Microstructrural aspects

Brittle fractures usually propagate by either or both of two fracture modes:

cleavage
intergranular

In most cases it is necessary to study the fracture surface with an electron microscope. Since very high magnifications are usually not necessary, a scanning electron microscope is usually preferred to a transmission electron microscope.
Cleavage fractures are characterized by splitting of the crystals, or grains, along specific crystallographic planes without respect to the grain boundaries. Since the fracture goes through the grains, this type of fracture is frequently referred to as transgranular, or transcrystalline. Cleavage fractures are the most common type of brittle fracture and are the normal mode of fracture unless the grain boundaries have been weakened by a specific environment or process.
It will be noted that the pattern is characterized by the joining together of microscopic ridges, much like the joining of tributaries of a river system to form the main stream of the river. This pattern reveals the direction that the fracture ran; the fracture propagated in the same direction that the water in a river flows: downstream.
Intergranular fractures are those that follow grain boundaries weakened for any of several reasons. An analogy may be made to a brick wall, which fractures through the mortar rather than through the bricks themselves. The mortar is analogous to the grain boundaries, while the bricks are analogous to the metal grains.
The reasons for weakened grain boundaries are frequently very subtle and poorly understood. Under certain conditions some metals are subject to migration or diffusion of embrittling elements or compounds to the grain boundaries.