Dissolution Condensation Mechanism of Stress Corrosion Cracking in Liquid Metals: Driving Force and Crack Kinetics

Abstract

Stress corrosion cracking (SCC) in aqueous solution is driven by exothermic reactions of metal oxidation. This stimulus, as well as classical mechanisms of SCC, does not apply to SCC in liquid metals (LMs). In the framework of the dissolution-condensation mechanism (DCM), we analyzed the driving force and crack kinetics for this nonelectrochemical mode of SCC that is loosely called ''liquid metal embrittlement'' (LME). According to DCM, a stress-induced increase in chemical potential at the crack tip acts as the driving force for out-of-the-tip diffusion mass transfer that is fast because diffusion in LMs is very fast and surface energy at the solid-liquid interface is small. In this article, we review two versions of DCM mechanism, discuss the major physics behind them, and develop DCM further. The refined mechanism is applied then to the experimental data on crack velocity V vs stress intensity factor, the activation energy of LME, and alloying effects. It is concluded that DCM provides a good conceptual framework for analysis of a unified kinetic mechanism of LME and may also contribute to SCC in aqueous solutions. CRYSTAL plasticity and strength are environmentally sensitive properties. Along with corrosion and stress corrosion cracking (SCC), complex but well amenable to scientific interpretation electrochemical phenomena, more abstruse nonelectrochemical effects in crystal plasticity were observed for solids of different molecular nature when under simultaneous action of tensile stress and wetting liquids. The most famous among these effects is liquid metal embrittlement (LME), which describes premature failure of metals under simultaneous action of tensile stress and wetting liquid metals. LME creates potential reliability concerns in many technologies, including nuclear energy, welding, soldering, protective coatings, and gas industry. [1][2][3][4][5][6][7][8][9][10] The fact that LME was reported not only for polycrystals but also for single-crystalline [1][2][3][4][5] and amorphous metals [11] suggests that the phenomenon can not be explained in terms of either grain boundary (GB) diffusion/grooving modified by stress or adsorptionenhanced dislocation injection/mobility. LME was reported to most strongly manifest itself in solid metal (SM)-liquid metal (LM) couples, which form simple eutectic phase diagrams without any intermetallics; [9] this suggests that the concept of formation and fracture of brittle surface films does not apply to LME. Figure 1 taken from Reference 5 shows that LME involves subcritical crack growth, the typical feature that distinguishes SCC from crack nucleation-controlled brittle fracture below the ductile-brittle transition temperature in inert atmosphere. Crack kinetics observations have made clear that LME is not a ductile-brittle transition but a special case of time-dependent process of SCC. The subcritical growth under LME starts at an apparent threshold stress intensity K TH that can be as small as~0.1 MPaAEm ½ ; the process of crack extension spans almost all the lifetim...

Multiscale investigation of quasi-brittle fracture characteristics in a 9Cr–1Mo ferritic–martensitic steel embrittled by liquid lead–bismuth under low cycle fatigue

“…So far, some promising LME models and mechanisms have been proposed, including adsorption induced reduction in surface energy [9], adsorption induced reduction in interatomic cohesion [10,11], adsorption induced dislocation emission [8,[12][13][14], grain boundary penetration [15,16], stress assisted dissolution-condensation [17,18] and adsorption enhanced work hardening [19]. The adsorption induced reduction in surface energy mechanism postulates that the surface energy of a solid metal is reduced by adsorption of a liquid metal, thereby lowering the stress required to fracture the solid.…”

Liquid metal embrittlement of a dual-phase Al0.7CoCrFeNi high-entropy alloy exposed to oxygen-saturated lead-bismuth eutectic

“…The other group needs external factors, e.g., stress, to aid the liquid metal to attack the grain boundaries or bulk of the solid metal, such as the T91/LBE and FeCrAl/LBE systems [40,41] . In this case, various models have been proposed to describe the LME mechanisms, including adsorption induced reduction in surface energy [42] , adsorption induced reduction in interatomic cohesion [43,44] , adsorption enhanced dislocation emission [45,46] , stress assisted dissolution/condensation [47,48] , and grain boundary diffusion under stress [49,50] , etc. Among them, the most widely accepted ones are the adsorption induced reduction in surface energy or interatomic cohesion models.…”

Environmental degradation of structural materials in liquid lead- and lead-bismuth eutectic-cooled reactors

“…In spite of this fact, several promising mechanisms or models have been proposed in the period between the 1960s and the 1980s that can complementarily cover most of the LME cases. These mechanisms and models have been extensively discussed in the LME studies of steel/HLM couples, and they include reduction in surface energy [296], adsorption-induced reduction in cohesion [361,362], adsorption-enhanced dislocation emission [285,[363][364][365], localised plasticity and GB penetration [366], stress-assisted dissolution [367,368], GB penetration via atomic diffusion [298], and enhanced work hardening [369]. Recent advances in understanding the LME effect are mainly concentrated on GB embrittlement caused by liquid metals, which is an important reference to facilitate the understanding of the LME effect in the steel/liquid Pb and steel/LBE couples, and thus it will be summarised in a later section.…”

“…This model was originally proposed by Robertson [367], and modified later by Glickman [368] (then labelled "a dissolutioncondensation model") to predict crack growth kinetics. There are three basic assumptions made in Robertson's model: first, the crack is perfectly filled with liquid metal and there are no barriers available inside the crack to obstruct the solid from dissolving into the liquid.…”