Anodic oxidation of Al–Ag alloys

The anodic oxidation of solution-treated and quenched Al–Ag alloys containing 0.3, 0.6, 0.9 and 1.2 at.%Ag, is examined in ammonium pentaborate electrolyte, which leads to growth of barrier-type anodic films. Enrichments of silver, 3.1×10¹⁵ Ag atoms cm⁻², are developed in the alloys immediately beneath the amorphous alumina films, with the level of enrichment not depending significantly upon the composition of the bulk alloy. The enrichment is relatively low due to clustering of silver atoms in the bulk alloy, which reduces the concentration of silver that is available to enrich from solid solution. Silver species are incorporated into the anodic film, where they migrate outward faster than Al³⁺ ions.     

Influence of current density in anodizing of an Al–W alloy

The influences of current density in development of amorphous, barrier anodic films on a metastable, solid solution, Al–6.5at.%W alloy is examined, with focus on enrichments in the alloy, oxidation processes at the alloy/film interface and migration of species in the film. The films were formed at current densities between 0.01–100 mA cm⁻² in sodium tungstate electrolyte and examined by transmission electron microscopy and Rutherford backscattering spectroscopy. Enrichments of tungsten in the alloy develop at each of the selected current densities, with enrichments in the range 2–4×10¹⁵ W atoms cm⁻². Incorporation of tungsten species into the films at the alloy/film interface proceeds non-uniformly, with cycles of local enrichment of the alloy, oxidation of the enriched tungsten and subsequent depletion of the alloy. The tungsten species migrate more slowly than Al³⁺ ions, leading to layering of the film composition. The cyclic incorporation of tungsten species into the film and the migration of tungsten species within the film are similar for the selected current densities, at the resolution of the experiments.     

Characterisation and modelling of precipitate evolution in an Al–Zn–Mg alloy during non-isothermal heat treatments

This paper describes the response of a precipitate microstructure to various types of non-isothermal temperature changes, namely reversion, ramp heating and thermal cycles, met in heat-affected zones (HAZs) of arc-welds, in an Al–Zn–Mg alloy in the T6 and T7 states. During these thermal histories, the precipitate size and volume fraction are quantitatively measured by small-angle X-ray scattering (SAXS). Reversion experiments are characterised by a fast dissolution stage at almost constant average precipitate size, followed by a coarsening stage that affects the surviving precipitates. Ramp heating experiments show a dissolution behaviour, which is temporarily interrupted for low heating rates, either due to phase transition when the initial precipitates are of a metastable nature (the case of T6 initial state), or due to a dynamic competition between the average and critical radii during the temperature increase (the case of the T7 material). The HAZ of arc-welds is characterised by a gradual increase of the dissolved precipitate fraction as one gets closer to the weld line. In the zone immediately before complete dissolution, precipitate coarsening is observed.

A simple model, based on the growth/dissolution of precipitates distributed in size classes, has been adapted to the present ternary alloy. This model, calibrated using the reversion experiments, has proven its predictive nature in all the other thermal cycles investigated (ramp heating and welding). The predictions of the model are used in order to improve the physical insight into the range of material behaviour observed in the experiments.     

Response to annealing treatments of twin-roll cast thin Al–Fe–Si strips

The new generation twin-roll casters are able to cast strips down to 1 mm and offer a notable increase in the casting speed and caster productivity. The thin strips, however, experience profoundly different temperature and deformation gradients in the caster roll gap and may thus require different down stream processing cycles. An attempt was made in the present work to identify the structural features of the thin-cast AlFeSi strip and its response to high temperature annealing treatments. The deformation introduced to the thin AlFeSi strip in the caster roll gap was largely restored by dynamic processes before coiling, producing recrystallized surface layers. When annealed as-cast, the grain structure was rearranged by an ordinary growth process at the surface and via recrystallization in the interior. The entire process was retarded due to the precipitation reactions, particularly in the immediate vicinity of the surface where the supersaturation of the aluminium matrix was the highest and the boundary mobility was severely impaired. The final strip structure was a coarse one. The entire section of the thin strip underwent recrystallization when the thin strip was annealed after a rolling pass. While some improvement appeared to be possible in this practice, a heterogeneous through thickness structure with relatively coarser surface grains prevailed.     

