Damage affected discontinuous plastic flow (DPF)

摘要

Evolution of micro-damage in the course of discontinuous plastic flow (DPF, serrated yielding) at extremely low temperatures is investigated. DPF is observed in many metals and alloys loaded in cryogenic conditions, within the temperature range specific of a given material and starting practically at absolute zero. The appearance of DPF is similar to dynamic strain ageing, however, its origin is attributed to the mechanism of local catastrophic failure of lattice barriers under the stress fields related to edge dislocation pile-ups. Failure of barriers, occurring in weakly excited lattice, leads to dynamic and massive motion of released dislocations. The phenomenon is accompanied by step-wise increase of the strain rate and drastic drop of stress during each serration. DPF has strong thermodynamic background consisting in the fact, that the plastic power dissipated in the course of serrations is partially converted to heat, which results in a local jump of temperature. It results from the so-called thermodynamic instability associated with vanishing specific heat when the temperature tends to absolute zero. The evolution of micro-damage affects loading and unloading moduli during each serration. This, in turn, results in gradual evolution of the amount of plastic slip accompanying each serration. The physically based constitutive model describes damage affected serrated yielding at the temperatures close to absolute zero. The model accounts for the thermodynamic background, including phonon mechanism of heat transport. Experimental identification of parameters of the constitutive model has been carried out based on a number of loading/unloading traction tests. A comparison between the experimental and the numerical results is presented and discussed. (c) 2017 Elsevier Ltd. All rights reserved.