Dislocation Mechanics Based Viscoplasticity Description of FCC, BCC and HCP Metal Deformation and Fracturing Behaviors

The separate dislocation mechanisms of thermally-activated overcoming of “forest” intersections as compared with the intrinsic Peierls-Nabarro stress for single dislocation movement provide a starting point for understanding the different temperature and strain rate dependencies of the stress-strain behaviors of FCC and BCC metals, respectively. Strong solute-dislocation interactions, especially for BCC metals but also for certain HCP and FCC metals, add the complication of yield point and strain aging behaviors that are increasingly important at finer polycrystal grain sizes. The grain size effect is accounted for through the Hall-Petch (H-P) microstructural stress intensity measured by the slope of stress versus the inverse square root of the polycrystal grain diameter. The H-P dependence, that is greater according to the segregation of solute at grain boundaries and the unavailability of deformation systems, applies for the total polycrystal stress-strain behavior, including the occurrence of deformation twinning or cleavage fracture in BCC and HCP metals, particularly, at low temperatures and high strain rates. At the highest strain rates, there are additional considerations to contend with of the appearance of a stress state dependent ductile-to-brittle transition, surprising evidence of added dislocation generation mechanisms occurring to enhance ductility, and, adiabatic heating effects relating to the onset of shear banding behavior.