Showing papers by "Robert S Averback published in 2013"

Atomic Mixing in Metals Under Shear Deformation

Abstract: The fundamental processes of shear-induced chemical mixing in heterogeneous Cu-based alloy systems have been studied by molecular dynamics computer simulations. These simulations reveal that two very disparate mechanisms operate depending on whether or not the two phases are coherent. For the coherent systems, mixing occurs as dislocations transfer across phase boundaries. The mixing in these systems is “superdiffusive,” and for spherical precipitates, the rate of mixing increases quadratically with precipitate radius. In systems that have incoherent phases, the mixing occurs by a local shuffling of atoms at the interface, and for them, the mixing is diffusive, with the mixing rates of spherical precipitates scaling linearly with particle radius. The morphologies of the interfaces for the two situations are also different. Coherent precipitates form rough interfaces that are relatively sharp, whereas the interfaces of incoherent precipitates are smooth but diffuse. These simulations also show that for incoherent precipitates, shear-induced mixing can be very different at different crystallographic interfacial planes as well as for different strain directions.

Complex nanoprecipitate structures induced by irradiation in immiscible alloy systems

Abstract: We investigate the fundamentals of compositional patterning induced by energetic particle irradiation in model $A\ensuremath{-}B$ substitutional binary alloys using kinetic Monte Carlo simulations. The study focuses on a type of nanostructure that was recently observed in dilute Cu-Fe and Cu-V alloys, where precipitates form within precipitates, a morphology that we term ``cherry-pit'' structures. The simulations show that the domain of stability of these cherry-pit structures depends on the thermodynamic and kinetic asymmetry between the $A$ and $B$ elements. In particular, both lower solubilities and diffusivities of $A$ in $B$ compared to those of $B$ in $A$ favor the stabilization of these cherry-pit structures for $A$-rich average compositions. The simulation results are rationalized by extending the analytic model introduced by Frost and Russell for irradiation-induced compositional patterning so as to include the possible formation of pits within precipitates. The simulations indicate also that the pits are dynamical structures that undergo nearly periodic cycles of nucleation, growth, and absorption by the matrix.

Compositional patterning in immiscible alloys subjected to severe plastic deformation

Abstract: 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.