Melting behaviour of cadmium particles embedded in an aluminium matrix

An Al-4.5% Cd alloy has been manufactured by melt spinning to produce a microstructure of 14–150 nm diameter faceted cadmium particles embedded in an aluminium matrix. The melting behaviour of the cadmium particles has been investigated by differential scanning calorimetry. The melting point of 20 and 14 nm diameter cadmium particles embedded are depressed by 7 and 9 K respectively, below the bulk equilibrium cadmium melting point, because of Gibbs-Thomson capillarity effects. The average solid cadmium particle/aluminium matrix interfacial energy is 27 mJ m^–2 higher than the average liquid cadmium particle/aluminium matrix interfacial energy. No significant superheating is needed to nucleate cadmium particle melting.     

Heterogeneous nucleation of solidification of cadmium particles embedded in an aluminium matrix

A hypomonotectic alloy of Al-4.5wt%Cd has been manufactured by melt spinning and the resulting microstructure examined by transmission electron microscopy. As-melt spun hypomonotectic Al-4.5wt%Cd consists of a homogeneous distribution of faceted 5 to 120 nm diameter cadmium particles embedded in a matrix of aluminium, formed during the monotectic solidification reaction. The cadmium particles exhibit an orientation relationship with the aluminium matrix of {111}_Al//{0001}_Cd and 110_AlAl//11¯20> _Cd, with four cadmium particle variants depending upon which of the four {111}_Al planes is parallel to {0001}_Cd. The cadmium particles exibit a distorted cuboctahedral shape, bounded by six curved {100}_Al//{20¯23}_Cd facets, six curved {111}_Al/{40¯43}_Cd facets and two flat {111}_Al//{0001}_Cd facets. The as-melt spun cadmium particle shape is metastable and the cadmium particles equilibrate during heat treatment below the cadmium melting point, becoming elongated to increase the surface area and decrease the separation of the {111}_Al//{0001}_Cd facets.The equilibrium cadmium particle shape and, therefore, the anisotropy of solid aluminium-solid cadmium and solid aluminium -liquid cadmium surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 420 °C. The anisotropy of solid aluminium-solid cadmium surface energy is constant between room temperature and the cadmium melting point, with the {100}_Al//{20¯23}_Cd surface energy on average 40% greater than the {111}_Al//{0001}_Cd surface energy, and 10% greater than the {111}_Al//{40¯43_Cd surface energy. When the cadmium particles melt at temperatures above 321 °C, the {100}_Al//{20¯23}_Cd facets disappear and the {111}_Al//{40¯43}_Cd and {111}_A1//{0001}_Cd surface energies become equal. The {111}_Al facets do not disappear when the cadmium particles melt, and the anisotropy of solid aluminium-liquid cadmium surface energy decreases gradually with increasing temperature above the cadmium melting point.The kinetics of cadmium solidification have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Cadmium particle solidification is nucleated catalytically by the surrounding aluminium matrix on the {111}_Al faceted surfaces, with an undercooling of 56 K and a contact angle of 42 °. The nucleation kinetics of cadmium particle solidification are in good agreement with the hemispherical cap model of heterogeneous nucleation.     

Solidification of droplets

The response of a crystallite to the thermal fields developing during the solidification of a droplet and the mode of growth on a S?L interface are discussed. Stresses are relieved by causing the crystallite to deform plastically. The dislocations and/or twins that arise alter the morphology of the S?L interface and can enhance its growth rate. The analysis concentrates on crystallites growing in the bulk or at the surface of a droplet. For the hexagonal metals Cd, Mg and Zn, when the temperature varies only through the thickness of a thin floating raft, the growth helixes arise mainly as a result of the relief of strain energy due to the action of surface tension forces. Their calculated height is lower than the height measured on spherical particles of Cd.     

Nickel particle embedded aluminium matrix composite with high ductility

Nickel particulate reinforced aluminium matrix composite was processed without formation of deleterious Al₃Ni intermetallic by friction stir processing (FSP). FSP resulted in uniform dispersion of nickel particles in the aluminium matrix with excellent interfacial bonding and also lead to grain refinement of the matrix. The composite exhibited a threefold increase in the yield stress (0.2% proof stress). The most novel feature of the composite is that an appreciable amount of ductility is retained while the strength increases significantly. The microstructure evolution was studied by transmission electron microscopy and electron backscattered diffraction analysis. EBSD analysis showed a dynamically recrystallized equiaxed microstructure having a considerable fraction of low-angle boundaries. TEM observations revealed that these low-angle boundaries are essentially subgrain boundaries formed by dislocation rearrangement and absorption during friction stir processing.     

Solidification of aluminium spray-formed billets

A critical part of the billet spray-forming process is the successive intermittent deposition of thin layers of semi-solid aluminium alloy at different points on the top surface of the billet. Each thin layer is made up of a large number of impacted semi-solid spray droplets. As successive layers of alloy are deposited significant re-melting and re-freezing of underlying layers can occur. If the layers become too dry, high porosity will result; if they are too wet, fluid dynamic surface instabilities are possible. In extreme cases no billet will form. The process is essentially incremental, so that heat fluxes within the deposit very close to the top surface play a major role in determining the final deposit microstructure.In this paper transient heat transfer and solidification processes in the billet are analysed. First, some general features of billet heat transfer are discussed. The focus then narrows onto a thin layer of the deposit, lying very close to the billet surface. A boundary layer approximation is derived and computational results from this approximation are used to answer a number of questions of high practical value.     

