Thermal and irradiation-induced phase separation in Fe-Ni based invar-type alloys

Abstract

We summarize and review critically the existing experimental and theoretical evidence concerning both thermal and irradiation-induced high-temperature miscibility gaps in Fe-Ni based Invar-type alloys. Independent data regarding phase separation are obtained from studies on magnetic, low-expansion Invar-type alloys and model austenitic Fe-Ni based alloys studied for potential nuclear applications. The response of these alloys to long-term thermal aging is found to be inconsistent with that of single-phase alloys predicted by most accepted or proposed phase diagrams. These alloys show anomalies in numerous properties which suggest compositional or magnetic heterogeneities or both. We herein model the kinetics and thermodynamics of spinodal decomposition and nucleation in these alloys under thermal conditions. The absence of models for surface energy and gradient energy in systems with negative departure from ideality severely limits our analysis of the kinetics of both nucleation and spinodal decomposition. We can combine our calculations with those of others and also with experimental studies of decomposition to reach a conclusion that suggests a high-temperature miscibility gap for Fe-Ni alloys in the Invar regime. The gap is found to be very narrow at high temperatures but to be broadened at low temperatures by magnetic effects. Alloys in the Invar composition range have been subjected to a variety of high fluence irradiation treatments in the 725 to 1000 K temperature range. The result in most cases was large-scale decomposition into approximately 25 and 50 pet Ni phases. The apparent miscibility gap under irradiation is much wider than that observed thermally. We discuss these observations in the light of existing theories of irradiation-induced or irradiation-altered alloy decomposition. We conclude that although irradiation-enhanced diffusion speeds up phase separation, other processes must be operating to produce the greatly widened miscibility gap.