The behaviour of double oxide film defects in Al–4.5 wt% Mg melt

The possibility of bonding of the two layers of a double oxide film defect when held in a liquid Al–4.5 wt% Mg alloy was investigated. The defect was modelled experimentally by maintaining two aluminium oxide layers in contact with each other in an Al–4.5 wt% Mg liquid alloy at 750 °C from 2 min to 16 h. Any changes in the composition and morphology of these layers were studied by SEM, EDX and XRD. The results showed that in contrast to previous studies reported in the literature on Al–0.3 wt% Mg in which the two layers bonded to each other after a holding time of 5 h, no bonding took place between the two oxide layers even after a holding time of 16 h. Based on the comparison between the two studies, it was concluded that a transformation involving rearrangement of atoms at the interface between the two oxide layers is essential for the bonding to take place between the two oxide layers. This criterion could be used to predict the bonding behaviour of oxide film defects when held in different liquid aluminium alloys, or when subjected to a HIPping process.     

Effect of aluminium content on the anodic behaviour of copper-aluminium alloys in 3.5 wt% NaCl solution

Anodic behaviour of heterogeneous Cu-Al alloys in 3.5 wt% NaCl solution has been investigated as a function of aluminium content by potentiodynamic polarization, cyclic voltammetry and potentiostatic current transient experiments and compared to pure Cu and homogeneous Cu-Al alloys. The specimens were subjected to various anodic potentials and subsequent analyses by scanning electron microscopy (SEM) supplemented with energy dispersion spectroscopy (EDS). The pure Cu and single -phase Cu-Al alloys showed that a relatively thick protective CuCl film forms at active-passive transition potentials and transforms into CuO or Cu(OH)₂ at the passivation potential, resulting in high limiting current densities. However, the Cu-10 wt% Al alloy is passivated to Al(OH)₃, impeding the transformation of the thick CuCl layer which contains AlCl or Al(OH)₂Cl salt into CuO or Cu(OH)₂. At high applied anodic potential, the Cu₂(OH)₃Cl phase forms on the deteriorated passive film Al(OH)₃.     

Deformation-mechanism map for Ti-6wt% Al alloy

A micro-computer program was developed to plot deformation mechanism maps for metals and substitutional solid solutions such as Ti-6 wt% Al. The package written in Turbo Basic is versatile and can be run to print out maps, using the related stored data files. In modelling the maps, the rate equations governing different deformation mechanisms were employed to find the dominant mechanisms and strain rate contours. In particular, the map for Ti-6 wt% Al was generated and it was found that the power law creep dominated over a wider range of stresses and temperatures than that of pure titanium. It is concluded that the strengthening effect of aluminium has shifted the iso-strain rate contours to higher stresses and temperatures.     

Influence of oxide film defects generated in filling on mechanical strength of aluminium alloy castings

Abstract

The influence of oxide film defects generated from the filling process on the mechanical strength of aluminium alloy castings has been investigated. Using numerical simulation and experimental validation, the investigation aims to reveal the relationships among the liquid aluminium flow behaviour in the filling, the likely oxide film defect distribution caused by surface turbulence and the final mechanical strength of the castings. CFD modelling was used to investigate the liquid metal flow behaviour and the likely oxide film defect distribution in the filling at different ingate velocities. In particular, a numerical algorithm - Oxide Film Entrainment Tracking (OFET, 2-D) has been proposed and developed for predicting such oxide film defect distribution in the liquid aluminium throughout the filling. Also, light microscopic and SEM techniques were used to identify the microstructures of oxide film casting defects. The Weibull statistics method was employed to quantify the effects of oxide film distributions on the mechanical strength and reliability of the acquired aluminium alloy castings for different runner systems. It was found from the numerical simulations that the ingate velocities acquired using different runner systems have significant influence on the distribution of oxide film defects generated by surface turbulence in the filling process, which results in the disparities of the final mechanical strength of the castings. The results of the mechanical property test and the SEM micro-structural analysis of the castings are consistent with the numerical simulations.     

