Nano-indentation near grain boundaries can exhibit very large 'pop-ins' or displacement bursts on the load-displacement curves. The phenomenon has been seen in bcc metals containing interstitials (carbon and/or nitrogen). Lowering the interstitial content, or increasing the dislocation density so that there are not enough interstitials to pin all dislocations removes the grain boundary pop-in effect.

The pop-in allows the resistance to slip transfer (K) to be assessed at individual grain boundaries. We have used this to show that K increases with misalignment of slip systems in the two grains at the boundary.