Analysis of the twin variant selection in polycrystalline cobalt

Abstract

The samples of polycrystalline cobalt were strained in tension in the second stage of work hardening in order to activate twinning. The identification of twins and their orientation was analyzed using electron backscatter diffraction. The study of twin variant selection was carried out based on two parameters, namely the Schmid factor (SF) and the geometric compatibility factor (m'). The results indicate that the grains contained the $$\left\{ {10\overline{1}2} \right\}$$ extension twins in most cases while the $$\left\{ {11\overline{2}1} \right\}$$ extension twin mode was present in few cases, as well. In the grains containing only one twin variant, the SF analysis correctly predicted the activated twins in most cases. In the grains with variant pairs in para-position, nearly half of the studied twins had the highest SF of all possible variants, which means that they followed the Schmid law. On the other hand, for twin pairs in meta- or ortho-position, a different behavior was observed. Twins in these configurations were selected rather by the strain compatibility criterion than by the Schmid law, as SF values of the activated twins were significantly lower than those of the twins in para-position. Of these two configurations, the ortho-position one is highly preferred. The combined analysis using SF and m' parameter, predicted the occurrence of specific twin modes and variants only partially. A local analysis considering the effect of neighboring grains should be developed to account for the cases where the two parameters fail to explain the occurrence of particular twins, which seems to be related to the presence of a high fraction of high-angle grain boundaries.