Solid solution strengthening
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Solid Solution Strengthening is a type of alloying that can be used to improve the strength of a pure metal. Atoms of one element are added to a crystalline lattice comprised of atoms of another. The alloying element will diffuse into the matrix, forming a "solid solution". In most binary systems, when alloyed above a certain concentration, a second phase will form and the material will enjoy the benefits of precipitation strengthening.
[edit] Types
Depending on the size of the alloying element, a substitutional solid solution or an interstitial solid solution can form.
In a substitutional solid solution, solute atoms replace solvent atoms in their lattice positions. Based on the Hume-Rathery Rule, solvent and solute atoms must differ in atomic size by less than 5% in order to form this type of solution. Because both elements exist in the same crystalline lattice, both elements in their pure form must be of the same crystal structure. Examples of substitutional solid solutions include the Cu-Ni and the Ag-Au binary systems.
When the solute atom is much smaller than the solvent atoms, an interstitial solid solution forms. This typically occurs when the solute atoms are less than half as small as the solvent atoms. Elements commonly used to form interstitial solid solutions include H, N, C, and O.
[edit] Strengthening Mechanisms
The strength of a material is a measurement of how easily dislocations in its crystal lattice can be propagated. These dislocations create stress fields within the material depending on their character. 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. This gain is a result of both the size and the modulus effect.
When solute and solvent atoms differ in size, local stress fields are created (if solute atom size is larger than solvent atom size, this field is compressive, and similarly, when solute atoms are smaller than solvent atoms, this field is tensile). Depending on their relative locations, solute atoms will either attract or repel dislocations in their vicinity, requiring an higher force to overcome the obstacle. This is known as the size effect. In substitutional solid solutions, these stress fields are spherically symmetric, meaning they have no shear stress component. As such, substitutional solute atoms do not interact with the shear stress fields characteristic of screw dislocations. Conversely, in interstitial solid solutions, solute atoms cause a tetragonal distortion, generating a shear field that can interact with both edge, screw, and mixed dislocations.
The energy density of a dislocation is dependent on its Burger's vector as well as the modulus of the local atoms. When the modulus of solute atoms differs from that of the host element, the local energy around the dislocation is changed, increasing the amount of force necessary to move past this energy well. This is known as the modulus effect.
