Copper-enriched Layer Research Articles

The direct observation of copper segregation at the broad faces of η′ and η precipitates in AA7010 aluminium alloy

High strength Al-Zn-Mg-Cu (7xxx) aluminium alloys obtain the required balance of mechanical and corrosion properties through complex heat treatments developed through empirical experiment. Understanding the influence of these heat treatments on the composition of the strengthening precipitates (η′ and η-phase) is critical to obtain a better mechanistic understanding of their effect. In this work, the composition of strengthening precipitates has been studied at the atomic scale in microstructures of commercially processed AA7010 alloy in the standard overaged temper state. Copper enrichment in the precipitates on over-aging is confirmed for both small (η′) and large (η) particles. It is demonstrated that in addition to entering the precipitates, a copper enriched layer forms at the particle/matrix interface on the broad faces of the precipitate plates. The implications of this structure for the mechanical and environmentally assisted cracking performance of the alloy are discussed.

Structure of the Copper–Enriched Layer Introduced by Anodic Oxidation of Copper-Containing Aluminium Alloy

This paper investigates the structure of the copper–enriched layer formed at the alloy/anodic film interface during anodizing of Al–2wt.% Cu binary alloy using transmission electron microscopy. It was revealed that θ′ phase was formed within the copper–enriched layer. For the copper–enriched layer formed on {100} aluminum planes, the interface between the aluminum matrix and the θ′ phase within the copper-enriched layer is coherent. For the copper–enriched layer formed on {110} and {111} aluminum planes, the interfaces between the aluminum matrix and the θ′ phase within the copper-enriched layer are semi-coherent or incoherent. The interfacial coherency influences the formation of oxygen gas bubbles within the resultant anodic films.

On the Origin of the Second Anodic Peak During the Polarization of Stainless Steel in Sulfuric Acid

The origins of the second anodic current peak in polarization curves of AISI 430 (UNS S43000) stainless steel in deaerated 0.1 M sulfuric acid (H2SO4) have been investigated by potentiodynamic polarization and atomic emission spectroelectrochemistry (AESEC). The elemental dissolution rates of Fe, Cr, Ni, Cu, and Mn were measured in real time during linear potential sweep voltammetry, revealing the formation and dissolution of a copper-rich corrosion product layer. The deposition and subsequent dissolution of this copper layer is proposed to be the main cause of the second anodic current peak. The negative current loop observed after the first anodic peak is attributed to the cathodic reduction of H+ ion on the copper-enriched layer. Other factors such as the oxidation of the adsorbed H atoms on the copper-enriched layer and the presence of chromium-impoverished grain boundaries are shown to have a negligible effect on the second anodic current peak.

Microstructural Modification Arising from Alkaline Etching and Its Effect on Anodizing Behavior of Al-Li-Cu Alloy

Surface and near-surface microstructures of alkaline etched AA2099-T8 aluminum alloy have been characterized. The effect of alkaline etching on the anodizing behavior of the alloy in both acidic and neutral electrolytes has been investigated by comparing the electrochemical responses and the resultant anodic films of mechanically polished and alkaline etched alloys. It was revealed that alkaline etching could locally remove intermetallics from the alloy surface, with development of a copper-enriched layer immediately beneath a residual alumina film of about 5 nm thickness. The “footprints” of alkaline etching, i.e. the removal of intermetallics and the formation of copper-enriched layer during alkaline etching, could affect subsequent anodizing behavior of the alloy, particularly in the early stages. The removal of intermetallics from the alloy surface could not only improve anodizing efficiency but also contribute to reduced discontinuities in the resultant anodic films, potentially benefiting corrosion and fatigue resistances of the anodized profiles. The difference in anodizing behavior caused by the copper-enriched layer developed during alkaline etching was considered limited to the early stages of anodizing, since a similar copper-enriched layer could be formed quickly during subsequent anodizing process, regardless of the surface pre-treatment.

