An energy analysis of nanovoid nucleation in nanocrystalline materials with grain boundary sliding accommodations
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Jianqiu Zhoua) Department of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, China; and Department of Mechanical Engineering, Wuhan Institute of Technology, Wuhan, Hubei 430070, China
Shu Zhang Department of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, China
Yingguang Liu Department of Energy and Power Engineering, North China Electric Power University, Baoding, Hebei 071003, China
Hongxi Liu, Ying Wang, and Shuhong Dong Department of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, China (Received 16 September 2013; accepted 19 November 2013)
A theoretical model of nanovoid nucleation at triple junctions in nanocrystalline materials is developed in this article. The sliding of grain boundaries (GBs) meeting at triple junctions, which can be attributed to the gliding of GB dislocations (GBDs), provides the driving force for nanovoid nucleation. The GB sliding is accommodated by the emission of partial dislocations from GBs as well as GB diffusion. The corresponding energy characteristics of the pile-ups of GBDs, the emission of partial dislocations from the GBs, and GB diffusion are calculated, respectively. Furthermore, an energy balance method to calculate the nucleation of nanovoid at triple junctions is studied. The analysis demonstrates that the nucleation of the triple junction nanovoid depends mainly on the applied stress, the GB length (length of the pile-up), the GB structures, and the GB sliding accommodations. I. INTRODUCTION
Nanocrystalline materials with average grain sizes ,100 nm show ultra-high yield and fracture strength, superior wear resistance, and enhanced hardness.1–5 However, the high strength is at the expense of both low tensile ductility and fracture toughness, which limit their utility in many applications.6–9 A large number of studies2,8,10 have shown that the low ductility and toughness can not only be attributed to their unique deformation mechanisms but also to additional plastic accommodation processes, such as cavitation and microcracking. In these circumstances, it will be of large interest in understanding the fundamental micromechanisms for the formation of voids as carriers of ductile fracture processes in nanocrystalline materials. In contrast to conventional coarse-grained polycrystals, deformation-induced nucleation of voids in nanocrystalline materials is strongly influenced by grain boundaries (GBs). The lattice dislocation slip serving as the dominant deformation mode in coarse-grained metals is suppressed in nanocrystalline materials.2,3 On the other hand, lattice a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.383 J. Mater. Res., Vol. 29, No. 2, Jan 28, 2014
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dislocations can be absorbed into GBs and dissociate into the glissile extrinsic GB dislocations (GBDs) and disordered networks of trapped sessile GBDs.11,12 The motion of the
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