Dual-phase strengthening and high-temperature performance of CoNiAlV medium-entropy alloy via in-situ VC formation from V2AlC

In this work, VC-reinforced CoNiAlV medium-entropy alloy (MEA) composites were synthesized by powder metallurgy, where an in-situ reaction between V₂AlC MAX-phase precursors and Co-Ni powders. The phase transformation mechanism, microstructure evolution, and mechanical properties were systematically investigated. The optimization of sintering parameters and the design of subsequent heat treatments were guided by differential scanning calorimetry (DSC) and CALPHAD (CALculation of PHAse Diagrams) simulations. The decomposition of V₂AlC produced uniformly distributed VC particles, while the released Al and V atoms diffused to form a CoNiAlV MEA solid-solution matrix. Subsequent aging led to the precipitation of coherent L1₂-(Co,Ni)₃(Al,V) phases. Transmission electron microscopy (TEM) revealed that both nanoscale and micron-sized VC particles hinder dislocation motion through Orowan bypassing and particle-dislocation interactions, while the L1₂ phase provides additional shear resistance. Density functional theory (DFT) calculations further confirmed a relatively low stacking fault energy (SFE) in the matrix, consistent with the observed annealing twins. The VC/CoNiAlV composite, with a relatively low density of 7.2 g/cm³, exhibited excellent mechanical properties over the entire testing temperature range, achieving a specific yield strength of 94 MPa g⁻¹ cm³ at 800 °C - surpassing most conventional superalloys, while retaining good ductility (fracture strain of 0.34). Fractography revealed shear-dominated failure with localized cracking, confirming its high plastic deformability. The enhanced mechanical performance is attributed to the combined effects of multi-scale reinforcement and low-SFE-induced twins, which together provide a favorable balance between strength and plasticity at elevated temperatures, offering valuable guidance for designing lightweight, high-performance composites for high-temperature applications.