Microstructure and martensitic transformation characteristics of CoNiGa high temperature shape memory alloys

• Published in: Acta materialia 2011-02, Vol.59 (3), p.1168-1183
• Main Authors: Dogan, E., Karaman, I., Chumlyakov, Y.I., Luo, Z.P.
• Format: Article
• Language: English
• Subjects: e/ a ratio; High temperature shape memory alloys; Hysteresis; Martensitic transformation; Martensitic transformations; Microstructure; Precipitates; Precipitation; Symmetry conforming short range order; Texture; Thermal cycling; Thermal stability; Transformation temperature
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2010.10.050

Microstructural evolution and martensitic transformation characteristics of Co 46Ni 27Ga 27 and Co 44Ni 26Ga 30 high temperature shape memory alloys were investigated in as-cast and hot-rolled conditions as a function of heat treatment. Heat treatments were selected to introduce single-, two-, and three-phase structures, where the precipitate phases do not martensitically transform. The effects of these precipitates, and associated compositional changes, on transformation temperatures, thermal hysteresis, and microstructural evolution during thermal cycling, were revealed . It was found that martensite start temperature linearly depends on the valence electron concentration ( e/ a) of the matrix, if the Ga content is constant. For a given e/ a, the higher the Ga content is, the higher the transformation temperatures become. The presence of γ′ precipitates and the volume fraction of γ phase were shown to have strong influence on transformation thermal hysteresis. The most cyclically stable compositions with narrow hysteresis (