Influence of the Ti:Al:Cr proportion on the structure and oxidation resistance of ternary intermetallic coatings produced by non-vacuum electron beam cladding

Abstract

Titanium is known to be a material with poor oxidation resistance. When heated, it reacts with atmospheric gases, leading to the formation of the TiO₂ oxide film on its surface. This film does not protect the material against further oxidation. To solve this problem, heat-resistant coatings, including TiAl-based ones, are applied to titanium. This study investigates ternary Ti-Al-Cr surface layers with different concentrations of components. The protective layers were fabricated on titanium substrates by non-vacuum electron beam cladding. This method provides the formation of thick coatings (up to 2 mm in a single pass of the electron beam); however, the structure of the cladding layers can be significantly inhomogeneous. To reveal the influence of microstructural features of the coatings and the uneven elemental distribution on their oxidation behavior, detailed structural studies combined with the CALPHAD method were performed both before and after oxidation testing. It was found that an Al-rich alloy consisting primarily of the γ-phase provided the best protection against oxidation. The introduction of high concentrations of both Al and Cr yielded contradictory results. Although the L1₂-and C14-phases formed in the coating are predicted to contribute to the development of a protective Al₂O₃ film, the oxidation resistance of this cladding layer was comparable to that of titanium. This result can be attributed to the inhomogeneity of the coating and the appearance of the B2-phase in local areas of the coating at oxidation temperature and the decomposition of the Cr-depleted C14-phase. Furthermore, the Cr-rich B2-phase provides better oxidation resistance than the α₂-phase of Al-lean cladding layers.