An analysis of creep deformation in nanocrystalline materials

The standard mechanism of diffusion creep involving mass transport through the grains (Nabarro-Herring) or along grain boundaries (Coble) is modified to allow for transport along triple lines. Transitions between the various diffusion creep processes are analyzed, and it is demonstrated that Triple Line diffusion creep is likely to be important in nanocrystalline materials. Furthermore, unlike conventional materials, nanocrystalline materials with a grain size below a critical value will not exhibit a transition from diffusion creep at low stresses to conventional intragranular power-law dislocation creep at high stresses, due to the difficulty in developing dislocation pile-ups in very fine grained materials. The enhanced matter transport associated with triple line diffusion is likely to contribute significantly to enhanced superplastic deformation in nanocrystalline materials.