Some factors affecting the shape recovery properties of NiTi SMA

Abstract

NiTi shape memory alloys (SMAs) have an excellent combination of properties including especi ally shape memory effect, superelasticity, anti -corrosion resistance, high damping capacity, high fatigue resistance, and the great kinetic output among many actuator materials. Thermomechanical treatments can greatly influence their shape recovery propert ies, modifying their transformation stress, strain, and temperatures. A treatment process commonly known as “ training ” where the specimen is repeatedly thermal cycled under constant stress /strain , is effective in developing stress -assisted two -way memory e ffect (SATWME). Subseque nt to training a two -way memory effect (TWME) is further developed when the stress is released. In this research, the effect of martensite deformation and constraint condition on the development of SATWME and TWME was investigated. The expe rimental results show that the SATWME an d TWME are affected by four major factors: pre -strain , total martensite strain, constrained stress, and thermal cycling. Increasing the de gree of martensite deformation leads to a martensite reorientati on and dislocation generation . Full utilization of martensite reorientation c apacity, be it achieved by the pre -stain ing or by the thermal cycling under constrained stresses , resulted in the development of maximum SATWME and TWME. The ma gnitude of constrained st ress i s critical in determi ning the generated internal stress and the internal plastic deformation. The constrained thermal cycling caused the rearrangement of microstructure s and a two step transformation which influences the development of SATWME and TWM E. Based on the differential scanning calorimetry measurement and thermodynamic analysis, although the thermomechanical mechanisms involved in the development of SATWME and TWME are similar at their maximum magnitudes, they are different before and after t he occurrence of maximum values.