The effect of annealing on NiAlFe B2 compounds

Abstract

Rods of both Ni30Al20Fe and Ni40Al30Fe were examined by optical microscopy and transmission electron microscopy in both the as-(double) extruded condition and after various heat treatments. Hardness, tensile and compression tests were performed on the alloys. The as-extruded and high temperature (750°C or higher) microstructures of Ni30Al20Fe were similar and consisted of B2 grains with a γ-Fe phase covering some boundaries; at 500°C the microstructure consists of α-Fe precipitates in a B2 matrix but no grain boundary phase was present. The microstructure of Ni40Al30Fe consisted of α-Fe precipitates in a B2 matrix at all temperatures, the precipitates coarsened with increasing annealing temperature. For short (200 min) anneals of both alloys yield strength, hardness maxima and ductility minima were found after annealing at 500°C. These effects were ascribed to the α-Fe precipitates. The presence of the γ-Fe phase on the grain boundaries of Ni30Al20Fe may have contributed to the ductility enhancement in the asextruded condition after high (750°C or higher) anneals.