Formation of Isothermal α”iso Phase and Transition to α Phase in Ti–10Mo–7Al Alloy

Yoshito Takemoto, Kohei Nojima, Kouki Morita, Yuki Suzuki, Jinta Arakawa and Ichiro Shimizu

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.

Vol. 67

No. 4

446

454