Wei Hsing Tuan

The effects of microstructure on the mechanical properties of Al2O3-NiAl composites

Abstract In the present study, the room-temperature mechanical properties of Al 2 O 3 -NiAl composites containing 0 to 100 vol.% NiAl are determined. The composites are prepared by attrition milling Al 2 O 3 and NiAl powders together followed by hot-pressing in vacuum. The Al 2 O 3 and NiAl grains in the composites constrain each others growth. Since the hardness of NiAl is lower than that of Al 2 O 3 , the hardness of the composites decreases with the increase of NiAl content. The strength and toughness of the Al 2 O 3 -NiAl composites are higher than the values predicted by the rule of mixtures. The strengthening effect is mainly contributed by the microstructural refinement. Because the Al 2 O 3 /NiAl interface is weak, cracks propagate mainly along the interfaces. The fracture toughness is thus enhanced.

Effect of surface grinding on the strength of NiAl and Al2O3/NiAl composites

Abstract For structural applications, dimensional tolerance has to be controlled tightly. Surface grinding is thus frequently applied to match the requirement. For brittle material, improper surface grinding can degrade their strength considerably. In the present study, an alumina wheel and a diamond wheel were used to grind NiAl and Al2O3/NiAl composites. The surface quality, strength and toughness after grinding are investigated. Cracks are formed on the surface of NiAl ground with the alumina wheel. However, there is no crack found on the surface of NiAl ground with the diamond wheel. The strength of the NiAl machined with diamond wheel is three times that of the NiAl machined with alumina wheel. As Al2O3 particles are added into NiAl, the presence of weak Al2O3/NiAl interfaces limits the formation of large flaws. The strength of the Al2O3/NiAl composites is thus less sensitive to the grinding conditions. The present study demonstrates that the Al2O3/NiAl composites are tolerant to the surface grinding conditions.

Preparation of Al2O3-ZrO2-Ni Nano-Composites by Spark Plasma Sintering

In the present study, a process to prepare Al2O3-ZrO2-Ni nano-composite is explored. The nano-sized nickel particles, around 1-10 nm, can be successfully prepared by using a solution and coating technique. These nano-sized Ni particles disperse uniformly onto the surface of micro-sized Al2O3 and ZrO2 particles after the coating treatment. The powder mixtures were then consolidated by employing a spark plasma sintering (SPS) technique at 1350C for 1 to 5 minutes. Dense composite is resulted after SPS, though a slight density variation within the specimen is also noted.

Effect of grinding parameters on the reliability of alumina

Abstract In the present study, the effect of grinding parameters on the reliability of alumina specimens is investigated. The reliability is characterized by using the Weibull statistics. The grinding parameters investigated are the wheel depth of cut (downfeed), the table speed and the peripheral speed of wheel (wheel speed). The downfeed is varied from 10 to 30 μm per pass, the table speed from 0.17 to 0.27 m/s, the wheel speed from 20 to 33 m/s. The effect of the grinding parameters can be combined into a single parameter, the maximum grit depth of cut. The reliability shows strong dependence on the maximum grit depth of cut. The Weibull modulus can be as high as 10.9 as a low value of the maximum grit depth of cut is used. Nevertheless, the Weibull modulus is only 6.9 for the case that a high value of the maximum grit of cut is applied.

Toughening Al2O3 with NiAl and NiAl(Fe) particles

Abstract Aluminia composites of Al2O3–NiAl and NiAl(Fe) were prepared by hot pressing mixtures of Al2O3with NiAl and NiAl(Fe) powders respectively to a density >98·5%theoretical for evaluation of toughness as a function of intermetallic content and comparison with hot pressed Al2O3 , NiAl and NiAl(Fe). The intermetallic grain size, in both cases, was found to be smaller than the as received powder particle size in alumina rich composites because of the pinning effect of alumina particles on grain boundaries, but increased with increase in intermetallic content due to grain growth as the pinning effect decreased at lower alumina contents. The toughness of hot pressed NiAl(Fe) was ∼50% greater than for NiAl. Both intermetallic compositions showed a steady increase in the toughness of the composites with increase in intermetallic content, with NiAl(Fe) composites being somewhat tougher at higher intermetallic contents. The Fe content in NiAl(Fe) appears to increase the interfacial strength in the composites with the result that, although Al2O3–NiAl(Fe) composites are the tougher of the two systems, the toughness enhancement over rule of mixtures values is less in the NiAl(Fe) system. Toughness enhancement in the Al2O3–NiAl system is attributed to a combination of crack deflection and crack bridging but in the Al2O3–NiAl(Fe) system it is mainly attributed to crack bridging.

Strengthening Zirconia by Adding Both Nickel and Alumina Particles

In the present study, the processing-properties relationships of the ZrO2/(Ni+Al2O3) composites are examined. Dense composites were prepared either by pulse electrical current sintering (PECS) at 1350C for 5 minutes or by pressureless sintering (PLS) at 1600C for 1 h. The size of Ni particles is as small as 30 nm to 50 nm. Though the size of ZrO2 grains in the matrix increases as alumina and nickel particles are added, the strength of the ZrO2/(Ni+Al2O3) composites is significantly higher than that of monolithic ZrO2.

Design of Multiphase Materials

In the present study, several principles are introduced as the guidelines to design multi- phased materials. Each phase in the multiphase material can offer one function or property to the material. The functions contributed from the phases within the multiphase material can interact with each other. Such interactions can be tailored by suitable microstructure design. The Al2O3-ZrO2-Ni multiphase material is used to demonstrate the applications of the design principles.