Microstructure and properties stability of Al-alloyed MoSi2 matrix composites

The development of structure and properties in the C40 Mo(Si,Al)2/Al2O3, Mo(Si,Al)2/SiC and Mo(Si,Al)2/ZrO2 composites prepared by SPS of MA powders, caused by the 5-h annealing at 1600 °C, was investigated. The structure coarsening and transformation of a retained phase were revealed in the annealed composites. Intergranular fracture, observed in the as-sintered composites, was completely transformed to transgranular cleavage failure after the annealing. A decrease in hardness and changes in fracture toughness were discussed in relation to the structure coarsening and transformation of the fracture mode. Surface oxidation of the Mo(Si,Al)2/Al2O3 and Mo(Si,Al)2/ZrO2 during the annealing led to the formation of Mo5Si3 near-surface layer containing inclusions of an aluminium silicate phase. Internal oxidation of SiC in the Mo(Si,Al)2/SiC composites prevented the oxidation of the Mo(Si,Al)2 phase in the near-surface region.