Microstructural origin of the magnetostriction deterioration in slowly cooled Fe81Ga19

Abstract Magnetostriction deterioration has been observed in slowly cooled Fe-Ga alloys when compared with the high temperature quenched ones. To reveal the underlying origin, here we investigated the microstructure of Fe81Ga19 thin-sheet samples quenched after slow cooling (at ∼0.3 K/min) to near and below the decomposition phase boundary between A2 and (A2’ + L12) phases. In comparison with the sample quenched at 1273 K (much higher than the decomposition phase boundary, ∼793 K) or the one quenched near the decomposition boundary (773 K), magnetostriction deterioration is observed for the one quenched below the decomposition boundary (673 K). Besides the A2 matrix phase, quenching at 673 K also promotes the formation of secondary L12 phase with negative magnetostriction and the phase transformation induced dislocations located close to the grain boundaries. Simultaneously, weak D03 ordering of the untransformed bcc matrix is also observed. All these microstructural changes deteriorate the magnetostriction. Our work provides important information why the magnetostriction of Fe81Ga19 greatly depends on the thermal history.