Influence of External Static Magnetic Fields on Properties of Metallic Functional Materials

Abstract

Influence of external static magnetic fields on solidification, solid phase transformation of metallic materials have been reviewed in terms of Lorentz force, convection, magnetization, orientation, diffusion, and so on. However, the influence of external static magnetic fields on properties of metallic functional materials is rarely reviewed. In this paper, the effect of static magnetic fields subjected in solidification and/or annealing on the properties of Fe–Ga magnetostrictive material, high strength high conductivity Cu-based material (Cu–Fe and Cu–Ag alloys), and Fe–Sn magnetic material were summarized. Both the positive and negative impacts from magnetic fields were found. Exploring to maximize the positive influence of magnetic fields is still a very meaningful and scientific issue in future.

1. Introduction

External static magnetic field (SMF) is one of the non-ambient thermodynamic parameters affecting thermal history during aspects of the processing of materials [1]. Lorentz force induced by the interaction between SMF and conductive melt is capable of suppressing natural convection, and thermoelectromagnetic convection can promote the convection [2]. Magnetization energy from external SMFs can change Gibbs free energy if there is a difference in magnetization energy between parent and product [3]. Thus, SMF can be introduced to modify microstructure in various materials by influencing phase equilibrium [4], phase transformation [5,6,7,8], solid solubility [9], diffusion [10], and precipitation [11,12]. There are several reviews concerning the effect of SMF on microstructure and affecting mechanisms in metallic materials [13,14,15,16,17], however, the resulting properties of metallic functional materials owing to the imposition of external SMFs are rarely reviewed. In this paper, we emphatically presented the changes in the properties of several metallic functional materials by applying external SMFs in solidification and/or annealing, and analyzed the positive trigger of SMFs.

2. Contributions of SMFs on Magnetostriction of Fe–Ga Alloy in Solidifying and Annealing

Owing to good interactive responses on reversible strains and applied magnetic fields, magnetostrictive materials are potentially used as acoustic sensors, generators, and actuators [18,19,20]. Fe–Ga alloys are firstly proposed by Clark et al. [21] as an alternative desired magnetostrictive material with high mechanical strength, large magnetostriction at low saturation fields, and high imposed stress levels, and low cost relative to the commercial Tb–DyFe₂ (Terfenol-D) alloys. However, the magnetostriction of Fe–Ga alloys (about 100 ppm) is only one tenth of the magnetostrictive coefficients of Terfenol-D alloys (about 1000 ppm). High temperature disordered (A2) phase due to Ga addition is the main contribution to the increase in the magnetostriction [22]. In view of significant contributions on the magnetostriction, controlling the phase constitutions of Fe–Ga alloys is an alternative way to improve their performances. Considering that the (100) crystalline direction of the A2 phase is not only the preferred growth direction but also the easy-magnetization direction, the strongly [100] textured Fe–Ga alloys might be obtained when applying an SMF to their solidification and/or annealing. Previous investigations show that an SMF can increase the nucleation of the melts [23] and enhance the texturing degree [24,25]. Solidification under an SMF was found to increase the magnetostriction of Terfenol-D alloy because of improving texture and phases [26,27,28]. Therefore, SMF has extensive applications in the solid-solid phase transformations to alter the transformation temperatures, the phase constitutions and selection, the phase morphology and disorder-order transition [29,30]. Moreover, magnetic field annealing has been confirmed to enhance the performances of some magnetostrictive materials [31,32]. It is the purpose of the present work, to compare the effects of SMFs to magnetostriction of Fe–Ga alloys in solidifying and/or annealing processing.

With respect to the solidification of Fe–Ga alloy, the Fe–27.5wt %Ga specimens were heated and furnace cooled under Ar atmosphere to room temperature with and without a 1-T SMF. With respect to the annealing, the Fe–27.5wt %Ga specimens were heated to 1273 K at the rate of 5 K/min, held for 60 min and then furnace cooled to room temperature or quenched in water. A 12-T SMF was subjected during the annealing process. More similar experimental information could be found in our papers [33,34]. The magnetostriction of the specimens was measured using a strain gauge. In a polycrystalline material having domains initially oriented randomly, the magnetostriction can be obtained by the formula [35]:

λ_total = λ_par − λ_per = 3/2λ_s

(1)

where λ_total, λ_par, λ_per, and λ_s are total magnetostriction, the magnetostriction parallel and perpendicular to longitudinal sections, and saturated magnetostriction, respectively.

