Microstructure And Property Research On TiAl-Based Alloy Sheet Fabricated By EB-PVD

Metallic thermal protection system (MTPS) is one of the key technologies for developing reusable launch vehicle (RLV). Compared with the ceramic tiles, the MTPS has many merits, such as more efficient internal insulation, lighter wight, more simple structure, easier installation and removal. Moreover, the most key part in the MTPS is surface material of resistance to elevated temperatures and oxidation. In this paper, TiAl, TiAl/Nb and TiAl/NiCoCrAl were prepared by electron beam physical vapor deposition (EB-PVD) technology. The microstructure and mechanical properties were investigated by means of modern testing and analysis methods( SEM,XRD,AFM,DTA etc.) for using those alloy in safe and reliable. Combining with the characteristics of EB-PVD technology, the formative cause of microstructure for TiAl alloy were analysed by thermodynamic theory. The oxidation resistance of TiAl alloy and TiAl/NiCoCrAl multilayer were studied.There is natural delamination inside the TiAl alloy sheet throughout the cross-section, the microstructure of which is columnar crystal. The formation of columnar crystal is related with the substrate temperature and deposition speed. TiAl layer and Nb(or NiCoCrAl) layer inside the laminated composites are evenly distributed throughout the cross-section. The interface of interlamination is transparent, but no delamination phenomenon is found in the TiAl layers. The short evaporated time and the retarded evaporation of Al element are the key causes for the phenomenon.The alterative microstructures of TiAl sheet before and after vacuum treatment were studied, the diffusion mechanics were determined by DTA. Combining with evaporated theory, the natural delamination inside the TiAl alloy was explained by thermodynamic theory.TiAl/NiCoCrAl multilayer composites have highest strength and best ductility among the TiAl-based alloys at room temperature. Except for the TiAl sheet, the multilayer composites behave yield phenomenon for tensiling test at 750℃, the toughening layers decrease the ductile-brittle transition temperature. From the fracture analysis, the strengthening mechanisms of the TiAl/NiCoCrAl multilayer composites are the fine grained dispersion strengthening, the toughening mechanisms are contributed by the crack deflection, the micro bridge join and the crack bend toughening.The continuous oxidation behavior of TiAl alloy and TiAl/NiCoCrAl multilayer at 850℃for 100h were studied. The oxidation gain in weight and oxide scale of the alloys are studied. Although their oxidation kinetics at 850℃obeyed approximately the parabolic law, TiAl/NiCoCrAl multilayer have higher oxidation resistance than TiAl alloy sheet. For analysising intuitively the oxidation resistance mechanics of TiAl/NiCoCrAl multilayer, diffusion mechanics of TiAl/NiCoCrAl multilayer were firstly studied. The results indicated that the mixed protective oxides of Cr2O3, NiAl2O4 were formed on the pure TiO2 oxide scale, which was previously formed on the alloy surface. It is the key reason for improving remarkably oxidation resistance.