Kinetics of Incipient Stages of Phase Transitions

Raymond Smith | 17-ERD-014


The primary goal of this project is to employ emerging experimental capabilities to provide a new class of in situ and time-resolved lattice level measurements at the onset of a pressure-induced structural phase transformation. Such atomic lattice transformations are common and can profoundly affect the mechanical and transport properties of a material. There is a strong programmatic need to develop predictive models that incorporate the timescales and pathways associated with the incipient stages of high-pressure and rapidly evolving structural phase transformations. However, new phase-nucleation processes are not well understood at high pressures and at rapid, i.e., less than nanosecond, timescales due to the lack of experimental data under these extreme conditions. As a result, kinetic models used to describe high-pressure and high-strain-rate phase transformations are typically based on large extrapolation from existing low-strain-rate data.

We used newly available high brightness x-ray sources to address this crucial lack of experimental data. We obtained new data on the timescales of structural phase transformations under shock or ramp compression and under isentropic pressure release, which reveals crystal structure orientation dependencies, compression rate phase metastability, and the correlation between fracture and the quenchibility (ability to recover) of high-pressure phases. We developed analytical tools to interpret the texture of in situ x-ray diffraction patterns to provide new information on the multistep mechanistic pathways associated with phase transformations. We also developed new analytical and experimental techniques needed for future high-repetition-rate laser operation.

Impact on Mission

Ongoing efforts within the lab to develop models that describe phase transformation kinetics urgently require lattice level data at high pressures and at high strain rates. Our research provided new in situ measurements of the timescales and processes involved in the incipient stages of new phase growth. These new data (on timescales comparable to molecular-dynamic simulations) represent a strong benefit to a range of programmatic goals. The experimental techniques will directly impact the nature of experiments planned on dynamic compression facilities. Our development of high repetition rate targets will greatly benefit future high-energy-density (HED) experiments operated on XFEL and other HED facilities. LDRD funding has allowed the Laboratory to deliver cutting edge science on new and emerging x-ray facilities.

Publications, Presentations, Etc.

Gorman, M., et al. 2018. "Femtosecond Diffraction Studies of Solid and Liquid Phase Changes in Shock-Compressed Bismuth." Scientific Reports 8, 1–8 (2018). LLNL-JRNL-761084.

––– . 2019. "Recovery of Metastable Dense Bi Synthesized by Shock Compression." Appl. Phys. Letts. 114, 120601 (2019). LLNL-JRNL-769387.

Smith, R., et al. 2018. "Equation of State of Iron Under Core Conditions of Large Rocky Exoplanets." Nature Astronomy 2, 452 (2018). LLNL-JRNL-777824.

Wicks, J., et al. 2018. "Crystal Structure and Equation of State of Fe-Si Alloys at Super-Earth Core Conditions." Science Advances 4, no. 4 (2018): eaao5864. LLNL-JRNL-777740.