Grain Boundary Penetration of Liquid Bi in Cu Polycrystals

Renaudot

Harabasz

et al. 2002

phys. stat. sol. (a)

The interest in ultra-high-purity (UHP) metals and alloys (including single crystals and bicrystals) in studying damage mechanisms is presented through three practical cases of environment or precipitation-induced damages in nuclear power plants. First, stress corrosion cracking mechanisms are assessed by direct measurements of hydrogen-dislocation interactions in pure nickel single crystals in low-cycle fatigue tests under cathodic potential. Hydrogen-induced softening is observed, that confirms an important assumption on the corrosion-enhanced plasticity model. Second, mechanisms of high-temperature intergranular cracking of bimetallic welds are analysed on a series of model materials including 302 H stainless steel and UHP stainless steel selectively doped with carbon. The key role of carbide precipitation and localised shearing in a chromium-depleted zone is pointed out. Finally, the analysis of intergranular penetration of liquid Bi-rich films in a polycrystalline solid Ni inducates the presence of very long and brittle films of nanometric thickness, which have to be taken into account in any model of liquid-metal embrittlement.

Section: Liquid-metal Embrittlement -The Study Of Intergranular Penetmentioning

confidence: 99%

The Use of Ultra-High-Purity Metals and Alloys to Study Environment-Sensitive Damage Mechanisms in Nuclear Power Plants

Renaudot

Harabasz

et al. 2002

phys. stat. sol. (a)

“…These observations, which were in general done on transverse polished sections by SEM or optical microscopy, were unsuccessful in describing the area ahead of the micrometric film tip, which is of major importance in the analysis of LME mechanisms. 15,16 This area was shown to induce strong intergranular room temperature brittleness in at least two systems: Cu-Bi 17 and Ni-Bi. 18 Mechanical testing was used to investigate the kinetics of depth embrittlement, which is frequently associated with the kinetics of intergranular penetration, i.e.…”

Section: Introductionmentioning

confidence: 98%

AES quantification of intergranular film thickness in the Ni–Bi system with respect to the liquid metal embrittlement phenomenon

Surface & Interface Analysis

Marié

Biscondi

2001

Intergranular penetration of liquid bismuth-rich alloy in solid polycrystalline nickel is investigated at 700• C. Short intergranular films of micrometric thickness with a very acute tip are observed by SEM. Intergranular penetration of Bi-Ni alloy induces strong intergranular brittleness that extends far ahead of the micrometric film tip. Very long intergranular films of nanometric thickness are identified by AES after in situ intergranular fracture. A quantitative model aimed at the determination of the actual thickness of these films is developed and described in detail. The measured bismuth/nickel intensity ratios (I Bi /I Ni ) are correlated to the thickness of the intergranular film. Detailed procedures related to the determination of the experimental I Bi /I Ni ratio as well as all assumptions related to the quantification model are clearly stated. Atomic densities, attenuation lengths and retrodiffusion factors are calculated whereas effective cross-sections, probabilities of de-excitation by the Auger process and the transmission efficiency of the analyser are suppressed from the final formulae due to reference measurements on the bulk Bi-Ni embrittling alloy, which is used as a bicomponent standard of known composition. Numerical application gives a value of 2-4 nm for the thickness of the film formed during 8 h at 700• C. The uncertainty mainly comes from the difficulty in determination of the Auger electron collection angle, due to the use of polycrystals. Interest for the use of bicrystals is underlined. It is postulated that the mechanisms of liquid metal embrittlement (LME) should be analysed with respect to the tip of this nanometric film.

“…The commonly admitted view of grain boundary penetration is that this is a phenomenon which occurs in some specific systems within the range of temperatures where grain boundary energy is higher than twice the solid metal / liquid metal interfacial energy [14]. This phenomenon is supposed to occur on a very short distance [7,15] with linear kinetics [16,17] and often results in the formation of nanometre-thick films [5,18,19]. While nanometre-thick films were effectively observed in several systems, including Ni-Bi, the existence of such a long transition profile together with the parabolic kinetics measured on both bicrystals and polycrystals, indicate a diffusion-based mechanism for the intergranular penetration phenomenon.…”

Section: Indications For a Diffusion-based Mechanism Of Igpmentioning

confidence: 99%

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

Marié

Laporte

et al. 2005

DDF

. This currently used definition describes only the result of the thermodynamic equilibrium and, consequently, there is no indication neither of the mechanism nor of the kinetics of this phenomenon. IGP might be critical in the long-term behaviour of a spallation target, where T-91 steel has to withstand the contact with liquid Pb-Bi eutectic alloy [3]. IGP can potentially lead to two modes of damage : first, liquid metal embrittlement (LME) at high temperature under simultaneous action of stress and liquid metal and, second, liquid metal induced embrittlement (LMIE) typically under the action of stress at room temperature when liquid metal has solidified. The prediction of the long-term behaviour of structural components under such conditions requires the knowledge of the mechanism of intergranular penetration. This is the main reason why we decided to study this phenomenon using a model solid Ni / liquid Bi system, in which IGP is known to occur at 700°C [4]. There are at least three advantages to use this model system : first IGP occurs in only a few hours (as compared to several months in systems of industrial interest), second, it results in LMIE and therefore gives access to the depth of embrittlement and finally it offers the possibility to analyse the bismuth composition profile on the fracture surface either by Auger Electron Spectrometry (AES) after "in situ" fracture or by Rutherford Backscattering Spectrometry (RBS) even after ex situ fractures. The main drawbacks are the existence of a peritectic