Coarsening behavior of an alpha-beta titanium alloy
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8/10/04
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Coarsening Behavior of an Alpha-Beta Titanium Alloy S.L. SEMIATIN, B.C. KIRBY, and G.A. SALISHCHEV The static-coarsening behavior of the alpha-beta titanium alloy, Ti-6Al-4V, was established via a series of heat treatments at typical forging-preheat and final-heat-treatment temperatures followed by quantitative metallography. For this purpose, samples of an ultra-fine-grain (UFG) size billet with a microstructure of equiaxed alpha in a beta matrix were heated at temperatures of 843 °C, 900 °C, 955 °C, and 982 °C for times between 0.25 and 144 hours followed by water quenching. The coarsening of the primary alpha particles was found to follow r 3-vs-time kinetics, typical of volume-diffusion-controlled behavior, at the three lower temperatures. At the highest temperature, the kinetics appeared to be fit equally well by an r 3 or r4 dependence on time. The observations were interpreted in terms of the modified LSW theory considering the effect of volume fraction on kinetics and the fact that the phases are not terminal solid solutions. Prior models, which take into account the overall source/sink effects of all particles on each other, provided the best description of the observed dependence of coarsening on the volume fraction of primary alpha. In addition, the volume-diffusion kinetics derived for the UFG material were found to be capable of describing the coarsening behavior observed for industrial-scale billet of Ti-6Al-4V with a coarser starting equiaxed-alpha microstructure.
I. INTRODUCTION
THE instability of microstructure in metallic materials during high-temperature exposure is of considerable interest from both a scientific and engineering perspective. For example, static grain growth, particle coarsening, spheroidization of a lamellar microstructure, and other processes may lead to substantial losses in both first and second tier mechanical properties, such as yield strength and creep resistance, during service exposure. Hence, a considerable amount of attention has focused on the phenomenological and mechanistic descriptions of such changes in microstructure.[1,2] In two-phase alloys, the coarsening of a precipitate phase during high-temperature exposure has been examined in great detail, both experimentally and theoretically. Driven by the reduction in energy associated with the matrix-precipitate interfaces, coarsening comprises the growth of large particles and the shrinkage of smaller particles. Such so-called Ostwaldripening processes were initially described in models developed by Greenwood,[3] Lifshitz and Slyosov,[4] and Wagner,[5] thus leading to a quantitative framework usually referred to as LSW theory. These approaches relied on a diffusion analysis in which the growth behavior of each particle is independent of the positions of all of the other particles and thus assumed a meanfield approximation in which the source/sink of solute is infinitely far away. The principal predictions of these models were (1) the cube of the average particle radius varies linearly w
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