Xin Li Wang

Microstructure evolution in metals induced by high density electric current pulses

Abstract In this paper, microstructure evolution in metals induced by high density electric current pulses (ECPs) treatment, such as grain refinement, formation of oriented microstructure and redistribution of inclusions, is carefully reviewed. The mechanism for these microstructure evolutions is discussed. It is suggested that there are usually several factors working during ECP: Joule heating (including temperature rise, the heating rate and the cooling rate), stress (including thermal compressive stress, skin effect, electron wind force) and free energy change caused by the current. The Joule heat and the stress usually affect the dynamics, enhance the microstructure evolutions rate, while the free energy change affects the thermodynamics and lowers the barrier of the phase transformation or recrystallisation. The experimental results indicate that high current density ECP provides an effective approach to refine the microstructure of polycrystalline materials and to improve the mechanical properties of the materials.

Refinement of inclusions in molten steel by electric current pulse

Abstract The refinement of inclusions in molten steel induced by a continuous electric current pulse was investigated at 1823 K. The results revealed that due to the application of electric current, the melted sulphide inclusions in molten steel were refined. Analysed from the thermodynamic theory, the refinement mechanism was ascribed to the decrease in the system free energy that resulted from the formation of the refined sulphide inclusions in molten steel at 1823 K. Hence, the electric current pulse treatment may be a new method to refine inclusions in molten metallic materials in the future.

Metal solidification–nucleation–rate model under coupling effects of shearing flow and vibration

This paper proposes the influence factors of coupling effects of shearing flow and vibration on diffusion coefficient and critical nucleation energy during metal solidification. Based on this proposal, a metal solidification–nucleation–rate model under coupling effects of shearing flow and vibration is established. Verification experiment using Al–7Si alloy is carried out. When vibration frequency and melt flow velocity are zero, the results calculated by the above model agree with that calculated by Turnbull’s theory. The results calculated by the above model under coupling effects of shearing flow and vibration agree with the experimental results, with the error within 0·2–14·3%. So the established model can calculate and explain the nucleation rate of melt under coupling effects of shearing flow and vibration.

Effect of Electric Current Direction on Texture Evolution in a Cold Rolled Fe-3%Si Steel under Electric Current Pulses Treatment

The effect of electric current on the recrystallization texture evolution with the rolling direction both parallel and perpendicular to the current flow during electric current pulses (ECP) treatment was investigated. The results showed that the exerted current direction played a great role on the formation of recrystallization texture {111}<112> and Goss texture {011}<100> at the primary stage of recrytallization induced by ECP treatment. However, with the current density increasing, the effect of current direction on texture evolution almost could be ignored and the final texture components in the two cases all are Goss texture.

Texture Evolution along the Layer Thickness in a Fe-3%Si Steel under Electropulsing Treatment

Effects of electric current densities on recrystallization texture evolution in cold-rolled Fe-3%Si steel were investigated by using a high current density electropulsing treatment. Results showed that the orientation density of α fiber and  fiber varied with the specimen thickness during current passing. However, with the current density increasing, the difference from layer thickness almost vanished. In addition, Goss component texture was the final sharper one but no relation with the specimen thickness at 9.96kAmm-2. By the texture evolution analysis, it was found that the preferred nucleation always occurred in the surface layer due to the high storied energy coursed by previous cold rolling. Combined with the corresponding microstructures, it could be found that though there was an apparent texture evolution along specimen thickness, the microstructure had no change with thickness. In addition, due to the application of electropulsing, the recrystallization nucleation was greatly increased.

Texture and Microstructural Evolution under Electric Current Pulses in a Fe-3%Si Steel

The evolution of the recrystallization texture under high current density electric current pulses (ECP) was investigated in a cold-rolled Fe-3%Si steel sheet. Results showed that the preferred nucleation always occurred in the direction parallel to the current direction at the primary stage of recrytallization. With the increment of the current density, the effect of current direction on texture and microstructural evolution was decreased. Due to the different texture component along the layer depth under different current densities, it was also found that the recrystallization nucleation was much easier to occur from the top surface.

Influence of Electric Current Pulses on the Microstructure and Mechanical Properties of X70 Pipeline Steel

The effect of high electric current pulse on the microstructure and macroscopic mechanical behavior of X70 pipeline steel was studied. With the increasing of current density, the grains become refined and the tensile strength has an evident improvement without a decrease in elongation rate. The theoretical analysis shows that two kinds of refinement mechanisms, recrystallization and phase transformation, compete when the exerted current densities differ. Therefore, the ECP treatment should provide a promising method to refine materials and to improve their physical properties by using different current densities.

Microstructural Evolution of AZ31 under the Application of High Density Electric Current Pulses

The microstructural evolution in annealed Mg-3Al-1Zn (AZ31) magnesium alloy during high density electric current pulses (ECP) treatment is investigated by using a same current density with different processing numbers. It is found when the processing number of the ECP treatment is not greater than four times, the grains are refined and more homogenized, and the texture intensity obtained from the (0002) pole figure appears an obvious enhancement from 11.22 to 22.88. However, increasing the repeated ECP processing number will cause the coarsening of grain size and the decreasing of the texture intensity. The mechanism of the microstructural evolution during ECP treatment is discussed from the point of view of grain boundary motion.

Martensitic Transformation and Crystal Structure Characterization of Ni-Fe-Ga-Co Ferromagnetic Shape Memory Alloys

In this paper, the martensitic transformation temperature, the microstructure and the crystal structure of the complicated martensitic phases of Ni56-xFe19Ga25Cox (x =0, 1.5, 3, 4.5, 6) alloys were investigated by DSC, XRD, SEM and TEM techniques. DSC results show that the martensitic transformation temperature Tm, which is above the room temperature, decreases with the increasing Co content. The microstructure of the Ni56-xFe19Ga25Cox (x =0, 1.5, 3, 4.5, 6) alloys is composed by the martensitic lath and randomly distributed γ phase. The 6M+14M mixed modulated martensite and the γ second phase were detected in the Ni53Fe19Ga25Co3 alloy by XRD and TEM tests.

Twins Evolution During the Recrystallization Induced by Electric Current Pulses in a Cu‐Zn Alloy

A high current density (up to 104 A•mm−2) electric current pulse (ECP) is used to cause recrystallization of a cold-rolled single phase Cu-Zn alloy directly. Since the whole treating time is the discharge time of ECP, and the duration time is very short, it is very easy to remain the non-equilibrium phase at high temperature to the room temperature to investigate the microstructural evolution during the recrystallization process. By the determination of TEM, it is found that due to the application of ECP, the original deformed twins are greatly decreased while a lot of incomplete annealing twins are emerged. Interestingly, the various morphologies of annealing twins are revealed, and the different growth directions are also observed. This phenomenon would contribute to study the variant selection of the twin in the early stage of recrystallization process.