Evolution of martensite start condition in general thermomechanical loads of Fe-based shape memory alloy

The martensitic transformation condition in an Fe-9%Cr-5%Ni-14%Mn-6%Si poly crystalline shape memory alloy is investigated during non-proportional tens ile/compressive-torsional loading. The initial martensite start condition i s not governed by the von Mises-type condition in the stress-temperature sp ace. A linear temperature dependence of the martensite start stress is obse rved. The martensite start stress increases, both in the initial loading an d in the subsequent loading, with the progress of prior martensitic transfo rmation. The concept of linear hardening in plasticity explains well the ex perimental results. On the contrary, the subsequent martensite start stress returns to the initial value, meaning the alloy exhibits the softening whi le the reverse transformation progresses. (C) 1999 Elsevier Science Ltd. Al l rights reserved.