratio of the parent and martensite phases.
When the spontaneous magnetization value
for the parent phase is estimated as being 68 emu/g, the spontaneous magnetization at 6 K due to the residual parent phase is estimated to be 6.3 emu/g in the 1M sample, which is not so far from the measured value of 15 emu/g in Fig. 5(a). Therefore, it is considered that the magnetization in 1M of Fig. 5(a) is mainly due to the residual parent phase and that the martensite phase is probably paramagnetic. Decrease in magnetization is also observed at around 420 K in 1M (Fig. 5(b)), which is apparently due to magnetic transition of the residual parent phase at TC. While the reverse martensitic transformation is obtained in the DSC measurement in Fig. 4, no obvious change in magnetization associated with the martensitic transformation is detected, which is probably caused by a slight difference in composition between samples for the DSC and VSM measurements, even though they were cut out of the same ingot.
Nevertheless, it is considered that in 1M the martensitic
transformation temperature is very close to TC and that the transformation is basically from paramagnetic parent phase near Curie temperature to paramagnetic martensite phase.
3-3. Superelastic property Figure 6(a) shows the results of cyclic compression tests of the 2P sample at 298 K. The grain size is large in this sample and it contains at least 11 grains with almost random orientation distribution, as shown in the inset of Fig. 6(a). Almost perfect superelasticity was obtained, and the martensitic transformation starting stress Ms and the reverse martensitic transformation finishing stress Af were about 167 MPa and 52 MPa, respectively, for the 1st cycle. The hardening rate slightly starts to increase at about 3.2% strain, and thus the superelastic strain is about 2%. Figure 6(b) shows the orientation dependence of compressive transformation strain calculated by the lattice deformation [26]
using the lattice constants obtained by the XRD measurements shown in Fig. 3, where the maximum and minimum transformation strain is expected to be 6.1% and 0%, respectively. It is known [27-30] that superelastic strain depends on mean grain size relative to specimen size because of the constraint from surrounding neighboring grains, remarkably reduced in a polycrystalline sample. The experimental superelastic strain is within the calculated values from 0 to 6.1 %, and the relatively small superelastic strain may have been influenced by the grain constraint.
4. Summary Thermoelastic martensitic transformation from the L21 parent to the D022 martensite phase with a small thermal hysteresis of about 23 K was found in an off-stoichiometric Co2Cr(AlSi) Heusler alloy. The parent and martensite phases of the alloy have ferromagnetic and (probably) paramagnetic properties and superelasticity with a shape recovery strain of about 2% was obtained in a compression test at room temperature for the polycrystalline sample.
Thus, although the off-stoichiometric
Co2Cr(Al,Si) Heusler alloy was confirmed to be a shape memory alloy showing superelasticity, no re-entrant transformation was found in the present alloys.
Acknowledgments This study was supported by a Grant-in-Aid for Scientific Research and Japan Society for the Promotion of Science (JSPS). A part of the experiments were performed at the Center for Low Temperature Science, Institute for Materials Research, Tohoku University.
