Thierry Magnin

Influence of critical surface defects and localized competition between anodic dissolution and hydrogen effects during stress corrosion cracking of a 7050 aluminium alloy

Abstract Aluminium alloys used in the aeronautical industry, and in particular the high strength Al-Zn-Mg(-Cu) alloys of 7XXX series, can be susceptible to stress corrosion cracking (SCC). In the numerous studies reported in the scientific literature on this subject, two basic mechanisms have been proposed to model SCC: anodic dissolution and hydrogen embrittlement. However, there is currently no consensus on the precise mechanism. By coupling discriminating slow strain rate tests and scanning electron microscopy (SEM) examinations, it is shown that: (1) both anodic dissolution and hydrogen embrittlement operate during the SCC process of a 7050 aluminium alloy stressed in a chloride solution (NaCl 3%); (2) the main role of anodic dissolution is to produce critical defects which promote subsequently localized hydrogen discharge, entry and embrittlement; and (3) the relative influence of these two mechanisms depends on the main parameters that govern cracking, i.e. the microstructure, the electrochemical potential and the strain rate.

Fatigue short crack behaviour in metastable austenitic stainless steels with different grain sizes

Abstract The martensitic transformation ( γ → α ′) induced by cyclic strains in a metastable austenitic stainless steel leads to significant modifications of fatigue mechanisms compared with stable alloys, in particular at the mesoscopic scale (short cracks). The present study has been carried out on a metastable high purity Fe–17Cr–13Ni steel with two different grain sizes. From the very beginning of cycling, the microstructural instability induces a strong modification of the plastic behaviour of the material by limiting the slip activity at surface. At same applied plastic strain amplitude, the grain size has a decisive influence on the amount of martensite formed and on the fatigue life which is almost twice longer in the fine microstructure with 20% α ′ than in the coarse one (2% α ′). Whatever the grain size and the martensite content, short cracks nucleate in transformed α ′ regions; γ → α ′ transformation in front of the crack tip precedes further propagation which takes place exclusively in the martensite. The beneficial effect of the grain size on the fatigue resistance of metastable steels is explained by the indirect influence of γ / γ grain boundaries on crack propagation rate, providing an original contribution to the problem of microstructural barriers in fatigue.

Microstructural factors of low cycle fatigue damage in two phase Al-Si alloys

Abstract Low cycle fatigue (LCF) tests in air have been carried out on smooth specimens of several model binary Al–Si alloys with microstructures characterised by a high degree of homogeneity and the absence of casting defects, owing to the processing involving directed solidification. The elementary damage events, namely short crack nucleation at Al–Si interfaces and propagation across microstructural barriers (layers of eutectic Al), have been identified. The microstructure dependent fatigue damage evolves through single or multiple cracking, with significantly longer fatigue lives in the second case. The microstructural parameter responsible for the activation of one of two observed damage modes has been identified and quantified. This parameter is related to the maximal linear extension of Si particles at the surface and to the distance between Si particles, rather than to the average parameters given by conventional image analysis. The validity of this approach is enhanced by extending an existing model of tensile fracture in Al–Si–Fe alloys to the case of low cycle fatigue.

Cyclic plastic deformation behaviour of Ni single crystals oriented for single slip as a function of hydrogen content

Ni single crystals oriented for single slip, exhibit well defined cyclic stress-strain curves at room temperature, corresponding, partly to the formation of persistent slip bands in a given plastic strain range. The aim of the paper is to study the influence of hydrogen on the cyclic plastic behaviour of such crystals. Tests on pre-charged specimens show a decrease of the saturation stress, in comparison with pure nickel samples. Such effects are discussed in terms of hydrogen-dislocation interactions, and reduced cross slip ability in presence of hydrogen.