Microstructure–property relations of a steel powder metal under varying temperatures, strain rates, and stress states

This study is a first to analyze the microstructure and associated plasticity and fracture characteristics under finite deformations for quantifying different applied stress state, strain rate, and temperature effects on a porous powder metal steel (FC-0205). The complexities of the inelastic deformation from the coupled plasticity-damage evolution induced stress-state differences (compression, tension, and torsion) up to 20% at just 2% strain when examining the stress–strain behavior. As far as the temperature effect, when the temperature increased, the yield stress and work hardening rate decreased as is typical in steel alloys. As the strain rate increased, the yield stress displayed an increase. The yield stress did exhibit an increase as strain rate increased for both porosity levels. However, when the strain rate increased, the work hardening did not increase as is typical in dense steel alloys. It might be that the competitions of the microbuckling and dislocation emission and interaction under compression have different rates of change, and hence the applied strain rate differences could induce these non-intuitive results. The fracture process was dominated by the stereological statistics of the porosity. The pore volume fraction (porosity) ranged from 9 to 19%, the average pore diameter was approximately 6 μm with a maximum of 160 μm, and the average nearest neighbor distance was approximately 10 μm with a maximum of 60 μm. Since the nearest neighbor distance was close to the pore size, the stress interaction between pores was almost immediate upon loading thus causing a coalescence interaction that was dominant for the tensile fracture process. Some minor cleavage planes were also observed in tension. The greater the initial porosity, the lower the elastic modulus, work hardening rate, and strain to failure. For compression, because of the large amounts of porosity, microbuckling of ligaments between pores was a key inelastic deformation mechanism along with dislocation emission and movement. For torsion, cleavage planes were very prevalent on the fracture surfaces.