Anomalous slip in high-purity niobium single crystals deformed at 770K in tension

Abstract High-purity niobium crystals with a centre-triangle axial orientation were deformed in steps at 77°K. Each crystal was cut to a square cross-section and a reference grid applied to two adjacent surfaces before deformation. At each deformation step the lengths and orientations of the grid sides were determined, and using this information the amounts of slip on eight independent systems of {110} type were calculated. The results showed that most of the deformation occurred on the lightly stressed systems (011) [111] and (011) [111] with very little slip on the primary (101) [111] and conjugate (101) [111] systems. This analysis confirms earlier observations on high purity niobium deformed at low temperatures, which showed that (011) slip lines predominated (anomalous slip). The present data show that the conjugate Burgers vector is almost twice as active as the primary. In the earlier stages of deformation the straining proceeded by a series of small load drops, but later the curve became smooth. The slip lines were very similar to those observed on round crystals. Anomalous (011) slip at large strains has only been observed in crystals of niobium purified by ultra-high vacuum annealing; other b.c.c. metals and alloys, and niobium purified by zone-refining do not exhibit this type of deformation. The origin of anomalous slip remains a mystery: it is possible that a solution will come out of the recent atomistic calculations on screw dislocations in a b.c.c. structure.