Ultrafine Grain Formation in a Ti-6Al-4V Alloy by Thermomechanical Processing of a Martensitic Microstructure
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INTRODUCTION
TITANIUM alloys are widely used in different industrial applications due to their unique combination of attractive properties such as high strength to weight ratio, exceptional corrosion resistance, and biocompatibility.[1] However, they are relatively expensive compared to other light metals (e.g., Mg alloys), which limits their use to specific applications. Therefore, there is an ongoing drive to develop new processing techniques to produce highstrength titanium alloys with improved properties at lower cost. Grain refinement is the most generally accepted approach to simultaneously improve the strength and toughness.[2,3] Moreover, the fine equiaxed grained structure also enhances the formability of titanium alloys at much lower temperature, which is beneficial for subsequent material processing and energy saving.[4,5] Early work on titanium alloys processing mainly focused on morphology modification of the initial a + b phase microstructure in Ti-6Al-4V alloy to convert the lamellar morphology to an equiaxed grain structure through thermomechanical processing (TMP).[5–14] Most of these investigations involved deformation at a relatively high temperature [i.e., typically over 1073 K (800 C)], which ultimately leads to limited grain refinement down to 2 lm at a strain range of 2 to 5.[7,9,10] Recently, considerable research[15–18] has focused on the development of new TMP routes to further enhance the level of grain refinement at relatively lower strains. The
QI CHAO, Ph.D. Student, PETER D. HODGSON, Australian Laureate Fellow, Alfred Deakin Professor and Director, and HOSSEIN BELADI, Senior Research Academic, are with the Institute for Frontier Materials, Geelong Technology Precinct, Deakin University, Waurn Ponds Campus, Geelong, VIC 3216, Australia. Contact e-mail: [email protected] Manuscript submitted July 12, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
presence of different phase allotriomorphs in titanium alloys provides a unique opportunity to potentially refine the grain structure at a lower strain level through plastic deformation in conjunction with solid–solid reactions such as recrystallization and phase transformation. The level of grain refinement in titanium alloys strongly depends on the initial microstructure and TMP route. Semiatin and co-workers[11,12] have shown that the final grain size and the required strain can be significantly reduced through lowering the deformation temperature and/or utilizing a finer initial microstructure (i.e., reducing the a platelet thickness in particular). These findings suggest that a fine martensite structure could be a good candidate as an initial microstructure to potentially enhance the grain refinement. However, the martensitic structure has very low formability, which makes it difficult to employ TMP. Inagaki[8,19] is among the first who successfully deformed a fine acicular martensite microstructure at a relatively low temperature of 1023 K (750 C) to a reduction of 95 pct to produce fine equiaxed a grains (
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