Influence of oxidation rate and alloy composition on alloy enrichments of anodized Al–W alloys

The effect of oxidation rate on enrichment of tungsten in metastable, solid-solution Al–W alloys during anodizing has been investigated by Rutherford backscattering spectroscopy and medium energy ion scattering. The oxidation rate has negligible influence upon enrichment for current densities in the range 0.01–100 mA cm⁻², with levels between 1.3 × 10¹⁵ and 1.5 × 10¹⁵ W atoms cm⁻² for an Al–0.7at.%W alloy. Further, enrichment factors, expressing the ratio of the enrichment of tungsten in the alloy, in units of 10¹⁵ atoms cm⁻², to the bulk composition of the alloy, in units of at.%, for alloys of compositions in the range 0.1–30 at.%W, decrease progressively in the approximate range 2.5–0.2.     

Influence of current density on transport numbers in anodic oxide films on an anodized Al–5.7at.%W alloy

The ionic transport numbers in barrier anodic films, formed on an Al–5.7at.%W alloy in 0.1 M sodium tungstate electrolyte at 293 K, have been measured for current densities from 0.1 to 1000 mA cm⁻². For this purpose, a xenon marker was ion implanted with a fluence of 1.5 × 10¹⁵ ions cm⁻² into a film formed to 10 V on the alloy, which was then further anodized to 150 V. The position of the marker layer in the final film was determined by transmission electron microscopy and Rutherford backscattering spectroscopy. The cation transport number was 0.38, with no dependence upon current density to an accuracy of 10%.     

Influences of ion migration and electric field on the layered anodic films on Al–Mg alloys

Anodizing of sputtering-deposited Al–Mg alloys containing 27 and 32 at.% magnesium in sodium hydroxide electrolyte is shown to develop two-layered anodic oxide films. The outer layer contains aluminium and magnesium species, and is enriched in the latter species relative to the alloy, particularly towards the film surface. The inner layer also contains the two alloy species but is depleted in magnesium, due to Mg²⁺ ions migrating to the outer layer faster than Al³⁺ ions. The ratio of the thickness of the outer layer to that of the film increases with increase of magnesium content of the alloy. The presence of aluminium species in the outer layer is attributed to the penetration of the outer layer by oxide of the inner layer with lower ionic resistance. This mechanism of film growth appears to be sustainable to alloy concentrations to 40 at.%Mg, when the inner layer may no longer form. Enrichment of alloying elements can accompany film growth on Al–Mg alloys, as shown by enrichment of tungsten to 2–3 × 10¹⁵ atoms cm⁻² in an Al–26 at.%Mg–1 at.%W alloy.     

Enrichment factors for copper in aluminium alloys following chemical and electrochemical surface treatments

Surface treatments, including chemical polishing, alkaline etching, acid pickling, and electropolishing, of aluminium and copper-containing aluminium alloys lead to enrichment of solid solution copper in the metal just beneath the residual oxide films of the treatment processes. The paper presents the enrichment factor for copper as a function of the copper content of the bulk matrix material. The factor is defined as the ratio of the copper enrichment, measured in units of 10¹⁵ atoms cm⁻², to the copper content of the matrix in at.%. Although absolute levels of enrichment increase with increase in copper content of the alloy, the enrichment factor increases in the opposite sense.     

Thermomechanical processing of a twin-roll cast Al–1Fe–0.2Si alloy

A sound homogenization practice is identified for a twin-roll cast Al–1Fe–0.2Si (AA8079) alloy, a popular foil stock with a higher Fe/Si ratio than most other AlFeSi commercial alloys. Homogenization below 793 K produces a very fine dispersion of _c particles which in turn yields a very coarse and heterogeneous grain structure upon interannealing. When the Al–1Fe–0.2Si strip is homogenized above 833 K, _c particles are replaced by relatively coarser Al₃Fe particles with a favorable effect on the recrystallized grain size. It is thus concluded that a homogenization temperature of at least 833 K must be employed to obtain a coarse particle dispersion which in turn would produce a fine grain size after interannealing.     

