Static electropulsing-induced phase transformations of a cold-deformed ZA27 alloy
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Xianming Liu Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei 230026, People’s Republic of China
Guoliang Hu and Qing Xu Graduate School at Shenzhen, Tsinghua University, Guangdong 518055, People’s Republic of China (Received 18 March 2011; accepted 17 May 2011)
Static electropulsing-induced phase transformations of a cold-drawn ZA27 alloy wire were studied by using x-ray diffraction, backscattered scanning electron microscopy, and electron backscattered diffraction (EBSD) techniques. By using EBSD, phases with close microstructure are discriminated, based on transmission electron microscopy determined lattice parameters of phases. Thus, it was quantitatively detected that electropulsing tremendously accelerated phase transformations in two stages: (i) g9S and e9T decomposed sequentially (in a way of quenching) and (ii) e9T and g9S formed via reverse decompositions (in a way of up-quenching). I. INTRODUCTION
As an advanced process, electropulsing treatment (EPT) has been found to be more powerful and more effective in accelerating microstructural changes and phase transformations1–6 and in improving properties of alloys, when compared to traditional thermal and thermomechanical processes.7–10 Previous studies reported that electropulsing tremendously accelerated phase transformations by a factor of at least 6000 times, compared with that achieved in the normal ageing process.4 It was also reported that with adequate electropulsing, the elongation of a Zn–Al-based alloy (ZA22) was increased by 437% at ambient temperature under a high strain rate, whereas the instantaneous tensile stress remained unchanged, compared with that of the non-EPT alloy.10 The fundamental mechanisms by which such significant changes of phase transformations and properties resulted from electropulsing are not yet understood. This study deals with static electropulsing-induced phase transformations in a cold-drawn Zn–Al-based alloy (ZA27), based on a systematic investigation on phase transformations and processing of the Zn–Al-based alloys.11–13 II. EXPERIMENTAL PROCEDURES
Ingots of a Zn–Al-based alloy (ZA27) of nominal composition Zn71.3Al26.5Cu2.2 (in mass%) were repeatedly extruded, Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2011.185
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1696
J. Mater. Res., Vol. 26, No. 14, Jul 28, 2011
http://journals.cambridge.org
Downloaded: 13 Mar 2015
tempered, and drawn to produce alloy wire of 1.18 mm in diameter. The cold-drawn Zn–Al-based alloy wire was cut into pieces of 100-mm length for static EPT (without external stress affect) at ambient temperature (28 °C). A self-made electropulsing generator was applied to discharge multiple positive pulses with current intensities: 5 A for 10 s (s), 30 s, and 60 s, and 10 A for 60 s. The frequency for the multiple electropulsing was selected as 50 Hz. The duration of each pulse was kept to about 2300 ls. The current densities (A/mm2) are 37.4 and 109
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