Figure 1 shows the resulting magnetostriction of Fe–27.5wt %Ga samples by applying SMFs in different processes. With increasing measured SMF, the magnetostriction of all samples increases (Figure 1a,b). When SMF is applied in the solidification process of Fe–Ga alloy (Figure 1a), the total magnetostriction of solidified Fe–27.5wt %Ga sample with an SMF is enhanced to 103 ppm from 46 ppm without an SMF (Figure 1c). That means that the saturation magnetostriction for polycrystalline Fe–27.5wt %Ga alloy solidified with 1-T SMF increases by about two times than that without an SMF. Though the magnetostrictive value of Fe–Ga alloys in our present work is not as large as the previous work from Zhou et al. [36], the imposed SMF to the solidification process of Fe–Ga alloys may be a simple but effective approach to fabricate the bulk Fe–Ga alloys. Comparing to the high saturation field of 12,000 Oe fabricated by rapid directional solidification from the undercooled melts [36], the saturation field in solidified Fe–Ga alloy with an SMF is remarkably lowered (about 3000 Oe listed in Figure 1c), which is an important factor during their applications at low fields. The improved magnetoelastic performances might to be attributed to the increasing volume fraction and texture of the disordered α-Fe phases, promoting more Ga atom clusters which was a result of the magnetic-field-undercooling and magnetic orientation induced by the applied SMF during the solidification process.

With respect to the annealing process, there are obvious decreases in the saturated magnetostriction of Fe–27.5wt %Ga alloys with a 10-T SMF compared to that without an SMF (Figure 1b (A1 and A2, or A3 and A4)). As the cooling rate increases (Figure 1b (A1 and A3, or A2 and A4)), the saturated magnetostriction of Fe–27.5wt %Ga alloys increases remarkably. The results demonstrated that applying a 10-T thermomagnetic processing to Fe–Ga alloys might increase the transformation from the disordered phases to the ordered phases, and enhance the magnetic anisotropy energy, thus decreasing the magnetostriction. Previous results from Terfenol-D alloys [37] indicated that the thermomagnetic processing introduced more additional anisotropy in the alloy with a large initial magnetocrystalline anisotropy. The increasing magnetic anisotropy energy broke the thermodynamics equilibrium of Fe–Ga system and promoted the formation of the ordered phases. Moreover, the nucleation stage rather than the growth stage during Fe–Pd ordering process induced by the external fields caused the phase transformation from the disordered A2 phases to the ordered L1₂ structure phases (Figure 1c confirmed it) [29]. The ordered phase products might be contributed to the decreasing magnetostriction of Fe–Ga alloys. Our result has a negative influence of high SMF on magnetostriction of Fe–27.5wt %Ga alloy, however, Wen et al. [31] found a positive influence that the saturation magnetostriction for Fe₈₁Ga₁₉ alloy annealed at 10 T increased by about five times compared to one without SMF. It might be resulted from the differences in composition and annealing temperature because the magnetostriction of Fe–Ga alloy is dependent on the two factors.

In summary, we applied SMFs in solidifying and annealing processes, and compared the influence of SMFs on the saturation magnetostriction. Solidifying with SMF increased the saturation magnetostriction, however, annealing with SMF decreased the magnetostriction. The changes in the saturation magnetostriction might be related to the texture, composite, thermal history. The work indicates that the imposed SMF to the solidification process of Fe–Ga alloys may be a simple but potential method to fabricate the bulk and practical Fe–Ga alloys at low fields.

3. High Strength High Conductivity Cu-Based Alloys Treated by SMFs

Conductors with high strength and high conductivity are required in construction and design of both direct current (DC) resistive and pulsed high-field magnets. Cu–Ag and Cu–Fe alloys are known to have an excellent combination of these properties. However, the increase in the strength is generally accompanied by a decrease in the electrical conductivity, and vice versa [38,39,40,41,42]. Our group applied SMFs to control the microstructures during the solidification process of the alloys in order to improve the tradeoffs between strength and electrical conductivity [43,44,45,46].

