Effects of grain boundaries on cyclic deformation behavior of copper bicrystals and columnar crystals

Abstract

In order to reveal the effects of grain boundaries (GBs), the cyclic deformation behavior of a copper columnar crystal containing low-angle GBs, a really grown copper bicrystal RB and a combined bicrystal CB was investigated. The results show that the cyclic stress–strain curve (CSSC) of the copper columnar crystal exhibited a plateau region with saturation resolved shear stress of about 29 MPa over a plastic resolved shear strain range from 6.5×10 −4 to 4.7×10 −3. The electron channeling contrast technique in scanning electron microscopy (SEM-ECC) was applied to observe the cyclically saturated dislocation structures and persistent slip bands (PSBs). In combining the CSSC with the associated saturation dislocation arrangements, it is seen that the strengthening effect of low-angle GBs is very limited. Meanwhile, the CSSC of the combined bicrystal CB was also found to exhibit a plateau region with the saturation resolved shear stress in the range of 29–30 MPa at the plastic resolved shear strain range from 9×10 −4 to 5.58×10 −3. However, the bicrystal RB did not display a plateau in its CSSC, and its saturation stress increased with increasing strain amplitude. Surface observations revealed a GB affected zone (GBAZ) with additional slip near the GB in the bicrystal RB. The width W GB and the volume fraction V GB of the GBAZ increased with increasing strain amplitude. By comparing the CSSCs of the copper bicrystal CB and RB as well as [ 135]//[ 135] , [ 135]//[ 235] and [ 235]//[ 235] copper bicrystals, the effects of large-angle and low-angle GBs on cyclic deformation behavior were discussed. A GB strengthening model was proposed by introducing a GB resistance Δ τ B as to PSBs and the mean stress σ GB as in the GBAZ.