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 alpha '' and body-centered-cubic (bcc) beta 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 alpha '' martensite in the beta 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 beta phase due to precipitation of isothermal omega phase, and the latter by suppression of alpha '' martensitic transformation. These results clearly show that the formation of alpha '' martensite and isothermal omega phases from the unstable beta phase influences shape memory behaviors in Ti-Nb-Sn alloys.