Optimization of <001> grain gene based on texture hereditary behavior of magnetic materials

Genetic engineering is the act of modifying the genetic makeup of an organism. Modifications can be generated by methods such as gene targeting, nuclear transplantation, transfection of synthetic chromosomes or viral insertion. In biology, the genetic characteristics of organisms are determined by genes [1,2]. For example, arrangement (sequencing) of DNA and RNA in human genes determines the main functions (traits) of the human body. Similarly, the properties and arrangement of atoms, including crystal structure and defects, determine the intrinsic properties of materials. When the similar idea is grafted into materials science, the similar behavior is reflected. For instance, face centered cubic (fcc) and body centered cubic (bcc) structures are usually more ductile than hexagonal close-packed (hcp) structures, and the formation of Goss-texture can improve the magnetic properties of materials and so on. Therefore, how to improve the material gene is very important to promote the material performance. With the increasingly serious global ecological problems in recent years, energy conservation and environmental protection has become one of the global strategic objectives. The electric energy, as an indispensable energy in modern society, is of great significance to the construction of ecological earth in reducing power consumption. Furthermore, magnetic materials used in electrical components play a decisive role in reducing power consumption. Much attention has been paid to the electromagnetic properties of materials due to a strong demand for soft magnetic material about high magnetic induction and low iron loss [[3], [4], [5]]. The arrangement of atoms in crystals along different directions determines the physical properties in this direction. The magnetization of iron-based soft magnetic crystals is anisotropic. Among them, <001> direction is the easiest direction to achieve saturated magnetic induction intensity, namely that, <001> is the maximum magnetic conductivity direction [6]. Goss-texture ({110} <001>) and Cube-texture ({100} <001>) in steel sheet is preferable for magnetic applications since the easy magnetization axis <001> is parallel to the rolling direction [[7], [8], [9], [10], [11]]. At present, there are many ways to control <001> orientation, while such efforts have achieved some success in terms of controlling orientation [[11], [12], [13]]. They have not developed any electrical steel products with high orientation density Goss-texture due to high requirements for initial material orientation, treatment temperature and time, protective atmosphere and heating rate. Previous research has established that texture formation during recrystallization and grain growth are affected by the magnetic field properties due to the magnetic ordering and magnetic field intensity [14]. Laser powder bed fusion is another promising approach for obtaining fcc/bcc metal with [001] orientations [15]. Accordingly, how to obtain more grains with orientation index of [001] is of great value to improve the magnetic properties of materials. Electropulsing treatment, as an instantaneous high-energy input method with high efficiency and low energy consumption, had been applied to the metallic materials to induce the transform of microstructure in a short time [[16], [17], [18], [19]]. The objective of this work is to take ultra-thin oriented silicon steel as the research object to explore optimization of <001> grain gene based on texture hereditary behavior of magnetic materials after being applied pulsed electric current. The sharpness of Goss-texture ({110} <001>) determines the excellent magnetic properties of the ultra-thin grain-oriented silicon steel. However, the development of the Goss-texture is challenging since the Goss orientation is not stable by traditional heat treatment [[20], [21], [22]]. In this letter, a possible formation mechanism to use electric current to induce a high magnetic conductivity from a low magnetic conductivity matrix has been proposed based on experimental observations. This provides a possibility to use electric current to promote the materials magnetic properties by improving the material gene, which would be of great interest and of physical importance if this kind of gene optimization was beneficial to high-quality materials prepared from low-performance raw materials by deep processing. Therefore, the technique is useful to many materials upgrading, such as aluminum alloy, nuclear power steel and new energy materials, where gene in base material needs to be optimized.

