MICROSTRUCTURE OF METALS – All you need to know
The microstructure describes the internal structure of materials. A distinction is made between the microstructure and the macrostructure, whereby the former can only be examined under a microscope, while the latter can also be seen with the naked eye. The individual crystalline components are called grains or crystallites. Since they are very small, the grains can only be made visible by using a microscope and suitable contrast medium. In metals, the structure, size and orientation of the grains is determined by the alloys of the materials. The manufacture of the workpiece also affects these parameters.
For metals, a distinction is made between the primary microstructure and the secondary microstructure.
The primary structure is formed naturally during the solidification of a melt and is also called the casting structure. If heat is removed from the molten metal, the energy of the constantly moving atoms in it decreases. The kinetic energy of the atoms reduces until the bonding forces, as they occur in a solid crystalline compound, gain the upper hand. When such atoms meet, they stay together in an ordered and crystalline configuration. This results in a so-called seed crystal. If further atoms now attach themselves to this seed crystal, the crystal grows.
Nucleation can be accelerated by so-called seeding. In this process, substances of the same or different composition are added to the melt. In addition to nucleation, this can also support the formation of grain boundaries.
With the help of metallographic methods, the primary structure can be made visible in order to determine the grain size or to analyse defects.
The secondary structure is formed out of the primary structure. It can be formed naturally, e.g. by solid-state reactions, or artificially, e.g. by forging, rolling, drawing, etc. Compared to the primary structure, the secondary structure is characterized by small and fine grains.
Types of microstructure using the example of iron materials
The microstructure of ferrous materials is differentiated by the carbon content:
- 0 % carbon corresponds to pure iron and is called ferrite or ɑ-iron.
- Ferrous materials with a carbon content below 0.8 % are called hypoeutectoid steels. They consist of pure ferrite grains and of ferrite grains interspersed with cementite. These are called pearlite grains.
- Eutectoid steel has a carbon content of 0.8%. It consists of a pure pearlite structure.
- Steel with a carbon content between 0.8 and 2.06 % is called hypereutectoid steel. The grain boundary cementite is a further structural component. The proportion increases with higher carbon content.
- Depending on the cooling rate and the alloying constituents, different types of microstructure can form in steels with a carbon content above 2.06 %. Slow cooling or the addition of silicon results in an iron-graphite composite. In this case, carbon forms a graphite phase. This is called the stable iron-carbon diagram. However, if the melt cools down quickly, it precipitates carbon primarily as cementite. The metastable iron-carbon diagram shows the microstructural transformations that takes place.
- At a carbon content of 4.3 %, the molten iron solidifies into a ledeburite structure.