Mechanisms of globularization of Ti-6Al-4V during static heat treatment
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LAMELLAR microstructures may be found in many systems including two-phase titanium alloys. Although such microstructures may promote resistance to fracture and hightemperature creep, a low-aspect ratio or globular microstructure is often desirable for applications requiring optimal strength or ductility. Lamellar microstructures in two-phase alloys tend to possess a high degree of stability even at high temperatures due to the absence of interface curvature. However, the stability of the microstructure is reduced by defects such as lamellar terminations, lamellar boundaries, and perturbations produced during deformation or casting. Given enough thermal energy, these defects can eventually lead to the breakdown of the lamellar microstructure to obtain a globular or spherical morphology.[1] The possible mechanisms by which lamellar microstructures coarsen or transform to a globular morphology have been studied by various investigators.[2–9] As a result, a number of mechanistic models have been proposed to describe the process. These include boundary splitting, lamellar termination migration, and various capillarityinduced instabilities based on Rayleigh perturbation theory.[8,10–16] Boundary splitting requires an internal boundary through the thickness of one of the lamellar phases (Figure 1). The boundary may be a pre-existing grain boundary or a subgrain boundary/deformation band introduced during cold or hot working (Figures 1(a) and (b)). Such a boundary creates an unstable (90 deg) dihedral angle. To lessen surface tension, the dihedral angle is reduced and stabilized by the diffusion N. STEFANSSON, formerly Visiting Scientist, Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, WrightPatterson Air Force Base, OH 45433-7817, is Research Engineer, Materials and Technology Group, Precision Cast Parts, Structural Division, Portland, OR 97206-0898. Contact e-mail: [email protected] S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM. Manuscript submitted November 13, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
of interlamellar phase into the boundaries (Figure 1(c)). Eventually, the diffusion of the second phase into the boundary (“thermal grooving”) results in a “pinch off” and two new terminations due to the elimination of the intralamellar boundary.[10] Termination migration consists of the transfer of mass from the curved surfaces of the lamellar terminations to the flat surfaces of the lamellae (Figure 2). The driving force is provided by curvature differences between the lamellar terminations and the flat lamellar interfaces.[8,11–13] Rayleigh perturbation theory describes the instability of a cylindrical shape caused by capillarity-induced perturbations. The mechanism by which the colony-alpha phase is converted to a globular morphology during deformation and postdeformation annealing of alpha-beta titanium alloys such as Ti-6Al-4V has
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