Compositional patterning in immiscible alloys subjected to severe plastic deformation
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Miao Wang, Robert S. Averback, and Pascal Bellon Department of Materials Science and Engineering, University of Illinois at Urbana-Champagin, Illinois 61801 (Received 2 April 2013; accepted 15 July 2013)
Compositional patterning in two-phase immiscible alloys during severe plastic deformation at elevated temperatures has been investigated. Kinetic Monte Carlo computer simulations were used to test the proposed idea that patterning derives from a dynamic competition between homogenization by forced chemical mixing and phase separation by thermally activated diffusion [P. Bellon and R.S. Averback, Phys. Rev. Lett. 74, 1819 (1995) and F. Wu et al., Acta Mater. 54, 2605 (2006)]. We utilize the concept of pair diffusion coefficients to compare thermal diffusion with forced chemical mixing and discuss the fundamentally different behavior with respect to pair separation distance in both mechanisms. While the general ideas of this model are verified and are in good quantitative agreement with our simulations, it is found that the dynamic processes of alloys under high-temperature shear are very complex, even in highly idealized systems, making experimental verification of this model very difficult. We illustrate our findings for a model AB alloy with properties similar to Cu–Ag by showing how alloy morphology and solubility depend on shear rate, temperature, and composition.
I. INTRODUCTION
Severe plastic deformation (SPD) is fundamental to numerous metallurgical technologies, ranging from materials processing via ball milling, accumulative roll bonding,1 or friction stir welding2 to in service degradation through wear,3 friction, and fatigue.4 In all of these situations, plastic deformation tends to cause changes in the alloy microstructure, not only through grain refinement and the creation of complex defect structures but also through chemical mixing and homogenization. Shear-induced mixing, in fact, has been likened to recoil mixing in materials subjected to energetic particle irradiation5; in both cases, atoms are forced down gradients in their atomic concentrations with little influence of their chemical potentials. In this sense, they are considered ballistic. At elevated temperatures, thermally activated diffusion also contributes to atomic mixing. This process is often enhanced by the creation of vacancies during shearing or irradiation.5–7 For alloy systems that are miscible, these two mechanisms act in parallel and enhance homogenization, whereas for immiscible alloys, the two mechanisms act in competition, with shearing tending to homogenize the alloy and thermally activated diffusion leading to phase separation. Two of the authors had shown previously using kinetic Monte a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.224 J. Mater. Res., Vol. 28, No. 19, Oct 14, 2013
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Carlo (KMC) simulations that these two mixing processes are not simply additive and that in immiscible alloys, their competition can lead to com
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