Triple junction diffusion and plastic flow in fine-grained materials

Abstract

A theoretical model is suggested which describes the yield stress dependence on grain size in fine-grained materials, based upon competition between conventional dislocation slip, grain boundary diffusional creep (Coble creep) and triple junction diffusional creep. In the framework of the model, the contribution of diffusional creep mechanisms to plastic deformation increases with reduction of grain size, causing the abnormal Hall-Petch dependence in the range of small grains. A grain size distribution is incorporated into the consideration to account for a distribution of grain sizes occurring in real specimens. The results of the model are compared with experimental data from Cu and shown to be in good agreement. Ó

Modeling the dependence of strength on grain sizes in nanocrystalline materials

“…We consider the grain size distribution which follows a log-normal function. To facilitate the analysis, the following assumptions similar to those specified in [4,8] are used. …”

“…'grain size softening' (also called the inverse Hall-Petch relationships) [2]. A number of theories and models have been developed to understand the deviation of the strength from the empirical Hall-Petch equation [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Recent molecular dynamics (MD) simulations have improved our understanding of the deformation of nanocrystalline materials [13].…”

Yield stress of nanocrystalline materials: role of grain-boundary dislocations, triple junctions and Coble creep

“…In fact, high-density ensembles of GBs serve as obstacles for conventional dislocation slip in nanocrystalline materials and, at the same time, open up several effective deformation modes that usually are not significant in coarse-grained polycrystals. These modes are GB diffusional creep (Masumura et al 1998, Kim et al 2000, Yamakov et al 2002, triple-junction diffusional creep (Fedorov et al 2002), rotational mode (occurring via movement of GB disclinations) (Ke et al 1995, Hackney et al 1996, Murayama et al 2002 and GB sliding (Hahn et al 1997, Hahn and Padmanabhan 1997, Konstantinidis and Aifantis 1998, Fedorov et al 2003) (for a review, see ). In these circumstances, grain refinement causes switching from conventional lattice dislocation slip to deformation modes conducted by GB dislocations, GB disclinations and point defects.…”

Grain-boundary dislocations and enhanced diffusion in nanocrystalline bulk materials and films

“…In particular, models (Masumura et al 1998, Kim et al 2000, Fedorov et al 2002, Yamakov et al 2002 based on the idea on the crucial contribution of grain-boundary diffusion to plastic flow of nanocrystalline materials should take into account enhancement of diffusion coefficient and its evolution in time, associated with transformations of grain boundary structures. Also, the low-temperature and high-strain-rate superplasticity (McFadden et al 1999, Islamgaliev et al 2001, Mukherjee 2002) of finegrained and nanocrystalline materials fabricated by severe plastic deformation can be strongly influenced by boundary diffusion enhanced owing to transformations of grain-boundary dislocation structures.…”

“…Also, the low-temperature and high-strain-rate superplasticity (McFadden et al 1999, Islamgaliev et al 2001, Mukherjee 2002) of finegrained and nanocrystalline materials fabricated by severe plastic deformation can be strongly influenced by boundary diffusion enhanced owing to transformations of grain-boundary dislocation structures. Therefore, grain-boundary junctions (exhibiting specific properties (King 1999)) can play a very essential role in the transformations and their effects on diffusion and plastic flow (for example, Fedorov et al (2002Fedorov et al ( , 2003). …”