Synergistic enhancement in strength and ductility of nitrogen-alloyed CoCrFeMnNi high-entropy alloys via multi-pass friction stir processing

Abstract

The dynamic microstructural evolution and tensile performances of CoCrFeMnNi high-entropy alloys (HEAs) with and without nitrogen alloying subjected to multi-pass friction stir processing (FSP) were comparatively investigated. Results showed that the strength and ductility of nitrogen-alloyed CoCrFeMnNi HEAs were synergistically enhanced by multi-pass FSP. Compared to free-nitrogen CoCrFeMnNi HEAs, the gains in the processed zone of the nitrogen-alloyed CoCrFeMnNi HEAs exhibited not only more refinement，but also a monotonic decrease in average gain size (AGS) with the increase in FSP pass. In addition, nitrogen-alloyed CoCrFeMnNi HEAs had higher fraction of low angle boundaries (LABs) than free-nitrogen CoCrFeMnNi HEAs after multi-pass FSP. The interaction between nitrogen atoms and LABs during multi-pass FSP was indirectly proved by the comparison of microstructural thermal-stability between two groups FSP HEAs. Meanwhile, the mechanism of synergistical enhancement in strength and ductility of nitrogen-alloyed CoCrFeMnNi HEAs was discussed.