Deformation Enhanced Diffusion in Aluminium Alloys
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HIGH-STRENGTH aluminium alloys rely on decomposition of a supersaturated solid solution to form precipitates that act as obstacles to dislocation motion. This process requires movement and aggregation of solute atoms, which is achieved during a heat treatment process by thermally activated diffusion. The scenario is made considerably more complex when precipitation occurs simultaneously with plastic deformation. In this case, solute transport can be accelerated either directly or indirectly by the dislocation motion that produces plasticity. This is of great practical importance in processes such as warm forming, creep age forming, high pressure torsion, and fatigue, where strong dynamic interactions can occur.[1–6] The nature of the interaction between precipitate evolution and deformation depends both on the initial microstructural state of the material (e.g. degree of supersaturation) and on the deformation conditions (temperature, strain rate, etc.). A full discussion of possible interactions is detailed elsewhere,[1,3,7] but important effects for aluminium alloys include direct precipitation on dislocations (dynamic strain ageing[1,8,9]), ballistic transfer of solute by slip,[1] and J.D. Robson is with the Department of Materials, University of Manchester, Manchester M13 9PL. Contact e-mail: [email protected] Manuscript submitted on June 3, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation-induced vacancies . Recent in situ studies suggest a dominant role for strain-induced excess vacancies in enhancing diffusivity, at least at temperatures less than approximately 473 K (200 °C).[2,3,5,10] The idea that part of the work applied during straining goes into the generation of excess vacancies is well established.[11,12] By estimating the fraction of applied work consumed in this process, a determination can be made of the excess vacancy concentration that is generated.[11,12] Excess vacancies will strongly influence diffusivity and other kinetic processes governing precipitation.[3] The deformation-induced vacancies can arise from a number of sources,[13,14] but the dominant one is considered to be non-conservative motion of conventional jogs on screw dislocations. Low-temperature deformation and annealing studies suggest that the deformation-induced excess vacancy concentrations approach 104 ,[15,16] orders of magnitude greater than the equilibrium thermal vacancy concentration and close to that expected near the melting point. Since the diffusivity of substitutional solute species is directly proportional to the vacancy concentration, this huge increase in the number of vacancies will result in a huge effect on self and solute diffusion rates. Simple models are available that attempt to capture dynamic effects on precipitate evolution. One of the most widely used models for the excess vacancy effect is due to Militzer et al.[17] Although not expected to be highly qualitatively accurate, such models are invaluable in helping to understand the fundamental dependencies
between process conditions
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