Uncovering the interstitial effects on the grain-refined high entropy alloy: A comparison between nitrogen and carbon

Grain refinement and interstitial strengthening are two effective ways to enhance the mechanical properties of high entropy alloys (HEAs). Carbon (C) and nitrogen (N) are two commonly used interstitials when it comes to interstitial strengthening. Yet, it is still not clear what the difference may lead to the microstructural evolution, mechanical properties, and associated deformation mechanisms these two interstitials may introduce to the fine-grained HEAs. In this work, we systematically investigate the effect of interstitials on the microstructural evolution, mechanical properties and deformation mechanisms of a non-equiatomic Cr₂₀Mn₂₄Fe₃₀Co₂₀Ni₆ HEA. The grain refinement is realized by cold rolling and subsequent short-time annealing. The two interstitials influence the matrix's phase stability and stacking fault energy (SFE) in different ways. C doping leads to the precipitation of the σ phase, while N doping results in a single face-centered cubic (FCC) structure. Both C-doped HEAs and N-doped HEAs show a decrease in yield strength and an increase in ductility. However, N is less sensitive to the temperature and generally shows better comprehensive mechanical properties. C-doping activates twinning-induced plasticity (TWIP) and FCC to hexagonal-closed packed (HCP) phase transformation-induced plasticity (TRIP). In the N-doped HEA, only the TWIP effect is observed. The findings in the present work can provide practical insight into the interstitial choice and the corresponding thermal-mechanical processing.