Unidirectional crystallization and high-temperature oxidation of in situ Ti3(Al,Si)–Ti5(Si,Al)3 composite

The paper is devoted to preparation and high-temperature oxidation of in situ Ti3(Al,Si)–Ti5(Si,Al)3 composite. The composite with nominal chemical composition of Ti–19 at.%Al–11 at.%Si was unidirectionally solidified using the optical floating zone technique. Two solidification rates were applied −5 and 18.8 mm/h. The composite comprises Ti5(Si,Al)3 particles dispersed in Ti3(Al,Si) matrix. Structural examination of unidirectionally crystallized samples shows that only crystallization rate of 5 mm/h produces fiber-like silicides with length up to 150 μm and diameter less than 10 μm that are well arranged in crystallization direction. Composite crystallized at 18.8 mm/h contains heterogeneous structure with coarse particles similar to the as-cast structure. By relating silicide interparticle spacing and growth rate, an equation of eutectic Ti3(Al,Si)–Ti5(Si,Al)3 composite growth is obtained: λ2.43R = 343 (interparticle spacing λ in μm, growth rate R in mm/h). Oxidation behaviour of the composite at 850 and 950 °C in air is compared to the Ti–60 at.% alloy (γ-TiAl intermetallic) and to the pure Ti. It is shown that both composite and intermetallic oxidize at almost same rate at 850 °C, while the composite oxidizes more rapidly at 950 °C. Scales on composite are stratified and contain at least three sub-layers—(1) alumina, (2) rutile with some amount of Si and (3) (Ti,Al,Si)N nitride. Mechanisms of sub-layers formation are discussed.