Heat Treatments of Fe-Mn-Si Based Alloys: Mechanical Properties and Related Shape Memory Phenomena

Heat treatments are usual means for modifying alloy microstructures and, consequently, to control mechanical properties. The aim of this work was to find suitable processes for improving the shape memory effect (SME) of Fe–Mn–Si-based alloys. In particular, we studied mechanisms that affect the plastic deformation of the austenite phase. A Fe–15Mn–5Si–9Cr–5Ni alloy was deformed by rolling at different temperatures and subsequently annealed at recovery- and recrystallization-temperature. The mechanical properties of the material after processing were evaluated by performing tension and flexure tests. The SME of room temperature deformed specimens was measured after heating them to 550°C for shape recovery. We found that the material rolled at 800°C followed by an annealing treatment at 650°C recovers nearly 95 % of a 3 % deformation. In this thermo-mechanical condition, the material has a yield stress of 450 MPa and an ultimate tensile strength of 880 MPa, corresponding to a total elongation of about 16 %. Optical and electron microscopy observations show that the matrix annealed at high temperature contains a low density of defects. As a consequence, there are fewer nucleation sites for martensite and the associated SME is low. On the other hand, annealing at intermediate temperatures (around 650°C) produces a favorable structure containing a large density of stacking faults.