Effect of heat treatment time on microstructure and mechanical properties of Ti-19Nb-9Zr (at%) shape memory alloy

The effect of heat treatment time on microstructure and shape memory effect of Ti-19Nb-9Zr (at%) alloy is investigated. The transformation energy and constituent phase show tremendous sensitivity to the heat treatment time. In the present work, the HT10 (Heat-treated for 10 min) TiNbZr specimen consists of a large amount of martensites and obtained the highest martensitic transformation energy (∼11 J/g) while the HT80 (Heat-treated for 80 min) TiNbZr shows a predominant β structure with very low martensitic transformation energy. Furthermore, athermal ω phase is found in the HT80 TiNbZr specimen. Since a longer heat treatment time followed by quenching (with larger grain size) is in favor of the formation of athermal ω phase, we suggested that the fine athermal ω phase may be attributed to the decrease of thermodynamic driving force of martensitic transformation. Finally, perfect shape memory effect is obtained after heating when the maximum pre-strains loading up to 6% in the precipitate free HT10 TiNbZr specimen.