Z. Fan

Microstructure of ceramic foams

This paper describes the preparation of ceramic foams by expansion of a ceramic suspension based on a polyurethane system. The microstructure and degree of reticulation of the foamed ceramic were examined and analysed with the help of a simple geometrical model. Like the porous ceramics prepared by the replica processing method, these foamed ceramics possess open cells in a nearly equiaxed shape but the cell size is much finer. The ratio of the window size to the cell size is a useful parameter for characterising the geometry of the foam and is related to the qualitative concept of degree of reticulation. For a face centred cubic array of cells it is related geometrically to the volume fraction of porosity and this relationship is tested using microstructural measurements for a range of ceramic foams.

Semi-solid processing of engineering alloys by a twin-screw rheomoulding process

Abstract Based on the extensive experience in injection moulding of polymeric materials, a twin-screw rheomoulding process has been developed in our laboratory for near net-shape production of engineering components. The rheomoulding equipment consists of a liquid metal feeder, a twin-screw extruder with closely intermeshing, self-wiping and co-rotating screws, a shot assembly and a central control unit. The fluid flow in the twin-screw rheomoulding process is characterised by high shear rate and high intensity of turbulence. The experimental results of rheomoulded Sn–15wt.% Pb and Mg–30wt.% Zn alloys have demonstrated that the twin-screw rheomoulding process is capable of producing small and near mono-sized solid particles distributed uniformly in a fine-grained eutectic matrix. Compared with other existing semi-solid metal processing techniques, the twin-screw rheomoulding process has the following advantages: small and spherical solid particles of near mono-size, chemical and microstructural uniformity throughout the component, accurate control over a large range of solid volume fractions, lower overall component cost due to low cost of feedstock materials, and shorter cycle time.

Bi-continuous metal matrix composites

Bi-continuous alumina:aluminium composites were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. The low density preforms were prepared from a polyurethane suspension of alumina powder which was pyrolysed and sintered after foaming. Higher density preforms consisted of ceramic foams with open cells. All these preforms were infiltrated with 6061 aluminium alloy using a modified squeeze caster fitted with a vacuum system and fine control of speed and pressure. The microstructure of the preform fitted an established relationship between the ratio of window diameter to cell diameter (k) and void volume fraction (Vp). Low k foams were infiltrated fully but on cooling below the solidus, interfacial debonding took place due to differential thermal contraction. This was overcome by modifying the processing conditions. High k foams which had high fractional porosity, retained sound interfacial bonding. The composites possess higher elastic modulus than conventional MMCs with a homogeneous reinforcement distribution at a given volume fraction. The loss of electrical conductivity is negligible in the lower volume fraction range because of the three dimensionally continuous aluminium phase. The experimental results are compared with a number of theoretical predictions.

Equilibrium pseudobinary Al–Mg2Si phase diagram

Abstract Preliminary experiments and phase diagram calculations were conducted to determine the equilibrium phase diagram of the Al–Mg2Si pseudobinary section. It was found that there is a narrow ternary phase field of Al+Mg2Si+liquid in the diagram. At the pseudoeutectic composition of Al–13.9 wt-%Mg2Si, a pseudoeutectic reaction takes place between the temperatures of 583.5 and 594°C. The solubility of Mg2Si in Al at 583.5°C is calculated as 1.91 wt-%.

Effect of cooling rate on the microstructure of hypereutectic Al-Mg2Si alloys

chinese acad sci, inst met res, shenyang 110015, peoples r china. brunel univ, dept mat engn, uxbridge ub8 3ph, middx, england.;zhang, j (reprint author), chinese acad sci, inst met res, shenyang 110015, peoples r china

Microstructural characterisation of interpenetrating nickel/alumina composites

Interpenetrating phase composites (IPCs) are a new class of composite materials in which both phases are three-dimensionally (3D) continuous. Such interpenetrating microstructures offer improved combinations of mechanical and physical properties and enhanced damage tolerance. However, identifying those composites containing interpenetrating phases has been a challenge. A study was performed on the microstructures of some nickel/alumina composites that had been produced by hot pressing powder blends of various volume fractions of nickel and alumina. The composite microstructures were characterised in terms of grain size and contiguity to establish the distribution of the phases. Two different techniques, preferential etching and resistivity measurements, were used to determine which compositions displayed interpenetrating phases.

