Effect of initial microstructure on plastic flow and dynamic globularization during hot working of Ti-6Al-4V
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INTRODUCTION
THE breakdown of transformed microstructures developed during cooling from the beta-phase field after hot working or heat treatment forms an important step in the production of mill products of alpha/beta titanium alloys. As such, a variety of investigations have been conducted to establish the phenomenology and kinetics of globularization. For example, Semiatin et al.[1] and Malcor et al.[2] used a hot compression/forging mode of deformation to establish the broad reductions for dynamic globularization, i.e., spheroidization during deformation, of Ti-6Al-2Sn-4Zr-2Mo (Ti6242) and Ti-6Al-4V (Ti-64), respectively, with a colony alpha preform microstructure. In other work, Weiss et al.[3] determined the effect of level of prior hot forging deformation on static globularization of Ti-64 quantified by the reduction in the aspect ratio of alpha platelets in colony microstructures with several different starting platelet thicknesses. The work of Rauch et al.[4] and Korshunov et al.[5] revealed a strong effect of mode of deformation on dynamic globularization kinetics. In the former effort, the breakup of a colony microstructure in Ti-6242 was found to require substantially larger strains during hot torsion than found previously for compressive modes of deformation. In the work of Korshunov et al., samples of the Russian alloy VT9 (Ti-6.6Al-3.5Mo-1.7Zr-0.27Si), also with a colony microstructure, were deformed in tension, torsion, concurrent tension and torsion, reversed torsion, alternating tension and torsion, and sequential upsetting along three orthogonal directions. The rate of dynamic globularization with strain E.B. SHELL, Graduate Student, is with the Dept. of Chemical Engineering, University of Dayton, Dayton, OH 45469. S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/ MLLM, Wright-Patterson Air Force Base, OH 45433-7817. Manuscript submitted January 29, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
was found to be the greatest for tension and concurrent tension and torsion. Monotonic torsion deformation yielded comparable kinetics at low strains but much lower rates at higher strains; these findings are in agreement with the observations of Rauch et al.[4] The most surprising result of the Korshunov work was the relatively low rate of globularization for the nonmonotonic loading paths and the particularly low rate for the sequential forging process. The work on Ti-64 by Margolin and Cohen[6,7] and Weiss et al.[8] gave insight into the mechanisms of globularization of colony microstructures, albeit primarily for static processes. Margolin and Cohen proposed a mechanism involving recrystallization within the alpha platelets followed by surface-tension driven penetration of the beta phase. In the work of Weiss et al., a similar mechanism of surface-energy driven penetration was proposed, but the postulated driving force was the development of intense shear bands or subgrains within the alpha
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