Effect of β grain growth on variant selection and texture memory effect during α → β → α phase transformation in Ti-6 Al-4 V

In the present study, the texture evolution and the role of beta grain growth on variant selection during beta -> alpha phase transformation have been investigated in Ti-6 Al-4 V with and without 0.4 wt.% yttrium addition. The aim of adding yttrium was to control beta grain growth above the beta transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate similar starting microstructures and crystallographic textures. Subsequently, both materials were solution-heat-treated above the beta transus followed by slow cooling to promote growth of the alpha lath structure from grain boundary alpha. Additional interrupted slow cooling experiments were carried out to identify the alpha lamellae that nucleate first from beta grain boundaries. Detailed electron backscatter diffraction analysis was carried out and it was found that the beta heat treatment did not generate new texture components although the intensities of the individual components changed dramatically depending on the alloy/beta grain size. Variant selection was assessed by comparing measured alpha texture components with predicted alpha texture components based on the high-temperature beta texture assuming equal variant selection. It was found that with increasing beta grain size variant selection intensified favouring the {phi 1, Phi, phi 2} {90 degrees, 30 degrees, 0 degrees} texture component. Interrupted cooling experiments revealed that alpha nucleates first on beta grain boundaries that are formed by two beta grains having a common (1 1 0) normal and that these alpha lamellae display almost exclusively a {phi 1, Phi, phi 2} {90 degrees, 30 degrees, 0 degrees} orientation. Consequently, the dominance of this variant with increasing beta grain size can be related to the relative free growth of this particular alpha texture component into an empty beta grain. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.