Diffusion kinetics and elemental transport in selective surface nitriding of CoCrFeNiTi0.5 high entropy powder

Abstract

Ceramic coated metal powders have recently found prominence owing to their prospective application in 3D microstructured metal matrix composite synthesis. To ensure a perfect matrix-reinforcement interface in these composites, the coating process needs to be in situ. To that end, we recently developed a process for in situ nitride surface coating of CoCrFeNi high entropy alloy (HEA) powder. This approach offers various advantages over other processes including ease of controlling the surface morphology and compositional control by adjusting the nitriding parameters. In this study, we aimed to elaborate the nitride layer formation mechanism, growth kinetics, and compositional control. Mechanically alloyed CoCrFeNiTi0.5 HEA powder was used as the starting powder. The nitride layer growth on the powder was observed at different temperatures of 973, 1073 and 1173 K for 1–12 h under partial nitrogen atmosphere. The nitrogen preferentially reacted with Ti to form a continuous layer of TiN on the powder surface. Further growth was found to be diffusion-controlled by the growth rate increase with increasing nitriding temperature. Morphological investigation revealed a strong efflux of Ti through the TiN grain boundaries resulting in a scale growth at the TiN/gas interface. This growth mechanism was validated by a high TiN layer growth activation of 240 kJ.mol−1. Finally it was revealed that the Cr dissolution within the TiN layer can be achieved at higher temperature. However, to retain the integrity of the surface layer, the Cr dissolution process needs better control