Multifunctional Characters Of Fe-Ga Alloys

In comparison with the brittle intermetallic compounds REFe2, magnetostrictive Fe-Ga solid solution (known as Galfenol) has a lower switching field, better malleability, lower cost, and better temperature stability. Consequently, the Fe-Ga alloys have drawn considerable interest as a new-type magnetostrictive material.The structure and magnetism of Fe-Ga solid solutions are very sensitive to both Ga concentration and heat treatment, due to the structural diversity. Because of the abundant solid phase transformations in this unique system, multi-functional characteristics are expected through controlling the magnetic /structural transitions.In this work, the diffusion-type structural transformation from BCC to FCC and the resultant novel properties have been investigated using XRD, TEM, thermal analysis and magnetic measurements. Interestingly,BCC+FCC composites formed by aging treatment are found to exhibit meta-magnetostrictions, piezomagnetic compensation and enhanced damping effects, which may broaden the application range of Fe-Ga alloy. Main results are as follows:First,the diffusive transformation process from BCC to FCC in Fe73Ga27 is revealed, providing useful principals to prepare magnetic composites. According to the Bain relationship, the transformation from BCC (DO3) to FCC (L12)requires not only significant lattice strain, but also atomic position exchanges between Fe and Ga. Therefore, such transformation need long-term heat-treatment to overcome the large energy barrier between two structures as well as the slow dynamics. Characterized by X-ray diffraction (XRD) and electron diffraction, the Fe73Ga27 alloy prepared in metastable state (ice-water quench from 1373 K) is of BCC average structure, but contains a large amount of ordered DO3 nanodomains.DSC, M-TG analysis and quasi in-situ XRD measurements reveal that the metastable BCC phase firstly experiences Curie transition at 698 K and undergoes subsequently magnetic/structural transitions D03para. (BCC) ? L12ferro. (FCC)?DO19para. (HCP)? B2para. (ordered BCC) at 720 K, 908.9 K and 949 K,respectively. Another Fe73Ga27 specimen subjected to long term aging for 1 month at 803 K, which is a phase mixture of L12 and remanent BCC phase, shows similar magnetic/structural transition sequence to the as-quenched one. Magnetic measurements show that the formation of L12 phase not only increases the magnetization, magnetocrystalline anisotropy and coercivity, but also changes the magnetostriction sign from positive to negative. Lattice parameter calculation indicates that remanent BCC phase in the long-term aged sample has a smaller Ga concentration than that before aging, which may deviate out of the narrow composition range where L12 phase can be formed. The above results strongly support that the BCC to FCC transformation is a diffusive process accompanied with obvious composition change. Consequently, it gets more clearer that single phase L12 Fe73Ga27 alloy cannot be obtained even aging for 1 month at 803 K.Second, novel properties, including meta-magnetostrictions, nearly-compensated piezomagnetic response, and enhanced damping capacity have been obtained in BCC + FCC Fe-Ga composites. Composites with different BCC/FCC volume fractional have been prepared through aging the samples for 12 h and 72 h at 803 K. Utilizing the competing domain switch paths between BCC phase with positive magnetostriction and FCC phase with negative magnetostriction, it is found that 1) BBC phase with smaller magnetocrystalline anisotropy exhibits 90°domain switches prior to the FCC phase with stronger magnetocrystalline anisotropy, resulting in that the magnetostriction changes from positive to negative during magnetization; 2) under uniaxial compressive stresses, magnetic moment of BCC phase rotates towards the perpendicular direction,meanwhile the FCC ones tend to align to the parallel axis, giving rise to piezomagnetic compensation effect;3) the competing domain switches between two phases also dissipate a large portion of elastic energy upon stress loading and unloading cycles, allowing one to get enhanced damping capacities.To summarize,the above two findings suggest that tuning the structural diversity of magnetic Fe-Ga alloy is promising to develop multi-functional materials.