简介概要

Magnetostriction and magnetization of <110> oriented Tb_0.27Dy_0.73Fe_1.95 alloys with different compressive prestresses

来源期刊：Rare Metals2020年第2期

论文作者：Xing Mu Hai-Jun Tang Xue-Xu Gao Xiao-Qian Bao Ji-Heng Li

文章页码：176 - 180

摘要：<110> oriented Tb_0.27Dy_0.73Fe_1.95 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.

稀有金属(英文版) 2020,39(02),176-180

Magnetostriction and magnetization of <110> oriented Tb_0.27Dy_0.73Fe_1.95 alloys with different compressive prestresses

Xing Mu Hai-Jun Tang Xue-Xu Gao Xiao-Qian Bao Ji-Heng Li

State Key Laboratory for Advanced Metals and Materials,University of Science and Technology Beijing

China Academy of Civil Aviation Science and Technology

作者简介：*Xue-Xu Gao,e-mail:gaox@skl.ustb.edu.cn;

收稿日期：7 January 2014

基金：financially supported by the National High Technology Research and Development Program of China (No.2011AA03A404);

Magnetostriction and magnetization of <110> oriented Tb_0.27Dy_0.73Fe_1.95 alloys with different compressive prestresses

Xing Mu Hai-Jun Tang Xue-Xu Gao Xiao-Qian Bao Ji-Heng Li

State Key Laboratory for Advanced Metals and Materials,University of Science and Technology Beijing

China Academy of Civil Aviation Science and Technology

Abstract：

<110> oriented Tb_0.27Dy_0.73Fe_1.95 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.

Keyword：

Tb-Dy-Fe; Compressive prestress; Magnetostriction;

Received： 7 January 2014

1 Introduction

The material Tb-Dy-Fe is well known for its excellent properties,e.g.,large magnetostriction,low anisotropy and high transition temperature [ 1] ,and it has been explored for many potential technological applications like actuators and sensors [ 2, 3, 4] .Application of compressive stress along the oriented direction can enhance the magnetostriction,thereby resulting in a sudden magnetostrictive rise known as jump effect.This effect,which arises from the magnetomechanical coupling between the magnetization and stress,was observed in the<110>-and<112>-oriented crystals by Mei et al. [ 5] and Clark et al. [ 6] ,respectively.Jiles and Thoelke [ 7] ,and Zhang [ 8] studied the magnetic behavior of Tb-Dy-Fe alloy under both the compressive stress and the filed with the Stoner-Wohlfarth model.A nonlinear magnetostrictive model proposed by Wang et al. [ 9] describes the relation of the magneto-thermo-mechanical properties,and the calculation result is consistent well with experiment at 20℃.The former research results indicated that the effect was a complicated magneto-mechanical phenomenon and should be researched in more detail.In this article,microstructure,magnetic properties,magnetostriction and compressive stress were investigated for<110>-oriented Tb_0.27Dy_0.73Fe_1.95 alloys.

2 Experimental

Starting materials of Tb_0.27Dy_0.73Fe_1.95 were prepared by vacuum induction co-melting of stoichiometric amounts of99.95 wt%pure Tb,Dy and Fe.<110>oriented crystals were grown under a very large temperature gradient by a directional solidification system.The sample rods,10.5 mm in diameter and 110 mm in length,weremachined by wire-cut electric discharge machine.Subsequently,these samples were heat-treated at 1293 K for 4 h in argon atmosphere followed by air cooling (Sample 1) or furnace cooling (Sample 2).The preferable orientation of these samples was determined to be<110>direction by the X-ray diffraction study on the transverse sections using a D/max-RB X-ray diffractometer (XRD).The polished surfaces of the samples were examined under a scanning electron microscope (SEM,FEI Quanta 200) fitted with a backscattered electron (BSE).The measurements of magnetos tric tive strain (λ),magnetic induction (B) and magnetic field (H) were carried out on a self-made instrument designated JDM-30.The differential magnetos tric tive coefficient (d₃₃=dλ/dH) was derived from theλ-H curves.The instrument utilized a standard resistance strain gauge technique to measureλ,a B-H tracer with a pickup coil to measure B,and a gas cell to supply an axial compressive load.

