Thermomechanical behaviour of FCC _HCP martensitic transformation in CoNi

Thermomechanical behaviour of the FCC HCP martensitic transformation in a Co-32wt%Ni single crystal was investigated. The measurement was carried out both by isothermal deformation testing and by heating and cooling under constant load. It was found that the alloy was capable of extensive tensite deformation. Under constant loads, FCC HCP martensitic transformation can be induced on cooling, whereas on heating the reverse transformation from HCP to FCC was induced. This is due to the multiplicity of the reverse transformation, which permits martensite to be further deformed in the direction of the extemal stress to austenite. This behaviour provides a unique case of a negative dependence of the critical transformation temperature on stress, in contrast to conventional thermal shape memory alloys.