RAGHAV MOHAN
Graduate Student
M.S in Technical Entrepreneurship and Management
University of Rochester
INTRODUCTION
Liquid metal embrittlement (LME) is a phenomenon of practical importance, where certain ductile metals experience drastic loss in tensile ductility or undergo brittle fracture when tested in the presence of specific liquid metals. Generally, a tensile stress or a residual stress is needed to induce embrittlement. Many mechanisms were proposed to explain the phenomenological characteristics of LME. The significance of liquid metal embrittlement is revealed by the practical observation of several structures experiencing ductility losses and cracking during hot dip galvanizing or during subsequent fabrication.
Liquid metal embrittlement effects can be observed even in solid state, when one of the metals is brought close to its melting point (e.g. cadmium-coated parts operating at high temperature). This phenomenon is known as solid metal embrittlement.
OBSERVATIONS OF LME
Mechanical structures are typically a space frame fabricated from parallel large diameter tubes, called chords, cross braced by smaller diameter perpendicular and diagonal tubes(e.g. sign bridge structures over freeway) .After welding, the structure is hot dip galvanized. The hot dip galvanizing process consists of submerging the structure in a bath of molten zinc. This leaves a relatively uniform zinc coating over the entire structure upon removal from the bath. This coating provides corrosion protection for the underlying steel tubes and welds.
Their inspection, after a while, revealed numerous cracks in the welds joining the small diameter tubes to the chords. The cracked areas were then cut from the structure and submitted to Metallurgical Associates Inc. (MAI) for analysis. Their examination of such sections revealed transverse cracks through the bottom chords at the welds joining these to the diagonal tubes (Fig 1). Polished metallographic cross sections were prepared from several of these cracks. Other cracks were sectioned from the chord/diagonal welds (Fig 2) and opened for examination of the fracture surfaces by Scanning Electron Microscopy (SEM) to identify the mode and origin of the cracking.
While these analyses were in progress, the physical properties and chemical composition of the tubes were determined by chemical analyses, hardness, tensile, and impact testing. These tests confirmed that the tubing was the correct and specified material and exhibited the required and appropriate strength. Since structures fabricated from grades of steel that had been successfully in use for many years, these results eliminated substandard or defective material as the root cause of the failure.
Examination of the polished metallographic cross sections revealed a heavy coating on the fracture surfaces. This sample was examined by SEM and the coating was analyzed by Energy Dispersive Spectroscopy (EDS) to determine its composition and source. The results of this analysis indicated that the coating on the crack surface was zinc. In order to characterize the mode, or type, of fracture by which the cracking had occurred, the coating was chemically removed using a buffered hydrochloric acid. The actual crack surface was then examined by SEM. This examination revealed that the cracking had occurred by intergranular fracture.
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Fig 1 and 2: Cracks at the weld joining the diagonal tube to the chord and the sectioned crack from the structure for examination of the fracture surface
INTERGRANULAR AND TRANSGANULAR CRACKING:
Virtually all industrial metals are composed of microscopic interconnected grains. When a metal component breaks, fracture occurs either through the grains (transgranular fracture) or along the grain boundaries (intergranular). Both fracture types are brittle fractures. Intergranular fracture results when the grain boundaries are the weakest area in the metal’s structure. This weakness may be due to one or more factors including chemical composition variations, corrosion, and hydrogen embrittlement.
In transgranular fractures, the grain boundaries are at least as strong as the grains themselves.
Transgranular fractures progress very rapidly and usually produces a loud cracking sound when they occur.
CAUSES OF LME
Cracking due to improper welding procedure is common and such cracks are often intergranular. Cracks due to improper welding would also be coated with zinc since the hot dip galvanizing process takes place after welding. However, cracks of this type usually grow, or propagate, once the structure has been placed in service. This growth usually occurs by fatigue, a transgranular fracture mode, and progresses from the weld into the base metal, in this case the tube. Furthermore, this crack growth would not be coated with zinc since it occurs after galvanizing. The cracking exhibited in such structures extended through the welds and into the base metal. Cracking in both locations was intergranular. Significantly, the intergranular crack through both the weld and the base metal was coated with zinc. The location, extent, and type of fracture, along with the fabrication and service history all pointed to the root cause of the failure-Liquid Metal Embrittlement.
