Severe plastic deformation of a Cu-Al-Ni-Mn shape memory alloy

Abstract

Shape memory alloys (SMA) have the ability to recover the initial shape after permanent deformation when heated up to a critical temperature called martensitic transformation temperature. In this temperature, the alloy undergoes a structural transformation which promotes the shape recovery. This "shape memory effect" has been used in several applications including sensors, actuators and damping devices. Among the most used SMA are the TiNi- and Cu-based alloys. The latter exhibit some advantages when compared with the former as lower cost and an easier processing. Nevertheless, these Cu-based SMA are susceptible to present an intergranular brittle fracture because their large elastic anisotropy, which results in stress concentration at the grain boundaries. This may cause a lower ductility and a lower shape recovery. By reducing the grain size, the intergranular fracture tendency is reduced and the fatigue life is extended. Among the several methods used to promote a microstructural refinement, there is the equal channel angular pressing - ECAP. This method enables to form nanometric-sized grains with a large amount of defects as dislocations, stacking faults and twinning, which make sometimes necessary to perform thermal treatments after processing in order to reduce these defects. Not only the microstructural refinement may affect the properties of shape memory alloys but also the large amount of defects generated. Only a few works were carried out about ECAP processing of Cu-based SMA. A systematic investigation is needed in order to investigate the effect of ECAP in the grain refinement and how the smaller grains and their large amount of defects may affect the thermal and mechanical properties of these alloys. Considering this, the present work aims to investigate the influence of ECAP processing in the microstructure, phase formation and mechanical and thermal properties of a Cu81.95Al11.85Ni3.2Mn3 (%wt.) SMA. Samples will be prepared by ECAP using different processing conditions and they will be characterized by optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, compression and hardness tests. (AU)