The precipitation hardening mechanisms and their dependence on precipitate size (rp) r p ) and precipitate/matrix lattice misfit (delta) delta ) in gamma'/gamma / gamma systems were studied in nickel-base superalloy using micro-indentations experiments and theoretical models. Metallurgical states were characterized by TEM, SEM-EBSD, XRD and APT. For the precipitate size range studied (radius rp p smaller than 110 nm), gamma' precipitates are ordered spherical Ni3(Al,Ti) 3 (Al,Ti) phase, the volume fraction remains relatively constant and equal to 20 % and lattice misfit delta is lower than 0.45 %. Prevalence of order strengthening by anti-phase boundary formation and Orowan bypassing was observed in various rp p range, whereas chemistry and coherency strengthening were found insignificant in the nickel-base superalloy studied (Waspaloy (R)). (R)). Shearing by weakly coupled dislocations was dominant for fine precipitates, i.e. rp p lower than 18 nm, shearing by strongly-coupled dislocations for an intermediate rp p range, i.e. from 18 nm to 98 nm and Orowan bypassing for coarse precipitates with a rp p larger than 98 nm.

This study provides some clarifications about the influence of microstructural parameters on the susceptibility of low-copper Al-Zn-Mg alloy to stress corrosion cracking (SCC). Tensile tests in air were carried out on AA7046 in T4 and T4 aged at 150 degrees C (named 150/20) metallurgical states after pre-exposure of the specimens to a chloride solution under mechanical loading. The results showed the predominant role of the corrosion-induced hydrogen during SCC process on the loss of elongation to failure. Scanning kelvin probe force microscopy (SKPFM) measurements were performed for the T4 specimens as well as for a 530 degrees C heat-treated T4 specimen with a coarse-grained microstructure; this allowed the contribution of hydrogen diffusion at the grain boundaries on the hydrogen distribution to be highlighted. The analysis of the fracture modes after tensile tests and hydrogen diffusion profiles obtained by SKPFM in the framework of previous studies investigating the microstructure-hydrogen and plasticity-hydrogen relationships allowed to propose a qualitative model to describe SCC phenomena. The detrimental role of hydrogen at the grain boundaries on the mechanical behaviour was highlighted; the outcome of the evaluation of results from the present study in combination with our previous studies and literature data suggested that it can be limited by hydrogen trapping on intragranular eta-MgZn2 precipitates. (C) 2021 Elsevier B.V. All rights reserved.

The influence of microstructure on the corrosion behaviour of commercially pure Al alloy in a chloride -containing sulphate solution was systematically investigated. An isothermal annealing at 300 ? led to an increase in the density of high-angle grain boundaries (HAGBs) and decrease in number density of intermetallic particles (IMPs). The grain size did not affect the pitting potential, but the corrosion current decreased with increase in the density of HAGBs. IMPs acted as pit initiation sites and influenced the pitting potential. After pit initiation, pit propagation occurred along HAGBs; pit width and depth were strongly dependent on the density of HAGBs.

Aluminium alloys are susceptible to stress corrosion cracking (SCC) in specific conditions, which can be associated with hydrogen embrittlement (HE). Interrupted SCC tests are often performed to evaluate the susceptibility of the samples to a specific environment under mechanical loading. Those tests consist in a pre-corrosion step, which can lead to hydrogen-precharging, followed by a tensile test in air. The present study, performed on a 7046 aluminium alloy, confirmed literature data showing that tensile tests in laboratory air on as-polished samples at sufficiently low strain rates led to a significant hydrogen ingress. Depending on the sample microstructure, various hydrogen - dislocations and dislocations - microstructure interactions could be effective leading to a change in the tensile behaviour of the samples. Such a phenomenon could lead to misinterpret the interrupted SCC tests performed on hydrogen-precharged samples. In the present study, a corrected SCC susceptibility factor was defined to better analyse the interrupted SCC tests.

The influence of equal channel angular pressing (ECAP) on the corrosion fatigue behaviour of an Al-Mg-Si alloy was studied. Preliminary fatigue tests in air showed an increase in fatigue lifetime for ECAP samples, as compared to as-received samples, related to the ECAP-induced grain refinement. After pre-corrosion, the fatigue lifetime was lower for ECAP samples than for as-received samples, because the fragmentation of coarse intermetallics during ECAP led to an increase in the density of corrosion defects. Corrosion fatigue tests demonstrated a synergy between cyclic mechanical loading and corrosion processes for all samples; a deleterious effect of ECAP was also noted.

