Grain refinement and formation of ultrafine-grained microstructure in a low-carbon steel under electropulsing
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High current electropulsing was applied to a low-carbon steel in the solid state. The relationship between grain size and experimental conditions was revealed. It was found that the ultrafine-grained (UFG) microstructure could be formed when electric current density, heating rate, and cooling rate all were high. The UFG samples prepared by applying electropulsing were free of porosity and contamination, and had no large microstrain. Also, their tensile strength was dramatically enhanced over that of their coarse-grained counterparts, without a decrease in ductility. The mechanism for grain refinement and formation of the UFG microstructure was discussed. It is proposed that the effect of a decrease in thermodynamic barrier and enhancement of nucleation rate in a current-carrying system cannot be neglected.
I. INTRODUCTION
It is well known that mechanical properties of metallic materials can be improved when grain size is reduced. Moreover, investigations conducted in recent years testify that ultrafine-grained (UFG; submicrometer and nanocrystalline) materials possess fundamentally different properties and behaviors from, and often superior to, those of their conventional coarse-grained polycrystalline counterparts.1–3 Therefore, the interest in UFG materials and grain-refinement techniques is dramatically increased. Recently, it was found that grains of a lowcarbon steel could be refined in the solid state by electropulsing treatment in our experiments.4 Zhang et al. found that little nanophases (volume fraction < 10%) can be formed in the conventional coarse-grained polycrystalline Cu–Zn alloy by applying high current electropulsing.5 The two phenomena give us an insight into whether UFG samples can be prepared by the control of conditions during electropulsing treatment. If it is possible, the ideal bulk UFG samples, namely pure (free of contamination) and dense (perfect bonding of crystallites and porosity free) bulk UFG samples with a minimized defect density (microstrain), will be obtained. In fact, the electropulsing technique is not a new subject in material science and technology. In the past fifty years, many researches have indicated that electric current can influence the behaviors of materials, such as the electromigration effect,6–8 the electroplastic effect,9–11 the solidification of metals and alloys,12–14 structural a
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 8, Aug 2002
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relaxation in amorphous solids and amorphous nanocrystallization,15–17 crack healing,18,19 and so on. In addition, many theoretical works have also been developed, such as the formulation of electron wind force,20,21 the theoretical investigation about the effect of electric current on phase transformation.22–27 Although the above-mentioned studies have been fruitful, the fact that the microstructure of solid metallic materials can be refined by electropulsing treatment has been nearly ignored. In the present work, the
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