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

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 x 10(-3)) and dynamic (10(3)-4 x 10(3)/ s) strain rates . Three mechanisms of deformation were identified and evaluated: slip, twin ning, and inter granular cracking. Whereas plastic flow by slip has conside rable strain-rate sensitivity in tungsten (which is found to be well repres ented by the Mechanical Threshold Stress constitutive equation) the cohesiv e strength of the grain boundaries was found to decrease with heat treatmen t 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 0.0 5 and 0.10. Slip and grain-boundary decohesion being competing deformation mechanisms, the material undergoes a ductile-to-brittle transition as the s train rate is increased from 10(-3) to 10(3)/s. Two failure modes are ident ified: debonding initiated by shear along a grain-boundary facet (similar t o the wing-crack mechanism) and debonding initiated at voids. The interacti ons between microcracks and twins are characterized, and there is both evid ence of fracture initiation at twins (intergranular cracks), and twin initi ation at cracks (transgranular cracks). Calculations based on existing wing -crack models enable the estimation of the grain-boundary cohesive energies . (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All ri ghts reserved.