Liquid Metal Embrittlement, or LME, is caused by a combination of two factors. These are: The presence of a specific liquid (molten) metal in contact with the affected component or structure, and an applied or residual tensile stress acting on the affected component or structure while in contact with the liquid metal. When these two conditions occur, the liquid metal is absorbed in the components’ grain boundaries in a manner similar to the capillary action of a paper towel absorbing water. A liquid-metal filled crack is produced as the boundary between grains absorbs the molten metal and breaks the bond between adjacent grains. The liquid metal cools and solidifies leaving, in the case of our sign bridge, cracks in a steel component filled with much weaker zinc. The absorption of the liquid metal occurs at an extremely rapid rate. LME crack rate velocities of 4.0 inches in one hundredth of a second have been recorded.
MECHANISM OF LME (MOLECULAR)
For many structures in which a liquid metal is in contact with a polycrystalline solid, deep liquid grooves form where the grain boundary meets the solid-liquid interface. By performing a series of molecular dynamics simulations of liquid Ga in contact with an Al bicrystal (by Ho-Seok Nam and David Srolovitz) have identified a novel mechanism for liquid metal embrittlement and have developed a new model for it.
The Al-Ga couple is a particularly well-known LME system. Experimental analysis showed that liquid Ga penetrates into grain boundaries in Al at a remarkable rate, leading to distinct channel morphologies. The penetration of liquid Ga along the grain boundaries produces wet layers with thickness ranging from several mono-layers to several hundred nanometers even in the absence of an applied load. Interestingly, the rate of propagation of such liquid layers is strongly influenced by very small stresses. These observations have led to the conclusion that liquid Ga embrittlement of Al is caused by rapid liquid Ga penetration.
Thermodynamically, wetting of a grain boundary by a liquid metal should be expected when the spreading coefficients S satisfies: S = γGB −2γSL ≥ 0, where γGB and γSL are the free energies of the grain boundary and solid-liquid interface, respectively. However, thermodynamic arguments do not explain the liquid channel morphology, the Ga penetration kinetics, and the atomistic mechanism of Ga penetration. The anomalously fast, time-independent penetration rate of very long nanometer-thick liquid films cannot be explained in terms of the classical Mullins grain boundary grooving or by normal grain boundary diffusion.
Various models have been proposed to explain the kinetics and atomistic mechanisms by which the liquid phase penetrates quickly along grain boundaries. While each of these approaches is capable of explaining one or more aspects of LME, each also leads to discrepancies with respect to other observed LME phenomena in the same materials system. For example, none of these approaches successfully explains the effects of stress on liquid film penetration. Nam and Srolovitz have studied LME by performing molecular dynamics (MD) simulations of an Al bicrystal in contact with liquid Ga (with and without an applied stress) and investigated how Ga penetrates along the grain boundaries during the early stages of the wetting process. Based on the simulation results, a new mechanism for LME is proposed showing excellent agreement with both simulation and experimental data.
