Mechanically alloyed nanocomposites

High-density nickel–dispersed-alumina (Al2O3/nickel) composites with superior mechanical properties were obtained by the hydrogen reduction and the hot pressing of alumina–nickel oxide (Al2O3/NiO) mixed powders. The mixtures were prepared by using NiO or nickel nitrate (Ni(NO3)2·nH2O) as a dispersion source of nickel metal. Microstructural investigations of the composite fabricated using nitrate powder revealed that fine nickel particles, } 100 nm in diameter, dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite. High strength (.1 GPa) and high-temperature hardness were registered for the composite that contained a small amount of nickel dispersion. The ferromagnetic properties of nickel, such as high coercive force, were observed, because of the fine magnetic dispersions, which indicates a functional value of structural composites.

Reduction and Sintering of a Nickel–Dispersed‐Alumina Composite and Its Properties

The use of chromium (III) acetylacetonate as a source of nanometre sized chromium particles for the production of Al 2 O 3 –5 vol.% Cr nanocomposites has been investigated. The details of the processing procedure are crucial in determining the mechanical properties of the composite. The highest strength and fracture toughness, 736±29 MPa and 4.0±0.2 MPa m 1/2 , respectively, were obtained for the nanocomposite hot pressed at 1450 °C. It is shown that the strengthening in Al 2 O 3 –5% Cr nanocomposites mainly results from microstructure refinement in that the mean alumina matrix grain size in the optimum composite was 0.68 μm compared with a grain size of 3.6 μm in the monolithic alumina hot pressed under identical conditions. Crack bridging and crack deflection by the nano-sized Cr particles did not occur to any significant extent. The slight improvement in fracture toughness may result from the observed change in fracture mode from intergranular fracture for monolithic alumina to transgranular failure for the nanocomposites.

Processing and mechanical properties of Al2O3–5 vol.% Cr nanocomposites

An optimum route to fabricate the Al 2 O 3 Cu nano-composite with sound microstructure and desired mechanical properties was investigated. Two methods for developing a uniform dispersion of Cu particles in Al 2 O 3 were compared on the basis of the resulting microstructures and mechanical properties. SEM and TEM analyses for the composites fabricated by reduction and sintering process using Al 2 O 3 CuO powder mixture showed that the nano-sized Cu particles were well distributed and situated on the grain boundaries of the Al 2 O 3 matrix. The composite, hot-pressed at 1450 °C, exhibited the maximum fracture strength and enhanced toughness compared with monolithic Al 2 O 3 . The strengthening was mainly attributed to the refinement of Al 2 O 3 matrix grains. The toughening mechanism in Al 2 O 3 Cu composite was discussed by the observed microstructural features and theoretical predictions based on crack bridging model and thermal residual stress effect.

Fabrication and mechanical properties of 5 vol% copper dispersed alumina nanocomposite

The role of additives has long been a theme in sintering science. Recently, it has been discovered that ceramics may contain multiple different grain boundary phases (complexions) that are chemically induced by certain additives. These complexions are useful in explaining a number of anomalous phenomena associated with sintering, such as abnormal grain growth. The current work investigates how transitions between these complexions occur and at which grain boundaries they are most likely to occur. The number of complexion transitions that occur increases linearly with grain size (grain boundary excess concentration), and exponentially with temperature. The results suggest that grain boundary energy and anisotropy are important in predicting which and how many grain boundaries will undergo such a transition.

Demystifying the role of sintering additives with “complexion”

Brittle solids can be toughened by the introduction of ductile metallic inclusions. In the present study, the mechanical properties and oxidation resistance of Al2O3/Ni composites are investigated. The oxidation resistance of the ceramic/metal composite (8 × 10−11 g2 cm−4 s−1 at 1300° C) is comparable to that of many silicon nitrides. The fracture toughness of the composite containing 13 vol.% nickel is twice that of alumina alone. The square of the toughness enhancement for composites containing various amounts of nickel exhibits a linear relationship with the product of volume fraction and inclusion size, as predicted in theoretical models. For the alumina/nickel composite system, it is demonstrated that dissolved oxygen in the nickel increases the yield strength of nickel and enhances the toughness of the composite.

W.H. Tuan

Papers

The toughening of alumina with nickel inclusions