Fabrication Of MoSi₂-Fe In- Situ Composites By SHS-Casting Technology

Molybdenum disilicides (MoSi₂) has high melting point, low density and good oxidation resistance in elevated temperature, thus it has a potential application in the field of aviation and aerospace. However, the low fracture toughness at ambient temperature is its largest drawback, which hinders its application. Recently, ductile-phase toughening has become an effective method to improve the toughness of ceramic and intermetallic compound. Therefore, in this thesis, a combustion synthesis (SHS) - casting technology was purposed to fabricate MoSi₂-Fe in-situ composites.By taking MoO3-Al-Si-Fe as a reaction system, MoSi₂-Fe in situ composites was fabricated by the proposed SHS-casting technology. The thermodynamics of the reaction, the melting and solidification process of the synthesized products were investigated. The results indicate that, when the Fe content in the synthesized MoSi₂-Fe composites is not higher than 45, the adiabatic combustion temperature of the reaction system is higher than the melting points of the synthesized products, thereby resulting in melting of the products . Thus, it's feasible to fabricate MoSi₂-Fe in-situ composites by the proposed SHS-casting technology. In addition, with the increasing of the Fe contents, the ignition time of SHS reaction increases, and the combustion model changes from thermal explosion into self-propagating model, in which the propagating velocity of combustion wave is decreased.The microstructures of the finally fabricated MoSi₂-Fe composites were investigated by OM, SEM and EPMA. The research results show that, the fabricated composites consist of MoSi₂ and Fe phase,which exists as the bonding phase and distributes on the boundary of MoSi₂. With an increase of Fe content, the shape of the synthesized MoSi₂ is transformed from tetragonal into near sphere, whereas its average size is decreased from 20μm to 2μm. The relative density and hardness of the composites is first increased and then decreased with Fe increased. When Fe content is 40 Fe, the fracture toughness of MoSi₂-Fe composites is about 4.48MPa?m-1/2. Crack-tip plastic blunting by a ductile metallic phase and crack deflection are the principal mechanisms of toughness improvement of MoSi₂-Fe composites.