Coarsening kinetics of metastable nanoprecipitates in a Fe-Ni-Al alloy

Microstructure evolution of a Fe - Ni - Al alloy has been examined during annealing at temperatures between about 700 and 800 degrees C. This material is brittle in the cast state but shows good strength with ductility after a stabilising anneal at 1100 degrees C when it has a duplex microstructure of B2 dendrites with fcc interdendritic phase. The 700 - 800 degrees C ageing leads to the formation of metastable bcc precipitates within the dendrites with less change within the interdendritic regions. The long- term coarsening of these precipitates is controlled by diffusion within the B2 phase. The composition of the B2 phase changes with annealing temperature, which is believed to modify the diffusion rate and, correspondingly, the rate of particle coarsening. The present coarsening study serves to define annealing conditions for preparation of optimum microstructure before material testing, as well as define upper temperature limits for possible long- term application, where stable microstructures are required.