Abstract: In-situ Ti₅Si₃/ Ti₃Al/TiAl composite coatings reinforced by high hardness Ti₅Si₃ intermetallic phase were fabricated on γ-TiAl substrates precoated with silicon powders by means of electron beam surface alloying. The microstructure characteristics of the composite coatings were investigated using optical microscope, electron probe microanalysis, energy spectrometer and X-ray diffraction. Micro-hardness was measured from the surface down to the substrate by a Vickers hardness tester. The results show that composite coatings are composed of TiAl, Ti₃Al and Ti₅Si₃ phases. More specifically, the morphology and distribution of Ti₅Si₃ phase change with gradient transition along the depth of the coatings. In the upper level of the coatings, a considerable amount of large hexagonal-pillar-shaped Ti₅Si₃ phases were observed, while in the middle and lower level of the coatings, the particles get smaller and denser because of faster cooling speed. The bond zone between the substrate and the coatings is wholly metallurgical bonding with no apparent interface. As for the hardness of the composite coatings, it is higher than that of the substrate, its maximum value reaches 895, nearly triple the value of the matrix one.

Key words: electron beam surface alloying; γ-TiAl alloy; Ti₅Si₃; microstructure; micro-hardness