Directional Solidification Of Bulk Undercooled Magnetostrictive Fe-Ga Alloys

Magnetostrictive materials are widely used in various sensor and micro-actuator applications as sonar transducers, weapon systems and robot devices. At present, the widely used RFe2 intermetallic compounds (where R refers to rare-earth elements) such as Tb-Dy-Fe alloys show large magnetostrictive strains and high Curie temperatures. However, Terfenal alloys are brittle, require large fields for saturation, and are expensive due to the high costs of Tb and Dy. Very recently, it was found that the magnetostriction of bcc Fe is greatly enhanced by the addition of Ga. A large magnetostriction of 400 ppm can be achieved in a single crystal [100] oriented Fe₈₁Ga₁₉ alloy which depends on the quenching conditions. In addition to their high magnetostriction, Fe-Ga alloys show high mechanical strength, good ductility, and low associated cost which render it a good candidate for commercial application.Researches on Fe-Ga magnetostrictive alloys show that proper composition range, metastable phase formation and [100] preferred orientation are crucial for the improving of magnetostrictive property. Therefore, a controllable solidification technique is urgently wanted to prepare bulk Fe-Ga magnetostrictive alloys which could achieve high texture and the metastable phase formed in rapid solidification. Our recent work indicates that Fe-Ga alloys produced by the technique of rapid directional solidification from the undercooled melts (UDS) meet all these expectations. UDS is a newly-developed rapid solidification technique. When the melt is triggering nucleated in a specific undercooling range, the dendrites grow rapidly into the undercooled melts with a preferred orientation at a speed of several m/s. So, directional solidified alloys with a homogenous composition can be obtained through the control of nucleation condition of crystal growth which renders UDS a potentially directional solidification technique. In this paper, the effect of denucleating glass composition on the undercooling of Fe-Ga alloy melts was investigated using the method of glass fluxing combined with superheating cycles. Microstructure evolution and MFM domain structures of undercooled Fe₈₁Ga₁₉ alloys were also studied with respect to different undercoolings. Then, based on the results of these experiments, UDS technique was applied in the preparation of directional solidified Fe-Ga alloys. The texture, microstructure and the magnetic properties of directional solidified Fe-Ga alloys were also investigated systematically. In addition, the effect of triggering undercooling on the texture and the magnetostriction property of Fe₈₁Ga₁₉ alloys prepared by UDS technique were discussed. The main conclusions are as follows: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: B₂O₃, NaSiCa+B₂O₃(simplified as Na-Si-Ca-Al-B) and Na-Si-Ca-Al-B+Na₂B₄O₇. The results showed that different glass has different denucleating mechanism. The purification of B₂O₃ glass is only a physical process, by which the stable bulk undercooling cannot be obtained during superheating-cooling cycles. While taking Na-Si-Ca-Al-B glass as purifying agent, its denucleating mechanism is a comprehensively physicochemical process. But the stability of undercooling is still undesirable because of the separation between melt and glass during cooling process in superheating cycling. A stable bulk undercooling of above 300 K can be obtained by physicochemical denucleating process in the case of 70% Na-Si-Ca-Al-10B+30%Na₂B₄O₇ molten glass owing to its suitable viscosity. Combined with the analyses of the effect of sample weight, superheating temperature and holding time on undercooling of Fe₈₁Ga₁₉ alloy, a stable experimental process was established which has been proved feasible.Secondly, high undercoolings up to 305 K have been successfully achieved in the bulk Fe₈₁Ga₁₉ magnetostrictive alloy melts by means of glass fluxing combined with superheating cycling method. When the undercooling is less than 50 K, the structures consist of coarse and broken dendrites with some cells present. The remelting of primary dendrites plays an important role in the crystal growth and the refinement. When the undercooling is in the rage of 100-200 K, the grain size decreases smoothly with increasing undercooling. Within the range of 230-260 K, abnormal large grains coexist with fine equiaxed grains. When the undercooling reaches 305 K, only large grains with an average size of 200μm exist. The competition between the strain energy stored in the rapid solidified crystals with the critical strain energy for recrystallization which results in different degrees of recrystallization explains the experiment results reasonable. In addition, the magnetic domain structure of undercooled Fe₈₁Ga₁₉ alloys has been found to be very sensitive to the variation of undercooling during the solidification process. With increasing the degree of undercooling, the evolution of domain patterns, magnetic contrast and the degree of the orientation were investigated which is relative to the magnetostriction. It should be noted that the grains tended to possess a preferred orientation within the undercooling range 150-200 K. Thirdly, bulk textured Fe₈₁Ga₁₉ alloy rods were successfully prepared using the technique of UDS by point and face triggering. The columnar grains of Fe₈₁Ga₁₉ rod directional solidified by point triggering have strong [100] texture. This preferred orientation of the [100] axis was approximately 10°from the rod direction. The microstructure of the rod was also studied by XRD, DSC and TEM. Results showed that the Fe₈₁Ga₁₉ alloy present in our sample is not a homogeneous bcc solution, but rather a nanodispersion of small Ga clusters within the A2 matrix which is corresponding to the modified DO3 structure. All these unique structural features have been attributed to the enhancement of magnetostrition of F81Ga19 alloy. The saturation magnetostriction is above 800 ppm, which is about 2 times as large as the maximum of the earlier report on Fe-Ga bulk samples. It has been ascribed to the high concentration of Ga-Ga atom pairs created by rapid solidification and their preferential orientation in (100) textured rod.Lastly, the effect of triggering undercooling on the texture and magnetostriction of directional solidified Fe₈₁Ga₁₉ alloy rods was discussed. Results showed that the [100] preferred orientation and the magnetostriction property were enhanced with the increasing of triggering undercooling in the rage of undercooling which could achieve directional growth crystals. The magnetostriction increases from 750 ppm for the triggering undercooling of 150 K to 885 ppm for the triggering undercooling of 210 K. In addition, the magnetostriction was found to decrease after it was saturated with the increasing applied field. It may be attributed to the fact that the [100] direction of textured grains tilted about 10°from the rod axis.