Dr Patrick Heighway

Abstract:

The prototypical α → ω phase transition in zirconium is an ideal test-bed for our understanding of polymorphism under extreme loading conditions. After half a century of study, a consensus had emerged that the transition is realized via one of two distinct displacive mechanisms, depending on the nature of the compression path. However, recent dynamic-compression experiments equipped with in situ diffraction diagnostics performed in the past few years have revealed new transition mechanisms, demonstrating that our understanding of the underlying atomistic dynamics and transition kinetics is in fact far from complete. We present classical molecular dynamics simulations of the α → ω phase transition in single-crystal zirconium shock-compressed along the [0001] axis using a machine-learning-class potential. The transition is predicted to proceed primarily via a modified version of the two-stage Usikov-Zilberstein mechanism, whereby the high-pressure ω-phase heterogeneously nucleates at boundaries between grains of an intermediate β-phase. We further observe the fomentation of atomistic disorder at the junctions between β grains, leading to the formation of highly defective interstitial material between the ω grains. We directly compare synthetic x-ray diffraction patterns generated from our simulations with those obtained using femtosecond diffraction in recent dynamic-compression experiments, and show that the simulations produce the same unique, anisotropic diffuse scattering signal unlike any previously seen from an elemental metal. Our simulations suggest that the diffuse signal arises from a combination of thermal diffuse scattering, nanoparticle-like scattering from residual kinetically stabilized α and β grains, and scattering from interstitial defective structures.

Abstract:

The response of solid matter to shock compression is complexified considerably by its strength, or its ability to withstand shear stress. Strength is challenging to measure experimentally under shock conditions and even harder to model, due to its being an extremely complicated function of the loading conditions. Our understanding of material strength and the way it manifests under dynamic loading thus remains, to a great extent, incomplete. This work presents studies of two phenomena arising from strength under the conditions of shock compression and release by means of multimillion-atom molecular dynamics simulations and femtosecond x-ray diffraction. The role of shock-induced grain interactions is first explored via simulations of elementary polycrystals. Such interactions are found to control the plasticity mechanisms activated under shock compression and the limiting shear stress state to which the polycrystal settles in the wake of the shock. A combined experimental-computational study of plastic-work heating under the conditions of shock release is then presented. An algorithm for extracting the temperature of released samples from their diffraction image is derived and verified on synthetic data. When applied to experimental data, the algorithm shows that the temperatures of shock-released tantalum foils vastly exceed those expected from a conventional isentropic release. The underlying microphysical processes responsible for the heating are then interrogated via large-scale simulations of crystals under shock and release. A heat equation is used to identify plastic-work heating owed to the sample’s exceptional strength during its rapid release as the culprit, thus challenging the conventional assumption that shock release is a universally isentropic process.