| Magnetostrictive material, of which the length or volume can be changed under external magnetic field, is a new function material and be widely used in various sensor and micro-actuator applications. At present, much more researchers put their attention to Tefenol-Då’ŒFe-Ga alloy in the field of magnetostrictive materials. Although Tefenol-D show large magnetostrictive strain, Tefenol-D are brittle and expensive due to the high costs of Tb and Dy. Very recently, it is reported that a large magnetostriction of 1300 can be achieved in melt-spun sample of Fe-Ga alloy. In addition to their high magnetostriction, Fe-Ga alloys show high mechanical behavior and low associated cost which render it a good candidate for engineering application.Employing glass fluxing combined with superheating cycling method, Fe-27.5 at.%Ga alloy obtained undercoolings ranging from 0 to 370 K. And Fe72.5Ga27.5 rod with undercooling of 80 K was fabricated using the technique of rapid directional solidification by triggering the undercooled Fe72.5Ga27.5 melts. In this paper, metallographic phase and microstructure evolution of undercooled Fe72.5Ga27.5 alloys were systematically studied with respect to different undercoolings. Then, microstructure and magnetostrictive performance of directional solidified Fe72.5Ga27.5 alloy rod were also studied.Firstly, adopting glass fluxing combined with superheating cycling method, the undercooling and its stability of Fe81Ga19 alloy melts were investigated using different kinds of denucleating glass: Na2B4O7, 90% Na-Si-Ca+10% B2O3 (simplified as Na-Si-Ca-B ) and 9.4% SiO2+84.6% Na2B4O7+6% CaO (simplified as Na-B-Si ). Fe72.5Ga27.5 alloys have great gallium loss when using Na-Si-Ca-B as the glass slag and Na2B4O7 can not produce a stable undercooling effect. For Na-B-Si glass, a stable large undercooling of above 300 K can be obtained for Fe72.5Ga27.5 alloy during the superheating-cooling cycles, and gallium loss can be effectively avoided. Additionally, the effect of other experimental factors on undercooling were also discussed.Secondly, metallographic phase character and microstructure evolution of undercooled Fe72.5Ga27.5 alloys were studied. With the undercooling range 0 to 200 K, the metallographic phase structure mainly consist of metastable phase D03 and L12, B2, A2 also exist. When the undercooling is in the range of 200-300K, L12 and A2 phases decrease sharply with increasing undercooling, and only D03 phase exist. When melts were undercooled larger than 300 K, metastable phase D03 coexist with a new phase. According to XRD and DSC analysis of the undercooled Fe72.5Ga27.5 alloy sample, this new phase may be the Modified-D03 which was referred in previous reports. The microstructure of undercooled Fe72.5Ga27.5 alloys evolutes with increasing undercooling. When the undercooling is less than 200 K, the structures consist of coarse and broken dendrites with some cells present. When the undercooling is in the rage of 200-300 K, equiaxed grains coexist with fine broken dendrites. When the undercooling reaches above 350 K, only a kind of abnormal large grain exists.Finally, microstructure and magnetostrictive performance of rapid directional solidified Fe72.5Ga27.5 alloy rod were also studied. It is found that along the growth direction microstructure consist of fine equiaxed grain zone, columnar grain zone and coarse equiaxed grain zone in turn. The relative intensity of (200) peak increases along growth direction and exceeds that of (110) peak at the top of rod. It is because that melts undercooling declines with the movement of solidification interface. The maximum magnetostrictive strain up to 296ppm was obtained in the columnar grain zone. It has been ascribed to the Modified-D03 phase created by rapid solidification and their preferential orientation in (100) textured rod. |