Here, 0.1 mm thick of an inhibitor-free Fe-3%Si electrical steel containing 70 ppm sulfur were prepared through cold-rolling processes, and the raw material is 0.3 mm low-grade grain-oriented silicon steel (with reduction of 66.7%). The chemical composition is 0.055C, 3.0Si, 0.104 Mn, 0.027 P, 0.0070S, 0.0284Al and 0.0078 N. The steel sheet was cut to the size of 30 mm × 5 mm, and the surface layer was polished and cleaned by ultrasonic wave to remove the surface insulation layer and stains. Magnesium oxide aqueous solution was evenly sprayed on the surface of the sample to form MgO protective layer, the steel samples were heat treated by muffle furnace at 300, 520, 650, 680, 750, 800, and 1200 °C, respectively, and the lofting system was adopted for heat treatment. The cold-rolled samples were coated with MgO and fixed at the output of the pulse power supply with copper clamps. The pulse current treatment was carried out by using a pulse generator, and contrast experiments have been carried out by adjusting the pulse parameters so that the surface temperature of the sample reached the same as the heat treatment. The current density J_e was optimized to be 4 × 10⁸ A/m², the pulse width is 50, 100, 150, 200 μs and the frequency is about 100 Hz. The temperature is uniform in the surface of pulsed samples to be measured. The samples (samples prepared by electro-polished using a solution of 10% nitric acid and 90% alcohol, at room temperature and 0.65 A) were characterized by EBSD and the data were analyzed by using HKL Channel 5 software analysis system. In addition, the resistivity values of the cold-rolled sample (deformed matrix) and the pulsed sample (the ideal recrystallized grains) were tested by physical property measurement system (PPMS-9VSM) at room temperature.

It can be seen the main cold-rolled texture type was {111} <112> texture after 66.7% cold rolling reduction deformation from Fig. 1, but there was still a little Goss component (Fig. 1(a) and 1(b)). It is well known that the cold-rolled texture of Goss-oriented single crystals is found to be {111} <112> texture after being cold-rolled to 60%-70% [23]. This rotation amounted to 35° around the <110> crystal direction, which is parallel to the transverse direction (TD). Here, EBSD data reflected the similar phenomena in Fig. 1(c) and (d). Furthermore, we treated the sample for 5 min at the temperature of 680 °C, and it was found that there was nucleation in the deformed structure in Fig. 2(a), but the recrystallization was low (about 8.3%). At the same time, the sample treated by pulsed current had completely recrystallized, and a sharp Goss-texture appeared (Fig. 2(c)). When the heat treatment was further performed to 800°C for 5 min (Fig. 2(b)), the degree of recrystallization was higher than that at 680°C, accompanied by the increase of grain size and the formation of weak Goss-texture. By contrast, the pulse current treated samples formed sharper Goss-texture (Fig. 2(d)). In addition, the cold-rolled silicon steel was treated in the temperature range of 300-1200°C by conventional heat treatment and electric pulse treatment (Fig. 3). Fig. 3(a) and (b) plots the change of Goss-orientation distribution function f(g) = f(φ1, Φ, φ2) and volume fraction of Goss grain as treatment temperature by heat and pulse, respectively (treatment time is 5 min). For electric pulse processing, the formation of Goss-texture became easier with increasing processing temperature caused by current. Particularly, the Goss-orientation density of pulse treated steel increased most obviously (about 10 times higher) after treated for 5 min at 800°C, compared with that of heat-treated steel. Meanwhile, the volume fraction of Goss-grain also increased greatly in pulse treated steel. It can be seen from the ODF figure and (200) polar figure in Fig. 3(c) and (d), when the sample temperature is 800°C, the main texture type of recrystallization was Goss-texture, but Goss-texture density of the pulsed sample was stronger. Under pulsed treatment, the Goss-texture developed with increasing misorientation distribution of the low-angle grain boundaries (2°-10°), and it can also be seen that most low-angle grain boundaries were found in the Goss grains, as indicated by the white lines in Fig. 2. In contrast, the high-angle grain boundaries accounted for a large proportion by heat treatment. This is another characteristic of the pulse-induced Goss-texture.

