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 polycrystalline shape memory alloy is investigated during non-proportional tensile/compressive-torsional loading. The initial martensite start condition is not governed by the von Mises-type condition in the stress–temperature space. A linear temperature dependence of the martensite start stress is observed. The martensite start stress increases, both in the initial loading and in the subsequent loading, with the progress of prior martensitic transformation. The concept of linear hardening in plasticity explains well the experimental results. On the contrary, the subsequent martensite start stress returns to the initial value, meaning the alloy exhibits the softening while the reverse transformation progresses.