Nanocrystalline (NC) Ni-40Cu and Ni-40Cu-0.6P (at.%) alloys were mechanically loaded in tension at ambient (23°C) and elevated (150°C) temperatures with the deformed nanostructure captured by in situ transmission electron microscopy coupled with digital image correlation for data mining high strain regions prior to catastrophic failure. The addition of the P provided grain boundary partitioning that stabilized the NC structure under both loading cases, which was not found to be the case for the binary alloy. While mechanical strength softening was observed in each of the alloys upon thermomechanical loading, the retention of strength was substantially higher in the ternary alloy than its binary counterpart. Digital image correlation was found to be a useful means for image mining to identify different regions where failure mechanisms were initiated. Intergranular failure was observed as the dominant failure mechanism in all specimens with the binary alloy revealing a coarser fracture profile as compared to a finer fracture profile in the ternary alloy. Atomistic simulations are used to understand P solute strengthening of the grain boundaries against this fracture failure mode.