The effect of a new alternative trivalent chromium conversion process on fatigue lifetime of a 2024-T3 aluminium alloy was investigated. The decrease in fatigue-life induced by coating process was related to the dissolution of coarse intermetallic particles during the deoxidation pre-treatment preceding the conversion layer growth.

Corrosion's detrimental effect on the fatigue life of materials has been recognized for a long time. The first studies in this domain date back to 1910. Nevertheless, the mechanisms involved in corrosion fatigue damage remain among the most difficult to identify and to take into account in engineering applications. Corrosion fatigue can be defined as the reaction of the material to cyclic loading (fatigue) combined with an aggressive environment (corrosion). It is important to emphasize the term “combined” in the definition because many studies have shown that neither cyclic loading in air, nor corrosion can reproduce the same damage mechanisms separately: it is a strong coupling. These couplings are observed in almost all environments and affect many industrial sectors. Corrosion fatigue research has been undertaken mainly in two environments: gaseous and aqueous. The approximation of these two environments is possible in the case where cracking is assisted by hydrogen for example.

Cu-lean Al-Zn-Mg alloys are known to be susceptible to hydrogen embrittlement (HE), which currently limits their use in automotive industry. Several works suggested that the resulting loss of mechanical properties was related to hydrogen trapping in different metallurgical sites. The present work attempts to provide a better understanding of the hydrogen-dislocations interactions to evaluate their influence on the loss of mechanical properties of hydrogen-embrittled Al-Zn-Mg alloys. Pre-strained samples of 7046 aluminium alloy (AA7046) were therefore prepared in order to increase the density of motionless dislocations. Tensile samples, pre-strained or not, were then corroded in 0.6 M NaCl and mechanically tested to evaluate their HE susceptibility and the role of dislocations on hydrogen diffusion. Results highlighted a significant improvement of the HE resistance of the alloy with the increase in the density of motionless dislocations induced by the pre-strain step. This was attributed to preferential hydrogen trapping on motionless dislocations leading to a decrease in the hydrogen amount in the grain boundaries. The measurements of hydrogen penetration depth by Scanning Kelvin Probe Force Microscopy (SKPFM) for cathodically charged samples provided further evidence to support these assumptions. (C) 2019 The Authors. Published by Elsevier Ltd.

This paper presents an investigation on the effects of ageing on the microstructure and the corresponding physical and mechanical properties of a 2024 aluminium alloy used in a civil transport aircraft wing structure in order to assess the residual resistance of the end of a service life. More precisely, heat treatments are applied in order to simulate thermal ageing actually endured by the structure during service. The results of characterisation of microstructural, physical and mechanical properties are compared not only to the data obtained on a pristine alloy, but also to the results obtained on coupons of a similar alloy coming from the teardown of an A320 aircraft using the same experimental procedure. The main findings are that, during a service life, no significant modification in fatigue resistance is noticed despite of changes in the precipitation structure.

Numerous studies have shown that Ni-based superalloy 718 may be sensitive to hydrogen embrittlement and have highlighted the dominant roles played by the hydrogen solubility and the hydrogen trapping. Samples were hydrogenated by cathodic polarization in molten salts under different conditions to vary the diffusible hydrogen content and to saturate the different hydrogen traps present in the microstructure strengthened by precipitation. Open circuit potential and galvanic coupling measurements were conducted in order to characterize the effect of diffusible and trapped hydrogen on electrochemical behavior and to discuss the possibility of galvanic coupling between zones with different hydrogen contents.

