Laser compression of monocrystalline tantalum

Monocrystalline tantalum with orientations [1 0 0] and [1 1 1] was subjected to laser-driven compression at energies of 350–684 J, generating shock amplitudes varying from 10 to 110 GPa. A stagnating reservoir driven by a laser beam with a spot radius of ∼800 μm created a crater of significant depth (∼80 to ∼200 μm) on the drive side of the Ta sample. The defects generated by the laser pulse were characterized by transmission and scanning electron microscopy, and are composed of dislocations at low pressures, and mechanical twins and a displacive phase transformation at higher pressures. The defect substructure is a function of distance from the energy deposition surface and correlates directly with the pressure. Directly under the bottom of the crater is an isentropic layer, approximately 40 μm thick, which shows few deformation markings. Lattice rotation was observed immediately beneath this layer. Further below this regime, a high density of twins and dislocations was observed. As the shock amplitude decayed to below ∼40 GPa, the incidence of twinning decreased dramatically, suggesting a critical threshold pressure. The twinning planes were primarily {1 1 2}, although some {1 2 3} twins were also observed. Body-centered cubic to hexagonal close-packed pressure induced-transformation was observed at high pressures (∼68 GPa).The experimentally measured dislocation densities and threshold stress for twinning are compared with predictions using analyses based on the constitutive response, and the similarities and differences are discussed in terms of the mechanisms of defect generation.