Embrittlement

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Embrittlement is a loss of ductility of a material, making it brittle. Embrittlement is used for any phenomena where a hostile environment compromises a stressed material's mechanical performance. Various materials have different mechanisms of embrittlement. Due to these various mechanisms, embrittlement manifests in a variety of ways, from slow crack growth to a reduction of tensile ductility and toughness. Often, cyclical stresses or environments lead to embrittlement.

Mechanisms[edit]

Embrittlement is a series complex mechanism that is not completely understood; the mechanisms can be driven by temperature, stresses, grain boundaries, or material composition. However, by studying the embrittlement process, preventative measures can be put in place to mitigate the effects. There are several ways to study the mechanisms. During metal embrittlement (ME), crack-growth rates can be measured. Computer simulations can also be used to enlighten the mechanisms behind embrittlement; this is helpful for understanding hydrogen embrittlement (HE), as the diffusion of hydrogen through materials can be modeled. The embrittler does not play a role in final fracture; it is mostly responsible for crack propagation. Cracks must first nucleate. Most embrittlement mechanisms can cause fracture transgranularly or intergranularly. For metal embrittlement, only certain combinations of metals, stresses, and temperatures are susceptible; this is contrasted to stress-corrosion cracking where virtually any metal can be susceptible given the correct environment. Yet this mechanism is much slower than that of liquid metal embrittlement (LME), suggesting that it directs a flow of atoms both towards and away from the crack. For neutron embrittlement, the main mechanism is collisions within the material from the fission byproducts.

Types of Embrittlement[edit]

  • Hydrogen embrittlement is the effect of hydrogen absorption on some metals and alloys. This happens in cathodes for electroplating, in a mechanism similar to SCC.
  • Stress corrosion cracking (SCC) is the embrittlement caused by exposure to aqueous, corrosive materials. It relies on both a corrosive environment and the presence of tensile (not compressive) stress.
  • Sulfide stress cracking is the embrittlement caused by absorption of hydrogen sulfide.
  • Adsorption embrittlement is the embrittlement caused by wetting.
  • Liquid metal embrittlement (LME) is the embrittlement caused by liquid metals.
  • Metal-induced embrittlement (MIE) is the embrittlement caused by diffusion of atoms of metal, either solid or liquid, into the material. For example, cadmium coating on high-strength steel, which was originally done to prevent corrosion.
  • Neutron embrittlement causes embrittlement of some materials, notably certain metals. neutron-induced swelling, and buildup of Wigner energy. This is a process especially important for neutron moderators and nuclear reactor vessels (see ductility). While this is not that aptly described as embrittlement, the resulting change in material composition due to this process changes the mechanical properties of the material in a similar way to more conventional embrittlement mechanisms. Due to the hostility of environments allowing for neutron embrittlement, many other mechanisms are at play, including creep aggravation.
  • The primary embrittlement mechanism of plastics is gradual loss of plasticizers, usually by overheating or aging.
  • The primary embrittlement mechanism of asphalt is by oxidation, which is most severe in warmer climates. Asphalt pavement embrittlement can lead to various forms of cracking patterns, including longitudinal, transverse, and block (hexagonal). Asphalt oxidation is related to polymer degradation, as these materials bear similarities in their chemical composition.

Glasses and Ceramics[edit]

Inorganic glass embrittlement can be manifested via static fatigue. Embrittlement in glasses, such as Pyrex, is a function of humidity. Growth rate of cracks vary linearly with humidity, suggesting a first-order kinetic relaltionship; the mechanisms are similar to those of metals.

Cryogenic embrittlement[edit]

Around cryogenic temperatures plastics and rubbers become brittle, which is known as the embrittlement temperature.

Embrittlement temperatures[1]
Material Temperature [°F] Temperature [°C]
Plastics
ABS −270 −168
Acetal −300 −184.4
Delrin -275 to -300 -171 to -184
Nylon -275 to -300 -171 to -184
Polytron −300 −184.4
Polypropylene -300 to -310 -184 to -190
Teflon −275 −171
Rubbers
Buna-N −225 −143
EPDM -275 to -300 -171 to -184
Ethylene propylene -275 to -300 -171 to -184
Hycar -210 to -275 -134 to -171
Natural rubber -225 to -275 -143 to -171
Neoprene -225 to -300 -143 to -184
Nitrile -275 to -310 -171 to -190
Nitrile-butadiene (ABS) -250 to -270 -157 to -168
Silicone −300 −184.4
Urethane -275 to -300 -171 to -184
Viton -275 to -300 -171 to -184
Metals
Zinc −200 −129
Steel −100 −73

See also[edit]

References[edit]

  1. ^ Gillespie, LaRoux K. (1999), Deburring and edge finishing handbook, SME, pp. 196–198, ISBN 978-0-87263-501-2.