01
One ultrafine-grained microstructure contains grain boundaries in two distinct states, not one.
02
Deformation-modified boundaries transport atoms markedly faster than relaxed boundaries.
03
The difference only appears when shallow and deep measurements are compared; either alone reports an average.
04
Annealing shifts the population toward the relaxed state, which is what changes the bulk diffusion response.
This study showed that ultrafine-grained Ni contains a hierarchy of grain-boundary states after severe plastic deformation. Using SIMS and radiotracer diffusion, the work separated relaxed 'slow' boundaries from deformation-modified 'fast' boundaries, revealing how processing-induced microstructure controls atomic transport and thermal stability.
The cross-scale argument
SIMS gives a local, depth-resolved picture; radiotracer diffusion gives an integral one over long distances. Running both on the same ECAP-processed microstructure is what makes the separation possible — the two techniques disagree in exactly the way you would predict if two distinct boundary populations coexist, and that disagreement is the measurement.
Why it matters for processing
If a nanostructured metal is going to be used at temperature, the fast boundaries set the recovery and grain-growth kinetics, not the average. Designing thermal stability therefore means designing the boundary state, not just the grain size.
