Image courtesy of Nano-Letters
The left figure shows computed positions for atoms at grain boundaries during deformation (blue arrows indicating the direction of force), with the black arrows signifying point-defect migration. The right graph shows that as the grain size is reduced, the relative number of “noncrystalline” atoms (those at the grain boundaries) increases and that leads to an increase in creep compliance. When the grain size is sufficiently small the behavior begins to match that of fully amorphous material.
Computer simulations were used to discover a new mechanism of how nanocrystalline materials deform when placed under load in an irradiation environment, such as typically occurs in a nuclear reactor. The disordered nature of atoms in the boundaries separating crystalline grains was shown to be a key contributor to this behavior.
The mechanism suggests new routes to self-healing of radiation damaged materials and allows a better understanding of the potential influence of grain size on the damage processes, an important advance for nuclear energy.
Materials used in nuclear energy reactors are bombarded by energetic particles that are inherent in generation of electricity with nuclear power. Additionally, these components are often under a mechanical load or “stress”. Irradiation by these energetic particles displaces atoms in the crystalline structure of the material and, in conjunction with the stress, causes the materials to slowly deform (“creep”), resulting in changes to the material’s dimensions and strength. A new mechanism for irradiation-enhanced creep has been identified in nanocrystalline materials. The mechanism is based on nanoscale flow of the atoms in the boundaries between the nanocrystals making up the material. The deformation is due to local atomic relaxations within the grain boundaries as they absorb point defects e.g., interstitials and vacancies, produced in the grain interior during irradiation. The process was modeled by inserting point defects into the grain boundaries and following the material’s subsequent response using molecular dynamics simulation as stress was applied to the system. The calculated strain rates of the material are found to be in good agreement with those experimentally measured in dilute nanocrystalline copper-tungsten alloys. Extrapolation of the calculations to vanishingly small grain sizes yields creep rates that agree very well with those found in amorphous materials, suggesting that under irradiation grain boundaries in nanocrystalline materials behave very much like an amorphous phase.
Robert S. Averback
Department of Materials Sciences and Engineering, Seitz Material Research Laboratory, University of Illinois at Urbana-Champaign
Urbana, Illinois, 610801
Department of Energy, Office of Science, Basic Energy Sciences program.
“Irradiation Induced Grain Boundary Flow – A new Creep Mechanism at the Nanoscale”, Y. Ashkenazy and R.S. Averback, Nano Letters, 12, 4084(2012).
Irradiation-induced creep in nanostructured Cu alloys, K. Tai, R. S. Averback, P. Bellon, Y. Ashkenazy, Scripta Materialia, 65, 163(2011).