Phase Transformation Behavior of Ti-10Mo-7Al Alloy with Heat Treatment

Abstract

  A novel phenomenon recently observed in the tempering of Ti-4Fe-7Al alloy, has attracted considerable attention. This phenomenon is induced by the β→α″ transformation during tempering, and its noteworthy features are rapid hardening and a shape change in a U-shaped specimen. It is of interest whether this phenomenon occurs in other alloy systems. In this study, Ti-10Mo-7Al alloy, which contains a quantity of molybdenum corresponding to the lower β limit, was prepared and its microstructure, aging behavior, and shape change during tempering were investigated. An acicular α″ martensites were observed preferentially at the β grain boundaries in the quenched specimen, but it became invisible at the optical microscope level upon tempering. However, TEM (transmission electron microscopy) observation of the tempered specimen revealed nano size martensites instead of micron-size martensites. The novel phenomenon observed in the tempering of Ti-4Fe-7Al, was also confirmed to occur in Ti-10Mo-7Al alloy. In the heating of a U-shaped specimen, shape recovery occured first, then a shape return into a U-shape followed, and a curvature progressed more with increasing of temperature. As a result, the phenomenon was clarified to be a universal one. The transformation mechanism through tempering is explained as follows: Though aluminum addition to β-alloy raises the M_S chemically, on the other hand, it causes a remarkable solution strengthening, which increases shear stress required for martensite transformation, and decreases the apparent M_S. Consequently, the β phase is maintained by a rapid cooling. However, temperature rise decreases the shear stress, and the apparent M_S increases to a chemically intrinsic one. If the M_S increases beyond the tempering temperature, martensite transformation occurs abruptly.