Achieving superior combination of strength and ductility via nano-twins and γ′ precipitates in a NiCoCr-based medium entropy alloy

In this work, the nano-twin/γʹ precipitate microstructure was achieved in the NiCoCr-based medium entropy alloy (MEA) via rolling at ambient temperature and subsequent ageing treatments. During prolonged ageing treatments at 700 °C, high density of nano-twins was remained, while the T/M lamellar thickness slight increased from 12 nm in the rolled sample to 25 nm in the aged-300 h sample. Ageing treatments resulted in the sufficient precipitation of γʹ phase, and the volume fraction of γʹ precipitate was kept at ∼14%. The γʹ-size slightly increased from 19 nm to 26 nm during the 300-h ageing. Abundant precipitations in nano-twins raised the yield strength to a peak value of 1450 MPa in the aged-50 h sample, and simultaneously recovered the uniform elongation to 6%, leading to a superior combination of yield strength and uniform elongation as compared to other aged samples. Detailed analyses indicated that the dislocation cutting mechanism with γʹ precipitate dominated the plastic deformation, which contributed to both high strength and improved strain hardening capabilities. The present work demonstrated that a superior combination of strength and uniform elongation of the nano-twin/γ′ precipitate microstructure in the NiCoCr-based MEA could be achieved by appropriate ageing treatments.