Intergranular fracture

In certain materials and under certain conditions, the boundaries between the grains are the weakest regions in the material. An intergranular fracture is a type of fracture in which the crack propagates along the grain boundaries of the material, usually when these grain boundaries are weakened. Intergranular fracture is to be distinguished from the more commonly seen transgranular fracture, where the crack grows through the material grains rather than at the grain boundaries. Though less frequently observed, intergranular cracking is likely to occur if there is a hostile environmental influence and is favored by larger grain sizes and higher stresses. Intergranular cracking is possible at a wide range of temperatures. While transgranular cracking is favored by strain localization (which in turn is encouraged by smaller grain sizes), intergranular fracture is promoted by strain homogenization resulting from coarse grains. In certain materials and under certain conditions, the boundaries between the grains are the weakest regions in the material. An intergranular fracture is a type of fracture in which the crack propagates along the grain boundaries of the material, usually when these grain boundaries are weakened. Intergranular fracture is to be distinguished from the more commonly seen transgranular fracture, where the crack grows through the material grains rather than at the grain boundaries. Though less frequently observed, intergranular cracking is likely to occur if there is a hostile environmental influence and is favored by larger grain sizes and higher stresses. Intergranular cracking is possible at a wide range of temperatures. While transgranular cracking is favored by strain localization (which in turn is encouraged by smaller grain sizes), intergranular fracture is promoted by strain homogenization resulting from coarse grains. Embrittlement, or loss of ductility, is often accompanied by a change in fracture mode from transgranular to intergranular fracture. This transition is particularly significant in the mechanism of impurity-atom embrittlement. Additionally, hydrogen embrittlement is a common category of embrittlement in which intergranular fracture can be observed. Intergranular fracture can occur in a wide variety of materials, including steel alloys, copper alloys, aluminum alloys, and ceramics. In a wall of bricks, intergranular fracture would correspond to a fracture that takes place in the mortar that keeps the bricks together. In metals with multiple lattice organizations, 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 shiny surface may be seen. In ceramics, interganular fractures propagate through grain boundaries, producing smooth bumpy surfaces where grains can be easily identified. Quench cracking, or crack growth following a quenching process, is another example of intergranular fracture and almost always occurs by intergranular processes. This process of quench cracking is promoted by weakened grain boundaries and large grain sizes and additionally influenced by the temperature gradient at which quenching occurs and volume expansion during transformation. 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. Though it is easy to identify integranular cracking, pinpointing the cause is more complex as the mechanisms are more varied, compared to transgranular fracture. There are several other processes that can lead to intergranular fracture, or preferential crack propagation at the grain boundaries: Intergranular fracture can be categorized into the following: At room temperature, intergranular fracture is commonly associated with altered cohesion resulting from segregation of solutes or impurities at the grain boundaries. 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. At high impurity levels, especially in the case of hydrogen embrittlement, the likelihood of intergranular fracture is greater. Solutes like hydrogen are hypothesized to stabilize and increase the density of strain-induced vacancies, leading to microcracks and microvoids at grain boundaries. 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. In a study, it was shown that cracking was never exhibited for boundaries with misorientation of up to 20 degrees, regardless of boundary type. 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.