Fracture behavior of precracked nanocrystalline materials with grain size gradients
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The fracture behavior of precracked nanocrystals with grain size gradients is simulated using the molecular dynamics method. A large grain size gradient is found to elevate resistance to crack propagation and transform the fracture mode from intergranular to intragranular when the crack is obstructed by a coarse grain. But the intragranular crack is nipped in its bud due to the difficulty of intragranular fracture. However, intergranular fractures can be always kept in nanocrystals with a small grain size gradient. Both the Schmid factors for the slip systems of grains near the crack tip and the critical stress intensity factors are calculated, and energy partitioning is conducted to analyze the mechanisms behind this phenomenon. The research exhibits the key role of grain size gradient in improving the antifracture ability of nanocrystals.
I. INTRODUCTION
Extensive investigations have been carried out upon polycrystalline materials with high strength. Reducing grain sizes is found to be an effective way of increasing material strength. However, when the grain sizes are reduced to the nanometer scale, certain exceptions could take place. Farkas,1 for example, found that the fracture resistance decreased when the mean grain size increased from 5 to 8 nm. Meyers et al.2 and Zhu and Li3 also reported that the strength of polycrystalline metals increased but their ductility was reduced with the increasing grain size. Grain size distribution is found to have a significant effect on the mechanical performance of nanocrystalline materials. Distinct from the low ductility of nanocrystals with uniform grain size, a bimodal grain size distribution can give the material both high strength and ductility.4–6 In the tensile experiments of the distributed grain size specimens, up to 10 times higher yield strength and excellent tensile plasticity were achieved by coating the coarse-grained copper substrate with a gradient grain-size transition film, and cracks were found to initiate preferentially in the coarse-grained rather than the fine-grained regions.7 The fatigue experiments indicated that grain refinement led to an increase in total life, but also had a deleterious effect on the resistance to fatigue crack growth.8 This prompted our investigations into the internal mechanisms behind the behavior of nanocrystalline materials with grain size gradients. Contributing Editor: Susan Sinnott a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.18 J. Mater. Res., 2015
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During the synthesis process, it is generally difficult to keep materials free of defects such as pores and impurities. These defects degenerate the inherent mechanical properties of nanocrystalline materials and can lead to brittle fracture under tensile loading. To improve the fracture resistance, some potential toughening mechanisms, such as a high strain hardening rate,4 plastic deformation induced by grain growth,7 and twin boundaries,9–12 have been investigated experim
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