Unveiling solid-state transformation toughening mechanism of yttria stabilized zirconia on brazed joint by in-situ observation

Yttria stabilized zirconia (YSZ) has proved to be the most promising strengthening particles in the composite filler to release the residual stress of Ti₃SiC₂/Cu brazed joint. However, like most of ceramic/metal brazed joints, the microstructural evolution during the serving process was hardly to be characterized directly, limited to the conventional techniques and their brittleness. Therefore, the solid-state transformation toughening mechanism of YSZ on Ti₃SiC₂/Cu brazed joint during the service process is still unclear. In this study, Ti₃SiC₂/Cu brazed joints were fabricated by 10 vol% 3YSZ-40 vol% Ti₂₅Zr₂₅Ni₂₅Cu₂₅-Ag and 50 vol% Ti₂₅Zr₂₅Ni₂₅Cu₂₅-Ag fillers respectively, and the influence of YSZ on microstructural evolution of Ti₃SiC₂/Cu brazed joints was investigated by in-situ three-point bending experiment for the first time. Combined with the transmission Kikuchi diffraction and transmission electron microscopy results, two kinds of toughening mechanism models were proposed. During the loading process, the solid-state transformation from tetragonal zirconia to monoclinic zirconia occurred, contributing to volume expansion. The volume expansion of particles at interface of reaction layer/brazing seam inhibited the propagation of secondary microcracks. While the volume expansion of particles near the prime crack induced microcracks, retarding the propagation of prime crack. As a result, the main crack in the joint brazed by 10 vol% 3YSZ composite filler occurred and propagated throughout the interface, instead of concentration on the ceramic side in the 3YSZ-free brazed joint, where the maximum bending force was only about 250 MPa, just 66.75 % of that of brazed joint containing 3YSZ particles. This study provides new methods to analyze fracture behaviors of brazed joint during its service process and proves the promising prospect of YSZ as toughening method in the brazed joints.