Interdiffusion structures and paths for multiphase Fe-Ni-Al diffusion couples at 1000 °C

Diffusion studies were carried out in the Fe-Ni-Al system at 1000 °C using solid-solid diffusion couples assembled with β (B 2 ), γ (fcc) single phase, and ( β + γ ) two-phase alloys. The diffusion couples were encapsulated in quartz tubes under vacuum and annealed for 48 hours. The diffusion structures were examined by optical and scanning electron microscopy. For all β vs ( β + γ ) couples, growth of the β phase was observed as the ( β + γ ) two-phase region recessed with the dissolution of the γ phase. For multiphase couples assembled with two ( β + γ ) terminal alloys, demixing of the ( β + γ ) two-phase alloys occurred with the formation of single-phase β and γ layers. The development of an interphase boundary between the ( β + β′ ) two-phase region and the γ phase is reported for the first time for a Fe-Ni-Al diffusion couple assembled with single-phase, β , and γ terminal alloys. Various diffusion structures for the couples were related to their diffusion paths constructed from concentration profiles determined by electron probe microanalysis. Interdiffusion fluxes of individual components were determined directly from the experimental concentration profiles and examined in light of diffusional interactions and the development of zero-flux planes and flux reversals. In addition, the boundaries for the miscibility gap between the ordered β and disordered β′ phases of the Fe-Ni-Al system at 1000 °C were determined with the aid of diffusion couples that developed β and β′ phases in the diffusion zone.