Low-Temperature Coarsening and Plastic Flow Behavior of an Alpha/Beta Titanium Billet Material with an Ultrafine Microst
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THE development of severe-plastic-deformation (SPD) methods to produce ultrafine microstructures in bulk metallic materials has spurred considerable research into enhanced secondary forming response and service properties.[1-3] The vast majority of this work has focused on single-phase fcc or bcc alloys with or without second-phase particles. In general, these efforts have demonstrated the ability to produce materials with superplastic forming (SPF) behavior at lower temperatures or higher strain rates than those required for conventional coarser-grain materials. Considerable increases in strength and at times an attractive balance of strength and ductility have also been achieved.[4] The SPD of more complex, two-phase alloys with a large fraction of second phase has received less G.A. SARGENT, Consultant, and P.N. FAGIN, Technologist, are with the Materials and Processes Division, UES, Inc., Dayton, OH 45432, Contact e-mail: [email protected] A.P. ZANE, Undergraduate Student, is with Chemistry Department, Wright-State University, Dayton, OH 45435. A.K. GHOSH, Professor, is with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109-2136. S.L. SEMIATIN, Senior Scientist, is with Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted April 16, 2008. Article published online September 17, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
attention. In part, this is due to the reduced workability of such materials at low processing temperatures and the special tooling required for both the SPD operation itself and the determination of final SPF properties. Alpha/beta titanium alloys comprise such a material class, for which low-temperature/high-strain rate SPF is particularly attractive.[5,6] Reduced temperature (for example, from ~900 C to ~775 C) and higher forming rates would enable utilization of less expensive tooling materials (e.g., stainless steel rather than superalloys), faster cycle times, and reduced material losses and machining costs associated with part contamination (alpha case). The production of alpha/beta titanium alloys such as Ti-6Al-4V with an ultrafine microstructure is usually accomplished via a series of steps consisting of annealing above the beta transus temperature (at which alpha + beta fi beta) and water quenching to develop a fine martensitic-alpha microstructure, followed by SPD at warm-working temperatures. In the past, SPD has been imparted by rolling of sheet,[7] multidirectional (‘‘abc’’) forging,[8,9] equal-channel angular extrusion,[10] severe compression under superimposed hydrostatic pressure,[11] and high-pressure torsion.[12] By this means, ultrafine Ti-6Al-4V having a strain rate sensitivity (m value) of the order of 0.35 to 0.65 and elongation between 400 and 1100 pct at temperatures between 700 C and 800 C has been produced.
U.S. GOVERNMENT WORK NOT PROTECTED BY U.S. COPYRIGHT
VOLUME 39A, DECEMBER 2008—2949
Despite the success in developing ul
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