References [1] K. Ullakko, J. K. Huang, C. Kantner, R. C. O’Handley and V. V. Kokorin. Appl. Phys. Lett., 69,
1966 (1996). [2] R. Kainuma, Y. Imano, W. Ito, Y. Sutou, H. Morito, S. Okamoto, O. Kitakami, K. Oikawa, A. Fujita, T. Kanomata and K. Ishida . Nature 439 (2006) 957. [3] W. Ito, Y. Imano, R. Kainuma, Y. Sutou, K. Oikawa, K. Ishida. Metall. Mater. Trans. A. 38 (2007) 759-766 [4] Y.Q. Ma, C.B. Jiang, G. Feng, H.B. Xu. Scripta materialia.., 48 (2003) 365-369 [5] H.B. Xu, Y. Li, C.B. Jiang. Mater. Sci. Eng.A. 438-440 (2006) 1065-1070 [6] Y. Sutou, N. Koeda, T. Omori, R. Kainuma, K. Oikawa, K. Ishida. Appl. Phys. Lett. 85 (2004) 4358 [7] R. Kainuma, S. Takahashi, K. Ishida. Metall. Mater. Trans.A. 27 (1996) 2187-2195 [8] Y. Sutou, T. Omori, J.J. Wang, R. Kainuma, K. Ishida. Mater. Sci. Eng.A. 378 (2004) 278-282 [9] T. Omori, K. Watanabe, R. Y. Umetsu, R. Kainuma, K. Ishida. Appl. Phys. Lett. 95 (2009) 082508 [10] T. Omori, K. Watanabe, X. Xu, R. Y. Umetsu, R. Kainuma, K. Ishida. Scripta Materialia.., 64 (2011) 669-672 [11] W. Zhu, E. K. Liu, L. Feng, X. D. Tang, J. L. Chen, G. H. Wu, H. Y. Liu, F. B. Meng, H. Z. Luo. Appl. Phys. Lett. 95 (2009) 082508 [12] A. Kellou, N. E. Feninenche, T. Grosdider, H. Aourag, C. Coddet. J. Appl. Phys., 94 (2003) 3292 [13] M. Terada, Y. Fujita, K. Endo. J. Phys. Soc. Jpn. 36 (1974) 620. [14] S. Fujii, S. Ishida, S. Asano, J. Rhys. Soc. Jpn, 58, 3657 (1989). [15] M. Wuttig, J. Li, C. Craciunescu, Scr. Mater. 44 (2001) 2393-2397. [16] K. Oikawa, T. Ota, F. Gejima, T. Omori, R. Kainuma, K. Ishida. Mater. Trans. 42 (2001) 2472-2475 [17] K. Oikawa, T. Ota, Y. Imano, T. Omori, R. Kainuma, K. Ishida. J. Phase Equilib.Diffus. 27 (2006)
75-82 [18] X. Xu, T. Omori, M. Nagasako, A. Okubo R. Umetsu, T. Kanomata, K. IshidaK. R. Kainuma. Appl. Phys. Lett. 103 (2013) 164104 [19] S. Lee, E. Takahashi, N. Nomura, A. Chiba. J. Japan Inst. Metals. 70 (2004) 260-264 [20] S. Lee, E. Takahashi, N. Nomura, A. Chiba. Acta Biomater. 8 (2012) 2856-2862 [21] L. Z. Zhuang, E. W. Langer. J. Mater. Sci. 24 (1989) 381-388 [22] K. Ishikawa, M. Ise, I. Ohnuma, R. Kainuma, K. Ishida. Ber. BunsenGes. Phys. Chem. 102 (1998) 1206 [23] R. Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida. Scripta materialia.., 58 (2008) 723-726 [24] R. Y. Umetsu, A. Okubo, X. Xu and R. Kainuma. J. Alloys Compd., 588 (2014) 153-157 [25] A. Arrott. Phys. Rev. 108 (1957) 1394 [26] T. Saburi and S. Nenno, in Proc. Int. Conf. Solid-Solid Phase Transformations, H. I. Aaronson, D. E. Laughlin, R. E. Sekerka, C. M. Wayman eds., Pittsburgh, 1981, pp. 1455-1479. [27]N. Ono, A. Sato, Trans. Japan Inst. Metals 29 (1988) 267 [28]N. Ono, A. Sato, H. Ohta. Trans. JIM 30 (1989) 756. [29]N. Ono, Mater. Trans. JIM 31 (1990) 381. [30]Y. Sutou, T. Omori, K. Yamauchi, N. Ono, R. Kainuma, K. Ishida. Acta Mater. 53 (2005) 4121.
Figure Captions Figure 1. Typical microstructures of parent and martensite phases taken by an optical microscope from (a) Co55Cr23(Al11Si11) (1P) and (b) Co57Cr21(Al11Si11) (1M) alloys solution-treated at 1473 K for 24 h. Figure 2. Bright-field images of (a) Co55Cr23(Al11Si11) (1P) and (b) Co57Cr21(Al11Si11) (1M) quenched from 1473 K observed using a transmission electron microscope and (insets) their corresponding selected-area diffraction patterns. Specimens were tilted to (a) the and (b) the
matrix zone axis of 1P
matrix zone axis of 1M, respectively.
