Dynamic precipitation and the resultant magnetostriction enhancement in [001]-oriented Fe-Ga alloys

Precipitation of dispersive nanoparticles has recently been found to yield superfunctional properties, such as the large and sensitive magnetostriction in body-centered-cubic (bcc) Fe-Ga alloys with face-centered-tetragonal (fct) nanoprecipitates, and applying this strategy to grain-aligned alloys may allow one to obtain better performance. However, the internal stress generated during directional solidification may alter the precipitation behaviors by accelerating atomic clustering, therefore, a careful analysis of the morphology of precipitates and solute partitioning is needed. Herein, we investigated the dynamic precipitation behavior in a directionally solidified Fe73Ga27 alloy with [001] orientation. Through comparisons with random polycrystalline sample subjected to the same aging treatment, we find that the [001]-oriented sample produces sparser fct nanoprecipitates and extra interfacial omega nanoprecipitates. The internal stress accelerates Ga partitioning between the fct nanoprecipitates and the matrix, hence reducing their nucleation sites. The internal stress also alters the mutual elastic interactions between matrix and precipitates, where the Bain strains of fct nanoprecipitates are mostly accommodated by forming Ga-enriched omega nanoprecipitates and {112}<111> stacking faults at the phase front, unlike that for random polycrystalline sample, where the Bain strains are accommodated by local tetragonal distortion of the matrix. As a result, the magnetostriction enhancement ratio is 40% for the grain-aligned sample, weaker than ~165 % for the random polycrystalline sample. Our results not only shed lights on the precipitation difference between stress-containing and stress-free aging conditions but also help to guide the microstructure design of superfunctional alloys in which the type, number density and size of nanoprecipitates should be carefully controlled.