Effects of aging time on the microstructural evolution and strengthening behavior of a VCoNiMo medium-entropy alloy

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Medium-entropy alloys exhibit remarkable strengthening effects owing to their large solid-solution strength, providing high potential for further improvement of the mechanical properties by distributing secondary strengthening phases. In this respect, an equiatomic VCoNi alloy is an appropriate model alloy owing to its yield strength of approximately 1 GPa, with contributions from lattice and grain boundaries. However, the adoption of precipitation hardening is challenging because the solid-solution phase region is limited, which is obtainable only at high temperatures. In this study, Mo is used to generate strengthening phases and expand the region of a face-centered cubic (FCC) solid-solution phase at lower temperatures. The 10 at% Mo, determined through thermodynamic calculations, has little effect on solid-solution states. In low-temperature aging treatments, an ordered FCC (L12) phase forms first followed by the formation of intermetallic μ and κ phases, which are developed under prolonged heat treatment of up to 48 h. L12, which cannot be expected by thermodynamic calculations as an intermediate phase, results in remarkable strengthening effects with a slight reduction in tensile ductility. However, the (Co,Ni)3V-κ phase grows into the grain interiors and along the FCC grain boundaries forming lamellar structures with μ phase, which reduces ductility and increases strength. Thus, this work suggests that the Mo substitution to V overcomes the limitations of the VCoNi alloy and provides second-phase strengthening by forming homogeneous L12 within a large fraction of the FCC matrix; however, an appropriate heat treatment is required to prevent brittleness by suppressing grain-boundary μ and κ phases.