What Is Solid Solution Strengthening?
Solid solution strengthening is a process used to strengthen pure metals by combining two metals, also known as alloying. The process works by introducing atoms of an alternative element (alloying element) to the lattice of the pure metal, which forms a solid solution.
The introduction of the alloying elements into the lattice structure leads to a non-uniform structure. The distortion in the structure creates locals strain fields and as a result interact with dislocations. The interactions of strain fields hinder their movement leading to an increase in strength. A great example of this is steel. For steel, carbon atoms are introduced to the iron lattice, making the material stronger.
Types of Solid Solution Strengthening
How alloying elements are introduced to the structure is predominantly determined by their size. If the alloying element is large enough to directly replace an atom in the lattice of the pure metal, this will result in substitutional alloying.
On the other hand, if the alloying element is small enough, generally up to 57% of the parent atom radii, it will fit in interstitial sites between the atoms of the pure metal. This is known as interstitial alloying.
Examples
An example of a substitutional alloy is brass (an alloy of copper and zinc). For brass, the main lattice structure consists of copper, and the zinc atoms replace some copper atoms in the structure.
Steel is an example of an interstitial alloy. Steels lattice structure consists of iron atoms with carbon atoms located at interstitial sites.
Case hardening is an example of solid solution strengthening. Case hardening processes include carburising and nitriding. During the process, the density of solute carbon atoms is increased at the surface of the steel, creating a carbon gradient throughout the metal and providing better mechanical properties.