Modelling of degradation of nickel-pigmented aluminium oxide photothermal collector coatings

Thermal degradation phenomena occurring in nickel-pigmented aluminium oxide coatings were investigated by combining detailed microstructural analysis with modelling of the optical properties. Scanning electron microscopy and Auger electron spectroscopy sputter profiling showed that the initial film consisted of a nickel-pigmented aluminium oxide layer close to the substrate. This layer supported a porous aluminium oxide layer that had a rough outer surface. While the solar absorptance degraded substantially (from 0.94 to 0.71) after heat treatment at 350°C for tens of hours, the aluminium oxide film morphology and thickness remained virtually unchanged and there was apparently no redistribution of nickel within the coating. Instead, the optical quality of the film degraded through oxidation of the nickel particles. These observations were supported by an optical model of the coating which produced the spectral reflectance properties measured both before and after the thermal ageing.     

Interface and fracture of carbon fibre reinforced Al-7 wt% Si alloy

The interface structure in an aluminium-7 wt% silicon alloy reinforced with carbon fibres has been investigated using analytical electron microscopy. Crystals of aluminium carbide (Al₄C₃) have been identified in interface regions and their structure and growth are discussed. Mechanical properties of the composite have been measured and fracture behaviour studied using acoustic emission analysis in parallel with microstructural examination. The results indicated that the aluminium carbide interfacial reaction had produced a strong fibre matrix bond, but reduced the fibre strength and embrittled the matrix. Consequently, whole fibre bundles failed in a brittle manner in the longitudinal direction with limited pull-out of individual fibres. The findings are discussed in relation to the method used to manufacture the composite.     

Spinodal decomposition in Co-3wt% Ti-1wt% Fe and Co-3 wt% Ti-2 wt% Fe alloys

The transformation on ageing Co-3 wt% Ti-1 wt% Fe and Co-3 wt% Ti-2 wt% Fe alloys has been followed by transmission electron microscopy and diffraction to establish that the solid solutions undergo spinodal decomposition at 823 and 973 K. The microstructural evolution has been correlated with the observed variations in hardness and yield strength. The coarsening of the modulations on ageing is seen to follow ³- ₀ ³ =Kt kinetic law. After long periods of ageing a discontinuous coarsening process sets in.     

Thermally induced surface changes in aluminium-iron oxide thermites

The aluminium and iron oxide surfaces of Al-Fe₃O₄ thermite in powder and pressed pellet form were studied before and after accelerated ageing at 180° C by X-ray photoelectron spectroscopy. The Al₂O₃ surface film thicknesses on aluminium metal were deduced from the intensity ratio of aluminium K L L Auger signals induced by X-ray radiation on Al₂O₃ and aluminium metal. Based on the mean free path of 1.65 nm for aluminium K L L Auger electrons, the oxide thickness on aluminium flakes before mixing with Fe₃O₄ was estimated to be 0.8 to 0.9 nm. A slight oxidation was observed on the aluminium surface after mixing with Fe₃O₄ at room temperature. Hot pressing of this mixture at 425° C for 7 min increased the oxide film to 3.1 nm. This surface oxide film seemed to protect the aluminium metal, and further ageing at 180° C did not cause significant oxide growth.Mound is operated by Monsanto Research Corporation for the US Department of Energy Under Contract No DE-AC04-76DP00053.     

Structure modification of rapidly solidified Al-11 wt% Si alloy by the addition of 2 wt% Fe

An investigation of the structure formation of as-cast Al-11 wt% Si (AlSi11) and Al-11 wt% Si-2 wt% Fe (AlSi11Fe2) rapidly solidified (RS) ribbons is performed. Ribbon samples are examined in detail using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). A nanoscale thickness layer of Si-precipitations, at both Al-grains boundary and oxide/Al-matrix interface, is found to be characteristic for “pure” AlSi11 ribbons. For Fe-doped ribbons, this layer comprises Fe-containing phase also, which appears to possess amorphous nature and is situated between the Si-grains and Al-matrix. A possible mechanism of altering the thermally activated volume diffusion of elements, due to the effect of Fe on the structure modification, is speculated based on the obtained results.     

