Phase-dependent corrosion behavior and thermomechanical analysis of Si₃N₄/AgCuTi/316L brazed joints via first-principles calculations

The mechanical properties and corrosion resistance of different phases in Si₃N₄/AgCuTi/316L joints were investigated based on density functional theory (DFT). When the temperature was increased from 298 K to 1173 K, the coefficient of thermal expansion (CTE) of the Ti₅Si₃ and TiN phases increased more rapidly than that of other phases. By contrast, the thermal expansion coefficient of Fe₂Ti exhibited a relatively slow increase. This result implied that during the cooling process of the joint, the residual stress was affected not only by the mismatch in thermal expansion coefficients between the Si₃N₄ and 316L substrates, but also that the residual stress concentration on the Si₃N₄ side was associated with the large variation rates of the thermal expansion coefficients of the Ti₅Si₃ and TiN phases. Meanwhile, the residual stress concentration on the 316L side was attributed to the large difference in thermal expansion coefficients between the Fe₂Ti phase and the surrounding phases, especially the significant mismatch with the 316L substrate. Furthermore, based on the calculated and experimental results of these phases, the corrosion resistance of the phases in the joint was inferred to decrease in the following order: TiN > Ti₅Si₃ > Ag (s.s) > Fe₂Ti > CuTi > Cu(s.s), and the corrosion mechanism at the filler/316L interface was revealed in detail.