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The extreme pressures achievable with dynamic compression holds great promise for studying planetary interiors. Phase stability of Fe-Si alloys, which are complex to address, is particularly relevant to understanding telluric planetary cores due to the widely varying properties produced by small changes in Si concentration. Here we report the study of phase stability of pure iron and Fe-Si alloys by x-ray diffraction measurements carried out on shocked samples using an x-ray free electron laser (XFEL). Our setup combined with the brilliance of the XFEL allows us to observe the rapid onset of high-pressure solid-solid phase transformation in Fe and Fe-Si8.5wt%; we observe no such evidence in Fe-Si16wt% up to 110 GPa on the nanosecond timescale. Density Functional Theory calculations provide the conceptual framework to rationalize these observations. Taken together our experiments and calculations support recent dynamic compression measurements and shed light on conflicting static compression results. Our work highlights the need to properly consider the differing intrinsic timescales of the static and dynamic experiments when comparing results, and the complementarity of the techniques in assessing phase diagram and transition mechanisms.

The Virtual Materials Marketplace (VIMMP) project, which develops an open platform for providing and accessing services related to materials modelling, is presented with a focus on its ontology development and data technology aspects. Within VIMMP, a system of marketplace-level ontologies is developed to characterize services, models, and interactions between users; the European Materials and Modelling Ontology (EMMO) is employed as a top-level ontology. The ontologies are used to annotate data that are stored in the ZONTAL Space component of VIMMP and to support the ingest and retrieval of data and metadata at the VIMMP marketplace frontend., The Virtual Materials Marketplace (VIMMP) project is funded from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 760907

Despite a number of previous nonequilibrium molecular dynamics (MD) studies into plasticity in face-centered-cubic metals, and phase transitions in body-centered-cubic (bcc) metals, the plastic response to rapid compression of bcc metals remains largely unexplored. Key questions remain as to the relative importance of dislocation motion and twinning in shear stress release and consequent strength. We present here large scale MD simulations of shock compressed bcc metal, using an extended Finnis-Sinclair potential for tantalum, and demonstrate the presence of significant deformation twinning for pressures above the Hugoniot elastic limit for shock waves propagating along the [001] direction. The twinned variants are separately identified by a per atom order parameter, allowing the strain and stress states of the rotated material to be studied. The atomic motion during twinning, and thus its mechanism, for this potential, is identified by use of a three-dimensional pair-correlation function. Fil: Higginbotham, A.. University Of Oxford. Department Of Physics; Reino Unido; Fil: Suggit, M. J.. University Of Oxford. Department Of Physic; Reino Unido Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mendoza; Argentina Fil: Erhart, P.. Lawrence Livermore National Laboratory; Estados Unidos Fil: Hawreliak, J. A.. Lawrence Livermore National Laboratory; Estados Unidos Fil: Mogni, G.. University Of Oxford. Department Of Physics; Reino Unido Fil: Park, N.. Atomic Weapons Establishmen; Reino Unido Fil: Wark, J. S.. University Of Oxford. Department Of Physics; Reino Unido

Under uniaxial high-stress shock compression it is believed that crystalline materials undergo complex, rapid, micro-structural changes to relieve the large applied shear stresses. Diagnosing the underlying mechanisms involved remains a significant challenge in the field of shock physics, and is critical for furthering our understanding of the fundamental lattice-level physics, and for the validation of multi-scale models of shock compression. Here we employ white-light X-ray Laue diffraction on a nanosecond timescale to make the first in situ observations of the stress relaxation mechanism in a laser-shocked crystal. The measurements were made on single-crystal copper, shocked along the [001] axis to peak stresses of order 50 GPa. The results demonstrate the presence of stress-dependent lattice rotations along specific crystallographic directions. The orientation of the rotations suggests that there is double slip on conjugate systems. In this model, the rotation magnitudes are consistent with defect densities of order 10(12) cm(-2).

We present molecular dynamics simulations using a Tersoff-like potential of single crystals of silicon shock compressed along the $\ensuremath{\langle}001\ensuremath{\rangle}$ direction. We find an elastic response up to a critical stress, above which the shear stress is relieved by an inelastic response associated with a partial transformation to a new high-pressure phase, where both the new phase (Imma) and the original cubic diamond phase are under close to hydrostatic conditions. We study how the fraction of the two phases is related to both their geometry and their enthalpy, and discuss the relevance of the results to previous experimental measurements of the response of silicon to shock compression. We note that the simulations are consistent with shear stress relief provided directly by the shock-induced phase transition itself, without an intermediate state of plastic deformation of the cubic diamond phase, but that the onset of inelastic behavior within the simulations still occurs at considerably higher stresses than found in experiments.