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Fig. 1.   Texture of the 66.7% deformed, initially finished Goss-oriented material. (a) and (b) Electron backscattered diffraction IPFZ (Inverse pole figure Z) maps show microstructure of cold-rolled specimen. (c) Orientation distribution function (ODF) of EBSD data are displayed in a φ₂ = 45° section. (d) Polar figure and inverse polar figure both represent that the texture component with the highest intensity is {111}<112> component. In addition, a weak Goss component is observed, with a texture intensity of about 0.7.

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Fig. 2.   Micro-texture under different treatment conditions. IPFZ maps of (a) and (b) conventional heat treatment for 5 min and (c) and (d) electric pulse treatment for 5 min. The sample treatment temperature is (a) and (c) 680°C and (b) and (d) 800°C. Boundaries with misorientations larger than 15° are superimposed as black lines and 5°-15° are white.

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Fig. 3.   The difference in Goss-orientation distribution function and volume fraction of Goss grain by electropulsing treatment (EPT) and traditional heat treatment (THT). (a) Goss-orientation distribution with treated temperature for 5 min by EPT and THT. (b) The volume fraction of Goss grain with treated temperature for 5 min by EPT and THT. (c) ODF and Polar figure after 800°C for 5 min by THT. (d) ODF and Polar figure after 800°C for 5 min by EPT. (e) Misorientation angle distributions of samples after 800°C for 5 min by EPT and THT.

As mentioned above, increasing the sample treatment temperature was conducive to the formation of recrystallized Goss-texture. Furthermore, cold-rolled sample treated at 1200°C for 2 h formed a sharp Goss-texture (Fig. 4(d) and (f)). Meanwhile, the orientation distribution function and the volume fraction of Goss-texture (Fig. 4(a) and (c)) formed by pulse treatment (750°C) for 2 min were similar to that of heat treatment (1200°C) for 2 h (Fig. 4(g)). Therefore, the electric pulse can induce the formation of Goss-texture efficiently at relatively low temperatures. This finding is of great significance to shorten the industrial production process of oriented silicon steel.

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Fig. 4.   Effects of temperature and time on Goss-texture under two conditions. (a)-(c) and (d)-(f) EBSD of cold-rolled specimen treated at 750°C for 2 min by EPT and 1200°C for 2 h by THT, respectively. (g) The orientation distribution function and the volume fraction of Goss-texture.

According to the texture evolution of Goss (110) [001] and Fe-3%Si (111) [112] single crystal annealed by cold rolling, it is shown that the formation of Goss-oriented grains originates from the Goss-oriented sub-structure in {111} <112> oriented shear band or deformed band [24]. The preferential nucleation theory [23] indicates that these Goss-oriented substructures can be preferentially transformed into recrystallized nuclei during annealing. Therefore, the formation of Goss-texture is closely related to recrystallization. With the recrystallization nucleation and grain growth, the Goss-oriented nuclei will grow in the {111} <112> oriented deformed matrix by the aid of its large angle grain boundary migration, so it is a thermal activation process, which is closely related to the growth temperature [25]. But compared with the conventional heat treatment, the pulse current can promote the recrystallization and induce Goss-texture of cold-rolled silicon steel at the same temperature. So how do we understand this phenomenon of ultrafast induction in recrystallization texture? The existence of the single atom storage energy E_a changes the energy barrier that iron atoms need to overcome when they jump across the grain boundary. The energy barrier to be overcome from the recrystallized nucleus to the deformed grain is E_a+E₀, and the energy barrier to be overcome from the deformed grain to the recrystallized nucleus is E_a-E₀. The velocity of grain boundary migration can be deduced [26]:

