High-volume-fraction Si3N4-Al-based composites have been fabricated by high-pressure casting method. The effects of chemical compositionof infiltrated Al alloys, microstructures of Si3N4 preforms, and test temperature on the mechanical properties were investigated.The maximum four-point bending strength and fracture toughness of the composites reached 924 and 8.2MPa(√m), respectively. Increasingsintering temperature above 1650℃ for Si3N4 preforms resulted in an enhancement of fracture toughness at the expense of degradation inflexural strength. However, an increase in sintering time for Si3N4 preforms at moderately sintering temperature yielded an improvement inboth flexural strength and fracture toughness of the corresponding composites. Both flexural strength and fracture toughness decreased withincreasing temperature and CIP pressure due to inhomogeneous distribution of Al phase and some defects introduced into the preforms duringthe casting process. The Si3N4–6061 Al composite exhibited the lowest strength, which may be attributed to the presence of porosities andinterfacial reactions.

Ti–Al3Ti laminated composites have been fabricated through reactive sintering in vacuum usingTi and Al foils with different initial thickness.The aluminum layer was completely consumed resulting in microstructures of well-bonded metal–intermetallic layered composites with Tiresidual metal layers alternating with the aluminide intermetallic layers. The MIL composites exhibit a very high degree of microstructuraldesign and control. Microstructure characterization by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and energy dispersivespectroscopy (EDX) has shown that Al3Ti is the only titanium aluminide phase due to the thermodynamics and phase selection of the reactionbetween Ti and Al through mass diffusion in the presence of liquid Al. The mechanical properties and fracture behavior of the fabricatedlaminated composites were examined through three-point bending test. The results indicated that the composites exhibited anisotropic features.When the load perpendicular to the laminates was applied, they displayed a step-like or saw-tooth load-displacement response and superiorflexural strength as well as fracture toughness, which is also dependent on the number and thickness of individual layers. A non-catastropicfracture was observed in the laminated composites due to the deflection of cracks along the Ti/Al3Ti interface. The Ti layer failed by cleavagemode, showing extensive plastic deformation during the bending process.

Ti-Al3Ti laminated composites have been fabricated through reactive sintering in vacuum using Ti and Al foils with differentinitial thicknesses. The aluminum layer is consumed by forming a titanium aluminide intermetallic compound. Thus, the final microstructureconsists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructural characterization by scanningelectron microscopy (SEM), X-ray diffractometry (XRD) and energy dispersive spectroscopy (EDX) has shown that only theintermetallic Al3Ti is formed, which can be rationalized in terms of thermodynamics and kinetics of phase selection, as well as by thediffusion processes occurring in the presence of liquid Al.

Fe3Al-based composites reinforced with micron-sized titanium carbide, tetragonal zirconia and nano-scaled alumina particles were hot-pressed, and the room temperature mechanical properties were investigated. Results show that nano-Al2O3 is not an efficient ceramic for the enhancementof mechanical properties of Fe3Al, primarily due to its inhomogeneous distribution and severe agglomeration within the matrix as well as the weakinterfacial bonding. By contrast, Fe3Al/5 wt.% ZrO2 composite had a strength as high as 1361 MPa and a fracture toughness of 19.0 MPa ffiffiffiffi mp. Theenhancing effect is attributed to the low thermal expansion mismatch between ZrO2 and Fe3Al matrix, and to the stress-induced transformation ofZrO2 particles. The flexural strength and fracture toughness for Fe3Al/10 wt.% TiC composite was 1086 and 20.0 MPa ffiffiffiffi mp, respectively. Themechanical properties of Fe3Al/TiC composites decreased with TiC content above 10 wt.% due to the suppression of plastic deformation, residualporosity and interfacial debonding.

Porous ceramic with a framework structure of aluminum borate (9Al2O3-2B2O3) whiskers was in situ synthesized by firing above 1150 8Ca green powder compact of a mixture of aluminum hydroxide, boric acid and an additive of nickel oxide. During sintering, the whiskers ofaluminum borate grew in situ in the compact, and were bonded together. The porous aluminum borate consisted solely of whiskers with aporosity of 54-58%. The average diameters of the whiskers increased from 0.2 to 2 mm with increasing sintering temperature from 1150 to1350 8C. However, the estimated whiskers aspect ratio decreased with increasing sintering temperature.

Mo-Si-Al-C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by meansof reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showedthat Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo-Si-Al-C systems. For the stoichiometricMo5(Si,Al)3C mixed powders with a molar ratio of Mo∶Si∶Al∶C as 5∶1.5∶1.5∶1, the sintered body contained Mo3Si, Mo3Al2C, and Mo5Si3C as themajor reaction products whereas and the minor phases consisted of MoSi2, Mo2C, and Mo(Si,Al)2 compounds. When the starting powder mixturewas off-stoichiometric with a small amount of excess Si, only Mo2C accounted for the minor product. Moreover, the relative contents of the formerthree major phases were affected by the changed Si/Al ratio, where the amounts of Mo3Al2C and Mo5Si3C compounds decreased and increased,respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity.In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo-Si-Al-C-basedmultiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454and 438MPa, 4.93 and 4.85MPa√m.

NiAl-based alloys and their composites reinforced with in situ formed TiC and externally added ceramic particles are fabricated by hot-pressing.Their microstructures and mechanical properties are evaluated. Comparatively, the Ti-and/or C-alloyed NiAl and the ceramic particulate reinforcedcomposites possess a significant improvement in both flexural strength fracture toughness at room temperature. Nevertheless, the NiAl-TiC-nano-Al2O3 composite has low strength, mainly due to the existence of residual porosity, inhomogeneous distribution and severe agglomeration ofnano-scaled Al2O3 particle within the NiAl matrix.

The processing, microstructures and mechanical properties of intermetallic alloy based on AleMoeZreCo (AMZC) and its composites reinforcedwith micro-sized TiC, partially stabilized zirconia (PSZ)-ZrO2 or SiC particulates were investigated. The results showed that the alloysystem exhibits multi-phase microstructures, composed of several aluminides including ZrAl2, Al5Co2, Al9Co2, AlMo3, Al8Mo3 and Zr2Al. TheAMZC/SiC composite showed poor mechanical properties, due to the existence of residual porosity and weak interfacial bonding. In contrast,the other two composites exhibited superiority in both flexural strength and fracture toughness at room temperature than the AleMoeZreCobasedmulti-phase alloy. Homogeneous distribution of ceramic particles and perfect interfacial bonding accounted for the improvement ofstrength. The addition of TiC or ZrO2 particle into the matrix alloy produced remarkable toughening effect.

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 additionof 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 flexuralstrength and fracture toughness to the AMZG multi-phase alloy, which is derived from the contribution of uniformly distributedand well-embedded harder particulates and the constrained plastic deformation of the matrix. The addition of hard ceramic particlessimultaneously yielded higher bulk Vickers hardness. The toughness enhancement in the composites was attributed to the increased tortuousityby crack deflection, branching and bridging. Moreover, the transformation of tetragonal zirconia particles into the monoclinicform might also partially contribute to the toughness enhancement in the AMZG/ZrO2 composite.

The ternary carbide Ti3AlC2 was reactively synthesized from the elemental powder mixtures of Ti, Al and activated carbon. Cu/Ti3AlC2 composites were fabricated by hot-pressing method. Their microstructures and properties were investigated. The resultsdemonstrated that Ti3AlC2 is a promising reinforcement for copper and the Cu/Ti3AlC2 composites exhibited lower electrical conductivityand much superior flexural strength to pure copper matrix without remarkable loss of fracture toughness.