Thermodynamics, microstructure evolution and mechanical properties of Al- and C-added CoFeMnNi multi-principal element alloys

Abstract

Navigating through the phase space of multi-principal element alloys (mpeas) with desired microstructures and outstanding properties is challenging due to extremely high degrees of freedom in alloy composition and processing conditions. Separate Al and C additions and the resulting precipitation of hard phases are commonly used to strengthen CoCrFeMnNi-type alloys. However, the combined alloying effect and especially the high hardening potential of C in solid solution of mpeas are rarely investigated, but the few existing investigations reveal a high potential. Therefore, the properties of the Al0−22.1C0−2.3(CoFeMnNi)100−75.6 (at.%) system were comprehensively investigated, where Cr was intentionally removed to keep C in solid solution. For theoretical and experimental screening of the system, a custom calphad database was extended, samples were manufactured by additive manufacturing (am) and annealed between temperatures of 1150 and 550 °C. Detailed microstructural and mechanical analysis on 40 different revealed the properties of the equiatomic CoFeMnNi alloy and the effect of the separate and combined additions of Al and C, including comprehensive precipitate characterization. The system was especially potent in its room temperature tensile properties in the Al- and C-added fcc-based region, where it caused the activation of twinning-induced plasticity (twip) and combined high strength and ductility increases.