Effects of minor Nb alloying on the thermal stability and mechanical responses of a γ/γ′ type high-entropy alloy with high Fe content

In this work, by equipping the high-entropy concept we designed a γ/γ′-type high-entropy alloy with high Fe content (25 at.%), and further alloyed Nb to evaluate its microstructural stability and mechanical responses. We found that after 850 °C annealing, 1.5 at.% Nb addition can completely suppress the discontinuous precipitation (DP) reaction. The high γ/γ′ structural stability even after one month ageing at 800 °C is believed to be originated from strong partitioning of Nb to γ′. The onset of continuous inter-mass-flow of Fe and Nb across the γ/γ′ interface further enhances the γ′ ripening resistance. The increased tensile strength in the Nb-alloyed HEA across a wide temperature range (23–900 °C) is because of the increased volume fraction and anti-phase boundary energy of γ′. The corresponding lowered ductility is then attributed to the absence of the DP colony that not only acts as a stress buffer zone but also introduces grain boundary serration to withstand intergranular cracking.