Phase Stability and Precipitation in L1₂-Strengthened CoCrNi Medium-Entropy Alloys at Intermediate Temperatures

Understanding phase stability and precipitation at intermediate temperatures is crucial for tailoring microstructures and mechanical properties of L1₂-strengthened multicomponent alloys. In this study, the precipitate type, morphology, and distribution of (CoCrNi)_100−2x(AlTi)_x (x = 3, 5, and 7 at.%) medium-entropy alloys (MEAs) at 600-900 °C were systematically investigated through a combination of scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, electron backscatter diffraction, and thermodynamic calculations. Our results reveal that the Al and Ti additions promote the destabilization of supersaturated fcc into L1₂ and σ phases, and the dominating phases of the MEAs change from fcc + L1₂ to fcc + L1₂ + σ and to L1₂ + σ + L2₁ phases as the Al and Ti concentrations increase. In addition, increasing the temperature leads to a change of precipitate morphology from lamellar to granular microstructures. The effects of alloying additions and aging temperature on the phase stability, precipitation behavior, and mechanical properties of the MEAs were discussed from the thermodynamic and kinetic points of view.