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 (Am−1BmO3m+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.