Figure 3. X-ray diffraction patterns measured at room temperature for Co55Cr23Al11Si11 (1P) and Co57Cr 21Al11Si11 (1M), where Co-K was used as the radiation source. Powder and bulk samples were used for 1M and 1P alloys, respectively. Figure 4. Differential scanning calorimetry (DSC) measurements for Co57Cr21Al11Si11 (1M) at 10 K/min. Martensitic transformation starting temperature (Ms), martensitic transformation finishing temperature (Mf), reverse transformation starting temperature (As), and reverse transformation finishing temperature (Af) were determined in the 1st cycle. Figure 5. (a) Magnetization (M-H ) curves measured at 6 K and (b) thermomagnetization (M-T ) curves under a magnetic field of 500 Oe for Co55Cr23Al11Si11 (1P) and Co57Cr21Al11Si11 (1M). Tc is the Curie temperature of the parent phase. Figure 6. (a) Stress-strain curves obtained by compression tests for Co55Cr25Al10Si10 (2P) alloy with large grains. The measurements were performed three times. The inset shows orientations in the compression direction measured by EBSD. (b) The orientation dependence of transformation strain (compression) is shown in stereo-graphic triangle, which was obtained using theoretical calculation of the lattice deformation [26] with the experimental lattice parameters shown in Fig. 3.
220
1P (Parent) ە
620
440 420
400
200
111
ە
112
ە
70
90
110
2 Theta (Deg.)
224
204
200
ە
50
1M (Martensite)
312
200
ە
ە
Intensity
ە
ە
30
L21 D022 fcc
130
150
1M (Martensite)
2nd cycle
g n oli
1st cycle
Co
Exothermic
Ms = 429 K Mf = 358 K Af = 452 K As = 398 K
Heating
300
350
400
450
Temperature (K)
500
Highlights Article : Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys ・Thermoelastic martensitic transformation was found in the Co2Cr(AlSi) alloys. ・The parent and martensite phases have L21 and L10 structures, respectively. ・The parent phase was determined to be in ferromagnetic state. ・The martensite phase was confirmed to have weakly magnetic state. ・Over 2% of superelasticity was obtained for the polycrystalline sample by compression test.
S0925-8388(15)01102-0 http://dx.doi.org/10.1016/j.jallcom.2015.03.264 JALCOM 33971
To appear in:
Journal of Alloys and Compounds
Received Date: Accepted Date:
22 February 2015 22 March 2015
Please cite this article as: K. Hirata, X. Xu, T. Omori, M. Nagasako, R. Kainuma, Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.03.264
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys
Kenji Hirata1, Xiao Xu, 1, a) Toshihiro Omori,1 Makoto Nagasako2 and Ryosuke Kainuma1 1
Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-02, Sendai 980-8579, Japan 2
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Abstract
Martensitic transformation behaviors and magnetic properties in off-stoichiometric Co2Cr(AlSi) Heusler alloys were investigated mainly by TEM, DSC, SQUID and mechanical test.
In
Co57Cr 21Al11Si11 alloy, martensitic transformation from Heusler-type L21 to D022 phase was confirmed, where Ms and Af temperatures were 429 and 452 K, respectively, suggesting thermoelastic martensitic transformation because of the small transformation temperature hysteresis.
In polycrystalline
Co57Cr 25Al10Si10 alloy with large grains, superelasticity with a recovery strain of over 2% was detected by compression test at 298 K. Thus, a novel Co-based shape memory alloy was discovered in the Co-Cr-Al-Si system.
Corresponding author’s mail address: [email protected]
1. Introduction A number of shape memory alloys (SMAs) have been reported in Ni-based [1-6], Cu-based [7, 8] and Fe-based [9-11] Heusler alloys with the L21 structure having a chemical composition of X2YZ. Co-based Heusler alloys with half-metallic behaviors, which have large spontaneous magnetizations as well as high Curie temperature, have been intensively studied in the field of spintronics [12]. However, there have been no reports on martensitic transformation in Co-based Heusler alloys except for non-half-metallic Co2NbSn alloys [13, 14], while Co-Ni-Ga ferromagnetic shape memory alloy with the B2 structure has been reported [15-17]. Very recently, an anomalous martensitic transformation has been reported in off-stoichiometric Co2Cr(GaSi) Heusler alloys [18]. In the alloys, after normal martensitic transformation during cooling, the martensite phase transforms back to the parent phase by further cooling, which is called re-entrant transformation.
