Jacques Foct

The effects of an Fe-Zn intermetallic-containing coating on the stress corrosion cracking behavior of a hot-dip galvanized steel

This study deals with the mechanical behavior of galvanized, interstitial free (IF) steel in air and sodium chloride water. Tensile tests conducted in air at different strain rates lead in general to a ductile behavior. It can become brittle when tested in a NaCl solution at particular strain rate values. Fractographic analysis shows a brittle fracture similar to hydrogen embrittlement (HE). A detailed analysis taking into account both the individual mechanical strength of the phases on the coating as well as the electrochemical reaction between Zn and water allow the explanation of this embrittlement. Hydrogen can be produced and can penetrate the cracks formed inside the intermetallic phases of the coating during loading, thus, reaching the steel substrate. The HE of the IF steel, assisted by zinc dissolution and galvanized coating low toughness, appears to be the cause of the loss of ductility observed in the presence of specific environmental conditions, strain rates and morphologies of the coating.

Discontinuous precipitation of Cr2N in a high nitrogen, chromium-manganese austenitic stainless steel

The main purpose of the present work is to study the effect of a high nitrogen content (1 wt% N), on the microstructural evolution of a Cr-Mn austenitic stainless steel aged over the [400–900 °C] temperature interval. Thermal treatments carried out between 700 and 900 °C lead to the decomposition of the nitrogen supersaturated austenitic matrix by discontinuous precipitation of Cr2N particles. The microstructural features of the reaction are described and analysed. In the present case, the cellular precipitation of Cr2N is a peculiar and complex phenomenon which involves two diffusion mechanisms: the diffusion of an interstitial element (nitrogen) and the diffusion of a substitutional one (chromium). The nucleation of the discontinuous precipitation arises from a reduction of the surface energy of the precipitates. Furthermore, the precipitation growth is a non-steady state process, because the reaction is governed at first by the intergranular diffusion of chromium, and then tends to be controlled by its bulk diffusion. Consequently, the features of this discontinuous precipitation do not fit in with the assumptions of usual theories, which have been established for binary substitutional systems that transform in steady state conditions. This discontinuous precipitation brings about a slight hardening. Then, the hardness of the aged samples can be described by an additive relationship between the hardness of the precipitation cells and that of the untransformed matrix. Beside the discontinuous precipitation of Cr2N, sigma phase forms with significant volume fractions.

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal. Experiments and electronic structure calculations

Abstract In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb–Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.

Thermodynamic study of the galvanizing process in a Zn–0.1%Ni bath

The addition of alloying elements to the zinc bath is one of the best solution to reduce overthicknesses of the coating when galvanizing silicon-killed steels. The use of nickel addition to the molten zinc offers industrial advantages, especially in the modification of the Sandelin curve related to the reactivity of steels towards the galvanizing bath. However, dross formation in a 0.1 wt% Ni bath is the main drawback of the process.Bending on thermodynamic description of the ternary Fe–Zn–Ni system at 450°C, the reaction between a pure iron substrate and a nickel added zinc bath is modelled at 450°C. The approach of the extension of the liquid phase in the quaternary Fe–Zn–Si–Ni system leads to understanding of the effect of nickel addition to the zinc bath when galvanizing reactive silicon steels. These thermodynamic interpretations lead to understanding of experimental phenomena during the galvanizing process: dross formation, reduction of the coating thickness, smoothing the solid–liquid interface when nickel is added to the zinc bath at 450°C. Based on morphological and kinetic observations, steel-zinc reactions are clearly investigated showing the important effect of alloying elements to the substrate or to the bath.

Role of the microstructure on fatigue properties of 475°C aged duplex stainless steels

Abstract This paper deals with the role of microstructure, nitrogen content and α/γ phases volume fraction, on fatigue properties of duplex stainless steels (DSS) aged at 475°C. The results obtained from this study show that the α/γ volume fraction plays an important role on fatigue lives. The fatigue resistance is not deteriorated in an aged DSS with 70% of austenite. The stress response to strain cycling is strongly affected by the difference of hardness between ferrite and austenite. A 475°C aged DSS containing 0.4% nitrogen and 70% of ‘hard austenite’ lead to interesting fatigue properties which do not strongly differ from that obtained in annealed DSS.