Influence of pre-treatment on the surface composition of Al-Zn alloys

The influences of pre-treatments on the composition of Al-Zn alloys, containing 0.6-1.9 at.%Zn, are investigated using scanning and transmission electron microscopies, glow discharge optical emission spectroscopy and ion beam analysis. Alkaline etching, anodic alkaline etching and electropolishing result in zinc enrichments, just beneath the oxide/hydroxide films on the alloy surfaces. The enriched alloy, about 4-5 nm thick, contains zinc in solid-solution with aluminium, up to about 18 at.%. The highest and lowest enrichments are produced by alkaline etching and electropolishing respectively. For a given pre-treatment, the enrichment increases with increase in zinc content of the bulk alloy.     

Cube recrystallization textures in a hot deformed Al–Mg–Si alloy

The formation of cube recrystallization textures has been studied on Al–Mg–Si polycrystals deformed at 400 °C by channel die compression tests and annealed at 510 °C. A cube recrystallization texture was observed on samples presenting a very low fraction of cube volumes before and after deformation.     

Behaviour of copper during alkaline corrosion of Al–Cu alloys

Enrichment of copper beneath amorphous anodic films on relatively dilute, solid-solution Al–Cu alloys is necessary before copper can be oxidized and incorporated into the oxide layer. A similar enrichment arises during electropolishing, which also develops an amorphous oxide. In these cases, external polarization is applied, usually generating a relatively high oxidation rate. In contrast, enrichment behaviour at the corrosion potential has received less attention. The present study examines the corrosion of Al–Cu alloys, containing up to 6.7 at.% Cu, in 0.1 M sodium hydroxide solution at 293 K. Copper is again found to enrich in the alloy, similarly to behaviour with anodic polarization. However, following enrichment, discrete copper-rich particles appear to be generated in the corrosion product. These are suggested to be nanoparticles of copper, since the corrosion potentials of the alloys are low relative to that required for oxidation of copper. The corrosion rate increases with increase of both time and copper content of the alloy, probably associated with a greater cathodic activity due to an increasing number of nanoparticles. The corrosion proceeds with loss of aluminium species to the sodium hydroxide solution, but with retention of copper in the layer of hydrated alumina corrosion product.     

Examination of surface films on aluminium and its alloys by low-voltage scanning and scanning transmission electron microscopy

The potential of low-voltage, high-resolution scanning and scanning transmission electron microscopy (SEM/STEM) for morphological characterization of various surface insulating films on aluminium and its alloys has been assessed by examination of porous anodic films, barrier anodic films and corrosion product layers. The characterization shows clearly the value of the approaches, particularly the ability to image directly fine details of appropriately-prepared aluminium surfaces that have usually required examination by transmission electron microscopy (TEM). Such ready characterization assists mechanistic understanding of the contributions of the macroscopic surface and flaws or second phase to the filming and corrosion processes. Further, the approaches are applicable to other materials where such understanding was limited by the sample preparation routes available for TEM.     

Structure and transformation behaviour of rapidly solidified Ni–Al–Hf alloys

Rapidly solidified Ni–Al–Hf alloys of ternary eutectic compositions were studied by X-ray diffractometry, transmission electron microscopy and differential scanning calorimetry. Ni₆₆Hf₂₀Al₁₄ amorphous alloy with relatively high Hf/Al ratio showed high tensile strength of 1600 MPa and high thermal stability against crystallization. The formation of a nanoscale metastable intermetallic compound having a body-centered cubic lattice with a lattice parameter of a=1.22 nm was observed in the alloys with higher Al than Hf content. The transformation of an amorphous+metastable cubic mixture to AlNi₃ produces AlNi₃ of the same composition as the AlNi₃ phase formed on rapid solidification. Pm m AlNi₃ phase can dissolve up to 11 at.% Hf.     

Measurement of anisotropy of crystal-melt interfacial energy for a binary Al–Cu alloy

The anisotropy of solid–liquid interfacial energy for an Al–4.0 wt% Cu alloy is determined through measurement of equilibrium shape. Measurements are performed using a fine dispersion of liquid particles entrained within a single-crystal single-phase solid. Specimens are sectioned normal to the 1 0 0 direction in the continuous phase, and the droplets are observed to exhibit four-fold symmetry on this plane. The radial variation on this plane is measured using optical and chemical techniques, and the normalized anisotropy parameter is determined to be 0.98%. Equilibrium conditions are validated, and quench effects are investigated using microchemical analysis techniques.     

