As a non-equilibrium processing, electric current (direct current, alternating current and electropulsing) shows its increasing applications in materials processing. Although the effects of electropulsing (ECP) on the microstructures of materials have been extensively investigated, these works mainly focused on the mechanism evolution, but few on ECP applications. In this dissertation, the formation of nanocrystalline surface layer of a Cu-Zn alloy under ECP surface treatment was investigated; the mechanical properties and microstructure evolution of an ECAPed AZ31 Mg alloy under ECP treatment was discussed; finally, the electromigration of SnSb Pb-free solder joints was reported.
A nanocrystalline surface layer of a Cu-Zn alloy was developed by ECP surface treatment. The average grain size was about 20nm on the top surface layer and gradually augmented with the increase in depth from the top surface. Nanoindentation measurements showed that the micro-hardness was significantly enhanced to about 4GPa on the top surface layer, and it decreased gradually with the increase in depth. The mechanism about the evolution of this structure and property was related to the solid-state phase transformation during the heating and cooling course, but the main reasons were two effects of the ECP itself: the skin effect which made the current density fall towards the inside when high frequency pulse current passed through a conductor; the nucleation rate enhanced by decreasing the thermodynamic barrier in a current-carrying system, and the nucleation rate increased with the current density.
ECP treatment was applied to an ECAPed AZ31 alloy. After the ECP treatment, the yield stress and ultimate tensile strength were almost not decreased while the elongation-to-failure was increased dramatically to about 57%, the grains were slightly increased and more homogeneous, and the texture was hardly changed. The evolution of the properties and structures was mainly attributed to the enhancement of the recrystallization nucleation rate, the retard of the subsequent grain growth, and the annihilation of the dislocations during the ECP treatment; the preservation of a strong ECAP texture plays an accessorial role in enhancement of the mechanical properties.
The interfacial reactions in Cu/90Sn-10Sb/Cu Pb-free solder joints were investigated under current stressing. The growth of Cu-Sn intermetallic compound (IMC) layers was enhanced at the anode and inhibited at the cathode compared with the annealing state, and the growth of the IMC at the anode followed a parabolic growth rule. Upon increasing the temperature to 140°C, the Cu6Sn5 IMC at the anode grew up dramatically, in which large Sn3Sb2 IMCs were included; and the content of Sb atoms was increased at the cathode and formed Sn3Sb2 IMC. It was considered that the polarity effect on the interfacial IMC formation was mainly owing to the Cu and Sn atoms migration to the anode; and the changes of the Sn3Sb2 IMC might be due to the Sb migration to the cathode under the effect of back stress.
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