Qian Yiyu

Microstructure and strength of the SiC/TiAl joint brazed with Ag-Cu-Ti filler metal

SiC ceramics are considered one of the most promising structural materials for special applications. The development of bonding technology is widening the application field of SiC ceramics. There have been many reports on diffusion bonding and brazing of SiC ceramics to metals [1–3]. TiAl intermetallics have a great potential to become important candidates for advanced applications in aerospace and military industries. The researches on diffusion bonding and brazing of TiAl intermetallics to other materials have progressed in recent years [4–6]. The concept of utilizing ceramic, intermetallic and metallic materials to attain one complete armor system by bonding process is a recent approach for defeating armor projectiles [7]. Therefore, a previous study of diffusion bonding of SiC to TiAl was carried out [8]. This letter aims to demonstrate the feasibility of brazing of SiC to TiAl, and the focus is placed on the microstructures and strengths of the SiC/TiAl joints brazed with Ag-Cu-Ti filler metal. The materials used in experiments were cylindrical SiC rods (diameter 6 mm, height 4 mm), and cylindrical TiAl rods (diameter 10 mm, height 4 mm) with an average composition of Ti-43Al-1.7Cr-1.7Nb (at.%). The chemical composition of the Ag-Cu-Ti filler metal foils (thickness 20 μm) was Ag-27Cu-4.5Ti in weight percentage. The surfaces to be brazed were ground and polished through diamond paste and cleaned in ethanol and acetone prior to brazing. The coaxial SiC/TiAl assemblies were brazed at 1173 K for 5–40 min under a vacuum of 6.6 mPa in a vacuum furnace (Centorr-3520). The cross-sections of the brazed SiC/TiAl joints were prepared for metallographic analysis by standard polishing techniques. The microstructures of the SiC/TiAl joints were examined by scanning electron microscopy (SEM, S-570), electron probe X-ray microanalyzer (EPMA, JXA-8600) and X-ray diffractometer (XRD, JDX-3530M). The room-temperature shear strengths of the SiC/TiAl joints were evaluated by means of a specially-designed fixture in an electron tension testing machine (Instron-1186), and the average strength of the three joints brazed under the same conditions was used. Fig. 1 shows the back-scattered electron image of the cross-section of the SiC/TiAl joint brazed at 1173 K for 15 min. It can be found from the figure that three kinds of different microstructural zones have occurred in the brazing seam between SiC and TiAl. For the sake of convenience, these zones are marked by D, E and F, respectively. Fig. 2 shows the concentration profiles of major elements across the brazing seam of the SiC/TiAl joint brazed at 1173 K for 15 min. The AB line in the figure indicates the position analyzed by EPMA. Obviously, the distribution of each element across the brazing seam is not even. There is almost no Ag in E zone, and the concentration profile of Ag is undulating in D zone and flat in F zone. Cu exists in all zones, and the concentration of Cu in D zone is lower than in E zone and much higher than in F zone. There is almost neither Ti nor Al in D and F zones, and the concentration profiles of Ti and Al are both flat in E zone. These results reveal that D zone is an Ag-rich and Cu-rich one, and F zone is