Synergistic doping of the grain interior and grain boundary alters deformation mechanisms and enables extreme strength in nanocrystalline Ni-Cr-Y alloys

Abstract

Solid solution addition and grain boundary segregation have been independently shown to enhance the strength of nanocrystalline alloys. In the present study, the interplay between these two effects is investigated in nanocrystalline Ni-Cr-Y sputtered films through systematic variation of alloying element contents with grain size kept constant. Cr is introduced into a solid solution and serves to strengthen the lattice, while Y segregates to the grain boundaries to stabilize these features. Nanoindentation is used to probe hardness, with unexpected trends and very high values observed. Cr additions led to nanocrystalline solid solution strengthening, yet saturation was observed at higher concentrations due to the emergence of grain boundary dominated processes, as evidenced by pile-up morphologies containing slip steps and grain rotation. Y enrichment was observed at the examined grain boundaries, enhancing boundary-mediated strengthening by pinning the dislocations and suppressing dislocation emission, grain boundary sliding, and grain rotation processes. With increasing Y concentration, the nanocrystalline solid solution strengthening effect induced by Cr addition becomes weaker. This phenomenon can be attributed to a reduced dislocation bowing distance caused by dopant pinning. Most notably, the strongest ternary Ni-Cr-Y alloy exhibited a hardness of 11.0 GPa, among the highest hardness values reported for single-phase nanocrystalline Ni alloys with grain sizes near 20 nm. These findings highlight how tuning grain and grain boundary chemistry offers a viable strategy to control dislocation mechanics and improve the strength of nanocrystalline metals.