Early Stages of Microstructure and Texture Evolution during Beta Annealing of Ti-6Al-4V
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INTRODUCTION
BETA annealing is often used for alpha/beta titanium alloys to develop a transformed microstructure for fracture-critical aerospace applications. Beta annealing is also applied as a solution treatment for beta and near-beta alloys prior to aging in the alpha/beta phase field. In both cases, the control of the beta grain size, resulting alpha colony size, and texture can be very important to mechanical properties. Because of the industrial importance of the workhorse alpha/beta titanium alloy, Ti-6Al-4V, numerous investigations of its beta-grain-growth characteristics have been performed. Some of these efforts have indicated a parabolic behavior which characterizes classical normal grain growth; i.e., dn ~ t, in which d denotes the average grain size, t is time, and n is the grain-growth exponent.[1] On the other hand, a number of measurements have shown substantial deviations from parabolic behavior.[2–6] Specifically, periods of rapid and slow growth have been observed during both isothermal heat treatments and processes involving continuous heating. Furthermore, grain-growth kinetics have been found to
A.L. PILCHAK and S.L. SEMIATIN are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXCM, Wright-Patterson Air Force Base, OH 45433. Contact e-mail: [email protected] G.A. SARGENT is a consultant for UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. Manuscript submitted May 15, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
vary quite noticeably in different lots of Ti-6Al-4V with nominally identical composition and initial microstructure, but different initial textures. These observations have been rationalized on the basis of the evolution of texture during grain growth. Such texture-controlled grain growth is a result of the anisotropy in energy and mobility of beta grain boundaries, whose effects have been quantified using deterministic, Monte-Carlo, and phase-field simulation methods.[7–11] Phenomenological measurements and modeling research on beta grain growth have been complemented by investigations which have shed light on the relation between the beta-phase texture developed during subtransus processing and that which evolves during annealing in the single-phase beta field. For example, Moustahfid, Divinski, and their coworkers[12,13] were perhaps the first to demonstrate that the beta-phase texture developed during beta annealing contains components that were present during subtransus processing. Thus, it was concluded that the beta grew from pre-existing regions of this phase. However, their results also revealed that certain texture components may grow preferentially during beta annealing. For rolled sheet of Ti-6Al-4V, for example, the data indicated that the beta phase tended to consist of f001gh110i and f111g 112 components after low temperature rolling, but preferential growth in the beta field resulted in a texture dominated by the latter component. The early insights of Moustahfid, Divinski et al.[12,13] were corroborated by Bhattacharyya et a
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