Solidification of aluminium spray-formed billets

Transient heat transfer and solidification within an axisymmetric spray-formed aluminium billet are investigated. The boundary of the solid billet grows outwards, due to deposition from a stream of atomised semi-solid metal droplets. Within the billet, it is necessary to determine the heat fluxes and, in particular, to determine the position of the solidus isotherm. Mathematically, one must solve a nonlinear two-dimensional parabolic initial-boundary-value problem in an irregular and expanding domain.The problem is formulated within the general framework of billet heat transfer. An effective numerical algorithm is developed and implemented. Results from the numerical algorithm are used to explore thermal transients in the start-up phase of billet spray-forming production runs, the phenomenem of steady-state heat flow in the billet crown and the complex dependence of heat flow on billet-surface movement.     

Wear behaviour of an aluminium matrix composite

The wear behaviour of a composite material consisting in AS12UNG alloy reinforced with 15% short fibres of alumina has been studied. The material composition and the wear test conditions were defined in order to evaluate the potential performance of automotive pistons produced with such composite composition. As initially expected, the results indicate that an increase in the sliding velocity lead to higher wear rates in the stationary stages, and higher applied loads also induced acceleration in the wear process. Also, reciprocating sliding movement is clearly more damaging than the circular. However, results have shown that wear rates at 150 °C are lower than those recorded at room temperature representing a promising result for the use of this material in components that operate in this condition. This advantageous behaviour is lost at temperatures near to 300 °C, when a marked increase in the wear rate and a signification contribution of adhesive wear were observed.     

Processing, microstructure and mechanical properties of nickel particles embedded aluminium matrix composite

Nickel particles were embedded into an Al matrix by friction stir processing (FSP) to produce metal particle reinforced composite. FSP resulted in uniform dispersion of nickel particles with excellent interfacial bonding with the Al matrix and also lead to significant grain refinement of the matrix. The novelty of the process is that the composite was processed in one step without any pretreatment being given to the constituents and no harmful intermetallic formed. The novel feature of the composite is that it shows a three fold increase in the yield strength while appreciable amount of ductility is retained. The hardness also improved significantly. The fracture surface showed a ductile failure mode and also revealed the superior bonding between the particles and the matrix. Electron backscattered diffraction (EBSD) and transmission electron microscopy analysis revealed a dynamically recrystallized equiaxed microstructure. A gradual increase in misorientation from sub-grain to high-angle boundaries is observed from EBSD analysis pointing towards a continuous type dynamic recrystallization mechanism.     

Wettability of aluminium nitride by tin–aluminium melts

The wettability of aluminium nitride by Sn–Al melts was studied by the sessile drop method in a vaccum of 2 × 10^–3 Pa at 1100 °C over the whole concentration region. The minimum interval on the contact-angle concentration dependence curve was observed at intermediate composition. For comparison, experiments were also performed on porous AlN. Wetting of porous nitride is worse than the dense nitride. The results have been analysed on the basis of the relation between wettability and the chemical interface reactivity in solid–liquid metal systems.     

Solidification behaviour of Pd–Rh droplets during spray atomization

Solidification microstructure in spray-atomized Pd–10 wt% Rh powders using high-pressure gas atomization was studied. The solidification cooling rate and the solidification front velocity were investigated using a transient heat-transfer finite element method. Two different atomization gases, nitrogen and helium, were considered in the modelling studies. On the basis of the results obtained, it was found that gas atomization using helium gas led to solidification cooling rates and solidification front velocities which were two times higher than those obtained using nitrogen gas. Moreover, the cooling rate and the solidification front velocity increased with decreasing powder size for both types of atomization gas. The numerically estimated solidification front velocity using finite element analysis for nitrogen gas atomization was found to be smaller than the analytically determined absolute stability velocity that is required to promote a segregation-free microstructure. This was noted to be consistent with the segregated microstructure that was experimentally observed in nitrogen gas atomized powders. In the case of helium gas atomization, however, the increased cooling rate and solidification front velocity are anticipated to promote the formation of a segregation-free microstructure in the gas-atomized powders. This revised version was published online in November 2006 with corrections to the Cover Date.     

The one-body density matrix in Helium droplets

The growth of the Bose condensate in Helium droplets is studied through variational Monte Carlo calculations of the one-body density matrix. Both the off-diagonal and the =0 component of the one-body density matrix show unmistakable finite range ordering within the core of the droplet.     

Optical properties of tin oxide on anodized aluminium

The solar selective properties of tin oxide on anodized aluminium have been investigated. It has been shown that the pyrolytic spray method can be used to apply an IR-reflecting coating on black pigmented anodized aluminium. This surface exhibits high solar selectivity and the excellent mechanical, chemical and thermal stability of this sandwich makes it an interesting alternative in solar energy applications.     

