What Does Intergranular Fracture Mean?
Intergranular fracture is the propagation of cracks along the grain boundaries of a metal or alloy. It is a fracture that follows the grains of the material.
Intergranular fractures travel along the grain boundaries, rather than through the actual grains. This usually occurs when the phase in the grain boundary is weak and brittle (such as cementite in iron's grain boundaries). This can be visualized as a 3-D puzzle: Transgranular fracture cuts through the puzzle pieces, while intergranular fracture travels along the precut edges of the puzzle pieces.
Crack initiation and propagation accompany fracture. The manner in which the crack propagates through the material provides great insight into the mode of fracture. In this fracture, cracks spread very rapidly with little or no plastic deformation. Once initiated, the cracks that propagate in a brittle material continue to grow and increase in magnitude.
Corrosionpedia Explains Intergranular Fracture
Intergranular fractures take place along the grain boundary of a material. Straight edges of the grain and a shiny surface may be noticeable. It is like a brick wall that fractures through mortar, rather than bricks.
Examples of intergranular fractures include:
- Season cracking of brass by ammonia
- Liquid metal embrittlement during soldering
- Sensitization of stainless steels
- Phosphorous or antimony in hard steels
Materials that fracture without any plastic deformation are called brittle materials. Examples include glass and most other ceramic materials. An intergranular fracture is one kind of brittle fracture. When brittle fracture occurs, the crack initiates and propagates through the material at high speeds (the speed of sound). Factors that favor crack initiation include:
- Larger grain size
- Lower temperatures
- Higher stress
There are several processes that can lead to intergranular fracture, as follows:
- Microvoid nucleation and coalescence at inclusions or second phase particles located along grain boundaries
- Grain boundary crack and cavity formation associated with conditions due to elevated temperature stress rupture
- Decohesion between contiguous grain, due to presence of impurity at grain boundaries, hydrogen and liquid metals
- Stress corrosion cracking associated with chemical dissolution along grain boundaries
- Cyclic loading when the material possesses an insufficient number of independent slips systems to accommodate plastic deformation between contiguous grain, leading to grain boundaries
At temperatures above 600°C, the oxygen grain boundary diffusivity becomes fast enough to cause a transition from transgranular to brittle intergranular fracture during slow fatigue, as well as static loading conditions. Fracture stress may be used as local criterion of fracture and a parameter for the identification of critical impurity levels that cause intergranular embrittlement.