Softening-resistant ultra-strong nanotwinned CoCrFeNi medium entropy alloy thin films

Nanostructured and particularly nanotwinned metals with superior mechanical performance have been developed for decades but still suffer from the drawback of detwinning-caused significant softening at an elevated temperature or under mechanical stress, especially for those with a twin spacing below 10 nm. In the present work, we synthesized an ultra-strong nanotwinned CoCrFeNi medium entropy alloy thin films, with an average twin plane spacing of only 2 nm but an ultrahigh softening resistance. Overwhelming coarse-grained and nanocrystalline medium or high entropy alloys, or other nanotwinned metals, the nanotwinned CoCrFeNi medium entropy alloy thin films exhibited a much higher hardness of 11.3 GPa, particularly at a low temperature of −80 °C. A slight decrease in hardness for only 10 % was noticed at 300 °C, and very thin localized detwinning was observed only at the top surface of the nanoindenting deformation zone, attributable to the highly stabilized nanotwinned structure of the alloy in a consequence of its low stacking fault energy. In addition, in the detwinning region, high density of Lomer-Cottrell locks was observed in the high-temperature case, which also provides a significant ability against the softening. The present work demonstrates that the synergy of nanotwinning and distorted structure provides medium entropy alloy thin films ultrahigh structural and mechanical stabilities over a wide range of temperatures. © 2025 Elsevier B.V.