Enhancement of ductility and improvement of abnormal Goss grain growth of magnetostrictive Fe–Ga rolled alloys

The influences of initial microstructures on the mechanical properties and the recrystallization texture of magnetostrictive 0.1 at% Nb C-doped Fe83 Ga17 alloys were investigated. The directionally solidified columnar-grained structure substantially enhanced the tensile elongation at intermediate temperatures by suppressing fracture along the transverse boundaries. Compared with tensile elongations of 1.0% at 300℃ and 12.0% at 500℃ of the hot-forged equiaxed-grained alloys, the columnar-grained alloys exhibited substantially increased tensile elongations of 21.6% at 300℃ and 46.6% at 500℃. In the slabs for rolling, the introduction of 〈001〉-oriented columnar grains also promotes the secondary recrystallization of Goss grains in the finally annealed sheets, resulting in an improvement of the saturation magnetostriction. For the columnar-grained specimens, the inhomogeneous microstructure and disadvantage in number and size of Goss grains are improved in the primarily annealed sheets, which is beneficial to the abnormal growth of Goss grains during the final annealing process.

Influence of yttrium on the structure and magnetostriction of Fe_(83)Ga_(17) alloy

Polycrystalline Fe83Ga17 alloy rods with various amounts of yttrium were prepared by high vacuum induction melting. It is found that yttrium addition has a significant effect on the structure and magnetostriction of Fes3Ga17 alloy. The small addition of yttrium alters the solidification character and the grain shape of Fe83Ga17 alloy, and as a result, columnar grains with the 〈100〉 preferential direction are pro- duced. Yttrium addition improves the magnetostrictive performance of the as-cast Fes3Ga17 alloy. The magnetostriction values of the as-cast alloy with 0.32at% and 0.64at% yttrium addition go up to 119×10^-6 and 137×10^-6 under 15 MPa compressive stress, respectively. The energy dispersive spectroscopy （EDS） result shows that almost all of the yttrium atoms exist in the Y2Fe17-xGax phase. A small amount of this kind of secondary phase cannot obviously increase the saturate magnetic field.

Influence of Al on the magnetostriction of Fe-Ga polycrystal alloys under compressive stress

Fe80Ga20-xAlx （x = 0, 6, 9, 14） ingots were prepared from high purity elements using a vacuum induction system. X-ray diffraction patterns show that the alloys are A2 disordered structures. The influence of the partial substitution of Ga in Fe-Ga alloys with A1 on their magnetostrictive properties was investigated, and the effects of different heat treatment conditions on the magnetostriction and microstructure of the alloy rods were also examined. The saturation magnetostriction value of FesoGa2o can reach to 240 x 10-6 under a compressive stress of 20 MPa. The Fe80GallA19 alloy has many good properties, such as low hysteresis, high linearity of the magnetostriction curve, and low saturated magnetic field, which make it a potential candidate for magnetostrictive actuator and transducer applications. It is found that subgrains have little influence on the magnetostriction of Fe-Ga alloys.

Microstructure and magnetostrictive performance of NbC-doped <100>oriented Fe-Ga alloys

The〈100〉oriented Fe83Ga17 alloy rods with various NbC contents less than 1at%were prepared by the directional solidification method at a growth rate of 720 mm·h^-1. Low NbC-content was found to affect the oriented grain growth and slightly improve the〈100〉ori-entation. Flat grain boundaries in the alloys with low NbC contents less than 0.2at%became greatly curved at higher NbC contents, and a large amount of Nb-rich precipitates were observed in the alloys with high NbC contents. Small amounts of NbC, less than 0.2at%, resulted in an increase in magnetostrictive strain due to the improvement of the〈100〉orientation, and a high magnetostrictive strain value of 335 ×10^-6 under a pre-stress of 15 MPa was obtained in the 0.1at%NbC-doped alloys. The magnetostrictive performance obviously decreased with the NbC addition higher than 0.5at%, and the strain sensitivity under no pre-stress was lower than that in the binary Fe？Ga alloy.

Secondary recrystallization of Goss texture in magnetostrictive Fe–Ga-based sheets

To avoid the unfavorable effect of precipitates on magnetic properties,low addition of 0.16 wt%NbC was employed in Fe_(82.5)Ga_(17.5)alloy,and 0.27 mm sheets were successfully prepared by a rolling process.Strong Goss texture was obtained in Fe-Ga sheets by two-stage annealing process.The sulfur annealing in quartz ampoules was made to produce the preferred growth of Goss texture,with a 51%increase in the observed saturation magnetostriction,from 87×10^(-6)in the Ar-annealed sheets to 132×10^(-6)in the S-annealed sheets.However,large amounts of S-rich and Nb-rich precipitates were formed in the S-annealed sheets.High-temperature annealing of the S-annealed sheets at 1,200℃under a flow hydrogenargon atmosphere could effectively eliminate the precipitates and promote the selective growth of(110)grains due to the low surface energy.Large grains(~2 cm)of secondary recrystallization were achieved in the final annealed sheets,with the maximum average saturation magnetostriction of 180×10^(-6).

