Evolution of high aspect ratio grains in a TiAl-based alloy by directional grain growth

Abstract

A concept of microstructural control named “directional grain growth” (DGG) has been proposed. In this method, local phase transition and grain growth are utilized for controlling grain shape and grain orientation in a restricted region for an inhomogeneous material in chemical composition which is given directionality by plastic deformation. In heat treatment for DGG, a seed grain grows preferentially in a shaped region without precipitates by being held at a relatively low temperature and then enlarges further by encroaching the surrounding region where precipitates vanish gradually with temperature elevating at a slow rate. Such a DGG heat treatment was applied to a Ti–45.6at %Al–1.7at.%Mn alloy fabricated by reactive sintering of an extruded mixture of elemental powders and contained inevitably directional inhomogeneity. As a result, a structure was obtained consisting of giant columnar grains of Al-bearing αTi phase of several mm in length, 6.0–7.0 in aspect ratio and a preferred orientation. The axis of the columnar grains was approximately perpendicular to [0001] α .