Suppression of Martensitic Transformation in Co 2 Cr(Ga,Si) Heusler Alloys by Thermal Cycling
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-based Heusler alloys have been extensively studied both theoretically and experimentally for they are good candidates of half-metallic ferromagnets (HMFs).[1–7] Some Co-based Heusler alloys exhibit martensitic transformation, and the first report of martensitic transformation (MT) in Co-based Heusler alloys was made in a non-half-metallic Co2NbSn alloy by Terada et al.[8] Recently, an unusual MT was reported in an off-stoichiometric Co2Cr(GaSi) Heusler alloy by Xu et al.[9] In this alloy, after normal MT from the high-temperature parent (H) phase (L21-type structure) to the martensite phase (D022-type structure), the martensite phase transforms back to the low-temperature parent (L)
XIAO LIANG and FEI XIAO are with the State Key Lab of Metal Matrix Composite, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P.R. China. Contact e-mail: [email protected] XUEJUN JIN is with the Institute of Advanced Steels and Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. TAKASHI FUKUDA and TOMOYUKI KAKESHITA are with the Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan. Manuscript submitted October 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
phase (L21, BCC) during the further cooling process. This unusual transformation behavior is termed as reentrant martensitic transformation (RMT).[9] Due to this RMT, Co2Cr(GaSi) shows shape recovery during the cooling process as well as conventional shape recovery in the heating process after deformation by detwinning in the martensite state. This new property could be used in applications that require multi-sensing, which is currently accomplished by complicated temperature-controlling systems.[9] Moreover, the RMT in Co2Cr(GaSi) alloys could also be applied on refrigeration areas by exploiting their magnetocaloric[10] and elastocaloric effects.[11] We recently reexamined the RMT in Co2Cr(GaSi) reported by Xu et al.[9] Through the study on RMT, we noticed that the transformation behavior was largely changed by repeating the successive L21(L)– D022–L21(H) transformation. However, as far as the authors are aware, there is no report on the effect of thermal cycling on the MT in Co2Cr(GaSi). In this paper, therefore, we make a brief report on the influence of thermal cycling on the MT of a Co2Cr(Ga,Si) ferromagnetic Heusler alloy. Ingot of Co51Cr27Ga11Si11 (at. pct) alloy was prepared by arc melting from high-purity Co bulk (99.98 pct), Cr piece (99.98 pct), Ga piece (99.99 pct), and Si sheet (99.999 pct) under an Ar atmosphere. Small pieces of specimens with the diameter of about 4 mm were cut. The cut specimens were solution-treated at 1373 K (1100 C) for 24 hours and quenched into water. The crystal structure of the as-quenched specimen was determined using a Rigaku SmartLab X-ray diffractometer (XRD) at room temperature [~296 K (23 C)], and Cu Ka was used as the
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