Phase I Research (2014 – 2018)

Mission Statement: To develop a fundamental understanding of energy dissipation mechanisms through electrons, phonons, and magnons, and ultimately control defect evolution in a radiation environment; and to yield new design principles for radiation–tolerant structural alloys for applications in nuclear energy.

Means of improving the performance of structural materials have been intensively investigated for many decades. Solid solution strengthening, traditionally by alloying minor elements into pure metals, is one of the most widely used methods to achieve specific desirable properties, including radiation tolerance. While it has long been recognized that specific compositions in binary and some ternary alloys have enhanced radiation resistance, it remains unclear how the atomic structure and chemistry affect defect formation and damage evolution. This knowledge gap has been a roadblock to future-generation energy technologies.

The evolution of radiation–induced defect concentration in alloys can be described by three simplified competing processes (Fig. 1): defect production from collision cascades (1st term), subsequent vacancy–interstitial recombination within the diffusion volume (2nd term), and absorption of point defects by extended defects (sinks) such as dislocations, additional phases, interfaces, grain boundaries, and precipitates (3rd term). This evolution has long been considered to involve primarily localized atomic displacements. However, a predictive model of radiation damage will require fundamental understanding of roles of electrons, phonons, and magnons in energy dissipation.

Fig. 1 Simplified terms controlling radiation–induced vacancy concentration. K0 is the defect production rate; Kiv the recombination rate coefficient; and Ci, Cv, Dv and Cs the interstitial and vacancy concentration, the vacancy–sink coefficient, and the sink density; respectively. Similar equations hold for interstitials and other defects.

A new class of materials shows great promise of synthesis with an atomic-level control: Single–Phase Concentrated Solid Solution Alloys (SP–CSAs) containing two to five or more multiple principal elements. To realize the potential of these transformative alloys, we must understand the roles of all constituents in their structural stability and their effects on energy dissipation mechanisms at the level of electrons and atoms. While most of the current research effort addresses the 3rd term to increase sink density, we focus on the reduction of damage accumulation by acting on the early stages of radiation effects (the 1st and 2nd terms). In the EDDE Center, we will advance progressively in alloy complexity from elemental Ni to quinternary SP–CSAs (Fig. 2). Studying this novel series of systems with increasing chemical complexity offers a powerful means to explore the influences of energy deposition and transport, as well as defect formation energies and migration barriers, on defect production and recombination.

The overarching goal of the EDDE EFRC is to develop a fundamental understanding of how the energy of radiation is dissipated, and ultimately to control defect dynamics and microstructural evolution in structural alloys. Specifically, we seek to understand and quantify the mechanisms of energy dissipation through electronic, vibrational, and magnetic excitation, and how these mechanisms are influenced by alloy complexity.

Fig. 2 Ever increasing complexity in fcc single–phase concentrated solid solution alloys.

Two thrusts are designed to test our hypothesis that alloy complexity can be tailored to minimize defect production and enhance recombination (Fig. 1). We will develop an understanding in Thrust 1 of excitation, relaxation, atomic motion, and defect production in model alloys for which detailed theoretical predictions can be tested with experiments. The focus in Thrust 2 is on energy dissipation processes and defect evolution in alloys with increasing complexity. We take advantage of recent successes in synthesis of the nontraditional multi-element alloys (Fig. 2) with compositions at or near equiatomic ratios, including high entropy alloys, to go beyond traditional alloy development. We will also take advantage of recent theoretical and computational developments, many of them by members of our Center, to explore for the first time a comprehensive electronic and atomic description of an irradiated material very far from equilibrium. State-of-the-art synthesis, controlled irradiations, in situ ion beam analysis, and post-irradiation microstructure characterization techniques will all be utilized, including channeling image scans, advanced analytical transmission electron microscopy, atom probe tomography, and X-ray and neutron scattering methods. Experimental results, especially on model alloys, will be quantitatively compared to predictions from specialized density functional theory, classical molecular dynamics, and kinetic Monte Carlo techniques.  The combination of experiments and these multi-scale computational approaches will offer us the possibility to probe and understand the critical knowledge for controlling and engineering material properties and performance at the ultimate scale – that of atoms and electrons.

The EDDE Center engages a diverse mix of principal investigators and senior/key personnel who have complementary experience and skills. Each thrust will incorporate both theory and experiment. Most participants will contribute to both thrusts, thus maximizing synergies and coordination. The strong university involvement will enhance educational outreach. The success of the EDDE Center will yield new design principles for radiation-tolerant structural alloys for applications in nuclear energy and new defect engineering paradigms for much broader science and technologies.

Energy Dissipation to Defect Evolution (EDDE)

Oak Ridge National Laboratory – Yanwen Zhang (Director), Hongbin Bei, German D. Samolyuk, Eliot D. Specht, G. Malcolm Stocks, Roger E. Stoller

Los Alamos National Laboratory – Magdalena Caro
Lawrence Livermore National Laboratory – Alfredo Correa
University of Michigan – Lumin Wang
University of Tennessee – William J. Weber
University of Wisconsin-Madison – Ian M. Robertson