Processing of immiscible metallic alloys by rheomixing process

Abstract Mixing immiscible alloys has been a long standing challenge to both materials scientists and processing engineers. Despite great efforts made worldwide, including extensive space experiments, no casting techniques so far can produce the desirable fine and uniform dispersed microstructure. Based on extensive experience in mixing immiscible organic liquids offered by the polymer processing community, the authors have successfully developed a rheomixing process for mixing immiscible alloys. The rheomixing process utilises first the intensive shear stress-strain field offered by a twin screw extruder to create a fine homogeneous liquid dispersion within the miscibility gap and then the viscous force offered by a semisolid slurry at a temperature below the monotectic temperature is used to counterbalance the gravitational force and the Marangoni effect. A laboratory scale rheomixer has been designed and constructed to realise this two step mixing strategy. The Ga–Pb and Zn–Pb systems were selected to demonstrate the principles of rheomixing. The experimental results showed that the rheomixing process developed is capable of creating a fine and uniform microstructure from immiscible alloys. This paper describes the rheomixing process in detail and the preliminary experimental results on rheomixing in the Ga–Pb and Zn–Pb systems are discussed.

Cellular arrays of alumina fibres

In conventional short fibre reinforced metal matrix composites, the quest is for a method of processing that will provide a homogeneous and preferably random arrangement of fibres. In contrast, recently developed contiguity models for multiphase composites on the one hand, and finite element modelling of structures on the other, independently predict that the modulus enhancement provided by short-fibre reinforcement can be improved if the fibres are arranged in a cellular structure. Furthermore, provided the metallic phase is continuous, the toughness of the composite may also thereby be enhanced. This paper, which is part of an attempt to explore the question of reinforcement arrangements, presents a method for making ceramic preforms for MMCs in which a polymeric foam is used to position the fibres in cellular array. The polymer is then removed by pyrolysis and the preform of fibres is strengthened by sintering. During high temperature sintering, phase changes and grain growth degraded the fibre. Methods of increasing the compressive strength of the preform by incorporation of alumina particles and by subsequent infiltration are described and compared.

Novel MMC microstructure with tailored distribution of the reinforcing phase

Saffil short fibre agglomerates with diameters of 0.4u2003mm to 1u2003mm have been prepared using a tumbling technique. These were packed and infiltrated with molten 6061 Al alloy to make a metal matrix composite (MMC) with a novel microstructure in which the composite spheres are randomly distributed in the fibre‐free aluminium matrix. In parallel, a commercial preform made of identical Saffil alumina short fibres and having the same fibre volume faction was used to prepare a conventional MMC by the same technique. Microstructural observation indicates that, within the composite spheres, the local volume fraction of fibre decreases from the outer layer to the centre region.

High‐resolution electron microscope observation of interface microstructure of a cast Al‐Mg‐Si‐Bi‐Pb(6262)/Al2O3p composite

High‐resolution electron microscopy was employed to characterize the interface structure of a cast Al‐Mg‐Si‐Bi‐Pb aluminium(6262)‐based composite reinforced by α alumina particles with a trace of β alumina in order to investigate the behaviour of alloying elements in cast composites. Except for a few primary Mg2Si particles, few reaction products were detected at the interface of Al/α‐Al2O3 due to the unfavourable reaction kinetics during the squeeze‐casting process. The Mg2Si particle has an orientation relationship with α‐Al2O3 of [011]Mg2Si//[1210]α‐Al2O3 (111)Mg2Si//(0006)α‐Al2O3. A significant amount of MgAl2O4 was found on the surface of the β‐Al2O3 particles, which is in contrast to the small degree of reaction found on α‐Al2O3 particles. MgAl2O4 and β‐Al2O3 particles have the following orientation relationship: [011]MgAl2O4//[1210]β‐Al2O3 (111) MgAl2O4//(0006)β‐Al2O3. The similar crystal structure of β‐Al2O3 to MgAl2O4 favours MgAl2O4 nucleation and growth on the surface of β‐Al2O3. Interfacial energy minimization dominates the atomic structure of the interface with the result that close packed planes and directions in the Al2O3 reinforcement and reaction products are parallel to the interfaces. Bi and Pb were found in the form of metallic nanometre particles between Al2O3 particles, or between the MgAl2O4 and Al2O3 particles, or in the open channels of β‐Al2O3 filled by the Al matrix.