Microstructure of diffusional zirconia–titanium and zirconia–(Ti–6 wt% Al–4 wt% V) alloy joints

Zirconia–titanium and zirconia–titanium alloy joints were made by diffusion bonding under an inert atmosphere at temperatures in the 1162–1494°C range. To inhibit the strong oxygen uptake by the titanium member a platinum insert was alternatively used. The microstructures and elemental profiles across the joints were investigated by scanning electron microscopy imaging and energy-dispersive spectroscopy or wavelength-dispersive spectroscopy microanalysis. It was found that direct ZrO2–Ti joining produces oxygen saturation in the Ti member and the formation of (Ti,Zr)2O at the interface. ZrO2/Pt/Ti joints present a complex layer sequence which at lower temperatures can be described on the basis of the Pt–Ti binary, except near the ceramic where a (Pt,Zr)-rich layer forms; at higher temperatures these joints develop an oxide layer of composition Ti2O3, this oxide probably resulting from local decomposition of the ceramic and reaction of oxygen with the incoming titanium. When Ti is replaced by the Ti–6 wt% Al–4 wt% V alloy in joints where Pt is present, the main consequences are the presence of liquid at lower joining temperatures and the earlier development of the oxide layer, now of nominal composition TiO. In all Pt-containing joints a phase of nominal composition Ti3Pt2 forms; it is advanced that this may be an equilibrium phase not predicted by the Pt–Ti diagrams available. All joints are weak, the fracture path running through the metal in the case of direct ZrO2–Ti joints and through the interface between the ceramic and the (Pt,Zr)-rich layer in joints where Pt is present.     

Prediction of surface oxidation weight gain on 7.8 wt% Cr-containing stainless steel electrode during electroslag remelting

The surface oxidation weight gains on 7.8 wt% Cr-containing stainless steel at different temperatures were studied by thermogravimetric analysis and scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS). The results show that there are two layers of oxide scales covering the surface of steel matrix after 100 min high-temperature oxidation. The inner oxide scale consists of (Fe, Cr) oxide and the outer oxide scale is composed of Fe oxide due to the stronger affinity of Cr with O. The growth of outer oxide scale is dominated by the combined effects of chemical reaction rate and mass transfer. Based on the study of the oxidation kinetics at different temperatures, a prediction model for surface oxidation weight gain on 7.8 wt% Cr-containing stainless steel electrode during electroslag remelting is acquired, providing a reference for the determination of the amount of deoxidiser added into the molten slag during electrode remelting.     

垂直磁记录介质薄膜研制的新途径(英文)

本文阐述了铝氧化电沉积法获得磁记录材料的基本原理,研究了铝交流氧化膜中沉积磁性金属及其合金制备垂直磁记录薄膜的新途径,建立了铝交流氧化电沉积法,探讨了氧化膜的结构和磁特性的关系。     

硅烷偶联剂KH550对Parylene C膜与金属铝基体结合强度的影响

采用拉开法分析了KH550(γ-氨丙基三乙氧基硅烷)偶联剂浓度对Parylene C膜与金属铝基体间结合强度的影响,采用拉曼光谱(Raman)和红外光谱(FT-IR)分析了偶联剂在膜和基体间的作用机理。研究结果表明,使用10%的KH550偶联剂可提高Parylene C膜与铝基体的结合强度到7.5MPa。其增强作用机理为KH550偶联剂的烷氧基团水解后,-Si-OH与基体的Al-O键发生化学作用生成-Si-O-Al键,-NH2能与Parylene C膜分子间形成氢键,从而使Parylene C膜与金属铝之间产生化学键桥连。     

Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co

Surface conditions similar to those found in aluminium alloys of practical use were assessed by cathodic deposition of transitions metals (Ni and Co) from different electrolytes. Fundamental aspects concerning with the growth of anodic oxide films at potentials lower than 10 V in neutral acetate buffer solution on these modified surfaces were analysed by common electrochemical techniques complemented with scanning electron microscopy and transmission electron microscopy. In both potentiodynamic and galvanostatic modes, the growth of aluminium oxide competes with the dissolution of deposited metal particles. The formation of a thin aluminium barrier oxide film beneath them shifts the dissolution potential over to 1.5 V towards more positive values. Some particles get progressively embedded in the matrix of the growing alumina and act as cation sources, increasing the film conductivity and diminishing the established electric field in the oxide. This effect is more pronounced with Co deposits due to its high active dissolution rate, before passivation occurs. Then, the generation of a two-layer film is explained in terms of the precipitation of metal hydroxide at the solution side on the oxide barrier film.     

Electrical conduction phenomena in thin tetracene films

Thin film aluminium oxide capacitors using anodic Al₂O₃ as the dielectric are described. The dependence of oxide thickness and dielectric loss on anodization voltage was studied. Variation of capacitance with temperature and frequency was also investigated. The capacitors were used in conjunction with tantalum resistors to fabricate an astable multivibrator circuit and the waveforms were recorded. The combination of aluminium oxide capacitors and tantalum resistors has some advantages over all-tantalum RC networks.     

