Effect of precipitation hardening and thermomechanical training on microstructure and shape memory properties of Ti50Ni15Pd25Cu10 high temperature shape memory alloys

Microstructural analysis, evolution of phase transformation temperatures and shape memory properties for Ti50Ni15Pd25Cu10 high temperature shape memory alloys were investigated in solution treated and aged conditions. It is shown that aging at 600 °C for 3 h resulted in the formation of two types of fine precipitates i.e. TiPdCu and Ti2Pd. It is observed that due to the formation of these precipitates, martensite start temperature under stress free and constrained (500 MPa) conditions are decreased by 32 °C and 29 °C respectively. Recovery ratio of 76% obtained for solution treated sample increased to 94% at 500 MPa for the 600 °C aged sample; resultantly the irrecoverable strain is decreased by 18%. Training for 5 thermal cycles was carried out under stress free condition resulted in the decrease of thermal hysteresis by 5 °C for the solution treated sample, whereas thermal hysteresis of the aged sample remained stable. Similarly thermomechanical training cycles at 500 MPa demonstrated an improvement in the recovery ratio by 14% and 5% for solution treated and 600 °C aged samples respectively. Aging at 600 °C for 3 h and thermomechanical training greatly improved the cyclic stability and shape memory properties of Ti50Ni15Pd25Cu10 high temperature shape memory alloys.