Crack resistance in Nb foam-reinforced Ti₂AlNb/Ti60 joints: Role of stress distribution from thermal expansion mismatch

This study explores a novel strategy to enhance the crack resistance and thermal stability of brazed joints using Ti-36.5Zr–10Ni–15Cu-0.5Co-0.5Nb filler with and without Nb foam as an interlayer, subjected to thermal exposure at 750 °C for up to 30 h. The research identifies the failure mechanisms in different joint configurations, revealing that thermal expansion mismatch between the phases in the brazed seam induces thermal stress, leading to microcrack formation, oxidation, and crack propagation. In contrast, the incorporation of Nb foam results in the formation of a Ti₄Nb phase, which significantly improves the toughness and plasticity of the joint. Notably, after 30 h of thermal exposure at 750 °C, (Ti, Zr)(Ni, Cu) is formed in the brazing seam and its thermal expansion coefficient is positioned between those of Ti₄Nb and (Ti, Zr)₂(Ni, Cu). This unique characteristic enables a more uniform stress distribution in the brazed joint. The differential thermal expansion coefficients drive a progressive stress distribution, effectively preventing stress concentration, inhibiting the initiation of microcracks, and blocking oxygen intrusion, thus remarkably improving the thermal stability of the joint. Moreover, the thermal expansion coefficient of Ti₄Nb lies between that of the parent materials Ti₂AlNb and Ti60, further contributing to a more homogeneous stress distribution. This innovative approach demonstrates how tailoring the thermal expansion mismatch through material design can improve the long-term stability and reliability of titanium alloy brazed joints in high-temperature environments. The study offers a promising solution for advancing brazing technology in aerospace, automotive, and other high-performance industries.