In-situ synthesis and oxidation behaviors of Ti-xAl coatings by high-frequency induction heated combustion

Ti-xAl coatings (x=25, 33.3, 50, 66.7, and 75 at%) were fabricated in situ using high-frequency induction heating synthesis (HFIHCS). The process parameters for preparing Ti-Al coatings via HFIHCS were orthogonally optimized employing the finite element method. The effect of the aluminum content on the microstructures and phase compositions of Ti-Al coatings fabricated by HFIHCS, as well as their oxidation behaviors at 900℃ for 100 h was investigated. The results showed that the distribution of interfacial temperature difference was influenced by induced current (62.11%), relative density of compact (9.86%), and indenter material (28.03%). The minimum interfacial temperature difference was achieved with an induced current of 300 A, a graphite indenter material, and a compact density of 82.9%. The Ti-xAl coatings exhibited a core (Ti)-shell (TixAly) ring structure. The rise in aluminum content in the Ti-xAl coatings led to a gradual reduction in the titanium-rich phase, accompanied by a progressive increase in the aluminum-rich phase and the interface thickness of zones Ⅰ and Ⅱ. The porosities of Ti-xAl coatings containing over 50 at% aluminum exhibited an order of magnitude higher than those of Ti-xAl coatings with 50% aluminum or lower. The mechanisms of reaction, pore-forming, and interfacial diffusion for Ti-xAl coatings were predominantly controlled by dissolution-precipitation, thermal expansion of intrinsic pores within the cold-pressed compact and voids formed after molten aluminum flow, and liquid-solid co-diffusion, respectively. The increase in aluminum content in the Ti-xAl coatings led to in a gradual decrease in its oxidative weight gain, a shift in the oxidation kinetics from a linear-parabolic to a parabolic law, and an enhancement in preservation of internal core-shell characteristics and the likelihood of forming a protective layer of thin Al2O3. The oxidation behaviors of Ti-xAl coatings with a core-shell structure were co-controlled by the aluminum content and the porosity affected by the aluminum content.