Martensitic transformations in Ti-Mo alloys

Abstract

A detailed investigation has been made of the structure of alloys of the Ti-Mo system containing up to 10wt% Mo, water-quenched from theβ-phase region. With increase in molybdenum content, the martensite structure changes from hexagonal (α′) to orthorhombic (α″) at ∼4 wt% Mo, and at 10 wt% Mo, the structure is completely retained β. For alloy compositions <4 wt% Mo, there is a diffusional component in the transformation of the β-phase at the quench rates employed. There is a transition, with increase in molybdenum content, in morphology (from massive to acicular) and in substructure (from dislocations to twins). However, the transitions in crystallography, morphology and sub-structure are not directly related to one another except for an abrupt loss of dislocation substructure at theα′/α″ transition. The α toα″ crystallographic transition has the characteristics of a second order transformation, and evidence has been obtained of the existence of a spinodal within the metastable orthorhombic system. The orthorhombic martensites of Ti-6 and 8 wt% Mo decompose during quenching producing a fine modulated structure within the martensite plates, consistent with a proposed spinodal mode of decomposition.