Comparing the Role of Dislocations during Plastic Deformation of Nanocrystalline Nickel and Coarse-Grained Nickel during
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1090-Z05-08
Comparing the Role of Dislocations during Plastic Deformation of Nanocrystalline Nickel and Coarse-Grained Nickel during Cold Rolling Andreas Kulovits, Scott M Mao, and Jorg M Wiezorek Department of Mechanical Engineering and Materials Science, University of Pittsburgh, 848 Benedum Hall, 3700 O Hara Street, Pittsburgh, PA, 15261 ABSTRACT We investigated plastic deformation of fully dense electrodeposited nano crystalline (NC) Ni with an average grain size of 30-40nm. We studied the microstructural evolution during cold rolling of NC Ni to a reduction in thickness up to 76% (true strain equivalent ~1.42). We determined changes in texture, grain morphology, grain boundary character and grain sizes as a function of cold rolling strain, using X-ray diffraction and transmission electron microscopy TEM. We compared our results of the NC Ni with our own results for cold rolled coarse grained (CG) Ni. Differences and similarities in deformation behavior are discussed with respect to well documented findings in the literature. INTRODUCTION Good quality bulk nano crystalline (NC) metals offer potential property enhancements compared to their coarse-grained (CG) counterparts [1]. However, effects of the reduction in grain size to the nano-scale involve changes in plasticity mechanisms [1]. Here we investigate plastic deformation of fully dense electrodeposited NC Ni with an average grain size of 3040nm. In this grain size regime dislocations mainly facilitate plastic deformation [1], [2], [3]. However, in contrast to CG Ni the dislocation density is uncommonly low [4], [5]. In the absence of in CG metals operating conventional regenerative sources [8], usually located in the grain interior, grain boundaries (GB’s) act as dislocation sources and sinks [6], [7] in NC metals. The constraints imposed by the small grain size limits dislocation activity to only single dislocation glide systems in the initial stages of plastic response of NC metals. Only individual dislocations can operate inside one grain at a given time [7], [9]. In the absence of other dislocations to interact with, dislocations therefore primarily react with GB’s during plastic deformation. These subtle but important differences impact microstructure and property evolution during plastic deformation to large strains. Meyers et al. [10] have presented simple models that show that dislocation-GB interactions are suitable to result in changes in grain boundary character, macroscopic texture and grain shape necessary to accommodate the externally applied stress. Here we study the effect of these two major differences on microstructural changes in NC Ni during cold rolling. We use X-ray diffraction (XRD) and transmission electron microscopy (TEM) to document the microstructural changes in NC Ni during cold rolling to reductions in thickness of up to 75% of the initial thickness. We report on changes in crystallographic macro-texture (XRD), local or micro-texture (TEM), grain shape and scale (TEM). Our results are discussed with respect to the microstructu
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