Observation of Microstructure of Materials under an Optical
According to their structural characteristics and carbon content, iron‑carbon alloys can be divided into three categories: industrial pure iron, steel and cast iron. Industrial pure iron contains less than 0.0218 wt% C. Iron‑carbon alloys with a carbon content below 2.11 wt% are defined as steel, and alloys with a carbon content above 2.11 wt% are cast iron.
At room temperature, the microstructures of carbon steel and white cast iron consist of two fundamental phases: ferrite (F) and cementite (Fe₃C). Differences in carbon content change the relative amounts, precipitation conditions and distribution of ferrite and cementite, resulting in various microstructural morphologies.
Ferrite is a solid solution of carbon in α‑iron, denoted by the symbol "F". Ferrite consists of equiaxed grains with a body‑centered cubic crystal lattice.
Cementite is an intermetallic compound of iron and carbon, denoted by "Fe₃C". Cementite can exist in different forms depending on its composition and formation conditions.
Pearlite is a mechanical mixture of ferrite and cementite, denoted by "P". Under normal annealing conditions, it is a lamellar structure formed by alternating layers of ferrite and cementite.
Etching of pure metals and single‑phase alloys is a chemical dissolution process. When a polished specimen comes into contact with etchant, the deformed disturbed layer on the polished surface is dissolved first, while the microstructure of the steel is not yet revealed. This is followed by preferential chemical dissolution at grain boundaries. The irregular atomic arrangement at grain boundaries is rapidly corroded to form grooves, revealing polygonal grains of the alloy. If etching continues, the etchant also attacks the grain interiors. Because the dissolution rate varies among grains, each grain exposes densely packed atomic planes after etching. Under vertical illumination, the grains show varying brightness.
Etching of two‑phase alloys is mainly an electrochemical process. Phases with different compositions and structures have different electrode potentials, forming numerous tiny local galvanic cells in the etchant. Ferrite acts as the anode with a higher electrode potential and dissolves during etching, appearing depressed and rough. Cementite serves as the cathode with a positive potential and is barely attacked. Under the optical microscope, ferrite appears dark and cementite appears bright white.
