Microstructures' Elasto-plasticity and Failure Analysis of Ti-6Al-4V/Si3N4 Dissimilar Joints Brazed with AgCuTi Fillers for Thruster Development.

The development of hybrid ceramic/metal thruster for next-generation propulsion system at ISAS/JAXA requires a robust and corrosion resistant dissimilar joint to endure severe thermal and vibrational stresses. Si₃N₄ ceramics will be utilized at the high-temperature combustion chamber, whereas the nozzle skirt exposed to a lower temperature will be made of thin Ti-6Al-4V plate. The operating temperature at the joining position is engineered to be approximately 873 K. In this study, the mechanical behavior of Ti-6Al-4V/Si₃N₄ joints brazed with AgCuTi fillers were investigated based on the understanding of elasto-plastic properties of interfacial microstructures and intrinsic properties of ceramics. Bending strengths obtained by thermo-mechanical finite element analysis based on the inputted as-received filler’s elasto-plastic properties are overestimated. Nanoindentation revealed that the hardness of as-received filler and brazed interface are significantly different; ductile Ag-Cu phase is replaced by the growth of hard Cu-Ti intermetallic compounds (IMCs) after brazing due to the excessive supply of Ti atoms from the Ti-substrate. The as-brazed microstructure resulted in a smaller plastically deformable region, thus causing higher residual stress and lower bending strength. The characteristic strength of ceramics was found to be the dominant intrinsic property that governs the failure of joints when subjected to thermo-mechanical stresses. Bending strength significantly improved with the insertion of a ductile Nb-interlayer, whereby it minimizes both mismatch in coefficient of thermal expansion and hard Cu-Ti IMCs next to the Si₃N₄. Although a relatively thick layer of Cu-Ti IMCs was formed between the Ti-substrate and Nb-interlayer, they did not show any sign of intrinsic brittleness at current stress level. Fracture initiated mainly from the Si₃N₄ due to residual stress. When tested at elevated temperatures up to 873 K under atmospheric condition, despite a decrease in filler’s intrinsic strength and effect of oxidation embrittlement, the overall strength improved attributed to the internal stress relaxation.