Formation of Isothermal α′′<sub>iso</sub> Phase and Transition to α Phase in Ti–10Mo–7Al Alloy

The aging behavior and microstructural changes of Ti–10Mo–7Al alloy at 250°C were investigated. The α′′Mq phase generated by quenching and the α′′Md phase induced by deformation underwent β-reverse transformation during short aging at 250°C. However, continued aging at 250°C led to the formation of α′′iso within 3 minutes. When a U-shaped bent specimen was aged at 250°C, shape recovery occurred, followed by shape change in the bending direction. Additionally, large undulations appeared even on the undeformed, mirror-finished specimen surface after aging at 250°C. These phenomena indicate that the β reverse transformation at 250°C is metastable and temporary. In the material aged at 250°C for 19.4 Ms, single-crystal regions several hundred microns in size formed, consisting of a single α′′iso variant. The formation of this microstructure suggests that compositional distribution of solute atoms is unnecessary for α′′iso formation in this alloy. After additional aging at 600°C, the α′′iso single-crystal region transformed into a lamellar structure of a single α variant and β phase with a Burgers orientation relationship, indicating the onset of molybdenum compositional distribution. The elongation direction of the lamella coincided with the lattice invariant line direction of the α and β phases, satisfying the Burgers orientation relationship.