Krzysztof Wolski

Intergranular penetration and embrittlement of solid nickel through bismuth vapour condensation at 700°C

Grain boundary penetration of liquid bismuth in polycrystalline nickel is investigated at 700°C. The contact between the two metals is ensured by bismuth transport through vapour phase. The formation of a nanometre-thick Bi-rich layer on external surfaces of solid nickel is revealed by glow discharge optical spectroscopy. This layer is the consequence of Bi vapour condensation on nickel substrate at 700°C. The liquid Bi/solid Ni contact leads to grain boundary penetration of liquid bismuth in the form of a film of nanometric thickness as revealed by Auger electron spectroscopy. The presence of these intergranular films causes strong room temperature brittleness of nickel, as shown by tensile and bending tests. The comparison with results due to the direct contact between solid nickel and bulk liquid bismuth-rich alloy indicates that in both cases intergranular penetration rate and embrittlement are of the same magnitude. Based on these results, a concept of a new device for liquid metal embrittlement (LME) tests is outlined.

Comparison of the High Temperature Surface Reactivity in Impure Helium of Two Materials for Gas Cooled Reactors

Nickel base alloys Haynes 230 and Inconel 617 are of interest for gas cooled reactors. At high temperature in impure helium, they generally form surface chromium-rich oxides. However above a critical temperature called TA, the scales are not stable anymore and the chromia destruction comes with a production of carbon monoxide. Reactivity tests on model alloys, with and without carbon, prove that chromia is reduced by the carbon from the alloy. TA vs P(CO) curves were also plotted for the two commercial alloys based on the experimental determination of TA in various atmospheres with increasing partial pressures of carbon monoxide. Unexpectedly, both materials exhibit an almost identical behavior although a basic equilibrium approach suggests that the chromia scale would be reduced in different conditions due to the thermodynamic particularity of the interfacial alloy/scale system.

Thermodynamic Modelling of the Destruction of the Surface Cr2O3 on Alloy 230 in the Impure Helium Atmosphere of a Gas Cooled Reactor

Above a given temperature called TA, the chromium rich oxide which has been developed on the surface of Haynes 230® and model NiCrWC alloys at a lower temperature becomes unstable in impure helium: carbon monoxide is released. Actually, oxide is reduced by carbon from the alloy. A thermodynamic model is developed to rationalize the variation of TA as a function of the partial pressure of CO in the gas phase. It was found that, at the early stages of the scale reduction, the relevant reaction occurs at the oxide/metal interface between chromia and carbon from the alloy. The interfacial activity of carbon in the alloy can be calculated based on measurements of the interfacial weight percentage of chromium and using ThermoCalc® software. Excellent agreement is observed between experimental values of TA and theoretical predictions.

Diffusion-controlled liquid bismuth induced intergranular embrittlement of copper

The consequences of the contact between liquid bismuth and a copper bicrystal are investigated at 500°C. Atoms of bismuth are shown to penetrate and embritlle the copper grain boundary. Grain boundary concentration profiles of bismuth are obtained on fracture surfaces by both Auger electron spectroscopy and He4+ Rutherford backscattering spectroscopy. The maximum bismuth intergranular concentration is calculated from experimental data to be about 1.7 monolayers (near the liquid bismuth / solid copper interface). The overall profiles are significantly different from typical erfc profiles and an interpretation is proposed, based on the coupling effect between grain boundary diffusion and non-linear segregation. These results allow us to conclude on the absence of grain boundary wetting for the Cu / Bi system at 500°C.

