Microstructure-dependent deformation behaviour of bcc-metals – indentation size effect and strain rate sensitivity

In this work, the indentation size effect and the influence of the microstructure on the time-dependent deformation behaviour of body-centred cubic (bcc) metals are studied by performing nanoindentation strain rate jump tests at room temperature. During these experiments, the strain rate is abruptly changed, and from the resulting hardness difference the local strain rate sensitivity has been derived. Single-crystalline materials exhibit a strong indentation size effect; ultrafine-grained metals have nearly a depth-independent hardness. Tungsten as a bcc metal shows the opposite behaviour as generally found for face-centered cubic metals. While for UFG-W only slightly enhanced strain rate sensitivity was observed, SX-W exhibits a pronounced influence of the strain rate on the resulting hardness at room temperature. This is due to the effects of the high lattice friction of bcc metals at low temperatures, where the thermally activated motion of screw dislocations is the dominating deformation mechanisms, which causes the enhanced strain rate sensitivity. For the SX-materials, it was found that the degree of the indentation size effect directly correlates with the homologous testing temperature and thus, the material specific parameter of the critical temperature T-c. However, for the resultant strain rate sensitivity no depth-dependent change was found