Surface texture formed on AA2099 Al–Li–Cu alloy during alkaline etching

The surface nanotexture developed on the lithium-containing aluminium alloy AA2099-T8 during alkaline etching has been investigated. The formation of the surface nanotexture is associated with copper-rich nanoparticles in the copper-enriched layer beneath the residual alumina film. Specifically, the copper-rich nanoparticles support local cathodic reactions, which support the anodic dissolution of the adjacent aluminium matrix, thereby resulting in the surface nanotexture. The initial development of the surface nanotexture is grain orientation dependent. Further, the presence of smut (etching products), which is porous in nature and rich in O, Mg, Mn, Cu and Zn species, also affects the surface nanotexture evolution.

Anodic film growth on Al–Li–Cu alloy AA2099-T8

AA2099-T8 aluminium alloy was anodized at a current density of 5mA/cm2 to selected voltages in 0.1M ammonium pentaborate electrolyte at 293K. It was found that the growth of barrier-type anodic film on the alloy was accompanied by oxidation of intermetallics, formation and rupture of oxygen gas-filled voids in the anodic film and healing of the anodic film at the sites of rupture. It was revealed that the formation of oxygen gas-filled voids was related to the oxidation of copper-rich nanoparticles in the copper-enriched layer at the film/alloy interface, and that the oxidation process of copper-rich nanoparticles depended on grain orientation. Further, the significantly reduced Pilling–Bedworth ratio for formation of anodic lithium oxide compared with that for formation of anodic alumina resulted in the formation of fine voids at the alloy/film interface and sequentially, detachment of the anodic film from the alloy surface.

SAE 1045 steel/WC–Co/Ni–Cu–Ni/SAE 1045 steel joints prepared by dynamic diffusion bonding: Microelectrochemical studies in 0.6 M NaCl solution

Corrosion of SAE 1045 steel/WC–Co/Ni–Cu–Ni/SAE 1045 steel interfaces was investigated in 0.6 M NaCl solution using an electrochemical microcell, which enables local electrochemical characterization at the micrometer scale. Two pieces of steel, one with a WC–Co coating covered with Ni (12 μm) and Cu (5 μm) layers, and the other with a Ni (15 μm) layer, were welded by dynamic diffusion bonding. A WC–Co coating was applied to the steel by the high velocity oxygen-fuel process, and Ni–Cu and Ni layers by electroplating. Polarization curves were recorded using an electrochemical microcell. Different regions of welded samples were investigated, including steel, cermet coating, and steel/cermet and steel/Ni–Cu–Ni/cermet interfaces. Optical and electronic microscopes were employed to study the corroded regions. Potentiodynamic polarization curves obtained using the microcell revealed that the base metal was more susceptible to corrosion than the cermet. In addition, cermet steel/cermet and steel/Ni–Cu–Ni/cermet joints exhibited different breakdown potentials. Steel was strongly corroded in the regions adjacent to the interfaces, while the cermet was less corroded. Iron oxides/hydroxides and chloride salts were the main corrosion products of steel. After removal of the superficial layer of corrosion products, iron oxides were mainly observed. Chloride ions were detected mainly on a copper-enriched layer placed between two Ni-enriched layers.

Undercooling in Co–Cu alloys and its effect on solidification structure

Undercooling behaviour and solidification morphology change of various Co–Cu alloys were examined. Each alloy was melted in an alumina crucible under an argon atmosphere by high-frequency induction, and the cyclic heating and cooling was repeated several times in the temperature range between 1300 and 1850 K. The temperature change during the experiment was analysed under the Newtonian cooling assumption. The temperature curve showed that the undercooling in a first few cycles was negligibly small but it increased remarkably. The alloy was undercooled below the metastable liquid miscibility gap after the next several cycles. In these samples, liquid separation was observed. The homogeneously mixed spherical grains of copper-enriched phase were observed in cobalt-enriched matrix for the samples solidified immediately after the liquid separation. The two melts became coarser after the separation by mutual coalescence. In the case of the slow start of the solidification after the separation, they formed a clear interface between the upper cobalt-enriched layer and the lower copper-enriched layer located in the lower part according to the density difference between the two melts. It depended on the cooling rate after liquid separation. The very fine duplex structure can be obtained by the rapid cooling of the melt at the initial stage of the separation.

