H. Kushida, K. Hata, D. Matsuwaka, T. Terai, T. Fukuda, T. Kakeshita, Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan
Ni2MnGa is known to transform from the parent phase (P-phase) to a martensite phase (M-phase) via an intermediate phase (I-phase). Recently, a new phase (X-phase) was found to appear under a compressive stress applied along [001]P (P stands for the parent phase). In this study, we have investigated details of the phase boundaries between the X-phase and other (P-, I- and M-) phases. The X-phase is induced by the compressive stress from the P-phase above 254 K, and also from the I-phase between 248 K and 206 K. The M-phase is induced from the X-phase above 206 K, while it is induced from the I-phase between 206 K and 199 K (the martensitic transformation temperature). No other stress-induced transformation has been detected up to the compressive stress of 300 MPa (the maximum stress of the present study). The slope of the critical stress of a transformation against temperature, dσ/dT is negative for the I → X transformation, while it is positive for other transformations. There is a triple point at which the M-, I- and X-phases coexist (206 K, 12 MPa). However, there is not a triple point at which the X-, I- and P-phases coexist, meaning that the X-phase appears even when the specimen is cooled without applying stress. That is, the transformation sequence of Ni2MnGa is P → X → I → M, being different from the P → I → M regarded so far.
Summary: Stress-temperature phase diagram of Ni
2MnGa has been investigated under a compressive stress applied along [001] of the parent phase. There is a triple point at which martensite, intermediate and X-phases coexist (where X-phase is a new phase found by our group before [Scr. Mater. 54 (2006), 585]). The X-phase has been found to appear even in the stress-free condition between the parent phase and intermediate phase in the cooling process.
