Metal-induced embrittlement (MIE) is the

embrittlement Embrittlement is a significant decrease of ductility of a material, which makes the material brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such as temperatu ...

caused by diffusion of metal, either solid or liquid, into the base material. Metal induced embrittlement occurs when metals are in contact with low-melting point metals while under tensile stress. The embrittler can be either solid ( SMIE) or liquid (

liquid metal embrittlement Liquid metal embrittlement (LME), also known as liquid metal induced embrittlement, is a phenomenon of practical importance, where certain ductile metals experience drastic loss in tensile ductility or undergo brittle fracture when exposed to speci ...

). Under sufficient

tensile stress In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

, MIE failure occurs instantaneously at temperatures just above melting point. For temperatures below the melting temperature of the embrittler, solid-state diffusion is the main transport mechanism.P. Gordon, "Metal-Induced embrittlement of metals—an evaluation of embrittler transport mechanisms" Metallurgical Transactions A, 9, p. 267 (1978). https://doi.org/10.1007/BF02646710 This occurs in the following ways: * Diffusion through grain boundaries near the crack of matrix * Diffusion of first monolayer heterogeneous surface embrittler atoms * Second monolayer heterogenous surface diffusion of embrittler * Surface diffusion of the embrittler over a layer of embrittler The main mechanism of transport for SMIE is surface self-diffusion of the embrittler over a layer of embrittler that’s thick enough to be characterized as self-diffusion at the crack tip.P. Gordon, "Metal-Induced embrittlement of metals—an evaluation of embrittler transport mechanisms" Metallurgical Transactions A, 9, p. 267 (1978). https://doi.org/10.1007/BF02646710 In comparison, LMIE dominant mechanism is bulk liquid flow that penetrates at the tips of cracks.

Examples

Studies have shown that Zn, Pb, Cd, Sn and In can embrittle steel at temperature below each embrittler’s melting point. *

Cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

can embrittle titanium at temperatures below its melting point.D.N. Fager and W.F. Spurr, "Solid cadmium embrittlement: Titanium alloys, Corrosion," 26, 409, (1970). * Hg can embrittle

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...

at temperatures below its melting point.N,A. Tiner, A study of fracturing behavior of cop- per and zinc coated with mercury, Trans. AIME, 221 (1961) 261. * Hg can embrittle

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...

at temperatures below its melting point.J.C. Lynn, W.R. Warke, P.Gordon, "Solid Metal-Induced Embrittlement of Steel," Materials Science and Engineering, Elsevier, 18, p. 51-62,,(1974) doi.org/10.1016/0025-5416(75)90072-5.

Mechanics and temperature dependence

Similar to

( LME), solid metal-induced embrittlement results in a decrease in fracture strength of a material. In addition, a decrease in tensile ductility over a temperature range is indicative of metal-induced embrittlement. Although SMIE is greatest just below the embrittler’s melting temperature, the range over which SMIE occurs ranges from to T, where T is the melting temperature of the embrittler. The reduction in ductility is caused by formation and propagation of stable, subcritical intergranular cracks. SMIE produces both intergranular and transgranular fracture surfaces in otherwise ductile materials.

Kinetics of crack onset and propagation via SMIE

Crack extension, as opposed to crack onset, is the rate determining step for solid induced-metal embrittlement. The main mechanism leading to solid metal induced embrittlement is multilayer surface self-diffusion of the embrittler at the crack tip. Propagation rate of a crack undergoing metal-induced embrittlement is a function of the supply of embrittler present at the crack tip. Crack velocities in SMIE are much slower than LMIE velocities. Catastrophic failure of a material via SMIE occurs as a result of the propagation of cracks to a critical point. To this end, the propagation of the crack is controlled by the transport rate and mechanisms of the embrittler at the tip of nucleated cracks. SMIE can be mitigated by increasing the

tortuosity Tortuosity is widely used as a critical parameter to predict transport properties of porous media, such as rocks and soils. But unlike other standard microstructural properties, the concept of tortuosity is vague with multiple definitions and vari ...

of crack paths such that resistance to intergranular cracking increases.

Susceptibility

SMIE is less common that LMIE and much less common that other failure mechanisms such as

hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...

fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...

, and

stress-corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC ...

. Still, embrittlement mechanisms can be introduced during fabrication,

coating A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powde ...

s, testing or during service of the material components. Susceptibility for SMIE increases with the following material characteristics: * Increase in strength of high-strength material * Increasing grain size * Materials with more planar-slip than wavy-slip

References

{{reflist Metals Corrosion