Coarsening Behavior and Plastic Flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si with an Ultrafine Microstructure
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ts of titanium alloys with a microstructure of fine, equiaxed-alpha (hcp) phase in a beta (bcc) matrix are widely used in the aerospace industry to make complex-shape parts via superplastic forming (SPF). For the most common Ti alloy, Ti-6Al-4V (Ti64), forming is typically done at ~1173 K (900 C). By reducing the size of the alpha particles from ~5 to 10 lm in diameter to ~1 to 3 lm, the forming temperature of this material can be reduced to ~1048 K (~775 C).[1–6] Reduced temperature and higher forming rates enable utilization of less-expensive tooling materials (e.g., stainless steel rather than superalloys), lead to
S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXCM, Wright-Patterson Air Force Base, OH 45433 Contact e-mail: [email protected]. F. SUN, Senior Principal Engineer, and E.M. CRIST, Director, Process & Product Development, are with Alcoa Titanium & Engineered Products, 1000 Warren Avenue, Niles, OH 44446. K.O. YU, Senior Director, Advanced Innovation and Technology (retired), was with Alcoa Titanium & Engineered Products. G.A. SARGENT, Consultant, is with UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45433. D.G. SANDERS, Senior Technical Fellow, is with Boeing Research & Technology, P.O. Box 3707, Seattle, WA 98124. S.L. Semiatin is employed by the Air Force Research Laboratory. U.S. Government work is not protected by U.S. Copyright. Manuscript submitted March 7, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
faster cycle times, and reduce material losses/machining costs associated with part contamination (alpha case). Such attractive features have thus driven mill suppliers to develop novel processes for manufacturing sheet products of Ti-64 (and similar alloys) with the desired ultrafine microstructure. Recently, development work has focused on the formulation of mill processes for producing sheet of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242S) with an ultrafine microstructure.[7] This alloy has a service temperature 100 K to 200 K (100 C to 200 C) higher than that of Ti64 and thus would be beneficial for parts such as engine nacelles, airframe tail components (located near the auxiliary power unit), and other thermal-protection systems. Assemblies used for these applications are exposed to higher than normal service temperatures of the order of 590 K (317 C) with excursions to 750 K (477 C). In extreme cases, part weakening due to environmental interactions, which can necessitate frequent inspection and occasional replacement, has been observed. The higher service temperatures that are possible result from increased strengthening due to substitutional alloying elements (such as Mo) and the formation of stable precipitates (silicides). However, the presence of Mo also reduces the rate of solute diffusion that plays a key role in controlling superplastic deformation, albeit it can also reduce the rate of (usually undesirable) dynamic coarsening of alpha particles. The objective of the
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