Specific features of two phase alloys response to cyclic deformation

Abstract The fatigue short crack propagation in polycrystalline metals and alloys is strongly dependent on microstructure because of the presence of interfaces (grain, twin or phase boundaries) which cannot be crossed without additional energy supply. The conventional approach to the problem of microstructure exclusively considered as interfaces appears insufficient to explain the cyclic response and the fatigue resistance of alloys with more complex microstructures. In the present paper, a generalised concept of microstructural barriers, valid both for single phase and multiphase materials, is proposed. It emphasises the role of physical properties of the grain (particle) adjacent to the element of microstructure in which a short crack has been nucleated and their evolution during cyclic straining. This concept is then applied to explain qualitatively the cyclic behaviour of two different types of two phase materials: austenitic–ferritic stainless steels and eutectic Al–Si alloys. On this basis, some general rules for the design of two phase alloys, taking into account the mechanical properties of both phases and the morphological parameters of microstructures, are drawn.

Crystallographic and electronic structure of CuXFe4-XN

Abstract In the present work, the electronic and the crystallographic structures of γ′-Cu X Fe 4− X N Perovskite nitrides have been determined. Mechanical alloying has been used to obtain these nitrides, meanwhile fully ordered γ′-CuFe 3 N single phase could not be obtained except up to ≈12 at% Cu (γ′-Cu 0.6 Fe 3.4 N). The measured lattice parameter a =0.378 nm is near the well known γ′-Fe 4 N nitride ( a =0.377 nm). Order phenomena during thermal aging have been established and discussed on the basis of Mossbauer results. Self-consistent LMTO calculations were performed for γ′-CuFe 3 N fully ordered compound. The calculated magnetic moment was calculated to be 6.18 ţ B per unit cell. Also theoretically determined results were compared with Mossbauer data (extrapolated for the ideal case) in terms of isomer shift and magnetic hyperfine field.

Mössbauer study of phase transformation induced by mechanosynthesis in Fe4N nitrides

Abstract γ′iron nitride Fe 4 N and mixtures of α - Fe + χ - Fe 2 N are ground and mechanically alloyed by ball-ball or ball-vial collisions. The powders are studied by X-ray diffraction and Mossbauer spectrometry. It is shown that grinding of γ′-Fe 4 N leads to a hexagonal-type e-Fe 4 N nitride which is characterized by a nearly random nitrogen distribution. By thermal ageing at 463 K the γ′ structure is restored. Mechanical aalloying of α - Fe + χ - Fe 2 N mixture leads to e-Fe 4 N similar to that obtained by grinding of γ′. The discussion of mechanisms involved in grinding of γ′, by alloying of α + χ and by ageing of e are mainly based on the evolution of Mossbauer spectra.

The structure of nitrogen-supersaturated ferrite produced by ball milling

Highly supersaturated solid solutions of nitrogen in ferrite (bcc) were produced by ball milling of various powder mixtures of α-iron and ε-Fe3N1.08. The microstructure and the crystal structure of the product phases were examined as a function of nitrogen content using X-ray powder diffraction, high-resolution electron microscopy and Mössbauer spectroscopy. It was found that the grain size decreases with increasing nitrogen content. Unexpected shifts of the reflections in the X-ray powder diffraction patterns of the supersaturated N-ferrites, depending on the hkl values of the reflections and nitrogen content, were observed. These shifts cannot be explained by tetragonal distortion of the bcc unit cell, but they are in accordance with the occurrence of a certain type of stacking faults on bcc {211} planes. This result, together with the observation of some isolated fcc crystals (by high-resolution electron microscopy) and a drop in microstrain for high nitrogen contents, demonstrates that unconventional deformation mechanisms are operative in these materials below a certain grain size, leading to a breakdown of the classical Hall–Petch relation for mechanical strengthening.

Ab initio calculations of some atomic and point defect interactions involving C and N in Fe

The critical importance of interstitial impurities in bcc metals has been well established over many years. Ab initio calculations based on the density functional theory offer a powerful means to revisit this subject and the present article is focused on the interactions of C and N atoms with point defects in Fe. The structures and relative stabilities of different configurations as well as their formation and binding energies have been determined. The consequences of the obtained results are discussed.