Flow stress of commercially pure niobium over a broad range of temperatures and strain rates

Abstract

The thermomechanical response of a commercially pure niobium (CP niobium) is investigated systematically in quasi-static (Instron, servohydraulic) and dynamic (UCSD's recovery Hopkinson) compression. Strains over 70% are achieved in these tests over a temperature range of 77–800 K and strain rates of 10 −3 to 8000 s −1. At the macroscopic level, the flow stress of CP niobium, within the plastic deformation regime, is dependent strongly on the temperature and strain rate. At temperatures below 296 K (room temperature), the flow stress of CP niobium at quasi-static strain rates is more sensitive to the temperature than it is at high strain rates. Dynamic strain aging occurs at ∼400 K at quasi-static strain rates. The microstructure of the deformed samples shows that adiabatic shearbands occur at low temperatures and high strain rates. Deformation twins are seen outside these shearbands. Results of the temperature- and strain-rate-jump tests show that, the corresponding microstructural changes are not significant. Finally, based on the concept of dislocation kinetics, paralleled with the systematic experimental investigation, a model describing the flow stress of CP niobium under high strain rates is developed by combining the effects of the long-range barriers to the motion of dislocations, with those of the short-range temperature- and strain rate-dependent barriers. The predicted flow stress and the experimental results are compared, attaining good agreement.