A pure metal consists of a single element, while an alloy is a mixture of a primary metal and one or more other elements. Hardness is defined as a material’s resistance to permanent deformation, such as scratching or indentation. The increased strength of alloys is a direct result of intentionally disrupting the highly ordered atomic arrangement of the base metal, a mechanism metallurgists call solid solution strengthening.
The Crystalline Structure of Pure Metals
Pure metals naturally arrange their atoms into highly uniform, repeating crystal lattices. This perfect, repetitive order makes pure metals relatively soft and ductile. Permanent deformation, or bending, occurs when layers of atoms slide past one another, a process known as slip. This sliding motion is facilitated by defects in the lattice called dislocations, which are line-like irregularities in the crystal structure. In a pure metal, the uniform size and spacing of the atoms allow these dislocations to move easily through the lattice under minimal stress. The low force required explains the ease with which pure metals, like copper or gold, can be shaped.
Creating Strain Through Atomic Size Mismatch
Forming an alloy introduces foreign atoms into the pure metal’s lattice, which disrupts the easy movement of dislocations. Solute atoms incorporate themselves into the host structure in two primary ways.
Substitutional Solid Solution
The impurity atom is roughly the same size as the host atom and replaces it on the lattice site.
Interstitial Solid Solution
A much smaller atom, like carbon, fits into the tiny spaces between the larger host atoms.
In nearly all cases, the size of the solute atom differs from the host atom, which is the source of the hardening effect. This atomic size mismatch generates localized distortions in the surrounding lattice structure, referred to as internal strain fields. A larger solute atom creates compressive strain, while a smaller solute atom creates tensile strain.
How Impurities Pin Dislocation Movement
These localized strain fields are the direct cause of the alloy’s increased hardness because they actively impede the motion of dislocations. When a moving dislocation encounters the distorted region surrounding a solute atom, the interaction between the dislocation’s strain field and the impurity atom’s strain field acts as a significant obstacle. The dislocation requires a much greater amount of applied shear stress to be pushed past the impurity atom.
This resistance to movement is known as “pinning,” where the impurity atom effectively anchors the dislocation in place. Blocking this motion forces the metal to withstand higher stress before it can permanently change shape. The greater the difference in atomic size between the host and solute atoms, and the higher the concentration of the solute, the stronger the resulting alloy becomes.
Real-World Examples of Solid Solution Strengthening
This fundamental principle is the basis for many common structural materials. Brass, for instance, is an alloy of copper and zinc. The addition of zinc atoms, which are a different size than the copper atoms, significantly increases the strength and hardness compared to pure copper.
Stainless steel also relies on this mechanism to achieve its strength. The small carbon atoms dissolve interstitially within the iron lattice, creating intense localized strain fields that are highly effective at pinning dislocations. Another element is aluminum alloyed with magnesium, where the magnesium atoms substitute for aluminum atoms, drastically improving the material’s mechanical properties for use in aerospace and automotive applications.