Past Research

  • Grain boundary sink strength

Grain boundaries (GBs) can act as either sinks or sources of the point defects that are produced in large numbers under irradiation damage. In polycrystalline materials, as the grain size decreases, more of the point defects resulting from irradiation damage annihilate at GBs. It is unknown, however, whether the GB sink efficiency will saturate after prolonged defect annihilation, particularly when the grain size is of nanoscale dimensions. Using a combination of molecular dynamics (MD) simulation and rate theory, the authors show that high-energy GBs in body-centered-cubic (BCC) Mo do not saturate as sinks of point defects. The MD simulations serve to provide direct measurement of defect evolution, and the rate theory serves both to test whether grain boundary sink strength is constant during prolonged defect annihilation, and to extend the MD results to realistic defect production rates. For more details please see: Y. Zhang, X. Y. Liu, P. C. Millett, M. Tonks, D. A. Andersson and B. Biner, Crack tip plasticity in single crystal UO2: Atomistic simulations, J. Nucl. Mater. 430, 96 (2012).

(Left) Molecular dynamics simulation cell for a sigma-29 GB and (right) evolution of vacancy and interstitial concentrations
  • Transformation of C15 phase interstitial particles to <100> prismatic loops

Radiation by high-energy particles produces vacancies and self-interstitial atoms (SIAs) in materials, which, together with their agglomerates, including voids and prismatic loops, lead to swelling and hardening. In body-centered cubic (bcc) Fe, SIA clusters at finite sizes usually form loops with either a <1 0 0> or a <1 1 1>/2 (referred to as <1 1 1>) Burgers vector. However, it is now clear how the <1 0 0> actually form during irradiation. Using molecular dynamics simulations, it is shown <100> may form by transformation of C15 Laves phase interstitial clusters. Molecular dynamics simulations and elasticity analysis show that, within a certain size range, C15 clusters are more stable than loops, but the relative stabilities are reversed beyond this range. C15 clusters can consistently grow by absorbing interstitials at small sizes and transforming into loops later. Both <100> and <111> loops may result from the transformation, revealing a new formation mechanism for <100> loops. For more details please see: Y Zhang, X. M. Bai, M. R. Tonks and S. B. Biner, Formation of prismatic loops from C15 Laves phase interstitial clusters in body-centered cubic iron, Scripta Mater. 98, 5 (2015).

Transformation process of a C15 phase interstitial cluster to dislocation loops. <1 0 0> dislocation segments are shown in red, and <1 1 1> segment in blue.
  • Hydrogen diffusion in hcp Zr

An accelerated kinetic Monte Carlo (KMC) method is developed to compute the diffusivity of hydrogen in hcp metals and alloys, considering both thermally activated hopping and quantum tunneling. The acceleration is achieved by replacing regular KMC jumps in trapping energy basins formed by neighboring tetrahedral interstitial sites, with analytical solutions for basin exiting time and probability. Parameterized by density functional theory (DFT) calculations, the accelerated KMC method is shown to be capable of efficiently calculating hydrogen diffusivity in α-Zr and Zircaloy, without altering the kinetics of long-range diffusion. Above room temperature, hydrogen diffusion in α-Zr and Zircaloy is dominated by thermal hopping, with negligible contribution from quantum tunneling. The diffusivity predicted by this DFT + KMC approach agrees well with that from previous independent experiments and theories, without using any data fitting. The diffusivity along <c> is found to be slightly higher than that along <a>, with the anisotropy saturated at about 1.20 at high temperatures, resolving contradictory results in previous experiments. Demonstrated using hydrogen diffusion in α-Zr, the same method can be extended for on-lattice diffusion in hcp metals, or systems with similar trapping basins. For more details please see: Y. Zhang, C. Jiang, and X. Bai, Anisotropic hydrogen diffusion in α-Zr and Zircaloy predicted by accelerated kinetic Monte Carlo simulations, Scientific Reports 7, 41033 (2017).

(Left) Schematic of H diffusion in hcp Zr. Hydrogen in the octahedral site is shown in red, and tetrahedral site in green. Zr atoms are shown in golden. (Middle) Schematic of a small barrier basin. (Right) Hydrogen diffusivity computed by KMC in comparison to experiments.



  • Deformation in nanocrystalline solids

This work presents a mechanism of deformation-twin-induced grain boundary failure, and demonstrates the mechanism using molecular dynamics simulations. Deformation twinning is observed as the dominant mechanism during tensile deformation of <1 1 0> columnar nanocrystalline body-centered cubic Mo. As a twin approaches a grain boundary, local stress concentration develops due to the incompatible plastic deformations in the two neighboring grains. The magnitude of the stress concentration increases as the twin widens, leading to grain boundary cracking by nucleation and coalescence of microcracks/voids. For more details please see: Y. Zhang, P. C. Millett, M. Tonks and B. Biner, Deformation-twin-induced grain boundary failure, Scripta Mater. 66, 117 (2012).

(a) Schematic of deformation twin induced strain on a grain boundary. (b)-(d) show the approaching and arresting of deformation twin by a grain boundary and the consequent fracture initiation.