Mechanical and magnetostrictive properties of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 alloys

MA Tian-yu(马天宇), JIANG Cheng-bao(蒋成保)

School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics,Beijing 100083, China

Received 20 April 2006; accepted 30 June 2006

Abstract: The magnetization, magnetostriction and compressive strengths of arc-cast polycrystalline and directionally solidified Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 (x=0, 0.05, 0.10, 0.15, 0.20) rods were investigated by VSM, standard strain gauge method and compressive tests, respectively. The results show that the magnetostriction λ_s, saturation magnetization M_s and magnetocrystalline anisotropy constant K₁ decrease with increasing the Mn concentration. The optical micrographs and XRD patterns show that the Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 alloys are composed of MgCu₂-type Laves phase as matrix and a small amount of rare-earth rich phase. It is found that the distribution of the rare-earth rich phase has an important effect on mechanical property of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 samples. For the arc-cast samples, smaller equal-axial grains are arranged irregularly, which results in higher compressive strengths. However, the rare-earth rich phase is arranged as parallel arrays in the directionally solidified samples, which leads to smaller compressive strengths.

Key words: magnetostriction; Laves phase; rare-earth rich phase; compressive strength

1 Introduction

The C15 Laves phase compounds Tb_xDy_1-xFe₂ have attracted much attention due to their large magnetostriction. The magnetocrystalline anisotropy of these compounds minimized at a specific temperature T_r with an accompanying magnetization rotation[1]. Previous study reported that T_r decreases with both increasing Tb/Dy ratio and substituting Mn for Fe[2]. Moreover, Mn containing compounds showed larger magnetostriction than that for a Mn-free compound at low temperature due to the transition metals contribution to the magnetocrystalline anisotropy[3]. However, these compounds have a poor tolerance to tensile and shear forces with maximum compressive stress of about 320 MPa for polycrystalline samples due to the brittleness of the RFe₂ (R indicates rare earth) Laves phase, which make them suitable to be used under compressive conditions[4]. In fact, these compounds are mainly composed of two phases: RFe₂ Laves phase as matrix and ductile rare earth-rich phase distributed at the boundaries of the Laves phase[5]. For grain-aligned Terfenol-D rods, narrow phase spacing helps to improve the strength and toughness because of the dense interconnected skeleton network of the ductile R-rich phase[6, 7].

In our previous work, Tb_0.36Dy_0.64Fe₂ exhibits giant magnetostriction over the temperature range from -80 to 100 ℃, among which the easy magnetization direction lies along 〈111〉[8]. In the present work, the effects of Mn substitution for Fe on magnetostrictive and mechanical properties in Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 alloys were investigated at room temperature.

2 Experimental

Samples with initial composition of Tb_0.36Dy_0.64- (Fe_1-xMn_x)_1.9 (x= 0, 0.05, 0.10, 0.15, 0.20) were prepared by arc-melting the appropriate constituent metals under the protection of high purity argon atmosphere. The ingots were cast into rods with the diameter of 6.8 mm. The grain-aligned rods were fabricated by zone melting in alumina crucibles (inner diameter 7 mm) at 240 and 480 mm/h growth rates with a super high temperature gradient.

The compressive tests were taken on both as-cast (AC) and directionally solidified(DS) samples with the length of 8 mm by using MTS 880 material test systems. The loading velocity was selected as low as 0.5 mm/min. Fig.1 shows the typical axial force—displacement curves of both AC and DS Tb_0.36Dy_0.64Fe_1.9 samples. The compressive stress of the samples can be calculated as

σ=F₀/S                                     (1)

where  F₀ is the maximum axial force as shown in Fig.1 and S refers to the area of the transverse section of the sample. For each alloy, the maximum compressive stress of five specimens was confirmed as the compressive strength, because the rupture value is very irreproducible from specimen to specimen.

Fig.1 Typical axial force—displacement curves of both as-cast and directionally solidified Tb_0.36Dy_0.64Fe_1.9 samples

The magnetization of the as-cast samples was measured with a PAR155-type vibrating sample magnetometer(VSM). The maximum magnetic field available is 800 kA/m. The saturation magnetization M_s and magnetocrystalline anisotropy constant K₁ were calculated according to the law of approach. X-ray diffraction data of samples in powder form were obtained at room temperature with Cu K_α radiation in a D/max 2200 pc X-ray diffractometer. The magnetostriction measurements were made on both AC and DS samples with the length of 25 mm using standard strain gauge techniques. A gas pressure cell was employed to produce 0 and 10 MPa axial compressive pre-stresses. The polished samples were examined for revealing the grain size and phase distribution by an optical microscopy. The ruptured micrographs of the samples were taken by a scanning electron microscopy(SEM).

