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.
Electron beam surface alloying of γ-TiAl intermetallic alloy for Ti5Si3/TiAl composite coatings
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.
图3 S试样表面改性层的显微组织 Fig.3 Microstructures of surface modification layer of sample S (a)—Macrostructure; (b)—Microstructure of upper surface; (c)—Microstructure of inner layer; (d)— Bond zone
图4 S试样表面改性层背散射电子像及各元素面分布 Fig.4 Backscattered electron image and EMPA face scanning data of precipitates in sample S (a)—Backscattered electron image; (b)—Ti; (c)—Al; (d)—Si
图5 S试样表面改性层显微硬度沿层深方向的分布 Fig.5 Distribution of microhardness along depth from surface of sample S