Revealing the impact of valence electron concentration on precipitation and tensile behavior of a FeMnCoCr-based high-entropy alloy

Abstract

This study systematically investigated the tensile behavior of the face-centered cubic high-entropy alloys (HEAs) with the composition of (Fe₅₀Mn₃₀Co₁₀Cr₁₀)_96−4xNi_4xAl₂Ti₂ (x = 3, 5, 8, at.%). Special attention is given to the effect of valence electron concentration (VEC) that increases from 7.7 to 8.0 and finally to 8.3 for the three alloys on their precipitation behavior, deformation mechanism, and mechanical property. For the different alloys, after aging treatment, L1₂ and B2/7M precipitates form within grains and along grain boundaries, respectively. With increasing VEC, the size and volume fraction of precipitates increase monotonically. The L1₂ precipitate evolves from a single rod-like morphology to a mixture of rod-like and spherical morphologies, while the B2 phase gradually transfers into the 7M martensite, resulting in an enhanced precipitation-induced strengthening. The plastic deformation mechanism associated with precipitation transfers from dislocation bypass in the alloy with a VEC of 7.7 to dislocation cutting through spherical L1₂ particles in those with higher VECs of 8.0 and 8.3. As the volume fraction of spherical precipitates increases, their interaction with dislocations becomes more pronounced, promoting uniform plastic deformation. The (Fe₅₀Mn₃₀Co₁₀Cr₁₀)₆₄Ni₃₂Al₂Ti₂ alloy with the highest VEC exhibits optimal mechanical properties, with its yield strength increasing from 269 to 655 MPa during aging while maintaining a uniform elongation of 21%. Especially, the work hardening rate dramatically increases from 1944 to 3456 MPa at 0.1 true strain. The significant improvement in yield strength is attributed to the synergistic strengthening from L1₂ precipitates and 7M martensite, whereas the excellent work hardening capability results from the frequent interaction between dislocations, as well as the transformation of the 7M martensite into the B2 phase during deformation. These findings provide guidance for the design and development of precipitation-strengthened HEAs with high strength and good ductility.