Texture-based magnetostriction calculation of oriented polycrystalline cobalt ferrites

Oriented polycrystalline cobalt ferrites with different alignments were prepared via a ceramic method. The maximum magnetostriction λs up to -270×10^-6 was achieved for oriented samples, when the oriented magnetic field was above 1.0 T. Textures of all samples were determined by electron back-scattered diffraction （EBSD） technique and were used to calculate magnetostriction values of oriented polycrystalline cobalt ferrite samples. Results show that a preferred 〈001〉 orientation has been obtained within oriented sample. Calculation was done by considering the magnetostriction value as the sum of the content in volume fraction-weighted single-crystal behavior along [100], [110], [210], [310], [311], [211] and [111] directions. The calculated magnetostriction values when compared with experimentally obtained values show identical behavior as a function of applied magnetization orientation field. Good consistency of change rules between calculated values and measured ones illustrates the successful application of this method in predicting magnetostriction values of polycrystalline materials.

Single Goss grain growth by isothermal annealing in rolled Fe–Al–Ga–Nb C sheets

The(Fe_(82)Al_(13.5)Ga_(4.5))_(99.9)(NbC)_(0.1) sheets with strong Goss texture and large magnetostriction were prepared by rolling and isothermal annealing processes.Single Goss grain growth was observed in sheets undergoing isothermal annealing at 1050 ℃ for 1 h using Ar as protecting gas.With annealing time increasing to 8 h at1050℃,a largest magnetostriction of 154×10^(-6) is obtained in sheet and a large λ_(//)(the maximum magnetostriction with magnetic field parallel to rolling direction)of 160×10^(-6) along the rolling direction is also observed.The fact that a λ_(//) of 160×10^(-6) along RD can be achieved with only 4.5 at% Ga in the Fe-18 at%(Al,Ga) sheet is of practical importance.Compared with previous studies in Fe-Al and Fe-Ga sheets,this paper provides a more efficient approach to prepare strong Goss textured sheets.In this way,a slow continuous heating process,sulfur-induced surface energy control and high-temperature annealing above 1200 ℃ are no more necessary.It shows a good prospect for industrial production of high-performance magnetostrictive alloy sheets.

Inhibition force of precipitates for promoting abnormal grain growth in magnetostrictive Fe83Ga17-(B,NbC) alloy sheets

Inhibition force of precipitate particles for pro- moting abnormal grain growth in magnetostrictive Feg3Ga17-（B,NbC） alloy sheets was investigated in this study. After a continuous heating and a high-temperature annealing, the Fe83Gat7 ＋ 0.5 at% B alloy sheets do not occur significant abnormal grain growth. Correspondingly, textures of { 111 }〈112〉 and { 100}〈00l〉 in addition to the Goss texture are obtained in the final annealed alloy sheets. By contrast, after the same annealing processes, the size of {110} textured grains is very large in the final annealed Fe83Gaj7 ＋ 0.5 at% NbC alloy sheets due to the abnormal grain growth, which results in a sharp Goss texture. BN precipitates were introduced into Fes3Ga17 ＋ 0.2 at% B alloy sheets by nitriding annealing at 800℃ for 2 min under NH3 atmosphere. The abnormal grain growth of Goss grains is achieved in 0.2 at% B-doped Fe83Ga17 alloy sheets after a high-temperature annealing, which is attributed to the enhanced inhibition force by introducing BN precipitates. During the recrystallization annealing process, Fe2B precipitates is easy to coarsen and decom- pose at high temperature due to the low thermal stability, resulting in a decrease or even disappearance of the inhi- bition force. For NbC and BN precipitates, the thermal stability and hardness of particles are both better than those of Fe2B precipitates, leading to strong inhibition force. Because of the preferred Goss texture, the magnetostriction of 2.05 × 10-4 and 1.81 × 10 4 is obtained in the sec- ondary recrystallized Fe83Gal7 ＋ 0.5 at% NbC and Fes3Gaw ＋ 0.2 at% B alloy sheets, respectively.

Magnetostriction of 〈100〉 oriented Fe–Ga rods with large diameter

The Fe83Ga17 alloy rods with h100 i preferential orientation was prepared by directional solidification method.The dimension of oriented Fe83Ga17 rods is large up to 40 mm greater than diameter in previous investigations of3–10 mm.The growth direction of columnar grain is nearly parallel to the solidification direction of rods.The deviation angle between axial direction and h100 i direction of columnar crystal was investigated by method of electron backscattering diffraction, and the deviation is smaller than10°.The large columnar grain and h100 i preferential orientation along axial direction of rods could be attributed to the calm solid–liquid interface during solidification process that is difficultly formed due to electromagnetism agitation in induction heating zone-melting method.The corresponding magnetostriction is large up to 290 9 10-6under pre-compressive press of 15 MPa.

Magnetostriction and magnetization of <110> oriented Tb0.27Dy0.73Fe1.95 alloys with different compressive prestresses

<110> oriented Tb0.27Dy0.73Fe1.95 alloys were studied under different applied compressive prestresses.Larger saturation magnetostriction by 500×10-6 is obtained for the sample under 5 MPa than that under no applied stress.To understand the effect of prestress on the magnetostrictive coefficient,differential magnetostrictive coefficient,relative permeability and coercive force were studied in detail.The results show that the maximum differential magnetostrictive coefficient is about 37 x 10-9 m·A-1,about two times that without stress.In addition,the saturation magnetostriction coefficient increases gradually with the stress increasing,while the maximum differential magnetostrictive coefficient decreases.The saturation inductions under different stresses are similar to a value of about 1.1 T.The relative permeability decreases with stress increasing.