Ti₂AlC is a typical MAX (M: early transition metal, A: main group element, and X: carbon and/or nitrogen) phase with ceramic and metallic properties due to its unique nano-layered structure. In order to investigate the interaction behavior between Ag and Ti₂AlC, a sessile drop experiment was conducted at 1080 °C for 5 min. The atomic rearrangement occurred at the Ag–Ti₂AlC interface was revealed using high-angle annular dark-field scanning transmission electron microscopy coupled with high-resolution transmission electron microscopy analysis. The results show that Ag nanoclusters generally appeared in most of the Ag–Ti₂AlC interaction regions thermally processed at 1080 °C. In addition, Ag can also substitute for Al and Ti atoms in the Ti₂AlC, promoting local structural decomposition of the Ti₂AlC and producing 4H–Ag with a hexagonal close-packed (hcp) structure. Additionally, Al atoms released from the Ti₂AlC lattices can dissolve locally into the liquid Ag, particularly at the grain boundaries. When the loss concentration of Al exceeded the critical level, the Ti₂AlC started to decompose and the residual Ti₆C octahedrons and Al atoms recombined, giving rise to the production of anti-perovskite Ti₃AlC with a cubic structure. Lastly, the discrepancy in substitution behavior of Ag in the Ti₂AlC was compared when thermally processed at different temperatures (1030 °C and 1080 °C). This work contributes to the understanding of the intrinsic stability of Ti₂AlC MAX ceramics under high-temperature treatment.

Micro-arc oxidation (MAO) is performed on Ti-6Al-4V electron-beam-welded joints, and the microstructure, phase composition, and corrosion resistance of the joint and surface coating are investigated systematically by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), as well as electrochemical and stress corrosion analyses. SEM analyses revealed that the joint undergoes a phase transformation. The coating morphology of the joint and base materials is similar, but the joint coating is denser and thicker. XRD analyses recognize the rutile and anatase phases in the coating. Polarization and electrochemical impedance spectroscopy (EIS) corrosion tests reveal that the MAO treatment results in a decrease of two orders of magnitude in the corrosion current density of the welded joint and an increment of corrosion resistance. Stress corrosion evaluation reveals that a dense layer is exposed to protect the joint after long-term exposure to a high-stress corrosion environment. No stress corrosion-induced cracking or defects are observed in the joints, indicating the corrosion resistance of the joint has significantly improved.

In this work, (Au₇₉Ni₁₇Pd₄)₉₆Ti₄ (wt.%) filler alloy was designed and employed to join SiC ceramics. The effects of brazing temperature and soaking time on the microstructure and fracture morphology of joints were investigated. The results show that the joint obtained can be described as SiC/reaction layer/braze/reaction layer/SiC. The reaction layer was composed of TiC and Au (Si, Ti). The wettability of the filler alloy toward the SiC ceramics was analyzed. The braze zone was mainly constituted by Pd₂Si, Ni₂Si, and Au (Ni, Si). A large number of nano-sized TiC particles were distributed within the Au (Ni, Si) layer. The formation mechanism of the braze containing different phases was discussed. The brazing temperature and soaking time had a significant effect on the reaction layer at the SiC/braze interface and TiC particles within the Au (Ni, Si) layer, while they showed a negligible effect on the Pd₂Si and Ni₂Si within the braze. The inherent reason was also clarified in detail. The joint fractography indicated that a good bonding was achieved between the filler alloy and SiC, while joint fracture was primarily induced by the thermal stresses residing after the brazing cycle.