3 Results and discussion

3.1 Effect of compressive prestress on magnetostrictive strain analysis

The magnetostriction curves under various compressive prestresses for Sample 1 are shown in Fig.1.A clear change ofλwith compressive stress increasing is observed.Without compressive stress,the magnetostriction of the sample increases slowly with the magnetic field increasing and reaches saturation magnetostrain with a small value ofλs=500×10^-6,in a very low magnetic field.The magnetostrain curve presents an abrupt jump around a low point of H=25 kA·m^-1 under a compressive prestress of5 MPa,which refers here to the jump effect.This effect was previously reported and considered as the result of the reorientation of initial magnetic domains along the rod axis under compressive prestress [ 10, 11] .Upon increasing stress,jump position shifts toward high magnetic field and the effect is gradually weakened until the stress is up to30 MPa where the effect becomes unclear.In addition,the application of compressive prestress enhances the saturation magnetostrain,while the increment ofλ_s is not obvious.Theλ-H curves of Sample 2 which is cooled in furnace are similar to that of Sample 1.

Fig.1 Magnetostrictive curves of Sample 1 under different external compressive prestresses

Figure 2 shows the effects of compressive prestress on the differential magnetostrictive coefficient (d₃₃) of Sample1.The maximum d₃₃ without external compressive is about15.8×10^-9 m·A^-1.Since the rapid rise of the magnetostrain,d₃₃ changes remarkably under stress.When a stress of 5 MPa was applied on both sides of the sample rod,the maximum d_33max reaches up to 37×10^-9 m·A^-1.However,continuous increase in stress will undermine the d₃₃,e.g.,d_33max=17×10^-9 m·A^-1 at 25 MPa.Another noteworthy point is that the peak of d₃₃ moves to higher magnetic field with stress increasing,confirming the impact of prestress on the jump effect position of (110) oriented Tb_0.27Dy_0.73Fe_1.95 alloys.In order to make full use of properties of the alloys,it is of great importance to choose the right compressive prestress and bias magnetic field.

3.2 Effect of compressive prestress on hysteresis loop

As described above,the compressive prestress as a key parameter can improve samples'performance.To understand the mechanism of the impacts of compressive stress on the Tb_0.27Dy_0.73Fe_1.95 alloys,the hysteresis loops under different compressive stresses were investigated.Figure 3is the hysteresis loops of Sample 1 under different compressive prestresses (0,5,10,20 MPa).The saturated magnetic induction does not change obviously under different applied stresses,which is about 1.1 T.In addition,the curves change from S shape to almost a straight line from 0 to 20 MPa.Furthermore,the shape change observed in Fig.3 implies that the application of stress could decrease the relative permeability,which shall be studied in following section.Moreover,compressive stress can reduce the magnetic hysteresis and hence improve its application performance,e.g.,the output accuracy of actuator.

Fig.2 Differential magnetostrictive coefficient (d₃₃) versus magnetic field of Sample 1 under different external compressive prestresses

Fig.3 Hysteresis loop of Sample 1 under different external com-pressive prestresses

3.3 Effect of compressive prestress on relative permeability (μr)

By differentiating the hysteresis loops versus magnetic field,theμ_r of (110) oriented Tb_0.27Dy_0.73Fe_1.95 alloys under different prestresses is obtained as shown in Figs.4and 5.In Fig.4,theμ_r of Sample 1 demonstrates that the maximumμ_r decreases with applied stress.When the compressive stress is below or equal to 5 MPa,theμ_r increases rapidly with stress increasing and then decreases gradually after the maximum peak which appears when stress increases to 5 MPa.Nevertheless,when the stress is larger than 10 MPa,μ_r starts with large values,quickly reaches the maximum,then decreases slowly with magnetic field increasing,and finally trends to the same level as the values when stresses are 0 and 5 MPa.Moreover,the maximum position moves to high magnetic field at 5 MPa and then reverses and moves to low magnetic field with prestress increasing.