Evidence for a Diffusion-Based Mechanism of Liquid Metal Intergranular Penetration: Case Study of a Ni-Bi Model System

A model Ni-Bi system has been used to investigate intergranular penetration (IGP) phenomenon. All experiments have been done on Ni 26°<110> bicrystal at 700°C using bismuth vapour condensation as a source of liquid bismuth. Such a procedure results at room temperature in either partial or total Liquid Metal Induced Embrittlement (LMIE) of a unique grain boundary, depending on the duration of liquid Bi / solid Ni contact at 700°C. Auger Electron Spectrometry (AES) and Rutherford Backscattering Spectrometry (RBS) have been used to measure the Bi concentration profile between the source of liquid bismuth and the penetration front. Two zones have been clearly identified : the first one of almost constant Bi concentration called nanometrethick film which is interpreted in terms of Fowler-Guggenheim multi-layer segregation under local equilibrium conditions and the second one with a progressive decrease of Bi concentration over a distance of the order of 20-200µm. Such a long transition zone, together with parabolic diffusion kinetics indicates diffusion-based mechanism of intergranular penetration as opposed to the direct grain boundary wetting.

Kinetics and Mechanism of High Temperature Internal Oxidation of Ni-14wt%W Alloy at 1000°C

Ni-14wt%W model-alloy has been used to study the kinetics and the mechanism of high temperature internal oxidation in presence of an external oxide scale. Oxidations have been done in air at 1000°C for different oxidation times and have allowed the evaluation of the oxidation kinetic from mass gain and from oxide thickness measurements. The oxidation kinetic which follows a parabolic oxidation rate indicates a diffusive behaviour of species during the oxide formation. Cross-section observations by Scanning Electron Microscopy (SEM) and analysis by Grazing Incidence X-Rays Diffraction (GIXRD) show a multi-layered structure of the oxide. Starting from the surface towards the bulk alloy, the scale is layered as follows : (i) a pure nickel oxide (NiO) in the outer part (ii) a porous NiO matrix containing NiWO4 second phase and (iii) an internal oxidation zone of tungsten, first forming WO3 sub-micrometer oxides that progressively transform into NiWO4 oxide precipitates in the alloy.

On the Depassivation Mechanism of Lean Duplex Stainless Steels and the Influence of the Partitioning of the Alloying Elements

The depassivation of UNS S32304, S32202, and S32101 lean duplex stainless steels (DSS) in 26 wt% sodium chloride (NaCl) was investigated by potentiodynamic polarization to understand the selective dissolution processes in localized corrosion phenomena. Results showed that the depassivation of the three grades proceeds in two stages. First, the austenite is the only phase depassivated within the range of no more than 0.3 pH units, providing a single corrosion peak on the polarization curves of the DSS. Subsequently, the ferrite is also depassivated and provides a second corrosion peak at a more cathodic potential. Results are discussed in terms of the partitioning of alloying elements.

The Transition from Internal to External Oxidation of Ni - X Wt.% Cr at 950°C

The papers focus is to establish the criterion for the transition from internal to external oxidation. This criterion is a simple value of oxide volume fraction where the coalescence of the nodular oxide in the continuous layer is considered as inevitable. It is obtained by the Wagners analytical solution and by the data of Giggins et al. [1], which give the experimental transition from internal to external oxidation between 10 and 11 weight percent of chromium in the initial alloy. This paper also enables to obtain the oxygen diffusion coefficient in pure nickel thanks to the experimental results of oxidation at 950 °C during 10 hours on Ni-0.2Cr, Ni-1Cr and Ni-5Cr model alloys.

Gallium Distribution in Gallium-Coated Aluminum for Brazing Application

This work deals with the study of a new aluminum brazing process, called Galluminium. This technique, consisting in using gallium as a solder, is advantageous since it enables to braze at ambient air without flux. Indeed if the gallium coating is done mechanically, it descales the alumina layer and acts as a barrier against further reoxidation. Nevertheless, liquid gallium causes a severe aluminum embrittlement since it penetrates into the aluminum grain boundaries. We will show in this paper that this phenomenon has only a little impact on the mechanical resistance of the brazed joints since the amount of deposited gallium is enough low to avoid a severe embrittlement and the heat treatment (from 200°C to 600°C during several minutes) during brazing process dissolves gallium in the bulk.