Lawrence Livermore National Laboratory

May 1, 2018

Two grain boundary phases.

Two grain boundary phases (b and c show closer views) in tungsten, coexisting in equilibrium at 2000 kelvin.

Grain boundaries govern many of the properties of engineering materials, but until recently, computational techniques were not sufficiently powerful to predict grain boundary properties at elevated temperature, where they can undergo transitions from one structure to another. The study of grain boundary phase transitions is in its infancy, largely dominated by experiments. Recent work by Tim Frolov (MSD) and coworkers at Lawrence Livermore; the University of Nevada, Las Vegas; and UC Davis produced a computational tool based on evolutionary algorithms that conducts grain boundary searches freed from many of the unphysical constraints imposed on earlier work. The question of grain boundary phase transitions in refractory metals is particularly interesting because those metals are used for high-temperature structural applications. Applying the search tool to tungsten and other refractory metals, the team, together with additional members from Pacific Northwest National Laboratory and UCLA, has identified new low-energy structures. These results were confirmed through first-principles calculations in some representative cases.

Through high-temperature molecular dynamics simulations, the team demonstrated that the grain boundary phases can coexist, further predicting that the flux of point defects due to irradiation will induce the transition from one phase to another. The grain boundary phases are not just of academic interest. Tungsten is being used as a plasma-facing material in magnetic fusion systems, where it must retain its mechanical integrity at high-temperatures and during exposure to radiation, and grain boundary phases can affect embrittlement.

This work was funded by the Office of Fusion Energy Sciences and the Laboratory Directed Research and Development Program (17-LW-012).

[T. Frolov, W. Setyawan, R. J. Kurtz, J. Marian, A. R. Oganov, R.E. Rudd, and Z. Zhu, Grain boundary phases in bcc metals, Nanoscale, available online on March 29, 2018, doi: 10.1039/C8NR00271A.]