Effect of precipitation hardening and thermomechanical training on microstructure and shape memory properties of Ti_(50)Ni_(15)Pd_(25)Cu_(10) high temperature shape memory alloys

摘要

Microstructural analysis, evolution of phase transformation temperatures and shape memory properties for Ti_(50)Ni_(15)Pd_(25)Cu_(10) 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 Ti_2Pd. 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 Ti_(50)Ni_(15)Pd_(25)Cu_(10) high temperature shape memory alloys.