Plasma spraying in-situ TiC reinforced AlCoCrFeNi composite coatings: Microstructure evolution and mechanical properties under post-annealing

In this work, AlCoCrFeNi reinforced with xTiC (x = 0, 5, 10 and 25 vol%) high-entropy alloy composite (HEAC) coatings were prepared by atmospheric plasma spraying (APS) and annealed at 600, 800, 1000 °C. The influence of TiC on microstructure and mechanical properties of HEA were studied. Results showed that the as-sprayed HEAC coatings were composed of BCC solid solution, TiC particles and minor oxides (Al2O3 + TiO2). The in-situ oxidation of C@Ti agglomerates significantly inhibited the oxidation of HEA powder during spraying. The embedded TiC particles with a size of 5–10 μm improved mechanical properties, showing a positive correlation between hardness and TiC content and reaching to 494 HV at 25 vol%. With increasing annealing temperature, the BCC → FCC phase transition occurred to strengthened coatings while TiC remained stable at all temperatures and during long-term annealing up to 24 h. The HEAC coating with 25% TiC addition reached highest hardness of 684 HV after annealed at 600 °C. Post-annealing stimulated intense phase transition of BCC → FCC at the TiC/HEA interface to form a continuous and ductile structure around TiC. Such maintain the good interface bonding to enhance thermal stability and resistance to softening at high temperatures.