Atomistic simulations of driven alloys: a study of effective temperature models for low and high temperature applications
Immiscible alloys under severe plastic deformation (SPD) or irradiation form non-equilibrium microstructures at steady-state with microstructural features of a characteristic length scale that are stable over prolonged processing times. Such alloys have been termed ‘driven alloys’ as these steady-state microstructures are formed and stabilized as a consequence of the dynamical competition between the applied extrinsic driving force that tends to homogenize the microstructure and the thermodynamic driving force restoring its phase-separated equilibrium state. In recent years, non-equilibrium processing of immiscible alloys by SPD or ion-beam mixing have attracted significant interest as they enable the synthesis of novel nano-composite alloys with desirable properties such as enhanced mechanical strength, high electrical/thermal conductivity, creep resistance and radiation tolerance. Nonetheless, some major roadblocks must be overcome before the promise of these alloys can be realized. Currently, there is no general theoretical framework that can predict the microstructures formed under driven conditions. Furthermore, the present understanding of the mechanisms underlying microstructural evolution in driven alloys is still rather limited. One approach to rationalize the novel microstructure that evolves in driven alloys has been the effective temperature model (ETM) which posits that the non-equilibrium phases formed during processing will correspond to the equilibrium phase at a different, typically higher, effective temperature different from the ambient one. The underlying intuition behind this model is that the extrinsic forcing via irradiation or SPD represents an entropy-like contribution to the free energy of the system. Thus, to a first approximation, the evolution of driven alloys will be determined by the relative magnitudes, captured in the forcing intensity parameter, of the chemical diffusivities corresponding to ballistic mixing and vacancy-mediated thermal diffusion (typically accelerated by the excess vacancies produced by the forcing mechanism). In this work, the validity of such ETM’s is investigated using atomistic simulations of two different alloy systems. Firstly, molecular dynamics simulations are used to study the phase evolution during low-temperature shear deformation of highly immiscible alloys (Ω=2 eV/atom). Key findings from this work that support the ETM concept are: (i) a two-phase microstructure evolves at steady-state despite the absence of thermal diffusion and significant co-deformation of the two phases; (ii) the observed steady-state supersaturation varies with the precipitate size reminiscent of Gibbs-Thomson-like behavior; (iii) the effective temperature scales with the shear modulus of the alloy, thereby supporting the proposal of a modified ETM (METM) for low-temperature SPD of immiscible alloys. The simulations are also shown to agree very well on a semiquantitative basis with experimental studies, providing new insights in understanding phase stability in real alloys, as well as elucidating the mechanisms underlying microstructural evolution during low-temperature SPD. The second part of the thesis employs kinetic Monte Carlo (KMC) simulations to investigate the evolution of moderately immiscible alloys (Ω=0.33 eV/atom) during irradiation at elevated temperatures. The effective temperature of these systems is shown to scale with the forcing intensity parameter as predicted by the ETM. Furthermore, it is shown that when the system is in the compositional patterning regime, defined by the presence of clusters of characteristic size distribution, the system follows an inverse Gibbs-Thomson relation upon increasing the nominal composition of the alloy, i.e. the steady-state solubility increases with increasing cluster size.
Driven Alloys, Effective Temperature Model, Atomistic Simulations, Molecular Dynamics, Kinetic Monte Carlo, Severe Plastic Deformation, Irradiation, Immiscible Alloys, Patterning, Non-equilibrium Processing
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign