Bulk Undercooling and Directional Growth of Fe-Ga Magnetostractive Alloy

Bulk Undercooling and Directional Growth of Fe-Ga Magnetostractive Alloy
　　Giant magnetostrictive materials have been widely applied in spaceflight and military field as the element of acoustic sensors, actuators, and other magnetomechanical devices. Recently, Fe-Ga system which is virtue of low-filed magnetostriction and good mechanical properties such as good ductility and high strength, has attracted increasing attentions as magnetostrictive materials. In this study, by way of molten glass denucleating and superheating-cooling cycles, the undercooling behavior of the Fe83Ga17 alloys was studied. The Fe83Ga17 bars with preferred strong magnets orientation were obtained using super-high temperature gradient directional solidification method.The Fe83Ga17 alloys were undercooled up to 287K through molten glass denucleating and superheating-cooling cycles. When the undercooling was less than 54K, the structures were composed of coarse dendrites. With increasing the undercooling up to 216K, the grains were refined. More particularly, the dendrites tended to possess a preferred orientation when the undercooling was in the range of 141-176K. When the undercooling exceeded 252K, abnormal large grains coexisting with fine equiaxed grains appeared. With a further increase of the undercooling up to 287K, only big equiaxed grains existed. The grain refinement with the increasing undercooling is due to the increased nucleation rate as the undercooling was less than 252K. When undercooling exceeded 252K, recrystallization occurred in the melt, giving rise to the strong magnets formation of big grains. Local shot-range ordering of Ga atoms was formed in the Fe83Ga17 alloy when the undercooling exceeded 176K, and the presence of clusters of near-neighbor pairs of Ga atoms contributed to the large magnetostriction.
　　The quenching process can improve the magnetostriction of Fe83Ga17 alloy by restricting the formation of DO3 phase. In the quenching process, small volume fraction of the b.c.c matrix turned to the Modified-DO3 phase which has pairs of Ga along [100]. On the other hand, in the annealing process, the development of long-rang DO3 structure was formed and the clusters of Ga atoms was disappeared, leading to a decrease in magnetostriction.Using super-high temperature gradient directional solidification method, the Fe83Ga17 alloy bars with [110] orientation were prepared. The experimental temperature gradient was 800K/cm. When the growth velocity was 6μm/s, several coarse columnar crystals along the longitudinal direction and even without transverse strong magnets interfaces were formed in the growth stability area. The preferred orientation of directional solidified sample was interrupted as the growth velocity was 10μm/s. The phenomena were explained by the relationship between growth velocity and the solid-liquid interface shape. In order to prepare samples with preferred orientation, it is necessary to make the growth velocity consistent with the temperature gradient, and to make sure the solid-liquid interface convex.Both bulk undercooling and directional growth can obtain structures with preferred orientation, and most importantly, the undercooling experiment improves the texture of [100] orientation, and provides a possibility to prepare magnetostriction alloys with [100] preferred orientation.