Light microscopy is a fundamental tool for microstructural studies aluminum alloys. It is also always applied before, how to use more time-consuming and expensive electronic optics.
What does an optical microscope see?
Light microscopy of aluminum alloys "works" to a magnification of 1500x, at which details up to 0,1 m. Light microscopy distinguishes most of the particles of the secondary phases with their sufficiently large size (more 1 μm), shows:
- size and distribution of soluble particles,
- grain or crystal structure of the matrix of aluminum solid solution.
A light microscope reveals such microscopic characteristics, as:
- coating thickness or diffusion,
- type and depth of corrosion damage,
- melting of low melting alloy components upon overheating,
- the presence of foreign metal inclusions or unwanted coarse intermetallic particles.
A light microscope is not able to show particles of precipitation of hardening phases, and he “does not see” dislocations and their structure. Special etching and other sample preparation are sometimes used to exert effects., which make it possible to draw some conclusions about these "invisible" characteristics. Generally speaking, research of these "invisibles" – prerogative electron microscopy.
Aluminum phases under a light microscope
The identification of various phases - chemical elements or intermetallic compounds - is an important task of light microscopy of aluminum alloys. These phases are the products of equilibrium or nonequilibrium reactions and can vary inside this alloy depending on the casting conditions., mechanical or heat treatment.
The chemical composition of these phases is associated with equilibrium and nonequilibrium state diagrams for double, triple, quadruple and even more complex systems. The crystal structure and chemical composition of these phases is known and the challenge is, to identify them by their optical characteristics or by etching behavior by various etchants.
For custom designs or, when there are doubts about the reliability of light microscopy data, it is supplemented or replaced by electron microscopy, which allows more accurate identification of various phases in aluminum alloys.
Light microscopy of deformable aluminum alloys
Products from wrought aluminum alloys are somehow made from cast ingots. These initial ingots are machined and heat treated., which make changes to the original foundry structure. These changes are usually relatively small for large items., obtained by hot deformation methods, such as forgings, thick plates and massive pressed products. Structural changes become more noticeable with an increase in hood during hot and cold processing of products, as well as using various types of heat treatment, such as annealing or solid hardening.
Visible changes in the microstructure of aluminum alloys include the following:
- Changes in the chemical composition and crystalline structure of phases due to peritectic reactions, which were suppressed during the casting of the original ingots.
- Dissolution of the most soluble phases, as well as spheroidization and coalescence of phases due to the desire to reduce their surface energy.
- Elevation at elevated temperature of elements, which were in a supersaturated solution of the foundry structure.
- Mechanical fragmentation of brittle intermetallic particles and stretching of these particles along the main directions of hot or cold machining.
- Deformation of the initial cast grain structure and subsequent processes of return and recrystallization.
Categories and groups of aluminum alloys
As known, wrought aluminum alloys are divided into seven main groups (series, classes) in accordance with their main alloying elements. Each of these groups has its own types of microstructure..
These seven groups of aluminum alloys combine, in turn, in two categories depending on, do they lend themselves to thermal hardening or not:
- alloys of 2xxx series, 6xxx and 7xxx - thermally hardenable,
- 1xxx series alloys, 3xxx, 4xxx and 5xxx - thermally unstrengthened.
These categories of aluminum alloys also have their own common features when observed under a light microscope..
Источник: Aluminum: Properties and Physical Metallurgy – ed. John E. Hatch – American Society for Metals – http://www.asminternational.org – 1984