Observation of the Microstructure of Materials Using an Optical Microscope
According to organizational characteristics and different carbon contents, iron carbon alloys can be divided into three categories: industrial pure iron, steel, and cast iron. Industrial pure iron with carbon content less than 0.0218% C and carbon content less than 2.11% is called steel, while alloys with carbon content greater than 2.11% are called cast iron.
The microstructure of carbon steel and white cast iron at room temperature is composed of two basic phases, ferrite (F) and cementite (Fe3C).
However, due to different carbon contents, the relative quantities, precipitation conditions, and distribution of ferrite and cementite vary, resulting in various different microstructure forms.
Ferrite is a solid solution of carbon in alpha iron, commonly represented by the symbol "F". The ferrite structure consists of equiaxed grains and a body centered cubic lattice.
Carbide is a compound formed by iron and carbon, commonly represented by the symbol "Fe3C". Depending on the composition and formation conditions, cementite can take on different forms.
Pearlite is a mechanical mixture of ferrite and cementite, commonly represented by the symbol "P". Under normal annealing conditions, it is a layered structure formed by the alternating arrangement of ferrite and cementite.
The etching of pure metals and single-phase alloys is a chemical dissolution process. When the polished sample is in contact with the etching agent, the deformation disturbance layer on the polished surface is first dissolved, and the microstructure of the steel is not exposed. Then, the chemical dissolution effect on the grain boundaries occurs, and the regularity of atomic arrangement on the grain boundaries is relatively poor, resulting in rapid corrosion and the formation of grooves. At this time, the alloy shows polygonal grains. If etching continues, the etching agent will dissolve the grains themselves. Due to the uneven dissolution rate of each grain, after etching, each grain will be exposed on the surface with the densest atomic arrangement. Under vertical light irradiation, grains with different brightness will be displayed.
The etching process of two-phase alloys is mainly electrochemical etching. Due to their different compositions and structures, different phases have different electrode potentials, forming many pairs of small local cells in the etching solution. Ferrite has a higher electrode potential as the anode, which dissolves and becomes low-lying and rough during etching, while cementite has a positive potential as the cathode and is basically not corroded. Ferrite appears dark black under a light microscope, while cementite appears bright white.
