The Formation and Evolution of Defects in Nanocrystalline Fe During Indentation: The Role of Twins in Pop-Ins

Nanoindentation is a most common experimental tool used for obtaining information on the mechanical behavior of materials. This is done by qualitatively relating the occurrence of pop-ins (in the load–displacement plots) to microstructural changes such as dislocation formation, fracture of surface oxides, or slip transmission. The present study takes a first approach in directly verifying the micro-plasticity processes that give rise to such pop-ins by performing molecular dynamics indentation simulations in BCC Fe-nanocrystals with a Σ5 symmetric tilt boundary. The simulations allow to track the material behavior throughout the indentation process, and illustrate that each pop-in is related to twin formation, twin growth, de-twinning, dislocation nucleation and glide, or dislocation–grain boundary interactions. For the particular Σ5 boundary considered, it is found that the pop-ins are most closely associated with twin formation. Although pop-ins have been related to dislocation nucleation, a direct correlation between twinning and pop-ins has not been shown before. Adding C segregants to the Fe sample, reduced the formation of twins after initial yielding, and allowed for dislocation activity to become the more dominant deformation mechanism.