Enhancing dislocation strengthening in face-centered cubic high-entropy alloys through interstitial nitrogen

Non-equiatomic high-entropy alloys (HEAs) that exhibit transformation-induced plasticity show great potential as a novel class of structural materials. To further strengthen such alloys, this study explores the synergetic effect of nitrogen doping and thermomechanical processing on Co20Cr20Fe34Mn20Ni6 alloy. The recrystallized specimens of original N-free and N-doped (0.3 at %) compositions represent single-phase face-centered cubic (FCC) microstructures. The alloys are processed by hot-caliber rolling at 800 and 1000 degrees C. Adding nitrogen not only enhances solid solution strengthening but also leads to significant strengthening upon hot-caliber rolling. Quantitative analysis of dislocation density via time-of-flight neutron diffraction measurements reveals a monotonic increase in dislocation density with rolling reduction, which governs the alloy strength of the hot-caliber-rolled alloys. Notably, adding trace nitrogen significantly increases the dislocation density to 1.6 x 10(15) m(-2) at 800 degrees C, attributed to interactions between dislocations and interstitial nitrogen atoms. While a higher rolling temperature decreases the dislocation density as slight grain refinement via dynamic recrystallization occurs, additional strengthening due to planar defects is activated. The addition of nitrogen suppressed early yielding, which, in the N-free alloys, occurs at lower stress levels than those predicted based on the measured dislocation density. Consequently, the hot-caliber-rolled N-doped alloys obtained at both rolling temperatures represent an excellent combination of high yield stress (similar to 1 GPa), four times greater than the original alloy, with similar to 40 % elongation-to-failure. This study provides insights into the processing and strengthening of HEAs at intersections, during alloy designs.