Microstructural evolution of Li-Ti ferrite/composite braze/Li-Ti ferrite joints reinforced by Bi₅(Ti₃Fe)O₁₅ whiskers

The composite brazes consisting of (i) Bi₂O₃-B₂O₃-SiO₂ glass and (ii) Li-Ti ferrite powder were used to join the Li-Ti ferrite. The microstructural evolution and resulting mechanical strength of the joint were systematically investigated. Both needle-like and plate-like Bi₅(Ti₃Fe)O₁₅ phases were observed in the joint. With a sufficiently pronounced Ti concentration gradient, the Bi₅(Ti₃Fe)O₁₅ crystal nucleuses evolve into whiskers along the c-axis orientation. Contrary to that, the Bi₅(Ti₃Fe)O₁₅ plates were observed when the Ti concentration gradient was small enough to make the driving force of the preferential growth markedly weaker in comparison to the attraction between the (Bi₂O₂)²⁺ and (A_m−1B_mO_3m+1)²⁻ domains in a bismuth layer-perovskite structure. The Bi₅(Ti₃Fe)O₁₅ whiskers having a large size could potentially absorb the fracture force by deflecting or stopping its propagation. The highest shear strength of 81.0 MPa was obtained for the joint with the composite braze containing 10.0 wt% of Li-Ti ferrite powder.