In-Situ Transmission Electron Microscopy Observation on the Phase Transformation of Ti-Nb-Sn Shape Memory Alloys

The microstructural changes accompanied by phase transformation during heating and cooling of quenched (Ti-35 wt pct Nb)-0 to 6 wt pct Sn alloys have been investigated using in-situ transmission electron microscopy (TEM). Reversible transformation between orthorhombic α″ and body-centered-cubic (bcc) β phases occurs on heating and cooling between 300 and 423 K in quenched (Ti-35 wt pct Nb)-3 and 5 wt pct Sn alloys, which consists of α″ martensite in the β matrix with diffused scattering after quenching. On the other hand, the reversibility was not observed in (Ti-35 wt pct Nb)-0 and 3 wt pct Sn alloys heated to 523 K or in quenched (Ti-35 wt pct Nb)-6 wt pct Sn alloy. The former is explained by stabilization of β phase due to precipitation of isothermal ω phase, and the latter by suppression of α″ martensitic transformation. These results clearly show that the formation of α″ martensite and isothermal ω phases from the unstable β phase influences shape memory behaviors in Ti-Nb-Sn alloys.