Embrittlement, or loss of ductility, is often accompanied by a change in fracture mode from transgranular to intergranular fracture.[4] This transition is particularly significant in the mechanism of impurity-atom embrittlement.[4] Additionally, hydrogen embrittlement is a common category of embrittlement in which intergranular fracture can be observed.[5]
Intergranular fracture can occur in a wide variety of materials, including steel alloys, copper alloys, aluminum alloys, and ceramics.[6][7][3] In metals with multiple lattice orientations, when one lattice ends and another begins, the fracture changes direction to follow the new grain. This results in a fairly jagged looking fracture with straight edges of the grain and a shiny surface may be seen. In ceramics, intergranular fractures propagate through grain boundaries, producing smooth bumpy surfaces where grains can be easily identified.
Though it is easy to identify intergranular cracking, pinpointing the cause is more complex as the mechanisms are more varied, compared to transgranular fracture.[6] There are several other processes that can lead to intergranular fracture or preferential crack propagation at the grain boundaries:[8][6]
From an energy standpoint, the energy released by intergranular crack propagation is higher than that predicted by Griffith theory, implying that the additional energy term to propagate a crack comes from a grain-boundary mechanism.[9]
At room temperature, intergranular fracture is commonly associated with altered cohesion resulting from segregation of solutes or impurities at the grain boundaries.[10] Examples of solutes known to influence intergranular fracture are sulfur, phosphorus, arsenic, and antimony specifically in steels, lead in aluminum alloys, and hydrogen in numerous structural alloys.[10] At high impurity levels, especially in the case of hydrogen embrittlement, the likelihood of intergranular fracture is greater.[6] Solutes like hydrogen are hypothesized to stabilize and increase the density of strain-induced vacancies,[11] leading to microcracks and microvoids at grain boundaries.[5]
Intergranular cracking is dependent on the relative orientation of the common boundary between two grains. The path of intergranular fracture typically occurs along the highest-angle grain boundary.[6] In a study, it was shown that cracking was never exhibited for boundaries with misorientation of up to 20 degrees, regardless of boundary type.[12] At greater angles, large areas of cracked, uncracked, and mixed behavior were seen. The results imply that the degree of grain boundary cracking, and hence intergranular fracture, is largely determined by boundary porosity, or the amount of atomic misfit.[12]
In materials science, intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides. (Cf. transgranular corrosion.)
These zones also act as local galvanic couples, causing local galvanic corrosion. This condition happens when the material is heated to temperatures around 700 ÂC for too long a time, and often occurs during welding or an improper heat treatment. When zones of such material form due to welding, the resulting corrosion is termed weld decay. Stainless steels can be stabilized against this behavior by addition of titanium, niobium, or tantalum, which form titanium carbide, niobium carbide and tantalum carbide preferentially to chromium carbide, by lowering the content of carbon in the steel and in case of welding also in the filler metal under 0.02%, or by heating the entire part above 1000 ÂC and quenching it in water, leading to dissolution of the chromium carbide in the grains and then preventing its precipitation. Another possibility is to keep the welded parts thin enough so that, upon cooling, the metal dissipates heat too quickly for chromium carbide to precipitate. The ASTM A923,[1] ASTM A262,[2] and other similar tests are often used to determine when stainless steels are susceptible to intergranular corrosion. The tests require etching with chemicals that reveal the presence of intermetallic particles, sometimes combined with Charpy V-Notch and other mechanical testing.
Another related kind of intergranular corrosion is termed knifeline attack (KLA). Knifeline attack impacts steels stabilized by niobium, such as 347 stainless steel. Titanium, niobium, and their carbides dissolve in steel at very high temperatures. At some cooling regimes (depending on the rate of cooling), niobium carbide does not precipitate and the steel then behaves like unstabilized steel, forming chromium carbide instead. This affects only a thin zone several millimeters wide in the very vicinity of the weld, making it difficult to spot and increasing the corrosion speed. Structures made of such steels have to be heated in a whole to about 1065 ÂC (1950 ÂF), when the chromium carbide dissolves and niobium carbide forms. The cooling rate after this treatment is not important, as the carbon that would otherwise pose risk of formation of chromium carbide is already sequestered as niobium carbide.[1]
Aluminium-based alloys may be sensitive to intergranular corrosion if there are layers of materials acting as anodes between the aluminium-rich crystals. High strength aluminium alloys, especially when extruded or otherwise subjected to high degree of working, can undergo exfoliation corrosion (metallurgy), where the corrosion products build up between the flat, elongated grains and separate them, resulting in lifting or leafing effect and often propagating from edges of the material through its entire structure. [2] Intergranular corrosion is a concern especially for alloys with high content of copper.
Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the steel or alloy to be susceptible to intergranular corrosion or intergranular stress corrosion cracking.
Certain alloys when exposed to a temperature characterized as a sensitizing temperature become particularly susceptible to intergranular corrosion. In a corrosive atmosphere, the grain interfaces of these sensitized alloys become very reactive and intergranular corrosion results. This is characterized by a localized attack at and adjacent to grain boundaries with relatively little corrosion of the grains themselves. The alloy disintegrates (grains fall out) and/or loses its strength.
Several methods have been used to control or minimize the intergranular corrosion of susceptible alloys, particularly of the austenitic stainless steels. For example, a high-temperature solution heat treatment, commonly termed solution-annealing, quench-annealing or solution-quenching, has been used. The alloy is heated to a temperature of about 1,060 ÂC to 1,120 ÂC and then water quenched. This method is generally unsuitable for treating large assemblies, and also ineffective where welding is subsequently used for making repairs or for attaching other structures.
Another control technique for preventing intergranular corrosion involves incorporating strong carbide formers or stabilizing elements such as niobium or titanium in the stainless steels. Such elements have a much greater affinity for carbon than does chromium; carbide formation with these elements reduces the carbon available in the alloy for formation of chromium carbides. Such a stabilized titanium-bearing austenitic chromium-nickel-copper stainless steel is shown in U.S. Pat. No. 3,562,781. Or the stainless steel may initially be reduced in carbon content below 0.03 percent so that insufficient carbon is provided for carbide formation. These techniques are expensive and only partially effective since sensitization may occur with time. The low-carbon steels also frequently exhibit lower strengths at high temperatures.
In any case the mechanical properties of the structure will be seriously affected. A classic example is the sensitization of stainless steels or weld decay. Chromium-rich grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes. Reheating a welded component during multi-pass welding is a common cause of this problem. In austenitic stainless steels, titanium or niobium can react with carbon to form carbides in the heat affected zone (HAZ) causing a specific type of intergranular corrosion known as knife-line attack. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. The problem of knife-line attack can be corrected by reheating the welded metal to allow diffusion to occur.
Many aluminum base alloys are susceptible to intergranular corrosion on account of either phases anodic to aluminum being present along grain boundaries or due to depleted zones of copper adjacent to grain boundaries in copper-containing alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage.
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