By the re-entrant
transformation, a cooling-induced shape memory effect and an inverse temperature dependence of superelastic stress has also been reported [18]. Co-based SMAs are attractive because of potential applications such as thermally- or magnetically-driven actuators, half-metallic spintronic materials and corrosion-resistant biomaterials such as Co-Cr-Mo alloys [19-21]. For practical applications of the Co2Cr(GaSi) SMAs, Ga is expensive and a substitution of Ga is desired. One candidate is Al, which is in the same group as Ga in the periodic table. Although unstable in the Co-Cr-Al system [22], the L21 phase is expected to be stabilized by the addition of Si in analogy with Co2Mn(Al1-xSix) [23] and Co2Cr(Ga1-xSix) [24] alloys. In this study, for some off-stoichiometric Co2Cr(AlSi) Heusler alloys, martensitic transformation and magnetic properties were investigated and whether the alloys show superelasticity and re-entrant transformation behaviors as is the case in the Co2Cr(GaSi) SMA was examined.
2. Experimental procedures Co57Cr 21Al11Si11 (denoted as 1M), Co55Cr23Al11Si11 (denoted as 1P) and Co55Cr 25Al10Si10 (denoted as 2P) alloys were prepared by induction melting under an argon atmosphere. Samples of these alloys were solution-treated at 1473 K for 24 h and quenched into water. After removing oxidation layers carefully, small pieces of the samples were cut out using a low-speed cutter. The microstructure was observed using an optical microscope (OM) and a transmission electron microscope (TEM: JEOL, JEM2100) at 200 kV. OM samples were prepared using an etchant composed of 4.8% hydrogen peroxide and 95.2% HCl. TEM samples were prepared by electropolishing with a solution of 72% acetic acid, 12% ethanol, 8% ethyleneglycol, and 8% perchloric acid at room temperature. The crystal structure was determined using X-ray diffraction (XRD) and TEM at room temperature. For XRD measurements, 1P and 1M were prepared as powder and bulk samples, respectively, where Co-K radiation source was used. After grinding the sample of 1P, the obtained powders were annealed at 1473 K for 2 min in order to remove introduced strain. Magnetic properties were investigated using a superconducting quantum interference device (SQUID) magnetometer in the range of 4.2 K to 300 K and using a vibrating sample magnetometer (VSM) in the range of 300 K to 500 K. Thermomagnetization measurements were performed at a heating/cooling rate of 2 K/min under a magnetic field of 500 Oe, and magnetization measurements were performed at 6 K up to 50 kOe. Martensitic transformation temperatures were determined using differential scanning calorimetry (DSC) at a heating and cooling rate of 10 K/min. Compression tests were performed at 298 K on a 2P sample with large grains obtained by prolonged heat treatment at 1473 K for 14 days, the crystal orientations being measured using the electron backscatter scanning diffraction (EBSD) method.
3. Results and discussion
3-1. Microstructures and crystal structures Typical microstructures of the 1P and 1M alloys are shown in Fig. 1. The 1P alloy has a parent phase with grain size up to 500 μm, and cracks along the grain boundaries can be found in Fig. 1(a), suggesting the brittleness of the present alloy. The 1M alloy has a martensite microstructure with thin plates. The 1P and 1M samples were subjected to TEM observations. Figure 2 (a) shows the TEM bright-field image of the 1P alloy tilted to the
matrix zone axis. The crystal structure was
determined to be the L21 structure in the selected area diffraction (SAD) pattern with a superlattice diffraction spot of (111). Figure 2(b) shows the TEM bright-field image of the 1M alloy with the SAD pattern shown in the inset, the incident direction being
. The corresponding diffraction
spots can be indexed as the D022 structure with the twin relation. Since 1P has a composition very close to 1M, the parent phase of 1M is considered to have the L21 structure. Therefore, 1M is concluded to show L21→D022 martensitic transformation, as reported in the Co-Cr-Ga-Si system [18]. Note that the tweed microstructure in the bright-field images and the streak in the SAD patterns are observed in Fig. 2, which may be related to spinodal phase decomposition, because such decomposition has been reported in stoichiometric Co2CrAl ternary alloy quenched from elevated temperatures [22]. Figure 3 shows the results of XRD measurements for 1P and 1M. The diffraction peaks were confirmed to be consistent with the L21 and D022 structures for the parent phase in 1P and the martensite phase in 1M, respectively. The lattice parameters were determined to be and
=0.378 nm and
=0.569 nm
=0.644 nm, respectively. A small amount of the fcc phase was
observed in the 1P powder sample, which is considered to have been formed during the cooling process. The weak peak of the parent phase, with lattice parameter of
=0.568 nm, was observed in
1M. From this XRD pattern, it was found that the martensitic transformation from the L21 structure
to the D022 structure was accompanied by a negative volume change (ΔV=－0.137%), which has the same tendency as in the Co-Cr-Ga-Si system [18].
