Abstract
Extensive experiments have shown that gradient nano-grained metals have outstanding synergy of strength and ductility. However, the deformation mechanisms of gradient metals are still not fully understood due to their complicated gradient microstructure. One of the difficulties is the accurate description of the deformation of the nanocrystalline surface layer of the gradient metals. Recent experiments with a closer inspection into the surface morphology of the gradient metals reported that shear bands (strain localization) occur at the surface of the materials even under a very small, applied strain, which is in contrast to previously suggested uniform deformation. Here, a dislocation density-based computational model is developed to investigate the shear band evolution in gradient Cu to overcome the above difficulty and to clarify the above debate. The Voronoi polygon is used to establish the irregular grain structure, which has a gradual increase in grain size from the material surface to the interior. It was found that the shear band occurs at a small applied strain in the surface region of the gradient structure, and multiple shear bands are gradually formed with increasing applied load. The early appearance of shear banding and the formation of abundant shear bands resulted from the constraint of the coarse-grained interior. The number of shear bands and the uniform elongation of the gradient material were positively related, both of which increased with decreasing grain size distribution index and gradient layer thickness or increasing surface grain size. The findings are in good agreement with recent experimental observations in terms of stress-strain responses and shear band evolution. We conclude that the enhanced ductility of gradient metals originated from the gradient deformation-induced stable shear band evolution during tension.
Highlights
The strength-ductility trade-off is a longstanding problem with metals, which means that the achievement of high strength in the materials is usually accompanied by low ductility, and vice versa [1,2,3,4]
The gradient Cu preserved a ductility comparable to that of the CG Cu. This excellent strength–ductility synergy of gradient nano-grained metals is believed to be attributed to unique gradient nanostructures, which combines the high strength of the nanograins and the high ductility of the coarse grains, and confers on the gradient metals extra strain hardening through hetero-deformation induced strengthening [12,13]
Cu withmodel a grainwas sizeestablished variation from tens ofshear nanometers formation in gradient nano-grained with a grain size variation from tens at the surface to tens of micrometersCu at the core
Summary
The strength-ductility trade-off is a longstanding problem with metals, which means that the achievement of high strength in the materials is usually accompanied by low ductility, and vice versa [1,2,3,4]. The deformation mechanisms of the gradient metals have been studied extensively [20,21,22,23,24], it is still not fully understood how the nano-grained (NG) layer at the surface of the gradient materials deforms during tension, especially considering that homogeneous NG metals usually have very limited ductility [25,26] and often fail through the formation of shear bands [27,28,29] that are initiated at a very small plastic strain, e.g., 0.3% [30], similar to the failure mechanism of brittle metallic glasses [31,32,33]. Experiments showed that the NG layer in the gradient metals does not deform uniformly under tension as suggested in the previous studies [6,34] Instead, strain localizations, such as shear bands, are formed at the surface of the gradient metals [35,36,37,38]. Our simulation results are in good agreement with experimental data in terms of both the stress-strain curve and shear band patterns
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