Intergranular crack propagation is associated with intergranular fracture. This type of fracture occurs when the grains of a material separate along the grain boundaries, creating a network of cracks.

Intergranular fracture

intergranular fracture, intergranular cracks or intergranular embrittlement occurs when a crack propagates along the grain limits of a material, usually when these grain boundaries are weakened. the most commonly seen transgranular fracture, occurs when the crack grows through the grains of the material. By analogy, in a brick wall, the intergranular fracture would correspond to a fracture that occurs in the mortar that holds the bricks together.

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 over a wide range of temperatures. Whereas transgranular cracking is favored by stress location (which in turn is encouraged by smaller grain sizes), intergranular fracture is promoted by stress homogenization resulting from coarse grains.

Intergranular fracture produced by crack propagation along grain boundaries

weakening, 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 impurity atom embrittlement mechanism. Furthermore, 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 metals with multiple lattice orientations, when one truss ends and another begins, the fracture changes direction to follow the new grain. This results in a very irregular fracture with straight grain edges and a shiny surface can be seen. In ceramics, intergranular fractures propagate across grain boundaries, producing smooth irregular surfaces where grains can be easily identified.

Intergranular fracture mechanisms

While it is easy to identify intergranular cracks, identifying 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 grain boundaries:

  • Nucleation and coalescence of microvoids in inclusions or second-phase particles located along grain boundaries
  • Grain boundary cracking and pitting associated with high temperature stress failure conditions
  • Decohesion between adjacent grains due to the presence of impure elements at the grain boundaries and in association with aggressive atmospheres such as hydrogen gas and liquid metals
  • stress corrosion processes associated with chemical dissolution along grain boundaries
  • Cyclic loading conditions
  • When the material has an insufficient number of independent sliding systems to accommodate plastic deformation between contiguous grains. This is also known as intercrystalline fracture or grain boundary separation.
  • Faster diffusion along grain boundaries than along grain interiors
  • Faster nucleation and growth of precipitates at grain boundaries
  • Suppress cracks or crack growth after a temper process, is another example of intergranular fracture and almost always occurs by intergranular processes. This quenching cracking process is promoted by weakened grain boundaries and large grain sizes and further influenced by the temperature gradient over which cooling and volume expansion occurs during transformation.

From an energetic point of view, the energy released by intergranular crack propagation is greater than predicted by Griffith’s theoryimplying that the additional energy term to propagate a crack comes from a grain boundary mechanism.

Types of intergranular fracture

Intergranular fractures can be classified as follows:

  • wavy intergranular fracture involves cases where microvoid coalescence occurs at grain boundaries as a result of creep cavitation or void nucleation in grain boundary precipitates. This fracture is characterized by ripples on the surface. Intergranular pitted fracture typically leads to poor macroscopic ductility, with pitted topology revealed in the grain facets when viewed at higher magnifications (1000 to 5000x). Impurities that adsorb on grain boundaries promote wavelike intergranular fracture.
  • intergranular brittle fracture involves cases in which the surfaces of the grains do not have undulations that signify coalescence of microvoids. Such a fracture is termed brittle because it fractures before plastic yielding. Causes include second-phase brittle particles at grain boundaries, impurity, or atom segregation in grain limitsand environmentally assisted weakening.
  • Intergranular fatigue fracture involves cases where integral fracture occurs as a result of cyclic loading, or fatigue. This specific type of intergranular fracture is often associated with improper processing of materials or adverse environmental conditions where the grains are severely weakened. Stress applied at elevated temperatures (creep), precipitations at grain boundaries, heat treatment causing segregation at grain boundaries, and environmentally assisted weakening of grain boundaries can all lead to intergranular fatigue.

Role of solutes and impurities

At room temperature, intergranular fracture is commonly associated with altered cohesion resulting from segregation of solutes or impurities at 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 levels of impurities, especially in the case of hydrogen embrittlement, the probability of intergranular fracture is greater. Solutes such as hydrogen are supposed to stabilize and increase the density of strain-induced vacancies, leading to microcracks and microvoids at grain boundaries.

Role of grain boundary guidance

Intergranular cracking depends on the relative orientation of the common boundary between two grains. The intergranular fracture path normally occurs along the highest angle grain boundary. In one study, it was demonstrated that the crack was never displayed for boundaries with misorientation up to 20 degrees, regardless of the boundary type. At larger angles, large areas of cracked, uncracked, and mixed behavior were seen. The results imply that the degree of cracking at the grain boundary, and therefore intergranular fracture, is largely determined by the porosity of the boundary or the amount of atomic mismatch.

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