Nucleation and growth of alpha phase in hot extruded Ti-46.6Al-2W-1.4W intermetallic alloy produced by hot extrusion of elemental powders

Abstract

The present study is concerned with nucleation and growth of alpha phase during heat treatment in Ti-Al-Mo-Mn intermetallic alloys processed by hot extrusion of elemental powders. TEM and X-ray diffraction analysis show that the microstructure of the as-hot extruded alloy consists of gamma, alpha(2) and B2 phases. The volume fractions of alpha(2) and B2 phases are 16 and 20%, respectively, which shows that B2 phases that contain significant amounts of Mo have an orientation relationship with gamma plates inside them. Microstructural evolution during heat treatment reveals that the alpha(2) phase nucleates and grows in lamellar form by edgewise and sidewise mechanism. Edgewise growth of the alpha(2) phase is more rapid than sidewise growth due to the coherent interface between alpha(2) and gamma phases along the sidewise direction. Sidewise growth of the alpha(2) phase occurs by a ledge mechanism. It has been shown that alpha phase nucleates in gamma plates located at the lamellar boundaries and grows along a direction that can produce an incoherent interface. gamma phase present in the B2 phase affects the growth of the alpha phase by providing a preferential site for alpha growth. (C) 2004 Elsevier B.V. All rights reserved.