Analysis of the strain=rate sensitivity of flow stresses in nanocrystalline FCC and BCC Metals

The dependence of the strain-rate sensitivity coefficient of flow stresses S = dln sigma/dln(epsilon) over dot on temperature, strain rate, and grain size in nanocrystalline (NC) metals is analyzed quantitatively in terms of the dislocation-kinetics approach taking into account the properties of grain boundaries as sources, sinks, and barriers for moving dislocations. The interaction of moving dislocations with a dislocation forest in nanograin boundaries is shown to be responsible for the fact that the values of this coefficient in NC fcc metals (Cu, Ni) are an order of magnitude greater than those in coarse-grained metals and for the strong dependence of the coefficient S on the above factors. This dependence is larg caused by the annihilation of lattice dislocations in grain boundaries controlled by the activation energy of grain boundary diffusion. The values of the coefficient S in NC bcc metals (alpha-Fe) are an order of magnitude lower than those in coarse-grained samples, because dislocations move in a Peierls relief in nanograins.