Nanoscale patterning of chemical order introduced by displacement cascades in irradiated alloys

In this thesis we perform simulation and modeling studies of the formation of patterning of order in ordered alloys under ion irradiation. Dense displacement cascades produced by irradiation with energetic particles lead to the formation of disordered zones in chemically ordered alloys. Under sustained irradiation, the competition between irradiation-introduced disordering and thermal ordering may drive the system into various steady states of order. Kinetic Monte Carlo (KMC) simulations are employed to identify these steady states in model alloys that form L12 or L10 ordered phases at equilibrium. Besides the expected long-range order (LRO) and disordered steady states, a new state, comprised of well ordered domains of finite size, is observed. We refer to this new steady state as a state of patterning of order. This steady-state is identified by direct visualizations of the configurations, and by rescaling the intensities of the structure factor. The patterning state becomes stable only when the disordered zones exceed a threshold size. Above this threshold size, reordering of cascade-induced disordered zones proceeds in two stages: new antiphase domains form first, and then shrink to the benefit of the matrix. This two-stage reordering is at the origin of the dynamical stabilization of patterns of order.;A continuum kinetic model is proposed to describe the coupled evolution under irradiation of the specific area covered by antiphase boundaries and of the volume fraction of the domains belonging to the four translation variants of the L12 structure. Combining this description with existing mean-field results for the order-disorder transition under irradiation, we construct a steady-state dynamical phase diagram, which is in good agreement with the one constructed from KMC simulations.;We test the predictions on the Ni3Al compound by combining molecular dynamics (MD) and KMC simulations. For 1MeV Kr and 70keV He ions, MD simulations are used to simulate the disordered zones, which are then incorporated into KMC simulations to reach long irradiation times. These simulations indicate that 1MeV Kr ion irradiation can lead to patterning, whereas 70keV He ion cannot. Experimentally, we propose to use HRTEM imaging to identify the patterning of chemical order in Ni3Al.