Isothermal Transformation of Ti-Mo Binary Alloy

Abstract

The aim of the present work is to obtain more detailed information on the phase transformation of metastable beta-type titanium alloys, Ti-(12∼20)Mo. Preliminary investigations revealed that an automatic transformation apparatus “Formastor-F” was applicable to titanium alloys. The phase changes of these alloys during isothermal transformation were mainly detected by the changes in dimension, hardness and microstructure using this apparatus, and then TTT diagrams were obtained. The start point of the contraction coincides with the point where quenched omega transforms to aged omega in the TTT diagram, and the starting point of expansion or the contraction end point also coincides with the point where softening due to the omega→alpha transformation occurs. The contraction and hardening of these three alloys were maximum at 400°∼450°C. In the Ti-15Mo specimens isothermally transformed at 650° and 700°C, the alpha phase precipitated along the grain boundary, while the alpha phase precipitated in the grain at 500°∼600°C. This indicates that the beta→alpha reaction is not so easy as the beta→omega→alpha reaction and occurs only at grain boundary at high temperature.