Exploiting the European XFEL for a Novel Generation of High Energy Density and Materials Science

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

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Description We have undertaken simulations that show that a long-held assumption in shock physics is wrong. It is normally assumed that when a shock reaches a free surface, then as the stress must be zero there, a rarefaction wave travels back into the target, that releases it isentropically, causing the material to significantly cool upon expansion. However, we have shown with multi-million atom molecular dynamics (MD) simulations that in a strong metal, the release strain rates within the last couple of microns of the surface are so high that the material sustains a very high tensile shear stress, and plastic work occurs at the surface on release, keeping the material at a temperature very close to that which it experience whilst shocked. The simulations are corroborated by experiments, where we use femtosecond X-ray diffraction to infer the temperature from the thermal expansion of the free surface. This work has been published in Physical Review Letters.

We have also undertaken molecular dynamics simulations to show that the rotation of the lattice owing plastic flow in a uniaxial shock or ramp compression experiment does not fulfil the Taylor of Schmid models normally used in materials science. We have developed a better model, and via MD proven it in the idealised cases of single or double slip. This work has been published in Journal of Applied Physics. Furthermore, as experiments have been delayed owing to COVID, we have spent considerable time looking at past data, and using MD simulations to look at precisely which dislocation slip systems are active in several of the previous experiments we have performed, and undertaken an analysis of how these particular plasticity mechanisms evince themselves in diffraction data. This work has been highly successfully, and has led to submissions to both Physical Review Letters, and a publication in Phys. Rev. Materials.

To date we have only been able to have one experiment on the European XFEL, owing to the delays caused by COVID. We used this to demonstrate the feasibility of inelastic scattering from phonons, with the ultimate aim of using the Stokes and anti-Stokes component ratios as an in situ temperature diagnostic of a shocked system. The 'non-shocked' target experiments were successful, and this work has been published in the Review of Scientific Instruments.
Exploitation Route Although the Oxford portion of this funding did not include PDRA support, a PDRA supported via the Department and AWE has been key to the success of the work to date. Based on his findings, we have submitted a further proposal to EPSRC for the continuation of the study of plasticity and material strength at ultra high strain rates. We are currently awaiting the outcome of that application.
Sectors Aerospace, Defence and Marine