Direct observation of nanoindentation-induced punching-out of prismatic dislocation loops in tungsten

While bulk bcc plasticity is traditionally governed by screw dislocation glide, prismatic dislocation loops are proposed as an essential pathway for strain accommodation under severe, local constraints. This experimental study on pure W, as a model bcc system, demonstrates that prismatic dislocation half-loops can develop and propagate in a triaxial stress field as imposed by nanoindentation. Using electron channeling contrast imaging, the Burgers vector of emitted dislocation half-loops is determined to be 1/2 〈 111 〉 and the punching-out process is studied during progressive stages of imposed strain. Long arrays of dislocation half-loops are emitted during the initial displacement burst (pop-in) and continue to propagate during subsequent loading. Although energetic considerations indicate that loop formation alone cannot account for the total plastic deformation energy—implying a concurrent contribution from glide mechanisms—these findings reveal that plastic flow under severe hydrostatic stress gradients can be accommodated via non-Schmid loop punching. Crucially, the unique slip system multiplicity of the bcc lattice yields highly mobile, circular loop geometries that stand in stark contrast to the faceted loops typical of fcc metals.