Solidification contact angles of molten droplets deposited on solid surfaces

Droplet impact and equilibrium contact angle have been extensively studied. However, solidification contact angle, which is the final contact angle formed by molten droplets impacting on cold surfaces, has never been a study focus. The formation of this type of contact angle was investigated by experimentally studying the deposition of micro-size droplets (∼39 μm in diameter) of molten wax ink on cold solid surfaces. Scanning Electron Microscope (SEM) was used to visualize dots formed by droplets impacted under various impact conditions, and parameters varied included droplet initial temperature, substrate temperature, flight distance of droplet, and type of substrate surface. It was found that the solidification contact angle was not single-valued for given droplet and substrate materials and substrate temperature, but was strongly dependent on the impact history of droplet. The angle decreased with increasing substrate and droplet temperatures. Smaller angles were formed on the surface with high wettability, and this wetting effect increased with increasing substrate temperature. Applying oil lubricant to solid surfaces could change solidification contact angle by affecting the local fluid dynamics near the contact line of spreading droplets. Assuming final shape as hemispheres did not give correct data of contact angles, since the final shape of deposited droplets significantly differs from a hemispherical shape.     

Multi-band silicon quantum dots embedded in an amorphous matrix of silicon carbide

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized by a magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy, glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used to characterize the chemical composition and the microstructural properties. The results show that the sizes and size distribution of silicon quantum dots can be tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon in the matrix. A physicochemical mechanism is proposed to demonstrate this formation process. Photoluminescence measurements indicate a multi-band configuration due to the quantum confinement effect of silicon quantum dots with different sizes. The PL spectra are further widened as a result of the existence of amorphous silicon quantum dots. This multi-band configuration would be extremely advantageous in improving the photoelectric conversion efficiency of photovoltaic solar cells.     

Compressive strength of a rigid-rod polymer fibre embedded in an isotropic matrix

The results are presented of an approximate elastic stability analysis for an anisotropic polymer fibre under compressive stress, which is embedded in an isotropic elastic matrix. This case, which thus far has not been treated properly, corresponds most closely to the experiments, which yield the best quantitative measurements of the compressive strength of high-modulus polymer fibres. Within the limits of a weak matrix, i.e. the shear modulus of the matrix is small compared to the shear modulus of the fibre, a simple analytical formula has been obtained for the compressive strength of the fibre in terms of its longitudinal Young's modulus, and the Poisson's ratio and shear modulus of the matrix. On the other hand, for a strong matrix the compressive strength of the fibre is solely determined by its shear modulus. For the intermediate regime, a simple but highly accurate interpolating expression has been constructed.     

Eccentric compression stability of multi-walled carbon nanotubes embedded in an elastic matrix

This paper reports the results of an investigation on the eccentric compression stability of multi-walled carbon nanotubes embedded in an elastic matrix. Based on continuum modeling, a multilayer shell model is presented for the eccentric compression buckling of multi-walled carbon nanotubes embedded in an elastic matrix, in which the effect of van der Waals forces between two adjacent tubes is taken into account. The critical bending moment and the eccentric compression mode for three types of multi-walled carbon nanotubes with different layer numbers and ratios of radius to thickness are calculated. Results obtained show that the eccentric compression buckling mode corresponding the critical bending moment is unique, and is different from the purely axial compression buckling of an individual multi-walled carbon nanotube. For different types of multi-walled carbon nanotubes, the effect of matrix stiffness on the critical bending moment of multi-walled carbon nanotubes under eccentric compression loading is obviously different, and is dependent on the innermost radius and layer numbers of the multi-walled carbon nanotubes. The critical bending stress exerted on the center tubes of nearly solid multi-walled carbon nanotubes does not change as the ratio of the axial compression loading to the bending membrane force increases. The new features and meaningful numerical results in this paper are helpful for the application and the design of nanostructures in which multi-walled carbon nanotubes act as basic elements.     

Fabrication of fibre composites using an aluminium superplastic alloy as matrix

An aluminium superplastic alloy has been used as the matrix for a variety of fibre reinforcements. It is shown that, by hot pressing in the superplastic regime of the alloy, a number of different reinforcements can be incorporated into the matrix. Tensile tests on composites with up to 25 vol % of reinforcement showed good agreement with the rule of mixtures.     

Structural studies of aluminium nitride embedded in amorphous carbon

Aluminum nitride containing diamond-like carbon was fabricated with pulsed laser deposition without post processing. The compositions of the targets used were varied at 1, 5, 10, 15 at.% and pure carbon was used as a reference. The films were comprehensively characterized with Atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM). Roughness analysis using AFM showed an increasing root-mean-square (RMS) roughness with increasing AIN content in target, while XPS analysis showed that the aluminum-nitrogen bonding was still present in the films after the fabrication process. Microstructural studies and selected area electron diffraction (SAED) pattern confirmed the presence of AIN crystals in DLC matrix. This nanostructured composite material is useful for luminescence applications.