Tailoring Grain Boundary and Resultant Plasticity of Pure Iron by Pulsed-Electric-Current Treatment
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INTRODUCTION
AS an important microstructural element, grain boundaries affect the mechanical, physical, electrical, and chemical properties of polycrystalline metals significantly.[1–3] A well-known example of this phenomenon is that coarse-grained polycrystalline metals are strengthened with the decreasing grain size, which implies the increasing total grain-boundary area, following the classical Hall–Petch relationship.[4] Another successful example is that in grain-boundary engineering, related properties can be mediated by tailoring grain boundaries.[5] In such a case, a grain-boundary interconnection (GBIC), which is expressed as {h1k1l1}/ {h2k2l2} by the Miller indices of the two interconnected crystallographic planes, is an appropriate approach to describe the characteristics of a grain boundary accurately.[6] According to crystallography rules, GBIC dominates the microstructures that contain dislocations, free volume, and the faceting mode [7] of a grain boundary. A manipulation of the distribution of characteristics for grain boundaries,[8] which implies the
C. YANG and Y.J. ZHAO are with the Guangdong Key Laboratory for Advanced Metallic Materials Processing, South China University of Technology, Guangzhou 510640, China and also with the National Engineering Research Center of Near-NetShape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640 China. Contact e-mail: [email protected] Z. WANG is with the Guangdong Key Laboratory for Advanced Metallic Materials Processing, South China University of Technology. S.G. QU, X.Q. LI, and W.W. ZHANG are with the National Engineering Research Center of NearNet-Shape Forming for Metallic Materials, South China University of Technology. L.C. ZHANG is with the School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, WA 6027, Australia. Manuscript submitted June 5, 2018. Article published online November 19, 2018 856—VOLUME 50A, FEBRUARY 2019
increasing fraction of special low-R (1 £ R £ 29) coincident-site-lattice (CSL) grain boundaries compared with random grain boundaries, can change the fracture mode, improve the mechanical properties of polycrystalline metals, and transform typical intergranular to transgranular fractures in Al-Li alloys, which results in its super-plasticity at elevated temperatures.[9] Thus far, considerable endeavors have been made to anneal twin (R3 boundaries)-induced grain-boundary engineering, mainly in face-centered cubic (fcc) metals with a low stacking-fault energy.[10,11] Technically, the method used in the current study is termed a thermal–mechanical process,[12] and includes a low-temperature deformation and a subsequent high-temperature annealing treatment. This scenario raises an interesting question: is there any processing method, other than the thermal–mechanical process, which can be introduced to tailor low-R boundaries and the resultant mechanical properties of polycrystalline metals? Recently, pulsed-electric-current (PEC) treatment was proposed as an effective meth
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