Jingdong Guo

Formation of a Nanostructure in a Low-carbon Steel Under High Current Density Electropulsing

The microstructure of a low-carbon steel after high current density electropulsing treatment was characterized by high-resolution transmission electron microscopy. It was found that nanostructured gamma-Fe could be formed in the coarse-grained steel after the electropulsing treatment. The mechanism of the formation of a nanostructure was discussed. It was thought that change of the thermodynamic barrier during phase transformation under electropulsing was a factor that cannot be neglected. It was reasonable to anticipate that a new method might be developed to produce nanostructured materials directly from the conventional coarse-grained crystalline materials by applying high current density electropulsing.

Grain refinement and formation of ultrafine-grained microstructure in a low-carbon steel under electropulsing

High current electropulsing was applied to a low-carbon steel in the solid state. The relationship between grain size and experimental conditions was revealed. It was found that the ultrafine-grained (UFG) microstructure could be formed when electric current density, heating rate, and cooling rate all were high. The UFG samples prepared by applying electropulsing were free of porosity and contamination, and had no large microstrain. Also, their tensile strength was dramatically enhanced over that of their coarse-grained counterparts, without a decrease in ductility. The mechanism for grain refinement and formation of the UFG microstructure was discussed. It is proposed that the effect of a decrease in thermodynamic barrier and enhancement of nucleation rate in a current-carrying system cannot be neglected.

Influence of electropulsing on nucleation during phase transformation

Phase transformation about precipitation in a Cu-Zn alloy was studied. It was found that with an electropulsing treatment the number of nuclei during phase transformation could be dramatically enhanced and nucleation of precipitates was more homogeneous. The phenomena did not result from the effect of rapid heating or rapid cooling during electropulsing but resulted from the electric current itself. The results were in good agreement with the theoretical model that electric current can increase nucleation by decreasing the thermodynamic barrier during phase transformation.

Effect of the electropulsing on mechanical properties and microstructure of an ECAPed AZ31 Mg alloy

The mechanical properties and corresponding microstructure development of the AZ31 Mg alloy after treatment with equal channel angular pressing (ECAP) and subsequent electropulsing (ECP) was investigated. Comparing the ECAP+ECP-treated AZ31 alloy with the ECAP-treated alloy, the elongation to failure was improved significantly, while the yield stress and the ultimate tensile strength were not decreased, the grain sizes were slightly increased and more homogeneous, and the texture was barely changed. The main mechanism for the evolution of the structures and properties might be ascribed to the increased nucleation rate on recrystallization and the decreased dislocation density during the ECP treatment. It was reason-able to expect that the ECAP+ECP treatment would provide a promising approach for enhancing the mechanical properties of the Mg alloys.

Diffusive phase transformation in a Cu–Zn alloy under rapid heating by electropulsing

Electropulsing with a damped oscillating waveform was employed to heat cycle a Cu–Zn alloy containing α and β′ phases. It was found that significant long-range diffusion and then a diffusive phase transformation α + β′→β can occur in the alloy during the course of heating, even when the heating rate during the electropulsing treatment is very high (106 K s−1). The phenomenon is different from the common cases of rapid heating and rapid cooling. It implies that electropulsing can dramatically enhance diffusion in the alloy.

Reverse polarity effect from effective charge disparity during electromigration in eutectic Sn-Zn solder interconnect

Polarity effect on the interfacial reactions from high-density electric currents was investigated in a solder interconnect with a large disparity in the effective charge between the solder constituents. A reverse polarity effect was found where the intermetallic compound layer at the cathode grew significantly thicker than that at the anode under electric loading. Such an abnormal polarity effect was shown to result from electromigrations of Sn and Zn along opposite directions as dictated by the disparity in their effective charges. As Sn migrated to the anode under electron wind force, the resulting back stress drove Zn atoms to drift to the cathode. A kinetic analysis of the Zn mass transport explained the differential growth of the intermetallic compounds at the two electrodes, in good agreement with the experimental data.

Phase transformation between Cu(In,Sn)(2) and Cu-2(In,Sn) compounds formed on single crystalline Cu substrate during solid state aging

Interfacial reactions between eutectic SnIn and single crystalline Cu during solid-state aging at low temperature were investigated systematically. Three types of phase transformations between Cu(In,Sn)2 layer and Cu2(In,Sn) layer were observed, which are Cu(In,Sn)2 grows and Cu2(In,Sn) consumes at 40 °C, Cu(In,Sn)2 and Cu2(In,Sn) grow simultaneously at 60 °C, as well as Cu(In,Sn)2 consumes and Cu2(In,Sn) grows at 80 and 100 °C. According to physicochemical approach, the chemical reactions at Cu/Cu2(In,Sn)/Cu(In,Sn)2/SnIn interfaces were discussed in detail. It was concluded that the diffusion ability of Cu and In atoms dominated different phase transformations. When diffusion constants k1In2 > 8/3k1Cu2 Cu(In,Sn)2 will grow, and if k1Cu2 ≫ k1In2 Cu2(In,Sn) will grow. Both Cu(In,Sn)2 and Cu2(In,Sn) can grow in the condition of k1In2 ≈ k1Cu2. The values of k1Cu2 and k1In2 at different temperatures on (100)Cu and (111)Cu substrate were also calculated or estimated by analyzing the growth kinetics of the comp...

Connection of bulk amorphous alloy Zr55Al10Ni5Cu30 by high current density electropulsing

Abstract Connection of bulk amorphous alloy Zr 55 Al 10 Ni 5 Cu 30 by high current density electropulsing was studied. The results show that the amorphous alloy can be connected together by using the electropulsing, and crystallization can be almost avoided. The reason for this phenomenon is discussed.

Improvement of mechanical properties in a saw blade by electropulsing treatment

Effects of electropulsing on mechanical properties of a saw blade were studied. It was found that the teeth of saw blade were hard but the back of saw blade was more ductile after electropulsing treatment. The strength and ductility of electropulsed samples were higher than that of normally tempered samples, when the former had the same or higher hardness compared with the latter. The decrease of hardness with rise of temperature in electropulsed samples was slower than that in tempered samples. Thus, samples could be treated under the conditions with higher temperature by electropulsing, which was helpful for improvement of brittle in samples