Microstructural change and phase transformation in each individual layer of a nano-multilayered AlCrTiSiN high-entropy alloy nitride coating upon annealing

Abstract

A high-performance AlCrTiSiN high-entropy alloy (HEA) nitride coating, consisting of a columnar CrN (BL-1), a nano-multilayered CrN/AlCrN (BL-2), a columnar AlCrN (TL) and a nano-multilayered AlCrTiSiN HEA nitride layer (HEA-NL) towards to surface, was isochronally annealed at 700 °C ∼ 900 °C. Microstructural change and phase transformation of each individual layer of the HEA nitride coating were studied, as well as microstructure-property relationship. Phase transformation sequence of CrN → Cr2N → Cr → σ-CrFe occurs in the BL-1 by nucleation and growth upon annealing, whereas the fcc-AlCrN solid solution of the BL-2 adopts a two-step phase transformation sequence: first to enriched-Al and enriched-Cr fcc-domains by interface-directed spinodal decomposition, and subsequently to hcp-AlN and σ-CrFe by recrystallization, accompanied by the dissolution of layered structure and the diffusion of Fe deriving from steel substrate. The TL shows two different features from that of the BL-2: (i) there is a precipitation process of hcp-AlN from the fcc-AlCrN; (ii) the spinodal decomposition is isotropic spinodal decomposition. The HEA-NL shows drastic differences in spinodal decomposition behavior and microstructure compared to the TL, in which interface-directed spinodal decomposition produces a triple-layer consisting of a rich-Cr/Ti layer fcc-domains confined by two enriched-AlN ones, simultaneously resulting in a notable self-hardening. Subsequently recrystallization results in a aging softening at 900 °C due to the dissolution of nano-multilayered structure, the phase transformation of enriched-Al fcc-domain to hcp-AlN and the formation of soft Cr. Moreover, both Cr2N and σ-phase layers deteriorate the adhesion strength.