The susceptibility to hydrogen embrittlement (HE) of the 7046 aluminium alloy (AA 7046) was investigated. Samples of AA 7046 corresponding to different ageing temperature/time couples were hydrogenated by cathodic charging in a H2SO4 solution. Scanning Kelvin Probe Force Microscopy (SKPFM) combined with global hydrogen amount measurements allowed apparent hydrogen diffusion coefficients (Dapp) to be measured: the decrease of the Dapp values with the increase of the ageing duration was attributed to hydrogen trapping by hardening η’ and η precipitates for the aged alloy. Additional SKPFM measurements were carried out on hydrogen charged samples after desorption at 25°C and combined with SEM observations of the fracture surfaces after tensile tests. Results showed that hydrogen could be trapped at the grain boundaries leading to brittle intergranular fracture. However, hardening precipitates could act as efficient trapping sites and reduce hydrogen trapping at the grain boundaries. Conclusion is that the most critical microstructural parameters for HE of AA 7046 correspond to the grain boundaries while ageing could contribute to improve the resistance to HE of the alloy by a well-controlled precipitation.

This work was performed in the framework of the NEPAL FUI project. CIRIMAT was financially supported by the French Ministry of Economy and industry (BPI-France), the Région Occitanie/Pyrénées-Méditerranée and the European Union (FEDER/ERDF). This work is a part of the NEPAL project (NEw Protections for ALuminum) that aims to replace hexavalent chromium treatments largely used in aeronautic industry by new trivalent chromium conversion layer. We studied here the influence of major alloying elements of the alloy on the growth on the conversion layer and therefore its corrosion properties. The study was carried out on an AA 2024 cold rolled 3 mm thin sheet. Three microstructures were studied: i) a T3 metallurgical state ii) a T3 state followed by a tempering at 190°C for 12 h to induce Cu precipitation (aged sample) and iii) a T3 state followed by a prolonged solution heat treatment at 494°C for 40 minutes to induce the formation of a Mg-depleted zone on the near surface of the alloy (solution heat treated sample). The microstructures were characterized by transmission electron microscopy (TEM). The corrosion behavior of both uncoated and coated samples was evaluated by corrosion potential (Ecorr) measurements, plotting of polarization curves and electrochemical impedance spectroscopy (EIS) measurements performed at Ecorr for various immersion times. For a better understanding of the conversion layer coated alloy corrosion properties, surface analyses of the AA 2024 samples at each step of the conversion layer process were performed by combining X-ray photoelectron spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and cyclic voltammetry measurements. Results showed the growth of a thick Mg-rich naturally formed oxide for the solution heat treated sample leading to a widening of the passivity plateau observed on the polarization curve for the uncoated sample compared to T3 uncoated sample. This was associated with the formation of a Mg-depleted zone on the near surface of the alloy; however, preferential dissolution of Mg occurred during the pre-treatment (degreasing and deoxidation) for both the T3 and solution heat treated samples so that the kinetics and mechanisms of the conversion layer growth and its corrosion properties were similar for both samples. For the aged alloy, TEM observations showed a dense precipitation of intergranular and intragranular Al2CuMg. These precipitates were assumed to contribute significantly to the Cu enrichment observed on the sample surface after degreasing and deoxidation leading to a Cu-rich conversion layer with a lower corrosion resistance compared to that formed on the T3 sample.

