Prediction of the kinetics of static globularization of Ti-6Al-4V
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Prediction of the Kinetics of Static Globularization of Ti-6Al-4V S.L. SEMIATIN, N. STEFANSSON, and R.D. DOHERTY The time for the static globularization of alpha lamellae in Ti-6Al-4V was predicted using analytical and finite-difference models based on the diffusion-controlled migration of the edges of remnant pancake-shaped particles. Similar to a previous two-dimensional analysis of the cylinderization of a semi-infinite, platelike second phase, the models quantified the influence of heat-treatment temperature, platelet thickness, and platelet diameter-to-thickness ratio on globularization kinetics. The present approaches were validated by comparison to previous observations of static globularization of Ti-6Al-4V at 955 °C.
Models for the evolution of microstructure during the thermomechanical processing of engineering alloys are of great interest from a scientific standpoint as well as with regard to the design of commercial production practices. Such models are especially useful for high-temperature aerospace alloys, which have a relatively narrow processing regime compared
S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433. N. STEFANSSON, Materials Engineer, is with Composite Materials Research and Development, Salem, OR 97302. R.D. DOHERTY, A.W. Grosvenor Professor of Materials Engineering, is with the Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104. Contact e-mail: [email protected] Manuscript submitted November 3, 2004. 1372—VOLUME 36A, MAY 2005
to conventional ferrous alloys. For example, when synthesized via an ingot-metallurgy route, alpha-beta titanium alloys are wrought using a series of hot-working and heat-treatment steps initially in the high-temperature, single-phase (beta) field and then at lower temperatures in the two-phase (alpha beta) field. By this means, a microstructure of colony (lamellar) alpha within large beta grains is converted to a structure comprising fine, equiaxed alpha particles in a matrix of beta (or transformed beta). The transformations that are used to effect this change in microstructure include static recrystallization of hot-worked beta (to produce a uniform and finer beta grain size) and dynamic and static spheroidization (or “globularization,” the more common term in the titanium industry) of the colony-alpha microstructure formed within the recrystallized beta grains during cooling from the beta phase field. Despite the importance of these processes, relatively little work has been conducted to obtain a quantitative description of them, let alone predictive capability, specifically for conventional titanium alloys. This situation contrasts sharply with extensive prior work to describe and model various shape instabilities as well as continuous/discontinuous coarsening of lamellar microstructures in a number of other nonferrous and ferrous alloys.[1–7] Relat
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