Flow stress of Nitronic-50 stainless steel over a wide range of strain rates and temperatures

To understand and model the thermomechanical response of Nitronic-50 stainless steel, uniaxial compression tests are performed on cylindrical samples, using an Instron servohydraulic testing machine and an enhanced Hopkinson technique. True strains exceeding 60% are achieved in these tests, over the range of strain rates from 0.001/s to 8000/s, and at initial temperatures from 77 K to 1000 K. To understand the underlying deformation mechanisms, the microstructures of the undeformed and deformed samples are examined by optical microscopy. These experimental results show that: (1) Nitronic-50 stainless steel displays good ductility (strain > 35%) for all considered strain rates, even at a 77 K temperature; (2) at high strain rates and 77 K initial temperature, adiabatic shearbands develop when the axial strain exceeds about 35%, leading to fracture; and (3) dynamic strain aging occurs at temperatures between 400 K and 1000 K for a strain rate of 0.001/s, but it is less pronounced at a strain rate of 3500/s or greater. Finally, based on the mechanism of dislocation motion and the experimental data, a physics-based model is developed for the deformation behavior of this material, including the effect of viscous drag on the motion of dislocations, but excluding the dynamic strain aging effects. The model predictions are compared with the results of the experiments. Good agreement between the theoretical predictions and experimental results is obtained. (C) 2006 Elsevier Ltd. All rights reserved.