Effect of directional solidification and heat treatments on the microstructure and mechanical properties of multiphase intermetallic Zr-doped Ni–Al–Cr–Ta–Mo alloy

The effect of directional solidification and heat treatments on the microstructure and mechanical properties of intermetallic Ni–21.29Al–7.04Cr–1.46Ta–0.64Mo–0.57Zr (at.%) alloy was studied. Increasing growth rate is found to decrease primary dendrite arm spacing and to increase volume fraction of β(NiAl)-based dendrites and low melting point γ′(Ni₃Al)/Ni₅Zr eutectic. Room-temperature tensile yield strength and ultimate tensile strength increase and plastic elongation to fracture decreases with the increasing growth rate. Two types of heat treatments of directionally solidified (DS) specimens including two-step ageing at temperatures of 1273 and 1123 K and two-step solution annealing at 1373 and 1493 K were performed. Ageing at 1273 and 1123 K decreases volume fractions of the dendrites and eutectic regions and leads to a coarsening of spherical -Cr and needle-like γ′ precipitates within the β-phase. Annealing at 1373 K for 100 h is shown to be sufficiently long to completely dissolve the eutectic regions. Compressive yield strength increases with increasing temperature reaching a peak value at about 1023 K and then decreases at higher temperatures. Minimum creep rate is found to depend strongly on the applied stress and temperature according to a power law. The power law stress exponent n is determined to be 5.1 and apparent activation for creep Q_a is measured to be 326 kJ/mol.     

Corrosion behaviour of Zn–Al–Mg coated steel sheet in sodium chloride-containing environment

Conventional hot-dip galvanised zinc coated (Z) and novel hot-dip galvanised Zn–Al–Mg alloy coated (ZM) steel sheet samples with a coating thickness of 7 μm each were exposed to standardised salt spray test and cross-sections of the corrosion samples were analysed by using SEM and EDS. On Z corrosion proceeds very fast and the steel substrate is attacked even after 100 h of exposure. ZM samples showed a different behaviour. The entire metallic ZM coating is converted into a stable, adherent aluminium-rich oxide layer, which protects the steel substrate against corrosive attacks. This layer is the main reason for the enhanced corrosion resistance of the ZM coating in sodium chloride-containing environment.     

Improvement of product strength and formability in stamping of Al–Mg–Si alloy sheets having bake hardenability by resistance heat and artificial aging treatments

Resistance heat and artificial aging treatments were introduced into a stamping operation to improve the product strength and formability of Al–Mg–Si alloy sheets having bake hardenability used for automobile body panels. In this treatment, the sheets undergo re-solution by resistance heat treatment, composed of resistance heating and water quenching just before the stamping. Stamped sheets are artificially aged just after the stamping to increase product strength. In the experiment, Al–0.60% Si–0.74% Mg alloy sheets were chosen as an example of Al–Mg–Si alloy sheets having bake hardenability. The re-solution solution treatment of the sheets was sufficiently accomplished by rapid resistance heat treatment, and formability of the sheets was improved. Hardness of the formed products was increased by artificial aging. It was found that the present process is effective in improving the product strength and formability of Al–Mg–Si alloy sheets having bake hardenability due to the compactness and rapidness.     

Rolling behaviour of steel backed spray deposited Al–Sn strip

Steel backed Al–10 wt%Sn strip has been prepared by the spray deposition of the molten Al–Sn alloy on a steel substrate, followed by rolling of the resulting composite strip. In the present paper the rolling behaviour of the composite strip at 473 K, together with the characteristics of the spray deposit have been discussed. It has been shown that the Al–Sn spray deposit is a coalesced network of particles having porosity. The particles in the network are of spherical, ligament and irregular shapes. The study shows that the composite metal strip can be rolled well at 473 K without any delamination or surface and edge cracking on the deposit layer. A rolling thickness reduction of 75% produces steel backed Al–10 wt%Sn strip having optimum properties. During rolling, densification in the deposit, deformation in the deposit and substrate, and bonding between the deposit and the substrate take place simultaneously. The mechanism of densification in the deposit layer during rolling has been discussed on the basis of the examination of the microstructure of the deposit at different stages of rolling.