A 718 superalloy, presenting a standard microstructure, was mechanically tested under uniaxial tensile loading at 80 degrees C and 300 degrees C in Light Water Reactor environment after an exposure at 300 degrees C for 200 h. Hydrogen embrittlement mechanism was clearly observed. In order to identify the most influent metallurgical parameters on hydrogen embrittlement, three "model" microstructures were synthesized to test the efficiency of carbides, delta, gamma' and gamma" precipitates to trap hydrogen at different temperatures. Results showed that gamma' and gamma" played the major role on the hydrogen embrittlement susceptibility of the alloy even though carbides and delta precipitates could also act as hydrogen traps and influence the final rupture mechanism. Results also characterized the influence of temperature on the fracture modes. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automotive industry is increasingly affected by standards requiring a major cut of polluting emissions. Rolled sheets of Cu-free 7000-series alloys offer a very attractive compromise between mechanical properties, corrosion resistance, forming and weight. With the continuing need for further down-gauging and light-weighting in transportation sector, as well as other applications, these alloys are being considered for large volume use. Nevertheless, their susceptibility to hydrogen embrittlement (HE) and to stress corrosion cracking (SCC) is a limiting factor. The present work concerns HE susceptibility of a 7046-T4 aluminium alloy exposed to a 0.6M NaCl solution. First results showed a strong decrease of the ultimate elongation for tensile samples after exposure to the chloride solution Surface observations revealed that the alloy was mildly affected by corrosion in this environment: pits initiated rapidly around intermetallic particles but only few pits propagated after several days of exposure. Cross-section of pre-corroded samples showed no intergranular corrosion and the maximal depth for the corrosion defects was about 100 nm which was not enough to lead to a noticeable loss of mechanical properties. Therefore, it was assumed that the decrease of the mechanical properties was related to the penetration of hydrogen inside the materials and the results gave proofs that hydrogen could enter and embrittle the 7046 aluminium alloy even without an applied stress during its exposure to the corrosive solution. Global hydrogen content measurements by melting method confirmed this assertion. Furthermore, comparison of the fracture surface of a healthy specimen and that of a sample pre-exposed to the NaCl solution showed that the ductile fracture mode observed for a healthy specimen was partially replaced by cleavage and brittle intergranular rupture when the sample was pre-corroded. Both fracture modes could be attributed to different hydrogen contents and different trapping sites in the microstructure. The hydrogen localized in the grain boundaries was believed to lead to intergranular decohesion, while the reticular and trapped hydrogen presumably induced cleavage rupture by exacerbating the localization of the plasticity. In addition, tensile tests were performed on samples pre-hydrogenated in a pH 2 sulfuric acid solution with a cathodic potential applied. The aim was to precisely control the hydrogen amount introduced in the samples. The depth and the nature of the fracture mode were then analyzed and related to the hydrogen amount. In parallel, pre-hydrogenated coupons were analyzed by SIMS in order to measure the maximal depth affected by hydrogen and to localize the trapping sites in the microstructure. The coupling of these two methods allowed to conclude about the role of the hydrogen generated by corrosion and to propose a mechanism to explain the embrittlement.

The effect of two sealing processes, i.e. an usual hydrothermal sealing and an innovating sealing process called (B1+B2), on fatigue behavior of anodized AA2024 was studied in air for as prepared and pre-corroded samples. Pre-corrosion exposure corresponded to salt-spray tests or continuous immersions. For salt-spray tests, the best corrosion resistance was related to the (B1+B2) sealing and, for continuous immersions, to the hydrothermal sealing. Fatigue life tests in air on pre-corroded samples revealed that anodized samples presented a decrease in fatigue life more pronounced than anodized and sealed samples in relation with a lower corrosion resistance; fatigue crack initiation was localized on pits issued from the degreasing and pickling steps. Independent of the sealing process, the fatigue behavior of the anodized and sealed samples depended on the pre-corrosion exposure. Corrosion fatigue tests induced an additional decrease in fatigue life for both sealing treatments. Crack initiation occurred preferentially on pits issued from degreasing and pickling but also on pits issued from interaction between cyclic loading and corrosive media, in relation with a mechanical damage of the sealed anodic film. The differences in sealed layer morphology could explain the difference in fatigue resistance between the sealed anodic films.

The effects of hydrogen during stress corrosion cracking mechanisms (SCC) have been highlighted for many years but hydrogen trapping mechanisms are not yet well understood for 7xxx aluminium alloys. The 7046-T4 Al-Zn-Mg alloy has been chosen for this study because its low corrosion susceptibility allows hydrogen embrittlement (HE) to be more easily distinguished during SCC tests. Tensile stress tests have been carried out at a strain rate of 10-3 s-1 on tensile samples after an exposure at their corrosion potential in a 0.6M chloride solution for 165 hours under an imposed loading of 80%Rp0.2. The results were compared to those obtained for samples pre-corroded without mechanical loading applied and healthy specimens. A loss of mechanical properties was observed for the pre-corroded samples and presumably attributed to the absorption, the diffusion and the trapping of hydrogen which affects a volume under the surface of the alloy and modifies its mechanical properties. Scanning electron microscope (SEM) observations highlighted a strong effect of hydrogen on fracture modes. The ductile-intergranular initial fracture mode observed on the healthy samples was partially replaced for the pre-corroded samples by a combination of two main fracture modes, i.e. brittle intergranular and cleavage, in relation with the nature of the hydrogen trapping sites and local stress state.