The atomistic mechanisms operating at the tip of the advancing Ga layers can be identified by analyzing the displacement and stress fields within the system. Figures 3 a - c show Ga concentration profile (left) and stress distribution (right) along a boundary at T=600 K at constant strains of 0, 0250 MPa and 500 MPa respectively. Although the liquid groove shapes and wetting angle are nearly the same in Figs. 3 a - c, the Ga penetration is strongly enhanced by the application of stress, forming nanometer-thick Ga-rich films. The Ga penetration rate is estimated by noting the depth at which the Ga concentration along the grain boundary exceeds a fixed value (one monolayer) at each time. This depth L versus time t is plotted in Fig. 3 d. In the absence of an applied stress, the rate at which Ga penetrates down the grain boundary (slope in Fig. 3 d) gradually decreases with time. However, when stress is applied, the Ga penetration rate becomes nearly time independent. Clearly, stress changes the fundamental nature of Ga penetration down grain boundaries in Al
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Fig 3 (a) 0 MPa (b) 250 MPa (c) 500 MPa
The effect of stress on Ga penetration can be examined by considering the stresses within the system; σxx is shown to the right of the Ga penetration figures in Fig 3a-c. Figure 3a shows that in the absence of an applied strain, the stresses in the system are small and random. However, when a strain is applied, we observe the formation of one (Fig. 3b) or more (Fig. 3c) patterns of concentrated stress at the grain boundary.
It is interesting to note that in the absence of an applied strain, no dislocation forms (Fig. 3a) and the Ga penetration rate decreases with time (Fig. 3d). However, when a strain is applied, dislocations form and climb at a fixed rate (Fig. 3b and c) and the Ga penetration rate is time independent (Fig. 3d). This suggests that the constant Ga penetration rate observed in the strained solid is associated with the fixed rate of "climb" of dislocations. A new picture of LME emerges. First, Ga diffuses down the grain boundary in Al below the liquid groove root and causes stresses large enough to nucleate a dislocation in the grain boundary. The first dislocation ‘climbs’ down by stress-enhanced Ga hopping across the dislocation core, leaving a trail of Ga behind. This Ga hopping leads to a constant dislocation climb rate that is independent of applied stress. Once the dislocation moves far enough from the groove root, another dislocation is nucleated. It too climbs down the grain boundary at the same rate, resulting in a uniform spacing of climbing dislocations. With Ga at the grain boundary, applied strains enhance the grain boundary opening and in turn more Ga is inserted from the liquid groove into the grain boundary to relieve the residual stress (i.e., Ga layer thickening process). The Ga penetration rate mirrors the dislocation climb rate and hence is time independent. In order for LME to occur, the solute must diffuse quickly in the grain boundary, a stress must be applied to nucleate dislocations and keep the grain boundary open, and the solute must be capable of creating grain boundary decohesion at sufficient concentrations.
CONCLUSIONS:
It is now clear that many mechanical structures like the sign bridge fail due to LME. The source of liquid metal, the molten zinc used in the galvanizing bath, was obvious. But as described above, LME also requires that the affected component is subjected to a tensile stress when in contact with the liquid metal. Careful measurements of the diagonal tubes in such structures revealed that a number of these were slightly longer than the design specification. In order to fit these diagonals into the structure without additional cutting, they were distorted by some means of leverage. This allowed the oversized diagonals to be fit and welded into place. However, this variation in size produced a tensile stress on diagonals of the correct length which had already been welded. Exposure of these stressed welds to the molten zinc galvanizing bath resulted in almost instantaneous cracking. By the time the structure was removed from the galvanizing bath it was not only cracked, but as the zinc solidified, the cracks were undetectable to visual inspection. Many of the welds were held together by virtually nothing more than a thin film of zinc. Such structures typically fail in less than three months.
Thus the mechanisms and effects of LME have to be carefully studied to control the failure of such indispensable structures.
REFERENCES: 1. ‘Embrittlement by Liquid Metals’ by W. Rostoker, J.M. McCaughey and H.Markus 2. ‘Dynamics and Cohesion of Materials Interfaces and Confined Phases Under Stress’ - Cooperative research team - Ho-Seok Nam and David Srolovitz 3. ‘Metallurgical Minutes on LME’ by Metallurgical Associates Inc.(MAI) 4. ‘Summary of Liquid Metal Embrittlement’ - en.wikipedia.org/wiki/Liquid_metal_embrittlement 5. ‘Intercrystalline Corrosion and Corrosion of Metals Under Stress’ edited by I.A. Levin
