The microstructure of high-temperature alloys is complex and varied, including solid solution, multi-phase solid solution, and dispersion strengthened solid solution. They are usually composed of a matrix and one or more types of precipitation phases, depending on the alloy system. The matrix structure varies with temperature and composition of the alloy. In many cases, the composition and structure of the matrix are designed to balance the strengthening and deformation properties of the alloy.
The common solid solution microstructure is composed of a gamma (γ) matrix, a threshold phase (α), and other minor elements. The γ matrix is the most heat-resistant and cold-worked part of the alloy. It is composed of Fe-Ni, Fe-Co and other alloy systems, which are generally metastable and can remain in an austenitic structure up to 600-1000℃. The α phase is a low temperature solid solution which has an l-shaped iron-iron binary diagram and an iron-nickel binary diagram. The structure is relatively hard and is used to improve the strength of the alloy. The other minor elements may include carbides, nitrides, borides, intermetallic compounds and other elements, which can improve the strength of the alloy.
The multi-phase solid solution structures are composed of two or more major phases. The γ matrix is still present, but the γ phase may be replaced by an austenite. The alpha (α) phase can also be present, as well as other minor phases. These multi-phase solid solution structures are very common in high-temperature alloys because the performance of the alloy can be greatly improved by combining different phases.
The dispersion strengthened solid solution structures are based on the multi-phase solid solution structures, but with the inclusion of small particulates dispersed in the solution. These elements can be used to resist the diffusion of atoms of other elements in the alloy, thereby improving the resistance of the alloy to deformation and embrittlement. The two most common dispersions are carbides and nitrides. The carbides are harder than the matrix and act as a source of agglomeration, while the nitrides are used to improve the oxidation resistance of the alloy.
In summary, high-temperature alloys are composed of complex microstructures, which vary with the matrix and additional phases present in the alloy. This microstructure is carefully designed to optimize the properties of the alloy, while maintaining the integrity of the material. The inclusion of dispersions can further enhance the performance of the alloy, providing increased strength and resistance to deformation and embrittlement.
