Effect of chemical inhomogeneity on the structure and properties of the two-layer TiAl-based coating obtained by non-vacuum electron beam cladding: Experimental investigations and density functional theory simulations

Non-vacuum electron beam cladding is a highly effective technology for producing protective coatings on metal workpieces. In this study, the 3.8 mm thick TiAl-based coating with Cr and Nb additions was obtained on the Ti alloy substrate in two passes of the electron beam. To evaluate inhomogeneity of the two-layer coating, energydispersive synchrotron X-ray diffraction (EDSXRD) in combination with energy-dispersive X-ray spectroscopy (EDX) was applied. It was found that in the direction from the top of the coating to the substrate, the dilution of the cladding layers with Ti increased. It influenced the phase constitution of the coating and led to a variation in the chemical composition of different phases (alpha 2-Ti3Al, beta-phase, and TiN). The properties of the first (lower) and the second (upper) cladding layers were evaluated separately. The upper layer exhibited greater oxidation resistance than the lower one due to the presence of the gamma-phase, which has superior high-temperature stability compared to the alpha 2-Ti3Al phase, predominant in the coating. The influence of varying chemical composition on the oxidation resistance was also estimated using density functional theory (DFT) simulations. It was found that decrease in Al content in the alpha 2 phase leads to an increase in oxygen absorption energy. This could potentially result in a decrease in oxidation resistance. Wear resistance of the first and the second cladding layers was at the same level mainly due to the low contribution of minor phases. Additionally, DFT simulations show that the variations in chemical composition of the major phase (Ti3Al) are not likely to impact the coating wear resistance.