We are investigating a new class of measurements for the onset of a material's phase transformation under dynamic loading conditions by exploiting the emerging experimental capabilities of advanced high-energy x-ray facilities. The data we will obtain are needed to support the development of predictive models of high-pressure and high-strain-rate material response.
Understanding the nature and time-dependence of structural phase transformations under dynamic loading conditions is key to condensed matter physics. New-phase nucleation processes are poorly understood at high pressures and at rapid (nanoseconds or less) timescales because of the lack of experimental data under these extreme conditions. Under compression, as the interatomic spacing decreases, overlapping electronic band structure and increased levels of pressure can fundamentally alter the transport, chemical, and mechanical characteristics of a solid. Depending on the thermodynamic compression pathway, many structural phase transformations can occur. Classical nucleation theory describes the dynamics of these transitions through a two-stage process: initial slow transformation (nucleation) followed by a rapid-growth regime in which the transforming interface can flow with velocities approaching the local speed of sound. Understanding of two-stage growth into the new phase is supported by experimental observations in colloidal systems where the transformation occurred over several hours. However, under dynamic compression (nanoseconds to microseconds) a nucleation period dependent on stress and strain rate prior to new-phase rapid growth has been inferred from interface profile analysis in many materials. These data suggest a nucleation period that is a sensitive function of pressure, strain rate, and crystal orientation. To develop a predictive understanding of phase-transformation processes, we need to address many fundamental questions. It is only with the recent emergence of a new generation of x-ray facilities, coupled to the availability of compact high-power laser drivers, that we can now make the necessary time-resolved in situ lattice-level measurements to answer these questions. We plan to provide a new class of these measurements at the onset of a phase transformation by exploiting the emerging experimental capabilities of advanced high-energy x-ray facilities. We will develop experimental techniques to enable femtosecond, lattice-level measurements for understanding the kinetics of these phase transformations. The data we will obtain are needed to support the development of predictive models of high-pressure and high-strain-rate material response. In addition, our research will directly inform experiments on the proposed MaRIE facility (Matter–Radiation Interactions in Extremes) planned for construction at Los Alamos National Laboratory, which is designed for time-dependent control of dynamic properties of materials.
We expect to obtain an experimentally based understanding to underpin the development of high-strain-rate and high-pressure-phase kinetics models. We intend to exploit emerging experimental capabilities at the x-ray free electron laser at the Linac Coherent Light Source at the SLAC National Accelerator Laboratory in Menlo Park, California; the Dynamic Compression Sector of the Advanced Photon Source at Argonne National Laboratory in Chicago, Illinois; and the OMEGA Laser Facility at the Laboratory for Laser Energetics in Rochester, New York. These facilities combine temporally shaped high-power laser drivers with ultra-bright femtosecond (or nanosecond) x-ray sources to diagnose the in situ transformation of material properties at the onset of a structural pressure-induced phase transformation. In our experiments we will measure, during compression across a phase boundary, the evolution of crystal structure, grain size, and texture as a function of phase boundary over-pressurization. Specifically, we will (1) perform direct imaging of new-phase grain growth, (2) determine the kinetics and pathways of solid–solid phase transformations, (3) determine the kinetics of melting and evolution of a liquid structure, and (4) perform measurements of temperature across a phase boundary. In addition, we will determine the forward and backward transformation timescale dependence on compression rate as well as the evolution of temperature and structure and the onset of melting. We will also develop high-repetition-rate laser target techniques for high-energy-density experiments that will allow experimental data to be integrated over long periods of time (many thousands of shots), which will dramatically increase the signal-to-noise over standard single-shot experiments. These targets will be of particular importance to low-signal phenomena such as liquid diffraction, extended x-ray absorption fine-structure spectroscopy, and scattering off low-atomic-number or low-symmetry materials. The development of rapid target replenishment will ultimately allow us to generate movies of transitions using variable time delays over many shots. Such targets are cost effective and will maximize data yields, significant for future high-repetition-rate, high-energy-density experiments.
Our research supports the Laboratory's high-energy-density science core competency by providing new in situ measurements of the timescales and processes involved in the incipient stages of new-phase growth for materials under dynamic loading conditions that will enable development of new, replenishable laser target concepts. This work is relevant to DOE goals to transform our understanding of nature and strengthen the connection between advances in fundamental science and technology innovation.
In FY17 we (1) performed experiments on Livermore's JANUS laser facility to develop techniques for phase-transformation kinetics experiments and to demonstrate the use of a cassette device for high-repetition-rate targets; (2) performed experiments on the Matter in Extreme Conditions end station at the Linac Coherent Light Source to study phase-transformation kinetics experiments on zirconium; (3) developed similar experiments on the Dynamic Compression Sector; (4) conducted campaigns on the OMEGA laser facility to study high-pressure phase transformations; and (5) began a collaboration with computer analysts aimed at handling the large amounts of data generated through our high-repetition-rate design.