Dissimilar Brazing of Alumina to Stainless Steel with a New MPEA Filler: Bonding Mechanisms and Residual Stress

The chemical and physical incompatibility between ceramics and metals presents a major challenge for dissimilar bonding and for maintaining a sound joint under harsh service conditions, such as high thermal cycling. This study developed a new Mn-rich ductile multi-principal element alloy (MPEA) filler for dissimilar brazing of alumina to 304 stainless-steel. The bonding mechanisms were investigated using scanning and transmission electron microscopy. A methodology combining in-situ high-energy synchrotron X-ray diffraction (XRD) and micro-computed tomography with the aid of finite element modeling was employed to examine residual stress evolution in the as-brazed condition and during thermal cycling up to 800°C, and revealed the impact of cooling rates (from 5 to 40°C/min) on interface integrity. Spinel, (Mn²⁺,Cr²⁺)(Mn³⁺,Cr³⁺,Al³⁺)₂O₄, and silicate phases, such as mullite, Al₆Si₂O₁₃, and spessartine, (Mn²⁺,Fe²⁺)₃Al₂(SiO₄)₃, were identified as the primary reaction products contributing to bond strength, incorporating chromium and iron from the stainless steel that migrated to the alumina faying surface. In the as-brazed condition, the joints were free of cracks, though tomography showed microvoids at the ceramic and filler interface. The joints withstood approximately 150 MPa of residual stress measured by synchrotron XRD, and the additional applied shear stress necessary to separate the joint was approximately 4 MPa. Upon thermal cycling, in-situ synchrotron XRD and postmortem tomography revealed beneficial stress relaxation above 400ºC during heating, and significant stress reaccumulation at temperatures below 200ºC upon cooling. Furthermore, the cooling rate was identified as a critical parameter for interface integrity. While rapid cooling at 40°C/min led to catastrophic joint failure at the conclusion of the heat treatment, a 5°C/min cooling rate maximized high-temperature stress relaxation, preventing low-temperature crack formation. This work demonstrates that limiting the cooling rate assists in maintaining joint integrity and that successful alumina–stainless steel joints can be produced without noble-metal fillers.