Interplay of slip and twinning in niobium single crystals compressed at 77 K

High-purity niobium single crystals of five different orientations were compressed at 77 K to 2–4% plastic strain to investigate the mechanisms operative in the initial stage of yielding. The crystals deformed in the direction close to the [001] axis exhibit predominant slip on the high-stressed (101) and a much lower stressed $\left( {0\bar{1}1} \right)$ plane. The expected slip on the $\left( {\bar{1}01} \right)$ plane is nearly homogeneously distributed with only a few sharp slip traces corresponding to localized slip. The samples compressed along center-triangle orientations and those close to the $\left[ {011} \right] - \left[ {\bar{1}11} \right]$ edge deform predominantly by twinning on {112}〈111〉 systems with some contribution from slip on the $\left( {\bar{1}01} \right)\left[ {\bar{1}\bar{1}\bar{1}} \right]$ system with the highest Schmid factor. A majority of twins exhibit internal contrast due to alternating slip on $\left( {\bar{1}01} \right)$ and $\left( {0\bar{1}1} \right)$ planes. No slip traces are observed in the matrix adjacent to the twin, which implies that twin boundaries are impenetrable obstacles for the motion of dislocations.