Theory 006/Physics & Matter /Radiation materials
Damage Clusters at Boundaries
Fine grains are supposed to soak up defects. They may also concentrate the heat that creates them.
Radiation damage accumulates at grain boundaries because thermal spikes are confined there, so nanostructured materials with more boundary area can be more susceptible rather than more resistant.
01The evidence
Nanocrystalline metals show both improved and degraded radiation tolerance depending on grain size, which is inconsistent with a monotonic boundary-sink model.
02The tension
Boundaries are modelled as sinks that absorb defects, predicting that more boundary is always better. The non-monotonic data are attributed to processing artefacts.
03The mechanism
A collision cascade produces a thermal spike. Near a boundary, the spike is confined by the change in thermal conductivity, which raises the local temperature and the fraction of defects that survive as clusters.
That makes boundary density a liability above a threshold, so there should be an optimum grain size balancing sink area against spike confinement.
04What this is not
Sink models predict monotonic improvement. This predicts an optimum, which explains the degraded cases.
05Predictions
- P1Damage should peak at an intermediate grain size.
- P2Thermal spike temperature should rise near boundaries.
- P3Conductivity contrast should predict the optimum.
06Refutation
Monotonic improvement with boundary density would restore the sink model.
07What it changes
It would change how radiation-tolerant alloys are designed.