Annealing-induced abnormal hardening in nanocrystalline NbMoTaW high-entropy alloy thin films

•Nanocrystalline NbMoTaW high-entropy alloy film deposited by magnetron sputtering.•Abnormal hardening effect observed in the NbMoTaW HEA films after annealing at 800 °C.•The oxygen segregation at grain boundaries contributes to the amorphous intergranular films (AIFs) formation. High temperature annealing of high-entropy alloys (HEA) typically results in reduced hardness due to grain coarsening, limiting its potential for applications at elevated temperatures. However, in this work, our nanocrystalline NbMoTaW HEA thin film exhibits enhanced hardness up to ~ 17.1 GPa after isothermal annealing at 800 °C for ~ 6 h (hrs). This phenomenon can be explained mainly by the formation of amorphous intergranular films (AIFs). This work does not only demonstrate a high-performance HEA film for potential high-temperature applications, but also provides a practical approach to tailor mechanical properties of nanostructured HEAs.