Morphotropic Phase Boundary Of Giant Magnetostrictive Materials TbDy(Ho)Fe

Giant magnetostrictive alloy TbDyFe is a kind of advanced intelligent material, which can rapidly convert magnetic energy into mechanical energy or vice versa. Due to the large magnetostriction, high strength, and high output force, this material has been applied in highly pressive transducers, actuators, sensors, and so on. The recently reported morphotropic phase boundary (MPB) in a number of giant magnetostrictive materials (GMMs) has drawn considerable interest to the local symmetry/structure near MPB region of these materials. Alloy with composition at MPB shows the flattened free energy profile state, coexistence of two low-symmetry ferromagnetic phases, and the significantly enhanced strain at MPB under low external fields. Consequently, this offers a new approach for the development of magnetostrictive materials. In this work, by in-situ X-ray diffraction (XRD) we show the susceptibility changes sluggishly across TMPB of Tb0.3Dy0.7Fe2, the typical composition of Terfenol-D GMMs. By high resolution transmission electron microscopy (HRTEM) we studied the local structure/symmetry and local domain structure near MPB. In addition, a quarternery Tb0.26Dy0.54Ho0.20Fe2 is developed on the basis of the MPB theory, by further reducing the magnetostriction hysteresis at room temperature. High performance materials with strong crystal texture have been successfully prepared by zone-melting unidirectional solidification and subsequent magnetic field annealing. Main achievements of this dissertation are as follows:We demonstrated that ferromagnetic MPB contains also nano-sized magnetic/structural domains that are responsible for the low field giant magnetostriction, being physically parallel to the ferroelectric MPB. The MPB that separates ferroelectric phases of different crystallographic symmetries in the composition-temperature phase diagram has drawn considerable interest to develop high performance piezoelectric materials for more than half a century. Very recently, MPB has been extended to another physically parallel ferroic system, the ferromagnets. Similar to ferroelectric MPBs, the already reported ferromagnet systems also show the flattened free energy landscape, which results in the coexistence of two low-symmetry phases and the significantly enhanced magnetostriction at MPB. Consequently, this offers a magneto-structural origin for the well-known Terfenol-D GMMs with compositions near MPB (typically, Tb0.3Dy0.7Fe2), which can generate large magnetostriction at low switching fields around room temperature. In the present work, by in-situ X-ray diffraction (XRD) and AC magnetic susceptibility measurements we discover that Tb0.3Dy0.7Fe2, the typical composition of Terfenol-D GMMs, is located near MPB (the MPB temperature, TMPB=258 K) in the rhombohedral phase at room temperature. High resolution transmission electronic microscopy (HRTEM) provides direct evidence for local rhombohedral symmetry of the ferromagnetic phase and reveals regular-shaped nanoscale domains with width below 10 nm. The nano-sized structural/magnetic domains are hierarchically inside a single micron-sized stripe-like domain with the same average magnetization direction. Such domain structures are consistent with the low magnetocrystalline anisotropy and easy magnetic/structural domain switching under magnetic field, thus generating large magnetostriction at low field.Tb0.26Dy0.54Ho0.20Fe2, which exhibits a remarkable magnetostriction performance with relatively low hysteresis had been successfully designed based on MPB theory. In comparison with the Ho-free Tb0.3Dy0.7Fe2 alloy, TMPB of Ho-containing alloy was raised to 268 K, which leads to the reduction in magnetostriction hysteresis at room temperature. Highly oriented polycrystals Tb0.26Dy0.54Ho0.20Fe2 have been prepared by zone-melting unidirectional solidification method with high temperature gradient at the solid-liquid interface, and magnetic field annealing is applied to improve its magnetostrictive performance by introducing additional unixial anisotropy. The saturation magnetostriction under free conditions can increase from 700 ppm for the thermal demagnetized state to 875 ppm for the magnetically annealed one. When subjected to a compressive pre-stress, magnetostrictive performance of the magnetically annealed specimen could be further enhanced. Under 30 MPa, the saturation magnetostriction can reach 1508 ppm. The transverse field annealing results in not only an increment in the area of hysteresis loops, but also a shift of the curve toward higher field regions, which is a evidence of the larger anisotropy because of the induced additional anisotropy by magnetic annealing. Just because the induced additional anisotropy tends to switch the magnetizations align along one or two principal easy axes, forming a uniaxial anisotropy, which is beneficial to the improvement of magnetostrictive performance. This induced additional anisotropy can also be reflected by the slight increase of the MPB temperature, i.e. magnetic annealing leads to the shift MPB in composition-temperature phase diagrams.