$v=bν_0x_0[exp(−\frac{E_0−E_a}{kT})−exp(−\frac{E_0+E_a}{kT})]=bν_0x_0exp(−\frac{E_0}{kT})[exp(\frac{E_a}{kT})−exp(\frac{−E_a}{kT})]$ (1)

where ν₀ is the natural frequencies of iron atoms (ν₀≈ 10¹³ s^-1), x₀ is the equilibrium vacancy concentration, k is the Boltzmann constant, T is the absolute temperature, E₀ is the grain boundary hopping energy barrier which can be expressed by the reduced system free energy. If an electric current is applied to the system, based on thermodynamics, the system free energy in this case includes chemical free energy G_chem, interfacial energy G_inter, strain-stress energy G_strain and electric current free energy G_e. Thus, the system free energy change (ΔG) is expressed as [[27], [28], [29]]:

ΔG=ΔG_chem+ΔG_inter+ΔG_strain+ΔG_e, (2)

The numerical calculation of ΔG_e is too complex, but ΔG_e can be simplified as [30,31]:

ΔG_e=$\frac{σ_1-σ_2}{2σ_1+σ_2}kj^2V$, (3)

where k is the geometric factor (k>0), j is the current density, V is the volume of a nucleus, σ₁ and σ₂ are the electrical conductivities of the deformed matrix and the ideal recrystallized grains, respectively. Here, the electrical resistivities of the deformed matrix and the ideal recrystallized grains are determined to be approximately 52.81 μΩ cm and 49.83 μΩ cm at room temperature, respectively. The system free energy ΔG will decrease when ΔG_e <0 due to σ₁ < σ₂. So the grain boundary hopping energy barrier E₀ decreases and the grain boundary migration rate increases according to Eq. (1). Therefore, the dynamic conditions for recrystallization Goss-texture formation treated by pulse current are much lower than those of heat treatment at the same temperature.

Meanwhile, the increase of Goss-texture means the enhancement of magnetic conductivity in <001> direction. A magnetic field is produced when current passes through a conductor. The change of free energy of the system δξ can be expressed by the magnetic field produced by the current [32]:

δξ=-∫δμ$\frac{H^2}{2}$dV, (4)

where μ is magnetic conductivity of material, H is the intensity of magnetic field. To determine the magnetic field of the current, the distribution of the current must satisfy the following equations [32]:

divB=0,rotH=j, (5)

where B is magnetic induction intensity and j is the current density. In this letter, cold-rolled Gamma texture is {111} <112> and recrystallization Goss-texture is {110} <001 > . According to the magnetocrystalline anisotropy properties calculation equation [33]:

$ε_k=ε_{uvw}-K_0≈K_1(α_1^2α_2^2+α_2^2α_3^2+α_3^2α_1^2)+K_2(α_1^2α_2^2α_3^2), $ (6)

where the angles between [uvw] direction and three basic crystal directions of [100], [010], [001] are α, β, γ, respectively. So, α₁=cosα, α₂=cosβ, α₃=cosγ. And the K₁ = 32,600 and K₂ = 10,600 in silicon steel [33], the numerical value is calculated in the Eq. (6), ε₁₀₀ = 0 J/m³ and ε₁₁₂ = 8346 J/m³, that is to say, μ_Gamma < μ_Goss. According to Eq. (4), it is easy to know the free energy of system decreases in the magnetic field. Similarly, the grain boundary hopping energy barrier is also reduced.

The Goss grain gene plays a key role in the sharpness of recrystallization Goss-texture. Simply, the preparation of ultra-thin grain-oriented silicon steel is the process of Goss-texture transforming into the cold-rolled gamma-texture, and then returning to Goss-texture. The low initial Goss-texture orientation caused by poor Goss grain gene does not match the specific orientation of cold rolled deformation matrix at the stage of recrystallization texture formation, which leads to the low sharpness of recrystallized Goss-texture. The imperfect $\ddot{g}$ene$\ddot{g}$ith low-orientation in the initial Goss grain was directly improved under the action of pulse current, and the efficient transformation from the initial Goss-texture to the recrystallized Goss-texture was accomplished by using its heredity. According to classical nucleation theory, the recrystallization nucleation rate (I_e) can be written as the following equation [31]:

