Interphases in Si3N4/Ni-Cr alloy joints

Microchem. and microstructures of interfaces in Si3N4/Ni-Cr alloy joints were investigated. The reaction products and resultant interphases have been characterized. These phases were related to the exptl. detd. phase diagrams of the Ni-Cr-Si-N system. Combined with thermodn. and diffusion data, predictions are made on the nature and extent of the reactions. Based on this information, the optimum bonding conditions and causal relations between joining practice and joint performance were established. [on SciFinder (R)

Formation of Structural Intermetallics by Reactive Metal Penetration of Ti and Ni Oxides and Aluminates

Alumina-aluminum composites can be prepared by reactive metal penetration (RMP) of mullite by aluminum. The process is driven by a strong negative free energy for the reaction (8 +x)Al + 3Al6Si2013 → 13Al2O3 + 6Si + xAl. Thermodynamic calculations reveal that titanium oxide, aluminum titanate, nickel oxide, and nickel aluminate all have a negative free energy of reaction with aluminum from 298 to 1800 K, indicating that it may be possible to form alumina-intermetallic composites by reactions of the type (2 +x)Al + (3/y) MOy → Al2O3 + AlxM3/y. Experiments revealed that aluminum reacts with titanium oxide, nickel oxide, and nickel aluminate, but not aluminum titanate, at 1673 K. Reaction with the stoichiometric amount of aluminum (x = 0) leads to the formation of alumina and either titanium or nickel. In some cases, reactions with excess aluminum (x \u3e 0) produce intermetallic compounds such as TiAl3 and NiAl

Microstructure and Properties of Al₂O₃-Al(Si) and Al₂O₃-Al(Si)-Si Composites Formed by in Situ Reaction of Al with Aluminosilicate Ceramics

Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites have been formed byin situ reaction of molten Al with aluminosilicate ceramics. This reactive metal penetration (RMP) process is driven by a strongly negative Gibbs energy for reaction. In the Al/mullite system, Al reduces mullite to produce α-Al2O3 and elemental Si. With excess Al (i.e., x \u3e 0), a composite of α-Al2O3, Al(Si) alloy, and Si can be formed. Ceramic-metal composites containing up to 30 vol pct Al(Si) were prepared by reacting molten Al with dense, aluminosilicate ceramic preforms or by reactively hot pressing Al and mullite powder mixtures. Both reactive metal-forming techniques produce ceramic composite bodies consisting of a fine-grained alumina skeleton with an interpenetrating Al(Si) metal phase. The rigid alumina ceramic skeletal structure dominates composite physical properties such as the Young’s modulus, hardness, and the coefficient of thermal expansion, while the interpenetrating ductile Al(Si) metal phase contributes to composite fracture toughness. Microstructural analysis of composite fracture surfaces shows evidence of ductile metal failure of Al(Si) ligaments. Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites produced byin situ reaction of aluminum with mullite have improved mechanical properties and increased stiffness relative to dense mullite, and composite fracture toughness increases with increasing Al(Si) content