Investigation of Long-range Ordering Kinetics of Ni-Cr-Fe Alloys with Kinetic Monte Carlo Simulations

Abstract Alloy 690 is widely used in pressurized water reactors, e.g. for steam generator tubes. Although most of the experimental data regarding thermal aging of heats of Alloy 690 relevant for PWRs indicates that long range ordering (LRO) does not form in these materials, some heats with low iron contents have presented LRO following thermal aging. LRO can affect the stress corrosion cracking resistance and ductility, and thus it is important to have tools to predict its occurrence at temperatures relevant for PWRs. In the present work, LRO of Ni-Cr-Fe ternary model alloys is modeled at the atomic scale to understand the expected thermodynamics and kinetics of ordering in Alloy 690. Pair interactions are fitted at 0 K to existing ab initio calculations of the formation enthalpies of ordered and disordered structures; and at high temperature to the Gibbs free energy of the γ Ni-Cr solid solution, as described in the available CALPHAD studies. Diffusion occurs by vacancy jumps, described by pair interactions fitted to ab initio calculation of vacancy formation and migration enthalpies, and to experimental data on tracer diffusion in dilute alloys and concentrated solid solutions at high temperatures. Monte Carlo simulations based on the pair interaction database enable us to establish phase diagrams and temperature-time-transformation diagrams for several Ni-Cr-Fe alloys. These are compared with results from experimental studies and the incubation times for LRO of Ni-Cr-Fe ternary alloys at temperatures relevant to PWRs are studied.