I_e=I₀⋅$(\frac{D}{λ^2})⋅exp(-{ΔG_0+ΔG_e}{R⋅T}),$ (7)

where I₀ is a constant, λ is the jump distance, D is the diffusivity, R is the Boltzmann constant, and T is the absolute temperature, ΔG₀ is the thermodynamic barrier without electropulsing, ΔG_e is the change of barrier caused by electropulsing. The relative nucleation rate can be given to determine the effect of current pulse on nucleation rate:

$\frac{I_e}{I_r}=exp(-\frac{ΔG_e}{R⋅T}),$ (8)

where I_r represents the nucleation rate of the deformed matrix without passed electropulsing and I_e represents the nucleation rate of the deformed matrix with passed electropulsing. The I_e/I_r >1 due to ΔG_e <0 (σ₁ < σ₂) and then I_e>I_r. Notably, the key reason for achieving good genetic inheritance is that the electric activation energy reduces the thermodynamic barrier and then promotes Goss grain nucleation and growth. Based on the above analysis and thermodynamic conditions for nucleation, the distribution of electromagnetic field promotes the nucleation of <001> oriented grains. Thus, the magnetic properties of the materials are improved. Meanwhile, the effect of rapid heating rate and inhibitor on the formation of Goss grain orientation can be ignored in this study.

Here, it is unavoidable to misunderstand the formation of Goss grain orientation induced by rapid heating rate when electric pulse processing technology is used. We also took it into account in the design of experiments, so thermocouples were used to measure the temperature in time. It was found that the heating rate of the sample treated by current pulse at room temperature was 20 °C/s. Meanwhile, we also adopted the Target temperature-lofting mechanism to ensure that the traditional heating rate was consistent with the pulse heating rate, and measured traditional heating rate was about 15 °C/s. As far as the heating rate is concerned, it is consistent with the lowest contrast condition of Park et al [34,35]. According to the research data, the volume fraction of Goss oriented grains increased from 1.48% to 2.25% under the condition of complete recrystallization from 20 to 150 °C/s, that is to say, the volume fraction of Goss oriented grains increased by about 52% [34,35]. However, in our experiments, we found that the volume fraction of Goss oriented grains increased from 14.7% to 70% under the condition of complete recrystallization although the heating rate is only 20 °C/s. Therefore, the electric effect in the pulsed treatment for the preparation of ultra-thin oriented silicon steel strip could not be ignored.

It is well known that inhibitor is an important factor to control the abnormal growth of Goss grain. In our experiment, the Goss oriented grains in primary recrystallization were controlled, and the abnormal growth of Goss grains in secondary recrystallization was not involved. It is also worth mentioning that the inhibitors have already ripened in the preparation of ultra-thin oriented silicon steel sheets from finished sheets due to the many annealing times [9]. At the same time, the decrease of thickness makes the surface effect more and more obvious. Many related studies have shown that these inhibitors have little effect on Goss oriented grains under ultra-thin oriented silicon steel.

The above analysis shows that the reduction of grain boundary hopping energy barrier caused by the distribution of electromagnetic field promotes <001> orientation grain nucleation and growth, which directly improve the initial <001> orientation grain gene. The inheritance of <001> orientation texture is used to control the formation of recrystallization texture in a short time at room temperature, thus improving magnetic properties of materials. Pulse treatment, as an efficient and green method, can greatly improve materials’ physical properties and shorten the preparation process of high-quality silicon steel. Therefore, it is possible to use this gene optimization technique in many materials upgrading such as metal materials and biological materials according to the differences in electromagnetic properties of microstructures.

Acknowledgments

The work was financially supported by the National Natural Science Foundation of China (Nos.51874023, 51601011andU1860206) and the Fundamental Research Funds for the Central Universities, Recruitment Program of Global Experts.