Dislocation mechanisms of low-temperature plasticity of nanocrystalline titanium: The role of impurity and grain boundary strengthening

The temperature dependences of the yield strength, the strain rate sensitivity of the deforming stress and the activation volume of the process of plastic deformation of nanocrystalline titanium VT1-0 under quasi-static tension in the temperature range 4.2–395 K are found. The nanoscale grain size was obtained using the method of cryomechanical grain fragmentation. A detailed thermal activation analysis of the experimental results was carried out and it was shown that the process of plastic deformation under conditions of ambient and low temperatures is determined by the overcoming of local impurity barriers by dislocations, and grain boundaries are a source of internal stresses. Empirical estimates are obtained for the parameters of the dislocation-impurity interaction. The unambiguous relationship between internal stresses and grain size established for monomodal nanocrystalline titanium made it possible to separate the effects of impurity and grain boundary hardening. The result obtained is an indirect basis for the conclusion about the impossibility of accumulation of dislocations in nanograins during plastic deformation. The decrease in the activation volume for a nanoscale grain at a constant impurity concentration is considered because of the manifestation of the dependence of the diameter of the dislocation loop generated by a grain boundary source on the grain size (confinement effect).