A new ordered orthorhombic phase in a Ti₃Al-Nb alloy

Abstract

An ordered orthorhombic phase has been detected in a Ti-25at.% Al-12.5at.%Nb alloy which is one of a family of Ti₃Al base alloys under development for high temperature applications. Convergent beam electron diffraction and channelling enhanced microanalysis have been used to assign a space group to the structure and determine atomic positions. The ordered orthorhombic phase has a Cmcm symmetry and is isomorphous to the NaHg type of structure. Preliminary studies on the effect of heat treatment on the formation of this new phase suggest that the Ti₃Al free-energy composition curve develops an inflection at low temperatures with Nb addition leading to a phase separation reaction in which the Nb rich component has the orthorhombic symmetry.