EFFECT OF STRAIN RATE ON PLASTIC FLOW AND FAILURE IN POLYCRYSTALLINE TUNGSTEN

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AbstractÐPolycrystalline tungsten (less than 100 p.p.m. impurities) was subjected to di€erent heat treat- ments to yield di€erent grain morphologies and tested at quasi-static (310 ÿ3) and dynamic (103±4103/ s) strain rates. Three mechanisms of deformation were identi®ed and evaluated: slip, twinning, and inter- granular cracking. Whereas plastic ¯ow 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 insensi- tive 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 ÿ3 to 103/s. Two failure modes are identi®ed: 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 (trans- granular cracks). Calculations based on existing wing-crack models enable the estimation of the grain- boundary cohesive energies.
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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