Liquid metal embrittlement is the reduction in the elongation to failure that can be produced when normally ductile solid metals are stressed while in contact with a liquid metal This review describes its principal characteristics and the several models which have been advanced in attempts to explain the occurrence and different features of the process Comparison between theory and experiment indicates that many, but not all, of its aspects are consistent with a mechanism which operates by reducing the fracture surface energy of the solid metal Literature reports show that liquid metal embrittlement can occur with a very wide range of material combinations, and while most of the data refer to laboratory studies, it is clear that the phenomenon is also of technological significance as a potential cause of plant damage

Liquid metal embrittlement

The Rinetics of A12O3 growth and spalling were observed for Ni-42 at. pct Al oxidized in air at 1100°C up to 1500 h. While crystallographic voids formed at the oxide-metal interface due to the oxidation process, the oxidation resistance was good in 1 h cycle tests. Spalling to bare metal was the predominant mode of oxide loss. The A12O3 grain size at the metal interface varied with a time-exponent of 0.2. In isothermal tests the oxide thicRness was nearly parabolic with time, having a time-exponent of 0.40, while the volume per void varied directly with residence time underneath intact oxide. The total void volume accounted for ~1/2 the aluminum needed to form the AI2O3 scale. In 1 h cycle tests the oxide thicRness and volume per void reached a plateau at ~150 h due to the spalling process. The correlation between total void volume and oxide volume was eventually obliterated by extensive cycling. A cyclic step-process spall model was used to predict the parabolic rate constant, Rp, and the oxide spall fraction, Rs, from gravimetric curves. Predicted values of Rp agreed well with experimental values, while predicted Rs values were often less than measured values. According to this model the severity of a long time test can be rated according to the factor fMe√ Rs · Rp · Δt mg/cm2 ·cycle, where fMe is the ratio of metal-to-oxygen in the oxide and Δt is the cycle time. Measured values of Rs in isothermal tests varied linearly with exposure time or approximately with (oxide thicRness).2 Cyclic tests showed more scatter and less dependence of Rs on oxide thicRness, presumably due to the complex oxide topography and relaxed stress states.

Oxide morphology and spalling model for NiAl

In situ investigation of liquid Ga penetration in Al bicrystal grain boundaries: grain boundary wetting or liquid metal embrittlement?

The advanced characteristics of synchrotron x-ray sources make it possible to implement radiology with powerful and innovative approaches. We review in simple terms the conceptual background of such approaches, then we present a number of selected examples. The practical tests concern life-sciences specimens as well as materials-science systems.

Coherence-enhanced synchrotron radiology: simple theory and practical applications

Liquid-metal embrittlement in an Al 6%Zn3%Mg alloy

Embrittlement and/or penetration of aluminium grain boundaries by liquid gallium have been investigated by a radiotracer technique and measurement of mechanical properties. A distinction is made between penetration without external stress and the embrittlement itself. However, the results show that a description of grain-boundary embrittlement must take into account the preliminary step of intergranular penetration by gallium. Moreover, two stages must be distinguished in the penetration of gallium; the driving force of the first stage seems to be a strong interaction between gallium and the grain boundaries. This interaction is usually described as intergranular adsorption. A general scheme for aluminium grain-boundary embrittlement and/or penetration is proposed. Some observations on the microstructure of the grain-boundary surfaces separated by liquid gallium are also reported. This technique of intergranular embrittlement by liquid gallium provides a new means of examining the structure of int...

The Influence of Testing Temperature and Thermal History on the Intergranular Embrittlement and Penetration of Aluminium by Liquid Gallium

Correlation of thermal pits and condensation cavities in aluminum

Mechanisms involved in decarburization and the fissurization of different carbon steels by liquid lithium were studied. The kinetics of liquid lithium penetration were found to depend strongly on the carbon content of the steel, the shape of cementite precipitates and the temperature and annealing time. The exact nature of the chemical reaction between cementite and lithium is established and a mechanism is proposed for propagation of the cracks.

Mechanisms of Carbon Steel Decarburization and Penetration by Liquid Lithium

In aluminium alloys, intergrantilar surfaces can be observed by optical microscopy or scanning electron microscopy after intergranular decohesion by liquid gallium. This experimental technique has been applied to aluminium–copper alloys (1 and 4 wt.-% copper). Two types of thermal treatment were carried out: water-quenching followed by reheating at different temperatures; air cooling followed by reheating at different temperatures. The results have shown that the following parameters control the morphology of the inter granular surfaces: cooling rate, solute content of the alloys, time and temperature of reheating, and grain boundary structure. After water-quenching no precipitates were observed, but after air-cooling, the interfaces were found to contain copper-rich precipitates. Reheating after air-cooling led to an increase in the number of copper precipitates. The density of the precipitates increased with increased copper content. Reheating after water-quenching revealed the presence of vacan...

Fractographic Investigation of Grain-Boundary Precipitates in Al–Cu Alloys (1 and 4 wt.-% Cu) by Gallium Embrittlement

Quench‐annealing experiments on aluminum (99.995% purity) have shown that vacancy condensation pits on the surface can only be formed when the annealing temperature is 200°C or more. In contrast to slow‐cooling treatments, the pit distribution is then uniform over the whole surface; no depleted zones are observed around grain boundaries and subboundaries. There is a strong disagreement between our own results and Tariyal and Ramaswami's paper which showed pit formation by quench annealing at 20 and 50°C. The reasons for such a disagreement are discussed.

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C. Roques-Carmes

Papers

The Influence of Testing Temperature and Thermal History on the Intergranular Embrittlement and Penetration of Aluminium by Liquid Gallium

Correlation of thermal pits and condensation cavities in aluminum

Mechanisms of Carbon Steel Decarburization and Penetration by Liquid Lithium

Fractographic Investigation of Grain-Boundary Precipitates in Al–Cu Alloys (1 and 4 wt.-% Cu) by Gallium Embrittlement

Comment on ``Formation of Vacancy Pits on the Surface of Aluminum on Quench Annealing''