In-situ investigation of dislocation tangle–untangle processes in small-sized body-centered cubic Nb single crystals

Abstract Time-resolved dislocation tangle and untangle processes in small-sized body-centered cubic (BCC) niobium single crystal samples were investigated using in-situ nanoindentation transmission electron microscopy. The results reveal that dislocations can not only undergo tangle processes but can also untangle during plastic deformation, which have been predicted theoretically but not observed experimentally. The formation of dislocation tangles is attributed to the high mobility of dislocations with a high density of sessile defects in the deformed BCC crystal, whereas the untangle process arises from increased stress. The strength of dislocation tangles in our experiments was calculated based on widely used models, which showed good agreement with previous simulations. Dislocation tangle–untangle processes lead to work-hardening and high strength in small-sized BCC metals.