An original approach coupling potentiostatic pulse tests and statistical analysis of the pitting features was carried out to compare the sensitivity to pitting corrosion of wrought and SLM 316L SS (stainless steels). The measurement and analysis of current densities during potentiostatic pulse tests has led to a better understanding of the pitting corrosion behavior of wrought and SLM 316L SS in 4 mol.l(-1) NaCl solution at 50 degrees C. The association of global measurements and local observations showed that the two stainless steels exhibit different pitting mechanisms. Moreover, a slightly different susceptibility to pitting corrosion is found between the direction of manufacture and the normal direction for the 316L SLM with these tests. The role of the microstructure and the passive film in the pitting corrosion behavior is discussed.

In the previous chapters we have presented the diversity and the complexity of the multiscale degradation mechanisms for mechanics-microstructure-environment coupling problems. The aim of this chapter is to provide a quantitative evaluation of the durability of materials and structures subjected to mechanical and environmental solicitations.

The different steps of stress corrosion have been described in the previous chapter. We will focus here on what is identified as the steps of rapid propagation, coalescence and ramification. In the first case, it involves studying the growth of a single crack propagating under the simultaneous effects of the environment and mechanical loads. Knowing the growth kinetics of these defects allows us to estimate and/or model the residual life of components on which they have been detected by non-destructive techniques. In contrast to initiation for which there are several definitions, the cracks concerned by the so called rapid propagation step are generally several millimeters in length. Many testing methodologies allow the growth of a network of cracks and therefore take an interest in the phenomena of coalescence and branching.

The electrochemical reactivity of Alloy 600 in the passive state was examined at the microstructural scale using a local-probe technique, the Scanning Electrochemical Microscopy (SECM). The complementarity of two modes of recording, the current maps and the approach curves, was demonstrated. This study allowed the qualitative determination of high-reactivity zones, but also the quantitative definition of kinetic and thermodynamic parameters, namely the intrinsic kinetic constant and the charge-transfer coefficient. Furthermore, the crystallographic orientation of the studied grains was determined by Electron Backscatter Diffraction (EBSD). The quantitative parameters obtained by SECM were thus assigned to specific crystallographic planes, revealing a relation between the crystallographic orientation of the grains and the reactivity measured on the passive film. The intrinsic kinetic constant increased with the misorientation angle between the grain normal and the 〈111〉 direction, which reveals the passive film anisotropy. These results show that the coupling of these two techniques is promising for the development of quantitative kinetic or corrosion models.

A local approach coupling atomic force microscopy (AFM) observations and polycrystal finite element calculations is proposed to provide data at the slip-band scale. From AFM measurements of slip bands emerging at the specimen surface during tensile loading, and from numerical results of strain fields at the grain scale, the method is able to determine at the local discrete scale the mean slip-band spacing and the number of dislocations emerging during the plastic strain. The methodology applied in the hydrogen embrittlement context highlights, quantitatively, at the grain scale an increased plastic strain localization with internal hydrogen.

Mechanical behavior analysis of the friction stir weld nugget of an aluminum alloy 2050 reveals a major role of the microstructure which varies with the distance to the weld surface. Three types of microstructure heterogeneities are considered namely grain size, precipitation state and textured bands. The grain size and the T1 precipitates density decrease with the distance from the weld surface. The density of T1 precipitates has a first order effect on micro-hardness variations and makes the Hall–Petch rule not valid in this case. Tensile tests combined with strain field identification done by digital image correlation measurements demonstrate the good correlation between textured bands and strain field heterogeneities. Crystal plasticity simulations by finite element method well account for the macroscopic mechanical behavior as well as strain field in textured bands.

The effect of microstructure-induced strain-stress heterogeneity on the evolution of hydrogen distribution was studied on 316 L stainless steel. A crystal plasticity-transient hydrogen diffusion finite element analysis was developed. To reach the stress-strain heterogeneity, synthetic microstructures were computed under uniaxial loading. The results show that hydrostatic stress gradients is the main cause of the hydrogen redistribution and the strain rate has a crucial effect on the redistribution degree. Statistical analysis of H-segregation reveals clustering of H-enriched zones at the mesoscopic scale.

