Plasticity of niobium and niobium-molybdenum single crystals in the superconducting and normal states

Abstract

Single crystals of niobium and niobium-molybdenum (1.2–7.6 at.% Mo), all of the same orientation suitable for single slip on (1̄01) [111], were annealed in an ultrahigh vacuum and tensile tests at 4.2°K were performed after a small prestrain at room temperature and 77°K. By the combination of ultrahigh vacuum purification and deformation at higher temperatures, stable plastic flow was possible at 4.2°K. During a constant strain rate test at 4.2°K, the superconducting and normal states were established alternately. The flow-stress change Δ τSN at the transition was studied as a function of strain, strain rate and alloy concentration. In all cases, Δ τSN decreases as the strain increases. For pure niobium, Δ τSN is independent of strain rate. Molybdenum solutes increase Δ τSN . The flow-stress change is discussed in terms of the different electronic drag coefficients in the two states and the stress and state dependence of the kink nucleation.