Effect of short-range ordering and grain boundary segregation on shear deformation of CoCrFeNi high-entropy alloys with Al addition

A method combining molecular dynamics (MD) and Monte Carlo (MC) simulation is used to analyze the short-range ordering and grain boundary (GB) segregation in the bi-crystals of equiatomic CoCrFeNi and Al (8 at%)-CoCrFeNi high-entropy alloys (HEAs). Based on the structures of the two HEAs obtained by the combined MC/MD method, their defect evolution and deformation mechanisms during shear deformation at 300 K are studied. In addition, the bi-crystals of the Al (8 at%)-CoCrFeNi HEA with the inclusion of B2 intermetallic AlNi particles at the GBs are considered. For the CoCrFeNi HEA, the Cr and Fe atoms are revealed to segregate to GBs. In contrast, it is observed in the Al (8 at%)-CoCrFeNi HEA that Al and Fe have a strong tendency to segregate to GBs, while local ordering results in the formation of Fe3Al clusters, which in turn increase the stacking fault energy of the alloy. The GB segregation and the deformation behaviour of the alloys are found to be highly sensitive to the crystallographic orientation of the bi-crystals. The GB segregation, especially by the Al atoms, stabilizes the GBs and resists the plastic deformation through GB sliding and the GB migration. Overall, the Al (8 at%)-CoCrFeNi HEA with Al-atom segregation at GBs demonstrates an increased shear yield strength compared with the material without the Al addition. On the other hand, the AlNi particles reduce the yield strength of the HEA owing to the formation of amorphous structure at the face-centered cubic/B2 interface and thus facilite the GB sliding. The obtained results provide insights into designing HEAs with improved mechanical properties through GB engineering.