Growth of spheroidal graphite: light versus scanning and transmission electron microscopies

International audienc

Mechanism of metal dusting corrosion by pitting of a chromia-forming alloy at atmospheric pressure and low gas velocity

FeNiCr samples (800HT) were exposed at 570 °C, 1 bar to a 47.25CO-47.25H2-5.5H2O atmosphere (ac = 33) flowing at 18 μm/s. Pitting corrosion was observed. Pits showed a flattened morphology and a constant pit diameter growth rate. Corrosion rings appeared successively at the surface during pit growth. A four-step mechanism is proposed which includes internal oxidation of carbides, graphitisation and localised enhanced graphitisation. Gas velocity and thermal cycling play key roles in pit morphology. Thermal cycling induces circular cracks. Low gas velocity induces the gas to evolve in crevices, due to local oxygen consumption

Growth of Spheroidal Graphite: Light Versus Scanning and Transmission Electron Microscopies

Much effort has been put in the past for describing the structure of graphite spheroids and for suggesting a growth mechanism from these observations. Many theories have emerged, but none of them is yet fully established and accepted 70 years after the patent on manufacturing spheroidal graphite cast irons. In the meantime, observations with optical microscopy have become more and more challenged by electron microscopy, either in scanning or in transmission mode. However, conclusions drawn from these various types of observations sometimes appear conflicting. This unsatisfactory situation is here investigated for three special features of spheroidal graphite, namely: (i) the crystalline quality of graphite in the spheroids; (ii) the curved leaf-like overgrowth at the outer surface of spheroids; (iii) the radial structure that is evidenced with optical microscopy. The present results lead to sustain that the mechanism of graphite growth remains the same during the whole solidification process of spheroidal cast irons

Solidification sequence and four-phase eutectic in AlSi6Cu4Fe2 alloy

International audienceDifferential thermal analyses of an AlSi6Cu4Fe2 alloy at various cooling rates from the liquidstate were performed which showed: i) possible discrepancies in the first solidification stepsand; ii) solidification ending with multiple peaks at low cooling rate. Metallographicobservation of the samples gave hint for understanding the discrepancies while the presenceof rounded multi-phase pools – or so-called rosettes - entrapped within dendrite armsexplained the multiple peaks because they solidified independently of each other.The very fine eutectic microstructure in these rosettes was investigated by scanning electronmicroscopy which showed the precipitation of a four-phase eutectic. Focused ions beammilling was then used to investigate the topology of the distribution of the four phases.Electron backscattered diffraction analysis was carried out to investigate their localcrystallographic orientations and transmission electron microscopy analysis was performed toidentify the very fine crystalline structures. This eutectic appears as one of the very fewregular four-phase eutectics reported in the literature

High strength-high conductivity carbon nanotube-copper wires with bimodal grain size distribution by spark plasma sintering and wire-drawing

Copper and 1 vol% carbon nanotube-copper cylinderswith a micrometric copper grain size and either a unimodal or a bimodal grain size distribution were prepared using spark plasma sintering. The cylinders served as starting materials for room temperature wire-drawing, enabling the preparation of conducting wires with ultrafine grains. The tensile strength for the carbon nanotube-copperwires is higher than for the corresponding pure copper wires. We show that the bimodal grain size distribution favors strengthening while limiting the increase in electrical resistivity of the wires, both for pure copper and for the composites

Influence of hydrogen on the propagation of intergranular corrosion defects in 2024 aluminium alloy

To study the influence of hydrogen on the intergranular corrosion mechanism of a 2024 aluminium alloy, samples were hydrogen precharged by cathodic polarisation and then exposed to a NaCl solution. EBSD analyses and SEM observations showed that hydrogen increased the number of corroded interfaces and led to the embrittlement of low-angle grain boundaries which were not susceptible to corrosion without hydrogen precharging. The increase of the reactivity of the 2024 aluminium alloy in the presence of hydrogen gave a new insight into the intergranular corrosion mechanism: corrosion-induced hydrogen promoted the intergranular corrosion propagation and partially controlled the corrosion defect morphology

Development of multi-component fluoropolymer based coating on simulated outdoor patina on quaternary bronze

Abstract Bronze reacts with oxygen, humidity, and pollutants in the atmosphere so that a patina forms. Natural exposure to an outdoor atmosphere can be simulated and accelerated in order to achieve a patina that mimics outdoor ancient patina. In order to avoid the uncontrolled dissolving of either the natural or artificially formed patina, protection of the patina is needed. In this study, a multi-component fluoropolymer based coating for the protection of bronze patina was developed. In order to provide various functionalities of the coating (such as the hydrophobicity of the coating surface, obtaining interactions within the coating itself as well as a bronze substrate and inhibiting the corrosion processes), a fluoroacrylate coating with appropriate adhesion promoter was suggested, with and without a silane modified benzotriazole inhibitor. The protective efficiency and durability of the applied coatings were investigated electrochemically using potentiodynamic tests and electrochemical impedance spectroscopy in a simulated acid rain solution. All of the developed coatings showed a significant decrease in the corrosion current density. The self-assembled single layer coating (FA-MS) also showed 100% inhibition efficiency. After ageing the coating remained transparent and did not change by UV exposure and/or thermal cycling. The patina and coating investigations using FIB-SEM and EDX showed that the latter coating (FA-MS) successfully covered the surface of the patinated bronze. The mechanism of the bonding was proposed and supported with the spectroscopic observation of a thin and even coating

Influence of the microstructure and laser shock processing (LSP) on the corrosion behaviour of the AA2050-T8 aluminium alloy

The corrosion behaviour of AA2050-T8 was studied after polishing and after laser shock processing (LSP) treatment using the electrochemical microcell technique and the SVET. After polishing, pitting at constituent particles and intergranular corrosion were observed. By contrast, no intergranular corrosion developed after LSP. Microcell measurements revealed that LSP increases the pitting potential. SVET measurements revealed that local anodic currents are systematically lower on LSP-treated surfaces than on polished ones. The current density on the LSP-treated surface remains constant around 50 μA cm−2 up to 123 min after immersion, while on the polished surface it reaches 200 μA cm−2

Corrosion behaviour of AA 1370 strands for wires: Identification of the critical metallurgical parameters

The corrosion behaviour of AA 1370 in different metallurgical states was studied in chloride-containing sulphate solutions. The microstructures of a rod, a deformed strand and an annealed strand constitutive of wires were examined using scanning electron microscopy, transmission electron microscopy and electron backscattered diffraction. Results offered evidence for the ultra-fine grain size, high density of interfaces and high level of misorientation of the interfaces as relevant explanations for the low corrosion resistance of the deformed strand compared to the rod and the annealed strand

First cold spraying of carbonated biomimetic nanocrystalline apatite on Ti6Al4V: physical-chemical, microstructural, and preliminary mechanical characterizations

Carbonated Biomimetic Nanocrystalline Apatite (BNAc) coatings are obtained for the first time by Cold Spray. The coatings are characterized by FTIR, Raman, XRD, and SEM and compared to the powders. No significant chemical and structural changes are detected and the nanostructure features of these very reactive BNAc are preserved in the coating. These results and preliminary mechanical assays show that Cold Spray can produce an operational biomimetic coatings offering a high potential for implants functionalization and osseointegration. However, these first results need further studies in order to understand the mechanism of adhesion and the interactions at the coating-substrate interface