Effect of strain rate on plastic flow and failure in polycrystalline tungsten

Abstract

Polycrystalline tungsten (less than 100 p.p.m. impurities) was subjected to different heat treatments to yield different grain morphologies and tested at quasi-static (3×10 −3) and dynamic (10 3–4×10 3/s) strain rates. Three mechanisms of deformation were identified and evaluated: slip, twinning, and intergranular cracking. Whereas plastic flow by slip has considerable strain-rate sensitivity in tungsten (which is found to be well represented by the Mechanical Threshold Stress constitutive equation) the cohesive strength of the grain boundaries was found to decrease with heat treatment temperature, but was insensitive to strain-rate changes. Low-strain-rate deformation yielded limited damage at strains as high as 0.25, whereas high-strain-rate deformation led to catastrophic failure at strains between 0.05 and 0.10. Slip and grain-boundary decohesion being competing deformation mechanisms, the material undergoes a ductile-to-brittle transition as the strain rate is increased from 10 −3 to 10 3/s. Two failure modes are identified: debonding initiated by shear along a grain-boundary facet (similar to the wing-crack mechanism) and debonding initiated at voids. The interactions between microcracks and twins are characterized, and there is both evidence of fracture initiation at twins (intergranular cracks), and twin initiation at cracks (transgranular cracks). Calculations based on existing wing-crack models enable the estimation of the grain-boundary cohesive energies.