Eutectic engineering via Nb addition in CoCrNiNb_x MEAs: Dual-phase (FCC + laves) microstructure for high strength and 900 °C oxidation stability

This study employed vacuum arc melting to prepare a series of CoCrNiNb_x (x = 0, 0.2, 0.3, 0.4, 0.5) medium entropy alloys (MEAs) through niobium (Nb) alloying design. The effects of Nb content on microstructure, mechanical properties, and high-temperature oxidation resistance at 900 °C were systematically investigated. The addition of Nb induced the formation of a lamellar eutectic structure composed of FCC and Laves phases, causing the microstructure to evolve from hypereutectic to eutectic and then to hypereutectic with increasing x. Among these, the CoCrNiNb_0.4 alloy with a complete eutectic microstructure exhibited optimal comprehensive properties, achieving a compressive strength of 2219 MPa and plastic strain retention of 20.2%. This strengthening primarily stemmed from the grain boundary strengthening induced by the eutectic lamellar structure. Under oxidation at 900 °C, all alloys followed a parabolic oxidation pattern, with oxidation rate constants (k) for Nb-containing alloys consistently below 10⁻² mg²·cm⁻⁴·h⁻¹, indicating the formation of a protective oxide film. Notably, the k value for CoCrNiNb_0.4 further decreased to the 10⁻³ order of magnitude. This is attributed to the in-situ formation of a continuous Nb-enriched Laves phase barrier at the oxide layer/alloy interface, which effectively inhibits cation outward diffusion. Concurrently, the dense Cr₂O₃ internal oxide layer generated within the alloy synergistically contributes to constructing an efficient oxidation resistance protection system. This study simultaneously achieves ‘eutectic structure strengthening’ and ‘interface barrier protection’ through single-element alloying, offering a novel approach for designing eutectic medium-entropy alloys with outstanding mechanical properties and high-temperature oxidation resistance.