Mechanically driven grain refinement in Mo by nanoindentation pop-ins: A case of transient instability

During load-controlled nanoindentation of pristine metals, the homogeneous dislocation nucleation manifests as a sudden displacement burst, or pop-in, associated with shear stresses approaching the theoretical strength, massive dislocation nucleation, and rapid yet uncontrolled indenter penetration. Here, we show that manually terminating nanoindentation immediately after the first pop-in enables direct access to a transient deformation and reveals unexpected microstructural transformations in molybdenum (Mo). Using single-crystal Mo with different surface orientations, we demonstrate pop-in-induced nucleation of new, highly misoriented grains at room temperature. This nucleation behavior is attributed to two mechanically driven processes: 1) activation of twinning followed by the evolution of twin boundaries into random high-angle grain boundaries (HAGBs) and 2) rearrangement of dislocations into HAGBs within regions of high local dislocation density during the abrupt deformation accompanying the pop-in. Displacement-controlled atomistic simulations reproduce both twin- and dislocation-mediated grain nucleation within an individual grain when a preexisting grain boundary blocks dislocation glide. However, in the absence of such a grain boundary, nucleated dislocations glide into the surrounding crystal and fail to form new grains. Continued indentation beyond pop-in mechanically erases newly formed grains, as confirmed in both experiments and simulations, explaining why this phenomenon was not detected in previous studies. Together, the observed nucleation and subsequent erasure of the grains establish the nanoindentation pop-in as a simple, widely accessible platform for qualitatively probing transient microstructural evolution in defect-scarce metals.