Shock-induced omega phase in tantalum

The deformation substructure developed within polycrystalline tantalum under a high shock pressure (45 GPa) with a 1.8 μs duration has been examined using transmission electron microscopy. A shock-induced β (bec) → ω (hexagonal) displacive transformation is observed for the first time within tantalum. Needle-or plate-like ω phase is found to form accompanied with the {112} type deformation twins within shock-recovered tantalum. The orientation relationships between the shock-induced ω and parent β phases are determined to be {1010}ω∥{211}β, [0001]ω∥<111>β and <1210>ω∥ [011]β. The lattice parameters of ω phase are aω = √2aβ = 0.468 nm and cω = (√3/2) aβ = 0.286 nm (c/a = 0.611). Both deformation twins and shock-induced ω phase are primarily formed along the {211}β slip planes with high resolved shear stresses, and have a common habit plane [i.e. the {211} plane] with the parent β matrix. It is suggested that shear deformation on the {211} planes is the major cause for the formation of shock-induced ω phase within tantalum.