On the ductile damage of nanotwinned copper crystal with prolate void defect at the twin boundary

Abstract

Void growth and coalescence is one of the primary causes that are responsible for the ductile damage of metals. Extensive literature works have been conducted for exploring the dislocation-based damage mechanisms due to cylindrical or spherical voids embedded in single or polycrystals. In this paper, molecular dynamics simulations were performed for elucidating the ductile damage mechanisms of nanotwinned copper crystal containing prolate void defects sitting along the twin boundary. The effects of length scale, strain rate, temperature and void fraction on the strength, stiffness and dislocation evolution of the proposed nanotwinned model are studied in detail. Starting at the critical yield stress, the successive nucleation, propagation and reaction of Shockley partial dislocations are responsible for the continuous void growth process and result in a dramatic stress drop immediately following the ultimate strength. An obvious dependence on length scale is found for both the yield and the peak stress. The continuum Lubarda model that was developed for characterizing the critical stress of spherically voided nanocrystals is found to appreciably underestimate the strength of nanotwinned copper embedded with prolate voids. Simulation results also reveal that twin boundaries function as a dislocation barrier that is able to prevent dislocations from penetrating through. It is by this mechanism that twin boundaries affect the strength of nanotwinned crystals. Keywords: Prolate nanovoid, Twin boundary, Size effect, Dislocation evolution, Molecular dynamics