The effects of the microstructure and mechanical fields on intergranular stress corrosion cracking (IGSCC) of the nugget zone of heat treated welds obtained by friction stir welding in the AA2050 aluminum alloy have been investigated at different scales. At low strain rate, in 1.0 NaCl aqueous solution, IGSCC develops in the microstructure, whereas only pitting corrosion is observed without any mechanical stress. Based on surface observations, EBSD analysis and X-ray tomography, the key role of sub-millimetric textured bands (induced by the welding process) on the IGSCC is demonstrated. Analyses at a more local scale show the grain boundary (low angle boundary, special coincident site lattice boundary or high angle boundary) do not have a significant effect on crack initiation. Crystal plasticity finite element calculations show that the threshold normal stress at grain boundaries for IGSCC development is about 80% of the macroscopic stress. It is also highlighted by crystal plasticity calculations that there is a drastic effect of the local stress field on the shape of cracks. Finally, it is shown that plasticity induced residual stresses are sufficient for the formation of IGSCC. (C) 2014 Elsevier Ltd. All rights reserved.

Ge 2 Sb 2 Te 5 is a phase change material candidate to constitute the active element of future nonvolatile memory devices. The evolution of the thermal resistance at the interface between an aluminum thin layer and Ge2Sb2Te5 is studied using the time resolved pump probe technique from room temperature to 400 °C. The thermal resistance is influenced by the amorphous to crystalline phase change occurring in Ge2Sb2Te5. The decrease in the thermal resistance from the amorphous to the crystalline phase is well explained by the diffuse mismatch model asymptotic form for high temperature. The large increase of the interface thermal resistance between fcc and hcp crystalline states is explained by the fast and significant grain growth and species inter-diffusion during this second phase change. This leads to the formation of an interfacial layer whose chemical and mechanical intrinsic properties have been measured in order to model the thermal resistance in the hcp state.

A local approach combining atomic force microscopy measurements and finite-element calculations at the grain scale is proposed to study the effects of hydrogen on the slip morphology of polycrystalline austenitic stainless steel. The definition of a localization index highlighted the particularly visible effect of hydrogen on plastic slip localization in relation to the crystallographic orientation and the intragranular plastic strain level. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Intergranular corrosion sensitivity is studied for the friction stir weld nugget of aluminum alloy 2050-T8. The weld nugget consists of fine equiaxed grains having average sizes from 4 to 20 mu m, the grain size increases with decreasing of the distance from the weld surface. The weld nugget contains a particular microstructure called "onion rings" due to crystallographic orientations. The effect of the "onion rings" on stress corrosion cracks initiation is studied for stress corrosion cracking tests at a strain rate of 2.10(-6) s(-1). EBSD cartographies allow showing that the initiation site of the biggest cracks is located at boundaries between the texture bands. Corrosion and stress corrosion tests in 1.0 M NaCl are performed in order to show stress effect on intergranular corrosion. Pitting corrosion is observed during corrosion tests, whereas intergranular cracks appear during stress corrosion tests. The medium crack length is 20 mu m and 168 cracks per mm(2) can be located. In these severe laboratory conditions, the order of magnitude of long crack growth rate is 5.10(-8) m.s(-1).

In the field of the ANR-PROD’HYGE project an original coaxial architecture for a High Temperature Electrolyser (French Atomic Energy Commission Patented) is studied which makes use of low-cost spring-like interconnects to cope with thermal dilatation. Components and materials used for this application must ensure high long-term chemical and mechanical stability which are investigated here. Oxidation resistance of EN1.4509 and CroFer22APUI iron based alloy as well as Haynes 230 nickel based alloy is investigated with thermogravimetric analysis (TGA) in humid conditions, with or without mixing with hydrogen. Experiments reveal that oxidation kinetics is increased by 30% volume of hydrogen in moisture. Oxide layer composition and morphology are then investigated using Energy Dispersive Spectroscopy (EDS), and Auger Electron Spectroscopy (AES). Preliminary tensile tests are performed at room temperature showing embrittlement due to rough oxide layer, and CroFer22APU softening due to grain growth at elevated temperature.

A statistical analysis of the effect of internal hydrogen on the surface slip morphology of relatively high nickel content AISI 316L type austenitic stainless steel was carried out on high resolution data obtained by atomic force microscopy. Surface plastic strain localisation was studied for different hydrogen contents, two grain sizes, and two plastic strain levels. The height and spacing of approximately 8000 slip bands, observed on 12 specimens, are shown to follow log-normal distributions. Hydrogen increased the mean slip-band height and the mean slip-band spacing for the two macroscopic plastic strain levels considered, and for the two hydrogen concentrations in coarse-grained specimens. The hydrogen effect was also observed for fine-grained specimens, but only for the highest hydrogen concentration. In addition, the emerging dislocation velocity increased by a factor 3 for high hydrogen content.