Low-temperature superplasticity of ultra-fine-grained Ti-6Al-4V processed by equal-channel angular pressing |
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Authors: | Y G Ko C S Lee D H Shin S L Semiatin |
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Affiliation: | (1) the Department of Materials Science and Engineering, Pohang University of Science and Technology, 790-784 Pohang, Korea;(2) the Department of Metallurgy and Materials Science, Hanyang University, 425-791 Gyeonggi-Do, Korea;(3) the Air Force Research Laboratory, AFRL/MLLM, 45433-7817 Wright-Patterson AFB, OH |
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Abstract: | The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and
strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume
fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable
during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting
(coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10−5 to 10−2 s−1. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG
specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and
10−4 s−1. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement
of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity
of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation
theory. |
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