3 Results and discussion

3.1 Structure

X-ray powder diffraction analysis confirms that the matrix of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 (x= 0, 0.05, 0.10, 0.15, 0.20) alloys is the cubic Laves phase with MgCu₂-type structure, as shown in Fig.2. The lattice constant a is calculated according to the well known Bragg formula. The results show that a increases from 0.733 1 nm for Tb_0.36Dy_0.64Fe_1.9 to 0.734 9 nm for Tb_0.36Dy_0.64(Fe_0.80- Mn_0.20)_1.9. It indicates that Mn containing alloys still keep the same structure with Mn free alloy, which is similar to Tb_0.5Dy_0.5(Fe_0.9Mn_0.1)₂ alloy[9]. The increase of the lattice constant a can be ascribed to the larger radii of the Mn atoms.

Fig.2 XRD patterns of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 polycrystalline samples in powder form

Fig.3 shows the typical microstructure of transverse sections of AC and DS Tb_0.36Dy_0.64(Fe_0.9Mn_0.1)_1.9 samples. The alloy is composed of a Laves phase as matrix and a small amount of rare earth-rich phase. The grains with the average size of about 20 μm are equal-axial and randomly arranged in the as-cast state, as shown in Fig.3(a). After directional solidification, dendritic morphologies have been formed and the rare earth-rich phase is distributed regularly as thin sheets. The average lamellar spacing of the directionally solidified sample is about 70 μm, as shown in Fig.3(b), which is far larger than that of the as-cast state.

3.2 Mechanical properties

The typical force—displacement curves show the presence of a brittle rupture in the Tb_0.36Dy_0.64Fe_1.9 alloy either in as-cast state or in directionally solidified state, as can be seen from Fig.1. The nonlinear segment of the curve under low axial forces is not a real plastic distortion, but caused by the irreversible magnetic domains rotation under compressive stresses. After all the magnetic domains rotate along the direction perpendicular to the rod axis, only elastic distortion occurs due to the brittleness of the matrix Laves phase. The compressive stresses of the as-cast and directionally solidified Tb_0.36Dy_0.64Fe_1.9 samples reach 613.1 and 373.1 MPa, respectively. The effect of Mn substitution on compressive strength of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 alloys can be seen from Table 1. It is not simple to draw a conclusion that the Mn-doped alloys have higher or lower compressive strength than Mn free alloy because the microstructure is various among samples even prepared in the same furnace.

Fig.3 Typical microstructures of transverse sections of Tb_0.36Dy_0.64(Fe_0.9Mn_0.1)_1.9 alloy: (a) As-cast; (b) Directionally solidified

Table 1 Compressive strength of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 samples in as-cast (AC) and directionally solidified (DS) states

The low compressive stress of the directionally solidified sample results from the formation of the preferred orientation along the rod axis (the compressive force direction). The microstructure reveals different distribution of the rare earth-rich phase in as-cast and directionally solidified samples, as can be seen in Fig.3. According to the well known Hall-Pitch equation, the higher compressive strength can be obtained by fining the grain size. The dense network shape rare earth-rich phase in the as-cast sample with small equal-axial grains is helpful to improving the strength. However, cracks in the directionally solidified samples with axial preferred orientation and large lamellar phase spacing would be grown quickly and easily along the grain aligned- direction. Fig.4 shows the optical and SEM back- scattered micrographs of the ruptured directionally solidified Tb_0.36Dy_0.64(Fe_0.9Mn_0.1)_1.9 sample. It is obvious that the rupture occurs along the longitudinal direction of the directionally solidified direction, as shown in Fig.4(a). The plastic distortion symbols such as slippage etc can not be seen from the SEM back-scattered micrograph, as shown in Fig.4(b), which indicates the presence of elastic rupture in such alloys.

