Theory and Simulation of 1D to 3D Plastic Relaxation in Tantalum

In plane shockwaves the uniaxial strain rate can greatly exceed the rate at which dislocation flow can relax the concomitant shear stress. The result is an overdriven plastic state in which the compression is 1D uniaxial initially and only after a period of time does the lattice relax to a more 3D compressed state due to plastic flow. Here we use an analytic calculation based on a generalization of the Gilman model of flow involving dislocation evolution to predict the phases of plastic relaxation and to derive an analytic estimate of the relaxation time, including a decomposition into incubation and flow times, suitable for comparison with in-situ x-ray diffraction. We use molecular dynamics (MD) to study the threshold for homogeneous nucleation both in shock compression of single crystal Ta 〈100〉. We find that shock heating on the Hugoniot substantially lowers the threshold pressure for homogeneous nucleation.