Metallography of Fracture Specimen: The metallurgical microscope is yet another instrument very useful to

the failure analyst.

After collecting all the information through fractography of the failed

component, a section of the component can be cut transverse to the

fracture surface.

This section is then polished and examined in the metallurgical

microscope, both before and after etching. Inclusions present in the

material are observed on the as-polished surface.

The inclusion rating can be determined by standard quantitative

microscopy techniques.

The polished specimen is then etched with suitable etchants to reveal

the microstructure of the material.

Abnormalities in the microstructure that may have been responsible for

the failure can be identified at this stage.

The path of a crack, whether it is intergranular or transgranular, and

branched or not branched, will be clear in the microstructure.

Cracks due to stress corrosion, hydrogen embrittlement, and liquid

metal embrittlement are generally intergranular with some exceptional

situations.

Fatigue cracks are transgranular. If a stress-corrosion crack

propagates by fatigue, the transition from intergranular to transgranular

mode can be seen in the microstructure.

Stress-corrosion cracks in certain stainless steels are transgranular

with extensive branching.

Plastic deformation of the component prior to fracture can be

recognized in the microstructure by the elongated grains.

Abnormal grain growth, segregation of brittle or weak phases at the

grain boundaries, and recrystallization are some of the other features

that can be identified by metallography.

Figure shows the intergranular and transgranular modes of crack

propagation, revealed by metallography.

Figure: Optical microstructure showing transgranular crack propagation

Figure: Optical microstructure showing intergranular crack propagation.