| 其他摘要 | Electric current pulses (ECP) treatment is known as an effective, high speed and short duration approach to heat a bulk material up to a high temperature. During the ECP process, the heating duration is so short but the temperature is so high, it is expected that after rapid cooling to the ambient temperature, the unstable solid state in the sample at such a high temperature can be held. Thus, the materials treated by ECP may possess excellent properties. As a rapid non-equilibrium processing, ECP finds its increasing applications. However, the underlying mechanism of ECP is not well understood hitherto. In the present work, the behavior of lead inclusion in a Cu-Zn alloy under a high density ECP treatment was investigated, and the formation of oriented nanotwins in a Cu-Zn alloy was also reported.
Lead inclusions in a Cu-Zn alloy could be dispersed and segregated along grain boundaries with a significant long-range atomic diffusion after ECP treatment. This phenomenon differed from the routine high-rate heating method and rapid cooling process, and could not be explained by the classical electromigration theory. Theoretical analysis revealed that, due to the difference between the electric conductivity of lead and that of the brass matrix, the homogeneous distribution of current was modified when an electric current passed. As a result, there appeared a free energy difference between these two configurations and the difference became larger and larger with increasing current density, until the lead inclusions became unstable at a certain critical value. Consequently, those inclusions were dispersed into small particles and transferred into grain boundaries or defects. With the rapid cooling, the unstable state was kept to room temperature after ECP. The results also indicated that the application of ECP greatly reduced the diffusion energy of lead atom in brass, thus, significantly enhanced the diffusion rate of lead atom.
High current density ECP was applied to a coarse-grained Cu-Zn alloy with -phase and ’-phase at ambient temperature. After the application of an ECP process, oriented nanotwins in Cu-Zn alloys were observed in phase by high-resolution transmission electron microscopy (HRTEM), whose twin plane was {111} and parallel to the ECP direction. In view that the nucleation rate of a phase with higher electric conductivity was greatly enhanced during solid-state phase transformation caused by ECP, it was proposed that the formation of oriented nanotwins was closely related with the anisotropy of conductivity in each crystal plane of a crystal. It was suggested that nucleation rate of -phase along the crystal plane ({011} planes) with higher electric conductivity greatly exceeded the other planes, then, lots of oriented nucleus may form along {011} crystal planes. As a result, lots of oriented nano/sub-micro -phase were obtained in the heating course. In addition, martensitic transformation from phase to -phase according to a certain relationship took place in the following cooling course, which could be ascribed to an increase of the beginning temperature for martensitic transformation after the ECP treatment. Consequently, numerous oriented nanotwins formed.
The effect of high density ECP on the microstructure and mechanical behavior of X70 pipeline steel was also investigated. It was found that with the increase of current density, the grains of X70 were refined and its tensile strength had an evident improvement without a decrease in elongation rate. It implies that two kinds of refinement mechanisms, recrystallization and phase transformation, competed when the exerted current densities differed. When the temperature caused by Joule heating was lower than the phase transformation temperature, the main reason for grain refinement was supposed to be recrystallization, and the enhancement of dislocation mobility caused by current could improve the recrystallization nucleation rate. When the temperature increased over that transformation temperature with the current density increasing, the grain refinement could be attributed to a rapid phase transformation, in which the decrease of thermodynamic barrier and enhancement of nucleation rate in a current-carrying system should be dominating. In addition, by comparing with the routine annealing treatment, the application of ECP could evidently enhance the recovering rate and shorten the recovering time without obvious change in grain size. |
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