The wetting of carbon by aluminium and aluminium alloys

The contact angle made by molten aluminium with vitreous carbon was measured by the sessile drop method in vacuum at temperatures up to 1100° C. The effect on wetting behaviour of the oxide layer on the molten metal was highlighted by using two samples of aluminium in different states of oxidation. The investigation involved the variation of certain parameters affecting the stability of the oxide film, e.g. the temperature, additions of Ti, Si, Cr, Be, Ca and Li to aluminium and the time held at a certain temperature. The state of the molten aluminium surface under various experimental conditions was determined indirectly by surface tension measurements.     

Structure and composition of oxide film on 5083 alloy at brazing temperature

The structure and composition of the surface oxide film on the 5083 aluminium alloy at a brazing temperature of 500°C were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy. The results showed that the original γ-Al₂O₃ film on the surface of the cold rolled 5083 aluminium alloy was transformed into a complex oxide film consisting of MgO, MgAl₂O₄, and free Al atoms after heating. The thickness of this oxide film, which could be divided into two distinct layers, was approximately 130 nm. The outer layer was mainly composed of an amorphous MgO phase, while the inner layer was MgO based, with a few free Al atoms and a small number of nanocrystalline MgAl₂O₄ particles distributed in it.     

Viscoelastic tensile and shear properties of the 62 wt% Sn–36 wt% Pb–2 wt% Ag solder alloy

In this paper the results of the experimental determination of the viscoelastic tensile and shear properties of the eutectic 62 wt% Sn–36 wt% Pb–2 wt% Ag solder alloy in the temperature range between –20 and 100 °C are presented. The dynamic tensile and shear moduli and the viscous damping coefficients were measured using a phase-sensitive resonance technique. The results show that the temperature dependence of the dynamic Young's and shear moduli can be approximated by linear functions and a quadratic polynomial can be used to define a lower bound of the temperature dependence of the viscous damping coefficients of the material. For a temperature rise from 20 to 65 °C the dynamic Young's and shear moduli decrease by approximately 6% and the damping coefficient increases by approximately 80%.     

Mechanism of scale growth on liquid aluminium

Abstract

Metal loss is an unavoidable consequence of the high production melting of aluminium and its alloys. Losses must be minimised, for reasons of economy and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting result primarily from the formation of dross, a mixture of oxide and melt. Results are presented from an initial study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the initial stages in oxide scale growth, a study of growth kinetics, and observations of the subsequent breakdown of these initial scales to form dross. It was found using TEM that a thin surface oxide of γ-alumina forms rapidly on melts of commercially pure aluminium at 750°C providing a highly effective barrier confining the molten aluminium. This thin surface oxide reduces scale growth to a low level before the onset of breakaway oxidation. Localised failure of this protective oxide film results in exudations forming on the melt surface, the size and number of which increase with exposure time. These exudations would appear to be the onset of dross formation. In parallel studies of the characteristics of wetting of aluminium to alumina it is shown that the reported non-wetting is a result of the presence of this thin alumina film on the melt surface. Once broken, wetting of the alumina takes place and it is shown that this accounts for the exudation of aluminium through the surface oxide and hence dross formation.

MST/990     

Anti-reflective optical coatings incorporating nanoparticles

This paper presents a simple approach for forming anti-reflective film stacks on plastic substrates employing aqueous colloidal dispersions of metal oxide nanoparticles. Results demonstrate that it is possible to fabricate a polymeric thin film of continuously tunable refractive index over a wide range by loading the film with varying concentrations of metal oxide nanoparticles. Specifically, the refractive index for the polymer film was tuned from 1.46 to 1.54 using silica nanoparticle loadings from 50 to 0?wt% and from 1.54 to 1.95 using ceria nanoparticle loadings from 0 to 90?wt%, respectively. The low and high refractive index layers are then combined to create an anti-reflective coating which exhibits a reflectance spectrum, abrasion resistance, haze and transmission values that compare well with those produced using state-of-the-art vacuum based techniques. Furthermore, the results show that it is possible to begin with aqueous dispersions and then dilute them with organic solvents for use in a spin coating method to prepare the polymer-metal oxide nanoparticle composite films.