Grain Refinement in Surficial Cryogenic Grinding
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application in the fabrication of structural materials with high strength, alloying needs to be improved in alloy element recycling.[1] Grain refinement is an alternative approach to replace alloying due to the Hall–Patch strengthening mechanism[2]: ry ¼ r0 þ kd1=2
½1
where ry is yield stress, r0 is lattice frictional resistance produced by a single dislocation, k is a constant, and d is the grain diameter. Nanostructured alloys obtained through severe plastic deformation (SPD) exhibit much higher strength than their coarse-grained counterparts. However, they show limited work hardening and unacceptable elongation.[3] Ductility and toughness frequently drop sharply as the grain size falls to 1 lm. As such, gradient structure was proposed to achieve both high strength and ductility synchronously.[4] In gradient structures, the combination of different-sized grains maintains strength and reduces
YANCHAO LIU, YONGXIAN HUANG, MENG GUAN, XIANGCHEN MENG, and YUMING XIE are with the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, P.R. China. Contact email: [email protected] Manuscript submitted November 27, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the loss of ductility of refined grains at the same time. Surface mechanical attrition treatment (SMAT) was carried out in AZ31B magnesium alloy to fabricate gradient structure. The structure exhibited high strength and good ductility because of the combination of refined grains at the surface and coherent interface between the refined grains and coarse-grained matrix.[5] Fang et al.[6] fabricated a gradient nanograined structure of copper via surface mechanical grinding treatment (SMGT). The processed sample exhibited high strength. Meanwhile, the tensile strain exceeded 100 pct without fractures. Gradient distribution of dislocation densities in high-purity zirconium sheets was achieved via asymmetrical rolling and subsequent partial annealing, which resulted in an extraordinary synergy of high strength and uniform tensile ductility.[7] In order to investigate the gradient structure deeply, many SPD methods were used to fabricate gradient structures, such as friction stir processing (FSP),[8,9] SMAT, and SMGT.[10] The mechanism of grain refinement in SMAT was identified in terms of the formation of dislocation cells and twin-matrix lamellae.[11] Zhang et al.[12] suggested that grain refinement in FSP was achieved through dynamic recovery and dynamic recrystallization (DRX). Nevertheless, to our best knowledge, there is no unified theory that can explain the different grain features in the same method. In this article, we adopted surficial cryogenic grinding as a simple method to obtain charming gradient ultrafine-grained structure. The formation and evolution of ultra-fine-grained microstructure were revealed. The modes of grain refinement were identified in detail, corresponding to different strain rates. Copper sheets (100 9 100 9 3 mm3) of 99.9 pct purity were used. The tool is illustrated in Figure 1(a); its major parts ar
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