Copper enrichment in Al-Cu alloys due to electropolishing and anodic oxidation

The average thickness and composition of the copper-enriched alloy layer that is present at the alloy/oxide interface during anodic oxidation of an eleetropolished Al-0.9 al.% Cu alloy at a constant current density of 50 A m −2 have been determined by Rutherford backscattering spectroscopy and transmission electron microscopy. The copper-enriched layer, of about 2 nm thickness, has an average composition of Al-40 at.% Cu and contains about 5.4×10 15 Cu atoms cm −2. The average composition and thickness of the layer do not change significantly during anodizing from 10 to 200 V- The essentially steady-state, copper-enriched layer is established mainly by the prior electropolishing of the alloy. As a consequence of the pre-enrichment of copper, both aluminium and copper atoms are oxidized immediately at the alloy/film interface on subsequent anodizing. Owing to the importance of copper enrichment in Al-Cu alloys to the oxidation of copper atoms, alloy pre-trealment has an important role in determining the initial oxidation behaviour.

Mobility of copper ions in anodic alumina films

Transmission electron microscopy of ultramicrotomed sections and Rutherford backscattering spectroscopy have been used to quantify the significantly faster mobility of copper ions compared with Al 3+ ions in anodic alumina films. To determine the migration rate of copper ions, an Al-0.4 at% Cu alloy film of ca. 35 nm thickness has been sputter deposited onto an electropolished superpure aluminium substrate; the specimen comprising the alloy layer superimposed on aluminium was then anodized at a constant current density to various voltages at high current efficiency. The anodic oxidation of the alloy film results in the prior oxidation of aluminium and the accumulation of copper in a layer of alloy, ca. 2 nm thick, just beneath the anodic film; consequently, no copper is incorporated into the alumina film during anodizing of the alloy. At ca. 49 V for the particular alloy thickness, the alloy film is totally consumed by anodizing and the copper-enriched layer beneath the anodic film is incorporated abruptly into the film, because of the presence of an air-modified electropolishing film sandwiched between the alloy and aluminium regions. With further anodizing, the incorporated copper ions migrate outwards at a rate ca. 3.2 times that of Al 3+ ions. The method employed provides a novel approach to determining the precise mobility of foreign ions incorporated into anodic alumina.

The importance of surface treatment to the anodic oxidation behaviour of AlCu alloys

The anodic oxidation of Al-0.9 at%Cu alloy has been investigated, focusing on the effect of a mechanical polishing pre-treatment on incorporation of copper ions into the anodic film and the accumulation of copper atoms in the alloy just beneath the alloy/film interface. For the particular alloy and anodizing conditions, a relatively pure alumina film is formed to thickness of about 170 nm with associated progressive accumulation of copper atoms in the alloy at the alloy/film interface. The thickness of the copper-enriched layer is about 2 nm and the average composition approaches about 42 at%Cu. Only on achieving this level of enrichment are copper ions incorporated into the anodic film. The copper ions are more mobile than aluminium ions and hence, are able to reach the film/electrolyte interface during subsequent film growth.

耐熱性ファインセラミックスの接合に関する研究 Ｖ Ｃｕ富化層による熱応力緩衝効果に関する検討

Effects of the insert metals on the crack initiation in Si3N4 to SCM435 joints were investigated by paying attention to the thickness of copper enriched layer and the diameter of specimen. The 10 mm diameter joints without crack using Cu-5%Cr, Cu-1%Nb, Cu-3%V, Cu-5%Ti and Cu-10%Zr insert metals were obtained in case that the thickness of copper enriched layers were more than about 0.2 mm, 0.2 mm, 0.3 mm, 0.5 mm and 0.8 mm, respectively. As for Cu-5%Cr insert metal, the 13 mm diameter joints without crack were obtained in case of the thickness of copper enriched layer being more than about 0.4 mm. On the basis of residual stress analysis in Si3N4 to SCM435 joint, the critical maximum principal stress in Si3N4 to prevent crack in the joint were almost the same level of 350-400MPa in any cases. It was concluded that copper enriched layer had the function of relieving the thermal stress occurred in the bonding process.