Simulation of deformation and failure process in bimodal Al alloys
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DUCTION
THE processing and properties of “nanostructured” materials have been widely investigated during the last two decades.[1,2] Available data suggest that bulk nanostructured materials typically display very high strength or hardness, but very poor ductility. The low ductility in nanostructured materials has been generally attributed either to the lack of the dislocation activity when the grain sizes fall in the nanoscale range or to the existence of processing flaws (e.g., porosity or poor interfacial bonding) based on available experimental results and theoretical studies.[3,4] Early studies by Tellkamp et al.[5] on the behavior of nanostructured Al alloys fabricated via a cryomilling (e.g., milling in liquid nitrogen) and consolidation approach suggested that the presence of submicron grains in the nanostructured regions might contribute to ductility via increased plasticity.[6,7,8] These early observations were subsequently confirmed by other investigators’ work on copper.[9] In this study, an ultrafine-grained (UFG) copper processed by rolling to a high value of percentage cold work (93 pct) at liquid nitrogen temperature exhibited high strength but poor elongation. Brief annealing at 473 K for 3 minutes led to a “bimodal” UFG microstructure, resulting in a pronounced work-hardening capacity, as well as an excellent combination of both high strength and high ductility. More recently, cryomilled aluminum alloys with a mixture of nanostructured grains and submicron grains (e.g., socalled “bimodal” microstructure) have been successfully fabricated via a powder metallurgy approach and the preliminary results on microstructure and mechanical properties have been reported in previous studies.[10,11] In these studies, cryomilled aluminum alloys, processed with a predetermined fraction of submicron grains, have revealed an improved ductility without much sacrifice in strength. Related studies[10,11] on the mechanical behavior and microstructure characteristics of cryomilled Al alloys have led to the following
specific observations: (1) cryomilled regions are well bonded to submicron-grained regions, and (2) the ductility of a bimodal material increases and yield strength decreases with increasing volume fraction of submicron grains. Despite these encouraging results reported for bimodal materials, the elastic and plastic deformation mechanisms are not fully understood. Of particular relevance are the deformation mechanisms that may take place among the various length scales (e.g., submicron and nanostructured grains). Computational micromechanics modeling may help to provide insight into the behavior of bimodal materials in a broad range of length scales.[12,13] For example, the stress field influences failure processes (yielding, crack initiation, and propagation) and determines the final fracture modes. The strain at failure is related to the conditions for nucleation, growth, and coalescence of cracks for ductile fracture. Accordingly, numerical simulation of the stress distribution, beyond yielding and prior
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