Electropulsing-induced phase transformations in a Zn-Al-based alloy
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Xingming Liu Hefei National Laboratory for Physical Science at Microscales, University of Science and Technology of China, Hefei 230026, China
Yanbin Jiang and Guoyi Tang Graduate School at ShenZhen, Tsinghua University, Tsinghua 518055, China (Received 7 January 2009; accepted 15 April 2009)
Microstructural changes and phase transformations of an electropulsing-treated (EPT) ZA22 alloy wire were studied using scanning electron microscopy and transmission electron-microscopy techniques. Two stages of phase transformation were detected in the EPT alloy: (i) quenching from the as-furnace cooled (FC) state to the final stable state and (ii) up-quenching from the final stable state back to the as-FC state through two reverse phase transformations: T0 +Z ! a+e and Z0 T +e+a ! Z0 FC. Electropulsing accelerated phase transformation tremendously. It was at least 1200 times faster than the aging process. The mechanism of the electropulsing-induced phase transformations is discussed from the point of view of Gibbs free energy and electropulsing kinetics.
I. INTRODUCTION
An electropulsing treatment (EPT) was found to be able to reduce residual stress in the 1960s.1 As an alternative to the traditional thermal and mechanical processes, EPT has been recognized for its high efficiency, and extensive studies in materials science and engineering have been carried out in such areas as electroplasticity,2–4 electromigation,5,6 recrystallization,7–9 phase transformations,10–15 and in the mechanism of interaction between electrons and lattice atoms and dislocation movement.16–18 EPT has also been applied to materials processing, in such areas as cold drawing, recovering cracks and flaws, and refining grain size, as well as in biology and environmental protection and in the medical domain.19–23 Previous studies indicated that EPT enhanced rates of both crystallization and recrystallization of alloys.7,9 Compared with the conventional processes, microstructure and grain size were remarkably refined, and the material properties were improved. It was recently reported that, with adequate electropulsing, the elongation of the Zn–Al-based alloy was increased by 437% at ambient temperature under high strain rate, whereas the instantaneous tensile stress remained unchanged compared with that of the non-EPT alloy.14 This was significant from a practical point of view. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0300 J. Mater. Res., Vol. 24, No. 8, Aug 2009
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However, little has been studied on the effects of electropulsing on the microstructural evolution and phase transformations of alloy materials. As one of the driving forces for phase transformation and processing, electropulsing is more powerful and more effective than thermal energy. Traditional thermodynamics are not sufficient to explain electropulsing behaviors. It is of important theoretical significance to explore the mechanism of EPT processing. Thus, this work was initia
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