Thermomechanical response of AL-6XN stainless steel over a wide range of strain rates and temperatures

To understand and model the thermomechanical response of AL-6XN stainless steel, uniaxial compression tests are performed on cylindrical samples, using an Instron servohydraulic testing machine and UCSD's enhanced Hopkinson technique. True strains exceeding 40% are achieved in these tests, over the range of strain rates from 0.001/s to about 8000/s, and at initial temperatures from 77 to 1000 K. In an effort to understand the underlying deformation mechanisms, some interrupted tests involving temperature and low- and high-strain rates, are also performed. The microstructure of the undeformed and deformed samples is observed by optical microscopy. The experimental results show: (1) AL-6XN stainless steel displays good ductility (strain > 40%) at low temperatures and high-strain rates, with its ductility increasing with temperature; (2) at high-strain rates and 77 K initial temperature, adiabatic shearbands develop at strains exceeding about 40%, and the sample breaks, while at low-strain rates and 77 K, axial microcracks develop at strains close to 50% or greater; (3) dynamic strain aging occurs at temperatures between 500 and 1000 K and at a strain rate of 0.001/s, with the peak value of the stress occurring at about 800 K, and becoming more pronounced with increasing strain and less pronounced with increasing strain rate; and (4) the microstructure of this material evolves with temperature, but is not very sensitive to the changes in the strain rate. Finally, based on the mechanism of dislocation motion, paralleled with a systematic experimental investigation, a physically 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. In order to verify the model independently of the experiments used in the modeling, additional compression tests at a strain rate of 8000/s and various initial temperatures, are performed, and the results are compared with the model predictions. Good correlation is observed. (C) 2001 Elsevier Science Ltd. All rights reserved.