3-2. Martensitic transformation and magnetic properties Figure 4 shows the results of DSC measurement for 1M alloy. Reverse martensitic transformation during heating and forward martensitic transformation during cooling were observed in the 1st cycle. The martensitic transformation temperatures were determined to be Ms = 429 K, Mf = 358 K, As = 398 K and Af = 452 K (Ms, martensitic transformation starting temperature; Mf, martensitic transformation finishing temperature; As, reverse transformation starting temperature; Af, reverse transformation finishing temperature), and the thermal hysteresis defined as Af - Ms was 23 K. This small thermal hysteresis is typical in thermoelastic martensitic transformation.
No re-entrant martensitic
transformation was observed in the present alloy. After the 1st cycle, martensitic transformation was hardly detected in the 2nd heating/cooling cycle, which may be attributed to the aging effect. Further studies on this phenomenon are required. In order to investigate the magnetic properties of the parent and martensite phases, magnetization and thermomagnetization measurements were conducted for 1P and 1M. Figure 5(a) shows the magnetization curves at 6 K. The spontaneous magnetization of the parent phase in the 1P sample at 6 K was determined by the Arrott plot [25] to be 68 emu/g. Figure 5(b) shows the thermomagnetization curves in a heating process under a magnetic field of 500 Oe, where the Curie temperature of the parent phase in 1P is 429 K. The spontaneous magnetization of the ferromagnetic parent phase is comparable to 70~80 emu/g of the Co-Cr-Ga-Si system, but the Curie temperature is lower than that (about 550 K) of Co-Cr-Ga-Si [18, 24]. On the other hand, in 1M alloy, a weak ferromagnetic behavior was observed in Fig. 5(a). The residual parent phase was detected by XRD
measurement (Fig. 3) and the volume fraction of the parent phase can be roughly estimated as 9.2% using the peak ratio of the parent and martensite phases.
When the spontaneous magnetization value
for the parent phase is estimated as being 68 emu/g, the spontaneous magnetization at 6 K due to the residual parent phase is estimated to be 6.3 emu/g in the 1M sample, which is not so far from the measured value of 15 emu/g in Fig. 5(a). Therefore, it is considered that the magnetization in 1M of Fig. 5(a) is mainly due to the residual parent phase and that the martensite phase is probably paramagnetic. Decrease in magnetization is also observed at around 420 K in 1M (Fig. 5(b)), which is apparently due to magnetic transition of the residual parent phase at TC. While the reverse martensitic transformation is obtained in the DSC measurement in Fig. 4, no obvious change in magnetization associated with the martensitic transformation is detected, which is probably caused by a slight difference in composition between samples for the DSC and VSM measurements, even though they were cut out of the same ingot.
Nevertheless, it is considered that in 1M the martensitic
transformation temperature is very close to TC and that the transformation is basically from paramagnetic parent phase near Curie temperature to paramagnetic martensite phase.
3-3. Superelastic property Figure 6(a) shows the results of cyclic compression tests of the 2P sample at 298 K. The grain size is large in this sample and it contains at least 11 grains with almost random orientation distribution, as shown in the inset of Fig. 6(a). Almost perfect superelasticity was obtained, and the martensitic transformation starting stress Ms and the reverse martensitic transformation finishing stress Af were about 167 MPa and 52 MPa, respectively, for the 1st cycle. The hardening rate slightly starts to increase at about 3.2% strain, and thus the superelastic strain is about 2%. Figure 6(b) shows the orientation dependence of compressive transformation strain calculated by the lattice deformation [26]
using the lattice constants obtained by the XRD measurements shown in Fig. 3, where the maximum and minimum transformation strain is expected to be 6.1% and 0%, respectively. It is known [27-30] that superelastic strain depends on mean grain size relative to specimen size because of the constraint from surrounding neighboring grains, remarkably reduced in a polycrystalline sample. The experimental superelastic strain is within the calculated values from 0 to 6.1 %, and the relatively small superelastic strain may have been influenced by the grain constraint.
