Microstructure and Texture Evolution During Symmetric and Asymmetric Rolling of a Martensitic Ti-6Al-4V Alloy
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THE production of a bulk submicron crystalline structure is a prospective route to achieve high structural efficiency in metals by simultaneously enhancing the strength and ductility. For titanium alloys, such as the most widely used Ti-6Al-4V, microstructural refinement is not only desirable to achieve a well-balanced property profile of fatigue strength and ductility, but also it is important for promoting superplastic forming at a relatively low temperature.[1,2] So far, many different approaches have been employed to refine the grain size of Ti-6Al-4V alloy. One of the most popular routes is to apply severe plastic deformation (SPD) through different techniques[3] such as equal channel angular pressing,[4] hydrostatic extrusion,[5] or multistep isothermal forging.[6] However, tremendous deformation energy is required to generate a sufficient plastic strain,[7] which may lead to considerable mechanical instability in a highly deformed UFG microstructure. For example, a sharp fall tends to be observed in the engineering tensile stress–strain curve after reaching the peak stress, suggesting a poor strain-hardening capability.[8,9]
QI CHAO, Research Associate, PETER D. HODGSON, Professor, and HOSSEIN BELADI, Senior Research Academic, are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Contact e-mails: [email protected]; qi.chao@ deakin.edu.au Manuscript submitted June 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
Recently, a novel thermomechanical processing (TMP) using a martensitic starting microstructure was developed and recognized as a promising approach to produce an UFG structure in the Ti-6Al-4V alloy without SPD.[10–12] This processing employs several concurrent refinement mechanisms including the development of continuous dynamic recrystallization in a¢ martensitic laths and the decomposition of supersaturated martensite (i.e., a¢) into a and b phases, which concurrently lead to a pronounced grain refinement in the range of 150 to 800 nm at a strain of 0.8.[10] This approach is much more effective than many other conventional processing routes (e.g., TMP of a lamellar a + b microstructure), where the required strain is relatively high (i.e., 2 to 5[13–15]), and the optimum grain refinement is also comparatively coarse (i.e., ~2 lm[16–18]). Despite the relative success of grain refinement through this approach, the research to date has been mainly conducted by uniaxial compression testing,[10,12,16–18] which is generally used to simulate the bulk forming operations such as forging and rolling.[19] The UFG microstructure formation is inhomogeneous throughout the uniaxial compression specimen, where the specimen center revealed the maximum formation of UFG.[10] This is because that the effective strain in this region is much higher than its nominal strain.[15,19] In addition to the deformation strain, the extent of UFG formation is also strongly affected by other TMP variables such as deformation temperature, strain rate, and deformation mode (e.g., shear deformation[12,1
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