Experimental approach of the martensitic transformation in shape-memory alloys and TRIP steels

Abstract
Several materials exhibit a transformation from austenite to martensite during a thermomechanical loading. This phase transformation is responsible for remarkable mechanical properties such as the superelasticity in shape memory alloys (SMA) or the excellent compromise strength/ductility in TRIP-steels (TRansformation Induced Plasticity). To optimize these properties, it is useful to model the behavior of these materials, which requires correlating the evolution of the microstructures with their mechanical behavior. Therefore, the local stress state has to be linked to the metallurgical variations. The present work focuses on the stress distribution in each phase of multi-phased materials with an evolving microstructure. Experimental results based on X-Ray diffraction are presented in the case of TRIP steels and SMA: in TRIP steels, it is shown that the austenite can be in tension or in compression after a tensile test, depending on the nature of other phases. In shape memory alloys, the austenite stress state was determined in a single grain of a polycrystalline sample. It was shown that the transformation of the austenite into martensite is associated with a rather constant stress function on the crystal orientation.