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 highstressed ( 101) and a much lower stressed (0 (1) over bar1) thorn plane. The expected slip on the ((1) over bar 01) thorn 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 [011] - [(1) over bar 11] edge deform predominantly by twinning on {112} < 111 > systems with some contribution from slip on the ((1) over bar 01) [(1) over bar(1) over bar(1) over bar] system with the highest Schmid factor. A majority of twins exhibit internal contrast due to alternating slip on ((1) over bar 01) thorn and (0 (1) over bar1) thorn 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.