Shock-induced deformation twinning and omega transformation in tantalum and tantalum–tungsten alloys

Abstract

Effects of high strain-rate and high plastic-strain deformation on the development of deformation substructures in tantalum and tantalum–tungsten alloys (Ta–2.5 wt.% W and Ta–10 wt.% W) shocked at 15 and 45 GPa have been investigated. In addition to dislocation cells/walls, and {112}<111>-type deformation twinning, a shock-induced omega phase (hexagonal) is also found within polycrystalline tantalum shocked at 45 GPa. The orientation relationships between the omega phase and parent (bcc) matrix are {10 1 ̄ 0} h ‖{211} b ,[0001] h ‖<111> b and <1 2 ̄ 10> h ‖<0 1 ̄ 1> b . The lattice parameters of omega phase are a h ≈ 2 a b =0.468 nm and c h ≈( 3 /2)a b =0.286 nm (c h /a h =0.611). Since both deformation twinning and omega transformation occur preferably in the {211} b planes with high resolved shear stresses, it is suggested that both can be considered as alternative paths for shear transformations in shock-deformed tantalum. A greater volume fraction of twin and omega phase formed in Ta–W than in pure Ta reveals that shock-induced shear transformations can be promoted by solid solution alloying. While deformation twinning is resulted from 1/6<1 1 ̄ 1 ̄ > homogeneous shear in consecutive {211} planes, omega transformation can be attributed to the 1/12<1 1 ̄ 1 ̄ >, 1/3<1 1 ̄ 1 ̄ > and 1/12<1 1 ̄ 1 ̄ > inhomogeneous shear in consecutive {211} planes. Dislocation mechanisms for shock-induced twinning and omega transformation are proposed and critically discussed.