An alloy is a material created by combining two or more elements, with at least one being a metal. This combination is typically achieved by melting the components together and allowing the mixture to solidify. The new material’s properties are often significantly different from its individual components, which is the primary reason for their creation. The arrangement of atoms in the final solid structure defines the two main classifications of alloys: substitutional and interstitial.
Why Engineers Create Alloys
Engineers rarely use pure metals because they often lack the performance characteristics required for industrial applications. Alloying is a systematic method for customizing the properties of a base metal to make it suitable for a specific purpose. The process is designed to enhance several characteristics, such as increasing a material’s resistance to surface deformation or abrasion.
The addition of an alloying element increases the base metal’s tensile strength, allowing it to withstand greater pulling forces without breaking. Alloying elements also reduce chemical reactivity, making the resulting material resistant to corrosion and degradation from the environment. Engineers can also manipulate the melting point of a metal through alloying, which is useful for specialized applications like soldering.
Substitutional Alloys
Substitutional alloys form when the atoms of the secondary element are similar in size to the atoms of the primary, or host, metal. For the two elements to mix successfully, the solute atoms must be within approximately 15% of the size of the solvent atoms. This size compatibility allows the alloying atoms to seamlessly replace, or substitute for, some of the host atoms directly within the existing crystal lattice structure.
The substitution creates a solid solution where the two types of atoms occupy the same positions within the crystal structure. This arrangement introduces minor localized strains into the lattice, which increases the material’s hardness and strength compared to the pure base metal. Brass, formed when zinc atoms replace copper atoms, and bronze, created when tin atoms substitute for copper, are common examples.
Interstitial Alloys
Interstitial alloys are structurally distinct because they involve atoms of vastly different sizes. In this case, the alloying elements are much smaller, non-metallic atoms, such as carbon, nitrogen, or boron, compared to the host metal atoms. Instead of replacing the host atoms, these small atoms fit into the tiny gaps or spaces, known as “interstices,” between the larger metal atoms in the crystal lattice.
The small atoms wedged into the interstices act as obstacles, restricting the movement of the metal atoms past one another. This structural interference blocks the deformation mechanism of the metal, resulting in increased hardness and strength. Steel, the most commercially significant example, is an alloy of iron and carbon, where the carbon atoms fit into the iron lattice, transforming soft, ductile iron into a stronger and harder material.