Figure 2 shows the comparison of Cu–Fe alloys by using SMFs in different processes. An external 1-T SMF was applied to the solidification process of the Cu–Fe alloy with different Fe contents [47]. The alloys solidified with and without an SMF were heavily deformed to Cu–Fe composite wires with different drawing ratios, respectively. Where, the drawing ratio is defined in term of the logarithm of the ratio of the original cross-sectional area A₀ to the final area A_f, functionally expressed by η = ln (A₀/A_f). When the drawing ratio is lower, the strength of the drawn Cu–Fe composites are lower when pre-solidified with a horizontal SMF (Figure 2a), the increase ratio in the strength of the drawn Cu–Fe composites with a 1-T SMF is quicker with increasing Fe fraction and the drawing ratio (Figure 2b). The conductivity of the drawn Cu–Fe composite wires is decreased with increasing the Fe content. The injected SMF has no effect on the electrical conductivity (EC) of the drawn wires (Figure 2b). With respect to the annealing process, with increasing annealing temperature, the ultimate tensile strength (UTS) of Cu-15wt %Fe composite increase no matter with or without SMFs (Figure 2c). Inverted “C” type tendency is found in the relationship between UTS and EC (Figure 2d) [48], which shows that the range from 450 °C to 500 °C is optimal annealing temperature for property optimization of Cu–Fe composites. SMF has a slight influence on the changes in UTS and EC.

Cu–6wt %Ag alloy was solidified with a 1.12-T SMF, and then cold rolled into Cu–Ag composites. The reduction ratio is defined as RA% = (t₀ − t_f)/t₀ × 100, where t₀ and t_f are the initial and final thickness of sample, respectively. Intermedia annealing at temperature of 300 °C was applied to improve the properties of Cu–Ag composites. Cu–6wt %Ag composite with SMF had a lower hardness at a lower reduction, but had a higher hardness at a higher reduction than that without an SMF (Figure 3). We believed that more dissolved Ag in proeutectic Cu under an SMF was precipitated during rolling at a higher reduction, thus enhancing the precipitation hardening. Ageing at 300 °C promoted Ag precipitating from proeutectic Cu matrix and enhanced both microhardness and EC of Cu–6%Ag composite. More details can referred to a previous publication [44].

MR is generally described by [R_{(H, T)} − R_{(0, T)}]/R_{(0, T)} = ΔR/R₀, where R is the resistance; H and T are the magnetic field strength and temperature, respectively; R₀ is the resistance at zero magnetic field. Both Cu–28wt %Ag alloy and Cu–72wt %Ag eutectic alloy were solidified with a 12-T SMF. The magnetoresistance was measured by a standard four-probe method under high magnetic fields up to 31.2 T at National High Magnetic Field Laboratory, USA. MRs of both Cu–72wt %Ag eutectic alloy and Cu–28wt %Ag alloy at 77 K are plotted as a function of the applied DC SMF in Figure 4. MR of both Cu72wt %Ag eutectic alloy and Cu–28wt %Ag alloy was increased with ramping DC SMF in the MR measurement. MR of both Cu–28wt %Ag and Cu–72wt %Ag eutectic alloys solidified under SMF were higher than that solidified without SMF. With respect to Cu–28wt %Ag alloy, the maximum MR value of Cu–28wt %Ag alloy SMF with and without SMF is 25.4% and 34.0%, respectively. Previous literatures reported that the maximum MR of drawn Cu–Ag composites at 77 K was 28% at 60 T [49] and 14% at 50 T [50]. However, their MR of as-cast Cu–Ag alloy was not reported. Previous work found revealed that MR of as-cast alloy had higher MR value than that of deformed alloy [51]. Thus, MR of as-cast Cu–Ag alloy at 77 K might be larger than 28%. Our results were almost consistent with reported results. The modeling results of Cu–28wt %Ag alloy according to the rule of mixture (ROM) were presented in Figure 4a. ROM curve is between the experimental data of sample with 12 T SMF and that without SMF. This indicates that the ROM cannot be directly used to predict the MR. The maximum MR of Cu–72%Ag eutectic alloy, as shown in Figure 4b, with and without SMF and in its absence were 25.5% and 25.2%, respectively. Our experimental results are in good agreement with the results from previous literature [50]. MR of Cu–28%Ag alloy was higher than that of the Cu–72%Ag eutectic alloy, which shows that there is a large SMF response in Cu–28%Ag alloy. More details can referred to a previous publication [52].

