Effects of Cyclic Loading Performance on Grain Boundary Motion of Nanocrystalline Ni
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NANOCRYSTALLINE materials with grain sizes smaller than 100 nm have superior mechanical properties, including strength and hardness. These properties can be attributed to a deformation mechanism transformation caused by the small grain size, and a high percentage of grain boundaries (GBs).[1] In general, GB-mediated mechanisms, such as GB sliding[2,3] and migration, are prevalent in nanocrystals. GB sliding nucleates preferentially to dislocation slips under tension in nanocrystalline Ni.[4] Stress-driven GB migration, manifesting as grain growth, has been found in stressed nanocrystals.[4,5] In recent years, both theoretical predictions and molecular dynamics (MD) simulations have been performed to capture important characteristics of coupled GB motion.[6–8] The GB configuration is particularly important in systematic studies of GBs. GBs with different misorientation angles will show different configurations and energies.[9] A strong connection between the energy barrier and the interfacial boundary energy has been shown by comparing the energy barrier of dislocation transmission through a GB and the energy barrier of dislocation nucleation from a GB, for various types of GBs.[10] A GB with low static interfacial energy provides
PENG WANG, XINHUA YANG, and DI PENG are with the Department of Mechanics, Huazhong University of Science and Technology, Wuhan 430074, P.R. China. Contact e-mail: yangxinh@ hust.edu.cn Manuscript submitted January 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
a strong barrier against slip transmission and nucleation at the GB. Changes to the GB misorientation angle generally result in different migration velocities, but can also change the direction of the coupled GB motion.[6,11,12] Non-planar GB structures in polycrystalline Cu favor dislocation emission, while planar GB structures in polycrystalline Pd essentially show GB sliding.[13] GB configurations are not immutable during deformation, but are affected by loading conditions. The non-equilibrium GB structure in nanocrystalline Ni-W alloys can be relaxed by low temperature annealing and the hardness of the alloys can also be effectively increased.[14] Structural phase transformations have been investigated in terms of changes to the GB atomic density, induced by varying temperature or injecting point defects into the GB region.[15] Dislocations may interact with the GB during deformation. This not only induces strain localization but also changes the GB configuration.[16–18] In addition, the effects of cyclic loading on GB structure modulation have recently been observed experimentally and substantial grain coarsening and loss of growth twins have been found in the fatigue crack paths.[19] Mechanical cycling could alter the GB network and lead to a considerable increase in the R3 boundaries at elevated temperature.[20] This effect could enhance the cycling plastic strain accumulation. Similar results have also been obtained from MD simulations of nanocrystalline Ni.[21] Coupled GB motion has attracted considerable attention in recent
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