embrittlement

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embrittlement

[‚em′brid·əl·mənt]
(mechanics)
Reduction or loss of ductility or toughness in a metal or plastic with little change in other mechanical properties.

Embrittlement

A general set of phenomena whereby materials suffer a marked decrease in their ability to deform (loss of ductility) or in their ability to absorb energy during fracture (loss of toughness), with little change in other mechanical properties, such as strength and hardness. Embrittlement can be induced by a variety of external or internal factors, for example, (1) a decreasing or an increasing temperature; (2) changes in the internal structure of the material, namely, changes in crystallite (grain) size, or in the presence and distribution of alloying elements and second-phase particles; (3) the introduction of an environment which is often, but not necessarily, corrosive in nature; (4) an increasing rate of application of load or extension; and (5) the presence of surface notches.

Low-temperature embrittlement results from a competition between deformation and brittle fracture, with the latter becoming preferred at a critical temperature. For a material to be useful structurally, it is desirable that this critical temperature be below the minimum anticipated service temperature; in most cases, this is room temperature. At high temperatures, internal structural changes that lead to intergranular embrittlement can occur. Embrittlement usually occurs in the creep temperature range, a temperature at which deformation can occur under very low stresses; and the two processes are believed to be connected.

In many metals, particularly structural steels, annealing or heat treating in certain temperature ranges sensitizes the grain boundaries in such a way that intergranular embrittlement subsequently occurs during service. To reduce the brittleness, the steel undergoes an annealing treatment called tempering, which, while decreasing the strength, usually increases the toughness. The exception to this trade-off occurs when the steel is tempered at 1000°F (538°C). This can lead to a mode of intergranular fracture called temper embrittlement; such a process has led to catastrophic failures in turbines, rotors, and other high-strength steel parts. In other metals, there are less specific but similar types of embrittlement resulting from critical heat treatments. See Heat treatment (metallurgy), Tempering

Metals can fracture catastrophically when exposed to a variety of environments. These environments can range from liquid metals to aqueous and nonaqueous solutions to gases such as hydrogen.

If a thin film of a liquid metal is placed on the oxide-free surface of a solid metal, the tensile properties of the solid metal will not be affected, but the fracture behavior can be markedly different from that observed in air. Although many different liquid metals are capable of inducing embrittlement in a variety of solid metals, some of the more common couples, many of which have important engineering and design consequences, are mercury embrittlement of brass, lead embrittlement of steel, and gallium embrittlement of aluminum.

Stress corrosion cracking can occur when a metal is stressed and simultaneously exposed to an environment which may be, but is not necessarily, corrosive in nature. Both stress and environment are required; if only one of these elements is present, the metal usually displays no embrittlement. See Corrosion

Hydrogen embrittlement is a form of embrittlement often considered to be a type of stress corrosion cracking. Hydrogen atoms can enter a metal, causing severe embrittlement, again with little effect on other mechanical properties. This phenomenon was originally observed, and is most critical in, steels, but it is not documented to occur in titanium and nickel alloys, and may lead to cracking in other alloy systems as well.

Factors such as notches and the rate of application of stress can modify the response of a material to a specific type of embrittlement. In general, notches or surface flaws always enhance embrittlement, both by acting as a stress raiser and by providing a preexisting crack.

References in periodicals archive ?
Unlike thermal attrition, which kills and embrittles all the clay binder, it preserves the desirable active clay, which cannot be simply knocked off through physical contact.
At 25 years old, cryogenic granulation is hardly new, but NPE '94 in Chicago did shine new light on this recycling method, which uses liquid nitrogen to embrittle reclaim before grinding.
The concept involves using liquid nitrogen at -320 F to embrittle heat-sensitive or hard-to-grind materials such as flexible PVC and elastomers.
Lump reduction and separation of metallics prior to a drying stage will embrittle the soda making it removable (consistently below 1%) by mechanical scrubbing.
In the case of straight unmodified asphalts the experience in these areas has been that ACs embrittle within two to four years.
While bismuth enhances the machinability of copper alloys with few health problems, it has also been known to embrittle copper when used alone at the levels needed for good machinability.
TFE/P does not embrittle or exhibit surface cracking after exposure to any of these fluids, has good resistance to all types of brake fluids, but is not recommended for most fuel applications because of the high volume swell obtained after exposure to these fluids.
By controlling the temperature of die parts, an operator can selectively embrittle to the point where an impact from tumbling the parts will break off only a thin section.
An increase in crystallinity on the surface may result in a higher scratch hardness PP, but an increase in crystallinity can also embrittle the material.
Particle production is possible by cryogenic pulverization (5), but the ability of the foam to act as an insulator requires large quantities of cryogen to sufficiently embrittle the material.
On the other hand, a tendency to become embrittle and to decrease in strength was observed with a rise in [T.
Unreinforced PET is generally not useful as an injection molding resin because of its slow crystallization rate (3) and the tendency to embrittle upon crystallization such as by thermal annealing (4).