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.

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.

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.

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.

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.