Variant selection during cooling after beta annealing of Ti-6Al-4V ingot material

Abstract

The selection of alpha variants during the cooling of Ti-6Al-4V from the beta-phase field was investigated. For this purpose, samples with a coarse, columnar beta-grain structure with a 100 fiber texture were extracted from an as-cast production-scale ingot. The alpha variants in the as-cast samples as well as those produced during several successive beta-annealing treatments were determined using electron backscatter diffraction (EBSD). The EBSD results indicated that a subset of the 12 possible variants was developed within each grain; the specific variants were a function of the cooling rate after beta heat treatment. Moreover, the generation of similar variants during successive heat treatments involving an identical cooling rate suggested a noticeable memory effect. The variant selection process was rationalized based on calculations of the strain associated with the beta-to-alpha transformation. These calculations revealed that the overall aggregate strain approached zero in both the as-cast condition as well as after beta heat treatment, suggesting the occurrence of a long-range self-accommodation mechanism. © 2012 The Minerals, Metals & Materials Society and ASM International.