Influence of electropulsing on nucleation during phase transformation
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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.
In the previous works,1–3 we reported that the microstructure of a low-carbon steel could be dramatically refined and an ultrafine-grained microstructure could be formed by applying electropulsing in the solid state. It was thought that the microstructure changes were related to a solid-state phase transformation following a nucleation and growth mechanism. It was proposed that the increase of nucleation in a current-carrying system was an important reason for grain refinement. However, the explanation that the nucleation of a solid-state phase transformation could be enhanced by applying electric current or electropulsing still lacked good experimental confirmation. In this work, experimental evidence for the influence of electric current on nucleation is shown to confirm the above explanation. A single ␣-phase Cu–Zn alloy with a composition of Cu 63.8 wt% and Zn 36.2 wt% was selected as the investigated material. The flat samples were cut from the alloy and divided into four groups: H, HT, HE, and HL. Sample H was not subjected to any treatment and was employed as reference sample. Sample HT was heated to 850 °C for 30 s in the molten salt and then quenched in water. Sample HE was treated by electropulsing. Electropulsing was performed under ambient conditions, by capacitor banks discharge. To obtain a higher cooling rate in the effective part of the sample after the passage of electropulsing, the sizes of the middle part and the two ends of sample HE were designed to be different. The size of the middle part was 5-mm long, 3-mm wide, and 1.5-mm thick. The size of the two ends of the sample was much larger than that of the middle part (effective part),
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 19 Aug 2014
which means the current density of the two ends was much less than that of the middle part. The two ends of each sample were put into copper electrodes during the electropulsing treatment. Then the temperature rise of the two ends was very small and could be regarded as room temperature. Thus, due to the cooling effect of the two cool ends, a higher cooling rate could be obtained in the effective part of sample during cooling. The applied electropulsing was only a single pulse. The waveform of electropulsing was detected to be a damped oscillation wave by a Rogowski coil and a TDS3012 digital storage oscilloscope (Tektronix
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