Dynamic shear deformation and failure behaviour of pure polycrystalline tungsten

This study investigates the dynamic shear deformation and fracture behaviour of pure polycrystalline tungsten. Both wrought and recrystallized specimens are subjected to simple shear at room temperature and at strain rates ranging from 2 x 10(-3) to 4 x 10(3) s(-1) by means of a torsional split Hopkinson bar. The effects of strain rate on flow response, strain rate sensitivities, thermal activation volume and deformation mechanisms are investigated. Fracture phenomena are analyzed by scanning electron microscopy to evaluate damage initiation and propagation. Compared with quasi-static rare tests performed at 2 x 10(-3) and 3 x 10(-3) s(-1), we find that strain rate obviously influences shear strength and ductility. Recrystallized tungsten exhibits better ductility and lower shear strength. Fracture feature observations show that, at low strain rate, recrystallized tungsten's equiaxial grain structure fractures by simple intergranular cleavage from grain boundary sources, whereas at high strain rate there is a mixture of intergranular and transgranular cleavage. Wrought tungsten's elongated fibrous structure, at low strain rate, fractures by an alternating mixture of intergranular and transgranular cleavage, but at high strain rate fractures by transgranular cleavage propagating across the whole specimen. We briefly discuss possible dynamic fracture mechanisms. (C) 1998 Elsevier Science S.A. All rights reserved.