Deformation twinning in vanadium single crystals tested in compression at 77 K

Vanadium is the least studied non-magnetic body centered cubic metal whose mechanism of plastic deformation is still a subject of controversy. Depending on the level of purity and spatial distribution of interstitial impurities, it has been shown to deform by anomalous slip on the low-stressed (01̅1) plane or by twinning along {112} planes in the 〈111〉 direction. In order to resolve this controversy, we test high-purity vanadium single crystals of four orientations at 77 K under a low strain rate to 1-2% of the plastic strain. All our samples contain spatially homogeneous distribution of impurities and deform predominantly by twinning on two {112}⟨111⟩ systems in which the {112} planes are sheared in the twinning sense. The secondary surface markings correspond to slip on {110}⟨111⟩ systems with the highest Schmid factors, whereas weak (01̅1) slip is found mostly in the vicinity of twins. These results support the argument made previously by Thompson and Carlson (1965) that a spatially homogenenous distribution of interstitial impurities in high purity vanadium single crystals leads to predominant twinning.