Microstructure and mechanical behavior of directionally solidified Fe35Ni15Mn25Al25

The effects of directional solidification (DS) processing on the microstructure and hardness of Fe35Ni15Mn25Al25 are examined. The microstructure consists of (Fe, Mn)-enriched B2-ordered (ordered b.c.c.) and (Ni, Al)-enriched L21-ordered (Heusler) phases aligned along <100> directions in a complex percolating morphology. Because of the slow cooling rate during DS processing, the phase size in the DS alloy was approximately 3 times greater than that of an arc-melted alloy. Precipitates were found within both phases of the as-DS alloy, but not in the as-arc-melted alloy. Upon annealing, the phase size increased but with no significant change in the composition of the phases, while the interphase interfaces appeared to become sharper. The hardness of the DS alloy increased monotonically upon annealing, and showed an inverse relationship with the phase size before the formation of large β–Mn-structured precipitates. Several theoretical models are considered that might explain this variation of hardness with phase width. Longer annealing resulted in growth of the β–Mn-structured precipitates, causing a further 50% hardness increase.