Multi-phase aluminide-based composites - fabrication, microstructure and properties

Microstructures of Al45-Mo25-Zr25-Ge5 (in at.%, AMZG) alloy, produced by reaction hot pressing of elemental powder mixtures, have shown co-existence of AlMo3, Al3Mo8, ZrAl2, Zr2Al, MoGe2 and ZrGe2. In addition, its composites were fabricated through addition of micro-sized TiC, partially stabilized zirconia (PSZ-ZrO2) or SiC particulates into the pulverized multi-phase aluminide powders. The presence of SiC particulates showed a much less significant contribution to the strength/toughness enhancement of AMZG alloy, due to the existence of residual porosity and weak interfacial bonding. In contrast, the other two composites were superior in both flexural strength and fracture toughness to the AMZG multi-phase alloy, which is derived from the contribution of uniformly distributed and well-embedded harder particulates and the constrained plastic deformation of the matrix. The addition of hard ceramic particles simultaneously yielded higher bulk Vickers hardness. The toughness enhancement in the composites was attributed to the increased tortuousity by crack deflection, branching and bridging. Moreover, the transformation of tetragonal zirconia particles into the monoclinic form might also partially contribute to the toughness enhancement in the AMZG/ZrO2 composite.