Fig.4 Relative permeability (μ_r) versus magnetic field for Sample 1under different external compressive prestresses

Fig.5 Relative permeability (μ_r) versus magnetic field for Sample 2under different external compressive prestresses

To investigate the influence of different cooling methods,Sample 2 annealed at 1293 K for 4 h and cooled following furnace cooling was studied.Theμ_r curves under different prestresses are shown in Fig.5.It is easy to see that the maximumμ_r descends with the increase in applied stress.The maximumμ_r value without stress is 16 and then drops dramatically to 9 while stress added increases to10 MPa.However,sequent increase in the prestress does not bring an obvious decrease in the maximumμ_r.In contrast to Sample 1 as shown in Fig.4,the peak position for Sample 2 shows a slight shift toward higher magnetic field with prestress increasing.

3.4 Effect of compressive prestress on coercive force

The relationships between compressive prestress and coercive force for Samples 1 and 2 are shown in Fig.6.Figure 6 shows that both coercive forces of Samples 1 and2 show similar variation trend with positive slope.However,the slope for air-cooled sample is lower than that for another one.

The properties of magnetoresponsive materials,such as magnetostriction,differential magnetostrictive coefficient,relative permeability and coercive force,relate partially to their microstructure.Verhoeven et al. [ 12] studied different cooling methods after annealing and considered that the distribution of the inner stress affected the alignment and rotation of magnetic moments.Jiang et al. [ 13] attributed the enhancement of the magnetostriction jump effect by annealing to the release of the internal stress located at the interface of rare earth phase and RFe₂ phase.Backscattered electron (BSE) measurements were carried out from the transverse section of the samples,as shown in Fig.7.By comparing the morphologies of the two samples,it is found that the rare earth-rich phase in Sample 1 shows more quantity and larger volume than that in Sample 2.Furthermore,the rare earth-rich phase located around the boundaries presents more breaks in Sample 2.Given that the rare earth-rich phase is nonmagnetic [ 14] ,its appearance would block the domain rotation,and the more the rare earth-rich phase there are,the more difficult the domain rotation is.Thus,the coercive force of Sample 2presents a lower slope.

Fig.6 Curves of coercive force versus compressive prestress for samples under different cooling methods

Fig.7 BSE images from cross sections of<110>oriented Tb_0.27Dy_0.73Fe_1.95 alloys with different cooling methods:a Sample1 with air cooling and b Sample 2 with furnace cooling

The influences of compressive prestress on<110) oriented Tb_0.27Dy_0.73Fe_1.95 alloys reveal the complex magneto-mechanical coupling that the magnetostriction and magnetization properties are greatly depended on the mechanical prestress and magnetic fields.Based on the view of energy minimum,the jump effect induced by compressive prestress is explained through the initial redistribution of magnetic domains and their volume changes under stress [ 15] .The saturated magnetostriction is larger than the sample under no prestress as initial volume fraction of 90°domains increases under prestress.And more90°domains rotation will cause lager magnetos tric tive coefficient and relative permeability.After all of the domains change to perpendicular to the direction of applied magnetic field,the maximum magnetostriction will not become larger with prestress.And larger stress will hinder domain wall motion and lead to larger coercive force.

4 Conclusion

The magnetostriction of (110) oriented Tb_0.27Dy_0.73Fe_1.95alloys under a compressive prestress of 5 MPa is larger than that without stress by a value of 500×10^-6.Accordingly,the maximum differential magnetostrictive coefficient is about 37×10^-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 shape of the hysteresis loops changes with applied prestress.Without stress,the curve adopts S shape;and with stress increasing,the curve tends to become straight line.The saturation inductions under different stresses are similar to a value of about 1.1 T.The relative permeability decreases with stress increasing.