Microstructures and transformation behavior of CoNiGa ferromagnetic shape memory alloys

摘要

TEM observations and transformation characteristics of Co_2Ni_(1-x)Ga_(1+x) ferromagnetic shape memory alloys will be presented. The dependence of the martensitic transformation start temperatures on the valence-electron atom ratio is identified to that of NiMnGa alloys. Optical microstructures show typical self-accommodated structures. The domain boundaries are straight indicating only elastic distortions in the product phase. High-density striations are observed ion the TEM bright field images of the martensite-phase. Selected area electron diffraction patterns and TEM dark field images revealed that they are thin twin plates. Tweed or striated contrast is observed in the TEM bright field images of the parent phase at temperatures about 70 K higher than the M_s temperature.