Résumé
Shape memory alloys (SMA) are materials whose reputation has grown significantly in recent decades in various fields, especially since their classification as smart materials due to their specific properties of shape memory effect (SME) and superelasticity (SE).The microstructure of metals and alloys consists of a group of grains, which are generally oriented randomly. The evolution and orientation of grains can be controlled by the manufacturing process and thermomechanical treatments to which the material is subjected. Optimizing the microstructure and surface properties of a material is very important to achieve efficient mechanical behaviour. The application of appropriate thermomechanical treatments is the key to optimize the material properties to a given application. Recently, especially for NiTi shape memory alloy, several studies have shown that the Surface Mechanical Attrition Treatment (SMAT) process is an effective technique for improving mechanical properties. The main objective of this study is to determine the effect of SMAT on the microstructure and on the mechanical behaviour of NiTi shape memory alloy. Equiatomic (50.0 at. %) NiTi samples have been analyzed before and after SMAT with several techniques. First of all, Differential Scanning Calorimetry (DSC) analyses have been achieved to determine the optimum temperature for heat treatment and their influence on the microstructure and properties of NiTi alloy. Thus, using InfraRed Thermography and Digital Image Correlation, tensile tests have been performed to determine the material mechanical and energy responses to identify phase transformation phenomena. Microstructural investigations have been carried through optical and scanning electron microscopy (OM and SEM). The technique of Electron Backscattered Diffraction (EBSD) was used to analyse more precisely the microstructure of the alloy and to evaluate the effect of SMAT treatment on grain size and grain orientation. These techniques are very useful to understand the general state of the microstructure and the experimental protocol used in this work provides a complete view of the thermomechanical properties of the studied shape memory alloys.