Origin of grain boundary motion during diffusion bonding by hot pressing

The microstructural changes during diffusion bonding of Al₂O₃ fibres with boron-doped Ni₃Al plates by hot pressing were investigated. Particular attention was paid to the conversion of the bonding surfaces to an interior grain boundary and its migration during the hot-pressing treatment. The microstructure changes were found to depend on the grain size of the matrix material prior to hot pressing. Fine-grained matrix material led to a fast migration of the bonded interface in order to establish force equilibrium at grain boundary junctions at the prior surfaces. In coarse-grained material, the bonded interface moved after much larger hot pressing time by strain-induced boundary migration due to the accumulated plastic-strain during prolonged hot-pressing treatment. The fibres were found to strongly impede migration of the bonded interface, which can interfere with the perfection of the bonding process.