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 {101̄0}h∥{211}b, [0001]h∥〈111〉b and 〈12̄10〉h∥〈01̄1〉b. The lattice parameters of omega phase are ah≈√2ab = 0.468 nm and ch≈(3/2)ab = 0.286 nm (ch/ah = 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〈11̄1̄〉 homogeneous shear in consecutive {211} planes, omega transformation can be attributed to the 1/12〈11̄1̄〉, 1/3〈11̄1̄〉 and 1/12〈1̄1̄〉 inhomogeneous shear in consecutive {211} planes. Dislocation mechanisms for shock-induced twinning and omega transformation are proposed and critically discussed.