The Influence of Grain Size on Twinning and Microstructure Refinement During Cold Rolling of Commercial-Purity Titanium
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TRODUCTION
THE initial grain size in metallic materials can have a considerable effect on microstructure evolution/refinement during large plastic strain. A coarse initial microstructure usually results in marked flow localization and heterogeneity of the final structure. By contrast, grain refinement to a micrometer scale improves the uniformity of deformation and facilitates the formation of homogeneous microstructures.[1] In addition, smaller-grained microstructures often give rise to more rapid kinetics of deformation-induced microstructure refinement.[2,3] For example, the rate of increase of the density of high-angle grain boundaries (and hence rate of microstructure refinement) during deformation of stainless steel was noticeably higher for an initial condition comprising a smaller grain size in comparison with its coarse-grain counterpart.[2]
S.V. ZHEREBTSOV, Head of Department, is with the Department of Materials Science and Nanotechnology, Belgorod State University, 85 Pobeda str, Belgorod, 308015, Russia. Contact e-mails: [email protected], [email protected] G.S. DYAKONOV, Researcher, is with the Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, K. Marks 12, Ufa 450000, Russia. G.A. SALISHCHEV, Head of Laboratory, is with the Laboratory of Bulk Nanostructured Materials, Belgorod State University. A.A. SALEM, President and CEO, is with Materials Resources LLC, Dayton, OH 45402. 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. S.L. Semiatin is employed by the Air Force Research Laboratory. U.S. Government work is not protected by U.S. Copyright. Manuscript submitted December 17, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
The effect of grain size on microstructure evolution can also be pronounced for polycrystalline hexagonal closepacked metals such as commercial-purity titanium (CP Ti). For these materials with a low-symmetry crystal lattice, there are two main mechanisms of deformation, slip and twinning, each of whose extent depends on grain size. This phenomenon is associated with the different dependences of critical resolved shear stress for slip and twinning on grain size.[4–7] The contribution of twinning decreases with a decrease in grain size. At a certain grain size, twinning is not observed. At room temperature, for instance, this critical grain size has been found to be 0.9 lm for CP Ti[8] and 1 lm for Ti-5Al.[9] Because deformation twinning results in microstructure refinement,[8] it can be surmised that the accommodation of large imposed strains by twinning vs slip could affect the nature and kinetics of the formation of ultrafine microstructures. However, there is relatively little quantitative data in the literature in this area. The aim of the present work, therefore, was to quantify the evolution of microstructure in CP Ti with various initial grain sizes (between 1 and 30 lm) subjected to multipass
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