4. Summary Thermoelastic martensitic transformation from the L21 parent to the D022 martensite phase with a small thermal hysteresis of about 23 K was found in an off-stoichiometric Co2Cr(AlSi) Heusler alloy. The parent and martensite phases of the alloy have ferromagnetic and (probably) paramagnetic properties and superelasticity with a shape recovery strain of about 2% was obtained in a compression test at room temperature for the polycrystalline sample.
Thus, although the off-stoichiometric
Co2Cr(Al,Si) Heusler alloy was confirmed to be a shape memory alloy showing superelasticity, no re-entrant transformation was found in the present alloys.
Acknowledgments This study was supported by a Grant-in-Aid for Scientific Research and Japan Society for the Promotion of Science (JSPS). A part of the experiments were performed at the Center for Low Temperature Science, Institute for Materials Research, Tohoku University.
References [1] K. Ullakko, J. K. Huang, C. Kantner, R. C. O’Handley and V. V. Kokorin. Appl. Phys. Lett., 69,
1966 (1996). [2] R. Kainuma, Y. Imano, W. Ito, Y. Sutou, H. Morito, S. Okamoto, O. Kitakami, K. Oikawa, A. Fujita, T. Kanomata and K. Ishida . Nature 439 (2006) 957. [3] W. Ito, Y. Imano, R. Kainuma, Y. Sutou, K. Oikawa, K. Ishida. Metall. Mater. Trans. A. 38 (2007) 759-766 [4] Y.Q. Ma, C.B. Jiang, G. Feng, H.B. Xu. Scripta materialia.., 48 (2003) 365-369 [5] H.B. Xu, Y. Li, C.B. Jiang. Mater. Sci. Eng.A. 438-440 (2006) 1065-1070 [6] Y. Sutou, N. Koeda, T. Omori, R. Kainuma, K. Oikawa, K. Ishida. Appl. Phys. Lett. 85 (2004) 4358 [7] R. Kainuma, S. Takahashi, K. Ishida. Metall. Mater. Trans.A. 27 (1996) 2187-2195 [8] Y. Sutou, T. Omori, J.J. Wang, R. Kainuma, K. Ishida. Mater. Sci. Eng.A. 378 (2004) 278-282 [9] T. Omori, K. Watanabe, R. Y. Umetsu, R. Kainuma, K. Ishida. Appl. Phys. Lett. 95 (2009) 082508 [10] T. Omori, K. Watanabe, X. Xu, R. Y. Umetsu, R. Kainuma, K. Ishida. Scripta Materialia.., 64 (2011) 669-672 [11] W. Zhu, E. K. Liu, L. Feng, X. D. Tang, J. L. Chen, G. H. Wu, H. Y. Liu, F. B. Meng, H. Z. Luo. Appl. Phys. Lett. 95 (2009) 082508 [12] A. Kellou, N. E. Feninenche, T. Grosdider, H. Aourag, C. Coddet. J. Appl. Phys., 94 (2003) 3292 [13] M. Terada, Y. Fujita, K. Endo. J. Phys. Soc. Jpn. 36 (1974) 620. [14] S. Fujii, S. Ishida, S. Asano, J. Rhys. Soc. Jpn, 58, 3657 (1989). [15] M. Wuttig, J. Li, C. Craciunescu, Scr. Mater. 44 (2001) 2393-2397. [16] K. Oikawa, T. Ota, F. Gejima, T. Omori, R. Kainuma, K. Ishida. Mater. Trans. 42 (2001) 2472-2475 [17] K. Oikawa, T. Ota, Y. Imano, T. Omori, R. Kainuma, K. Ishida. J. Phase Equilib.Diffus. 27 (2006)
75-82 [18] X. Xu, T. Omori, M. Nagasako, A. Okubo R. Umetsu, T. Kanomata, K. IshidaK. R. Kainuma. Appl. Phys. Lett. 103 (2013) 164104 [19] S. Lee, E. Takahashi, N. Nomura, A. Chiba. J. Japan Inst. Metals. 70 (2004) 260-264 [20] S. Lee, E. Takahashi, N. Nomura, A. Chiba. Acta Biomater. 8 (2012) 2856-2862 [21] L. Z. Zhuang, E. W. Langer. J. Mater. Sci. 24 (1989) 381-388 [22] K. Ishikawa, M. Ise, I. Ohnuma, R. Kainuma, K. Ishida. Ber. BunsenGes. Phys. Chem. 102 (1998) 1206 [23] R. Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida. Scripta materialia.., 58 (2008) 723-726 [24] R. Y. Umetsu, A. Okubo, X. Xu and R. Kainuma. J. Alloys Compd., 588 (2014) 153-157 [25] A. Arrott. Phys. Rev. 108 (1957) 1394 [26] T. Saburi and S. Nenno, in Proc. Int. Conf. Solid-Solid Phase Transformations, H. I. Aaronson, D. E. Laughlin, R. E. Sekerka, C. M. Wayman eds., Pittsburgh, 1981, pp. 1455-1479. [27]N. Ono, A. Sato, Trans. Japan Inst. Metals 29 (1988) 267 [28]N. Ono, A. Sato, H. Ohta. Trans. JIM 30 (1989) 756. [29]N. Ono, Mater. Trans. JIM 31 (1990) 381. [30]Y. Sutou, T. Omori, K. Yamauchi, N. Ono, R. Kainuma, K. Ishida. Acta Mater. 53 (2005) 4121.
Figure Captions Figure 1. Typical microstructures of parent and martensite phases taken by an optical microscope from (a) Co55Cr23(Al11Si11) (1P) and (b) Co57Cr21(Al11Si11) (1M) alloys solution-treated at 1473 K for 24 h. Figure 2. Bright-field images of (a) Co55Cr23(Al11Si11) (1P) and (b) Co57Cr21(Al11Si11) (1M) quenched from 1473 K observed using a transmission electron microscope and (insets) their corresponding selected-area diffraction patterns. Specimens were tilted to (a) the and (b) the
matrix zone axis of 1P
matrix zone axis of 1M, respectively.
Figure 3. X-ray diffraction patterns measured at room temperature for Co55Cr23Al11Si11 (1P) and Co57Cr 21Al11Si11 (1M), where Co-K was used as the radiation source. Powder and bulk samples were used for 1M and 1P alloys, respectively. Figure 4. Differential scanning calorimetry (DSC) measurements for Co57Cr21Al11Si11 (1M) at 10 K/min. Martensitic transformation starting temperature (Ms), martensitic transformation finishing temperature (Mf), reverse transformation starting temperature (As), and reverse transformation finishing temperature (Af) were determined in the 1st cycle. Figure 5. (a) Magnetization (M-H ) curves measured at 6 K and (b) thermomagnetization (M-T ) curves under a magnetic field of 500 Oe for Co55Cr23Al11Si11 (1P) and Co57Cr21Al11Si11 (1M). Tc is the Curie temperature of the parent phase. Figure 6. (a) Stress-strain curves obtained by compression tests for Co55Cr25Al10Si10 (2P) alloy with large grains. The measurements were performed three times. The inset shows orientations in the compression direction measured by EBSD. (b) The orientation dependence of transformation strain (compression) is shown in stereo-graphic triangle, which was obtained using theoretical calculation of the lattice deformation [26] with the experimental lattice parameters shown in Fig. 3.
220
1P (Parent) ە
620
440 420
400
200
111
ە
112
ە
70
90
110
2 Theta (Deg.)
224
204
200
ە
50
1M (Martensite)
312
200
ە
ە
Intensity
ە
ە
30
L21 D022 fcc
130
150
1M (Martensite)
2nd cycle
g n oli
1st cycle
Co
Exothermic
Ms = 429 K Mf = 358 K Af = 452 K As = 398 K
Heating
300
350
400
450
Temperature (K)
500
Highlights Article : Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys ・Thermoelastic martensitic transformation was found in the Co2Cr(AlSi) alloys. ・The parent and martensite phases have L21 and L10 structures, respectively. ・The parent phase was determined to be in ferromagnetic state. ・The martensite phase was confirmed to have weakly magnetic state. ・Over 2% of superelasticity was obtained for the polycrystalline sample by compression test.