Enhancement of Shape Memory Properties through Precipitation Hardening in a Ti-Rich Ti-Ni-Pd High Temperature Shape Memory Alloy

Abstract

Effects of aging on martensitic transformation temperature, microstructure and shape memory characteristics were investigated for Ti₅₀Ni₂₀Pd₃₀ and Ti₅₂Ni₁₈Pd₃₀ alloys. There was no significant change in the microstructure and transformation temperature by aging, irrespective of aging temperature, in case of the Ti₅₀Ni₂₀Pd₃₀ alloy. On the other hand, fine Ti₂Pd type precipitates were formed upon aging in the Ti₅₂Ni₁₈Pd₃₀ alloy within the aging temperature range of 673-1073 K. The transformation temperature of the Ti₅₂Ni₁₈Pd₃₀ alloy was sensitively dependent on the aging temperature. The Ti₂Pd type nanoscaled precipitates were found to be very effective in enhancing the high temperature dimensional stability during shape memory cycling. Excellent shape memory properties with a large recovery strain and a small plastic strain even at 500 MPa were observed in the Ti₅₂Ni₁₈Pd₃₀ alloy solution treated and aged at lower temperatures of 673 and 773 K owing to the formation of fine and dense Ti₂Pd type precipitates.