Solid solution strengthening
In metallurgy, solid solution strengthening is a type of alloying that can be used to improve the strength of a pure metal.
The local nonuniformity in the lattice due to the alloying element makes plastic deformation more difficult by impeding dislocation motion through stress fields.
In contrast, alloying beyond the solubility limit can form a second phase, leading to strengthening via other mechanisms (e.g. the precipitation of intermetallic compounds).
[2] In both cases, atoms are visualised as rigid spheres where the overall crystal structure is essentially unchanged.
Elements commonly used to form interstitial solid solutions include H, Li, Na, N, C, and O.
When solute atoms are introduced, local stress fields are formed that interact with those of the dislocations, impeding their motion and causing an increase in the yield stress of the material, which means an increase in strength of the material.
When solute and solvent atoms differ in size, local stress fields are created that can attract or repel dislocations in their vicinity.
By relieving tensile or compressive strain in the lattice, the solute size mismatch can put the dislocation in a lower energy state.
As such, substitutional solute atoms do not interact with the shear stress fields characteristic of screw dislocations.
For example, consider an edge dislocation encountering a smaller solute atom above its slip plane.
In this case, the interaction energy is negative, resulting in attraction of the dislocation to the solute.
In contrast, Umodulus is positive for a “hard” solute, which results in lower total interaction energy than a soft atom.
For ionic solids where electrostatic interaction dictates bond strength, charge effect is also important.
For example, addition of divalent ion to a monovalent material may strengthen the electrostatic interaction between the solute and the charged matrix atoms that comprise a dislocation.
This lowers the stacking fault energy, leading to repulsion of the partial dislocations, which thus makes the material stronger.
[3] Surface carburizing, or case hardening, is one example of solid solution strengthening in which the density of solute carbon atoms is increased close to the surface of the steel, resulting in a gradient of carbon atoms throughout the material.
This provides superior mechanical properties to the surface of the steel without having to use a higher-cost material for the component.
where c is the concentration of the solute atoms, G is the shear modulus, b is the magnitude of the Burger's vector, and
The greater the difference in lattice parameter, the higher the local stress fields introduced by alloying.
Alloying with elements of higher shear modulus or of very different lattice parameters will increase the stiffness and introduce local stress fields respectively.
In either case, the dislocation propagation will be hindered at these sites, impeding plasticity and increasing yield strength proportionally with solute concentration.
This occurs if the concentration of the solute reaches a certain critical point given by the binary system phase diagram.
This critical concentration therefore puts a limit to the amount of solid solution strengthening that can be achieved with a given material.
The most popular example is the Inconel family, where many of these alloys contain chromium and iron and some other additions of cobalt, molybdenum, niobium, and titanium.
[11] This reduced strain rate is extremely important for turbine blade operation because they undergo significant mechanical stress and high temperatures which can lead to the onset of creep deformation.
Therefore, the precise control of grain size in nickel-based superalloys is key to creep resistance and mechanical reliability and longevity.
Some ways to control the grain size lie in the manufacturing techniques like directional solidification and single crystal casting.
Solid solution strengthening of steel is one of the mechanisms used to enhance the properties of the alloy.
[13] It is being used mostly for cookware, kitchen equipment, and in marine applications for its good corrosion properties in saline environments.
