Transformation behavior and microstructures of PtNiMnGa ferromagnetic shape memory alloys

Transformation characteristics and optical microscope (OM) and transmission electron microscopy (TEM) microstructures of Pt50Mn25Ga25 and Pt10Ni40Mn25Ga25 ferromagnetic shape memory alloys (FMSMAs) have been examined. Ingots of those alloys were prepared by argon arc-melting, and the ingots were homogenized at 1173 K for 72 h and then furnace-cooled at 10 K/min. Transformation temperatures were examined by an internal friction technique and/or a differential scanning calorimeter. OM microstructures taken by a polarized microscope showed typical self-accommodated martensitic structures. TEM observations were carried out by using a JEOL-2000FX microscope operated at 200 kV. High-density of striations were observed in the martensites of TEM bright field images. Selected area electron diffraction (SAED) patterns and TEM dark field images revealed the striations to be thin twin plates. The analyses of SAED patterns indicated that the Heusler-type bcc parent phase transforms to a tetragonal or orthorhombic martensite phase. The tetragonal or orthorhombic distortions were estimated to be c/a≈0.94 and/or c/b≈0.96.