Strain-path effects during hot working of Ti-6Al-4V with a colony-alpha microstructure
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rain-Path Effects during Hot Working of Ti-6Al-4V with a ColonyAlpha Microstructure S.L. SEMIATIN, J.O. BROWN, T.M. BROWN, D.P. DeLO, T.R. BIELER, and J.H. BEYNON The conversion of titanium ingot materials to wrought mill products often involves a series of hot-working and heat-treatment steps.[1] One of the most important consists of the breakdown of the lamellar colony or acicular microstructure formed during cooling after beta working or recrystallization heat treatment. This breakdown operation usually comprises upsetting or cogging operations followed by annealing in the alpha-beta phase field to produce a globularized-alpha structure. Recently, DeLo et al.[2,3] have established that the equal channel angular extrusion (ECAE) process can also be
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. J.O. BROWN and T.M. BROWN, Senior Technologists, are with UES, Inc., Dayton, OH 45432. D.P. DeLO, Director of Technology, is with Extrude Hone, Irwin, PA 15642. T.R. BIELER, Associate Professor, is with the Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226. J.H. BEYNON, Professor, is with the Department of Mechanical Engineering, The University of Sheffield, Sheffield, S1 3JD, United Kingdom. Manuscript submitted September 5, 2000. 1556—VOLUME 32A, JUNE 2001
used effectively to convert the colony microstructure in a typical alpha/beta alloy Ti-6Al-4V. However, it was shown that the successful application of the method requires an increment of upsetting deformation prior to the shear that is imposed in the ECAE deformation zone. The efficacy of this approach was rationalized on the basis of the dependence of flow-softening rate on strain. At low strains ( ⬍ 0.5), the flow-softening rate is high for Ti-6Al-4V (and similar alloys) with a colony microstructure, and the tendency for flow localization in shear is very high. At larger strains, at which flow softening is much lower, flow localization is less likely. Hence, the use of an initial increment of upset deformation, which is inherently more stable than shear because of geometric hardening, was thought to be beneficial through its ability to predeform the material uniformly to a strain at which flow softening and the tendency for flow localization in shear were considerably diminished. The primary objective of the present work was to provide a basis for an alternate, but equally plausible, explanation for the stabilizing effect of an upset prestrain on flow stability during ECAE. Specifically, the effect of a strainpath change on flow softening was established. A second objective was to establish the precise prestrain required to stabilize flow during ECAE and to relate this strain to the strain-path observations. The effect of a strain-path change on flow response and flow stability was assessed via two sets of experiments. Both made use of Ti-6Al-4V mater
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