4. Magnetic Properties of Fe–Sn Monotectic Alloys Solidified by SMFs

Monotectic alloys have attracted attentions for their unique potential applications, including bearing materials, catalysts, permanent magnets, fine particle superconductors, and so on [53]. Most efforts have focused on obtaining homogeneous microstructure from materials having different densities which are common in monotectic systems, and on obtaining steady state coupled growth conditions in hypermonotectic alloys by directional solidification, and on modeling the monotectic growth process [54]. The SMF [55,56] and other microgravity conditions [57,58] were applied to attain controlled coupled microstructures by suppressing the melt convection. But these investigations are focused on fundamental research. Few developments are made in the applications of monotectic alloy.

Fe–49wt %Sn (mass fraction) alloys used in this work were solidified with and without SMF [33,59]. The magnetic anisotropy energies of the specimens, which are subjected to the applied magnetic fields, were estimated from the magnetization curves in the parallel direction and the perpendicular direction according to the equation:

$$E_{\mathrm{ani}}={\displaystyle {\int }_0^H\left(M_{//}-M_{\perp }\right)\mathrm{d}H}$$

(2)

where E_ani is the magnetic anisotropy energy. M_∥ and M_⊥ are the magnetization of the parallel and the perpendicular to the sample axis. H is the magnetic field strength used in measuring property.

The magnetic property of the as-solidified samples (Figure 5) shows the anisotropy induced by SMF increases with increasing of the applied magnetic flux density during solidification process. Magnetic anisotropy in a system consisted of single domain (SD) magnetic grains is a factor which makes the coercivity Hc, and the remanent magnetization Mr increase. According to the experimental results, it is apparent that imposition of SMF leads to enhancement of Ms, therefore Mr is increased.

5. Conclusions and Outlook

The changes in the properties of several metallic functional materials by applying external SMFs in solidification and annealing were summarized.

(1) External SMF in solidification of Fe–27.5wt %Ga alloy increased the saturated magnetostriction, which was resulted from the induced magnetic-field-undercooling and magnetic orientation by the application of SMFs in solidification. However, SMFs in annealing decreased the saturated magnetostriction compared to those without an SMF, which was attributed to the disorder-order phase transformation induced by the application of SMFs in annealing.

(2) External SMF in solidification of Cu–Fe with over than 12% and above the drawing ratio of 3.8 will increase the UTS of Cu–Fe composite. The injected SMF has no effect on the electrical conductivity and UTS no matter which annealing temperate and SMF.

(3) Cu–6wt %Ag composite solidified with SMF and ageing at 300 °C enhanced both microhardness and electrical conductivity. MR of both Cu–28wt %Ag and Cu–72wt %Ag eutectic alloys solidified under SMF were higher than that solidified without SMF.

(4) The imposition of SMF in solidification of Fe–49wt %Sn alloy enhanced remanent magnetization and saturated magnetization, which is related to the magnetic anisotropy energies.

An SMF subjected in solidification can affect the fluid flow of the melt to modify grain orientation (Fe–Ga in this work) [60,61], primary phases (proeutectic Cu in Cu–Ag alloy, and primary Fe dendrites in Cu–Fe alloy and Fe-Sn alloy) [62,63,64]. A large fraction of grains and primary phases generally played important roles in the properties of functional metallic materials. However, when an SMF was imposed in the annealing, the magnetic energy was generally smaller than the thermal energy at room temperature or activation energy in the non-ferromagnetic systems. It was not enough to change chemical equilibrium [3], which might be one of the reasons that the properties of the four functional metallic materials were incapable of changing obviously by external SMFs. Therefore, exploring to maximize the positive influence of magnetic fields is still a very meaningful and scientific issue in future.