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

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) over bar0)(h)parallel to (211)(b),[0001](h)parallel to [11 1](b) and [1 (2) over bar 10](h)parallel to [0 (1) over bar1](b). The lattice parameters of omega phase are a(h)approximate to root 2a(b) = 0.468 nm and c(h)approximate to(root3/2)a(b) = 0.286 nm (c(h)/a(h) = 0.611). Since both deformation twinning and omega trans formation occur preferably in the (211)(b) planes with high resolved shear stresses, it is suggested that both can be considered as alternative paths for sheer 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) over bar(1) over bar] homogeneous shear in consecutive (211) planes, omega transformation can be attributed to the 1/12 [1 (1) over bar(1) over bar], 1/3[1 (1) over bar(1) over bar] and 1/12[1 (1) over bar(1) over bar] inhomogeneous shear in consecutive (211) planes. Dislocation mechanisms for shock-induced twinning and omega transformation are proposed and critically discussed. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.