Fig.4 Microstructures of ruptured directionally solidified Tb_0.36Dy_0.64(Fe_0.9Mn_0.1)_1.9 sample: (a) Optical graph; (b) SEM back-scattered micrograph

3.3 Magnetic and magnetostrictive properties

Fig.5 shows the magnetization hysteresis loops of as-cast Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 alloys at room temperature. The coercivity H_c, remnant magnetization M_r, saturation magnetization M_s and magnetocrystalline anisotropy constant K₁ calculated from the law of approach are listed in Table 2. Both the values of M_s and K₁ decrease with increasing the Mn concentration, which indicates the reduction of the magnetic moment and the magnetocrystalline anisotropy of the R(Fe, Mn)₂ (R=Tb, Dy) Laves phase.

Fig.5 Magnetization hysteresis loops of as-cast Tb_0.36Dy_0.64- (Fe_1-xMn_x)_1.9 samples: (a) x=0, 0.10, 0.20; (b) x=0.05, 0.15

Table 2 Coercivity H_c, remnant magnetization M_r, saturation magnetization M_s and magnetocrystalline anisotropy constant K₁ of as-cast Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 samples

The composition dependence of M_s can be understood in terms of the reduction of the magnetic moment of 3d sublattice. In general, the coupling between Mn-Mn or Mn-Fe would be antiferromagnetic if the average distance of the Mn-Mn atoms is less than 0.28 nm[10]. The distance of the 3d atoms is  in the C15 type Laves phase. Then the maximum 3d atoms distance in Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 samples is 0.259 nm for x=0.20, which is less than 0.28 nm, indicating the presence of the antiferromagnetic coupling between Mn-Mn and Mn-Fe atoms. The 3d magnetic moment decreases with the addition of Mn because of the opposite magnetic coupling of Mn-Fe and Fe-Fe atoms.

The magnetocrystalline anisotropy energy for Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 compounds is practically determined by rare earth element contributions because the rare earth elements have larger magnetocrystalline anisotropy than 3d transition elements. It has been reported that Mn substitution shifts the magneto- crystalline anisotropy compensation to higher Tb concentration in Tb_xDy_1-xFe_1.95 compounds[11]. As known that TbFe₂ has a large negative anisotropy and DyFe₂ has a large positive anisotropy, the low magnetocrystalline anisotropy in high Tb concentration compounds must result from the positive anisotropy contribution of Mn addition. So, the decrease of magnetocrystalline anisotropy constant K₁ in Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 compounds with increasing x can be understood in terms of the positive anisotropy contribution of Mn addition.

Fig.6 shows the magnetostrictive properties of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 as-cast samples at room temperature. As can be seen from Fig.5, the samples are approaching to saturation in 400 kA/m magnetic field. The magnetostriction in 400 kA/m is confirmed as the saturation magnetostriction λ_s for each sample. It is obvious that λ_s decreases with increasing the Mn addition. According to the single ion model[12], the saturation magnetostriction is proportional to, where M_SR is the saturation magnetization of the rare earth sublattice. The decrease of λ_s must result from the decrease of M_SR with increasing x, which indicates that Mn substitution also has an effect on the rare earth sublattice, similar to Sm_0.9Pr_0.1(Fe_1-xMn_x)₂ systems[13].

Fig.6 Magnetostriction of Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 as-cast samples

4 Conclusions

Mn substitution for Fe has an important effect on Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 compounds. The decrease of saturation magnetization can be explained on the basis of the antiferromagnetic coupling between Mn-Mn or Mn-Fe atoms in (Tb, Dy)(Fe, Mn)₂ Laves phase. The magnetocrystalline anisotropy decreases with increasing Mn concentration because of the positive anisotropy contribution. The decrease of the saturation magnetostriction demonstrates the Mn substitution decreases the magnetization of the rare earth sublattice.

The rare earth-rich phase has an important effect on the mechanical property of as-cast and directionally solidified Tb_0.36Dy_0.64(Fe_1-xMn_x)_1.9 samples. Higher compressive strengths are obtained in as-cast samples due to the small grain size of the equal-axial shaped morphologies. The optical and SEM back-scattered micrographs of the ruptured directionally solidified sample reveal that the cracks grow along the rod axis direction without any plastic distortion.

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(Edited by YUAN Sai-qian)

Foundation item: Project(60534020) supported by the National Natural Science Foundation of China; Project(03G51019) supported by the Aeronautics Foundation of China; Project(04-0165) supported by the New Century Programme for Excellent Talents of the Ministry of Education of China; Project(400152) supported by the Innovation Research Foundation for PhD student of BUAA

Corresponding author: JIANG Cheng-bao; Tel: +86-10-82317117; E-mail: jiangcb@buaa.edu.cn