Oxide Dissolution and Oxygen Diffusion in Solid-State Recycled Ti-6Al-4V: Numerical Modeling, Verification by Nanoindent
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TION
TITANIUM (Ti) alloys, especially Ti-6Al-4V, have an excellent combination of properties including high strength-to-weight ratio, excellent corrosion resistance, and good heat resistance. However, Ti remains expensive for many applications due to high production and manufacturing costs. For aerospace and biomedical applications, a large portion of feed material may end up as waste chips during machining operations, adding to the problem. Recycling Ti machining chips is thus highly desirable. Recently, it has been shown that solid-state recycling of chips of pure Ti[1,2] and Ti-6Al-4V followed by standard mill annealing
E.W. LUI and K. XIA are with the Department of Mechanical Engineering, University of Melbourne, Parkville, VIC 3010, Australia and also with the Defence Materials Technology Centre, Hawthorn, VIC 3122, Australia. Contact e-mail: [email protected]; [email protected] S. PALANISAMY is with the Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia and also with the Defence Materials Technology Centre. M.S. DARGUSCH is with the School of Mechanical and Mining Engineering, University of Queensland, St Lucia, QLD 4072, Australia and also with the Defence Materials Technology Centre. Manuscript submitted January 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
treatment[3–5] is effective in producing bulk materials with good mechanical properties. In the above examples, solid-state recycling is performed using equal-channel angular pressing (ECAP), a severe plastic deformation-based process. The machining chips are sheared, breaking up the brittle surface oxide and allowing direct metal-to-metal bonding. The oxide on the surface of the chips, however, remains in the microstructure and may significantly reduce ductility when loaded in tension. It has been shown that the titanium oxide (TiO2) on the surface readily dissociates into Ti and oxygen (O) at high temperatures and the O atoms subsequently dissolve and diffuse in the Ti matrix. For Ti-6Al-4V recycling, this process takes place above 873 K (600 C), either in the a + b phase field or above the b transus.[3,4] As the oxide dissolves, its immediate vicinity will have a high concentration of oxygen which diffuses away with time at high temperatures.[6] These oxygen-enriched areas have higher strength due to solution strengthening[7,8] as well as slower grain growth due to solute dragging.[9] In addition to practical implications for engineering applications, the situation provides a good opportunity to model and verify oxide dissolution and oxygen diffusion in Ti during annealing. There are several physical models describing the dissolution of titanium oxide when the surface oxide is subjected to diffusion bonding, including the oxygen diffusion-controlled model by Munir[10] and the reaction diffusion model
by Takahashi et al.[6] In the case of solid-state recycling of Ti-6Al-4V chips,[4] a modified version of the aforementioned models is developed to show that the oxide embedded in the m
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