An alloy is a mixture of two or more elements, where at least one of the elements is a metal. The resulting material possesses properties different from those of the constituent elements. Brass is a classic example of an alloy, specifically a mixture of copper (Cu) and zinc (Zn).

Brass is formed by melting copper and zinc together in specific proportions. The typical composition is around 60% copper and 40% zinc, although this can vary. This mixture is then allowed to cool and solidify. The properties of brass are different from both copper and zinc. For example:

• Strength: Brass is significantly stronger and harder than pure copper. The addition of zinc increases its tensile strength.
• Ductility: While not as ductile as pure copper, brass retains a reasonable level of ductility, making it workable.
• Corrosion Resistance: Brass is more resistant to corrosion than pure copper, particularly in saltwater environments. The zinc acts as a sacrificial anode, corroding preferentially to protect the copper.
• Malleability: Brass is malleable, meaning it can be hammered or rolled into different shapes.

The properties of brass are a direct result of the interaction between the copper and zinc atoms in the alloy's crystal structure. The presence of zinc disrupts the regular arrangement of copper atoms, leading to increased strength and corrosion resistance.

Alloying can significantly improve the strength and ductility of a metal by disrupting the regular arrangement of atoms in the metal's crystal lattice. This disruption makes it more difficult for the atoms to slide past each other when a force is applied, increasing strength. Ductility, the ability of a metal to be drawn into a wire, is also enhanced because the irregular lattice makes it harder for the metal to fracture. The alloying element introduces different sized atoms or interferes with the regular arrangement, hindering dislocation movement.

A good example is copper alloyed with tin to produce bronze. Pure copper is relatively soft and not very strong. Adding tin to copper creates bronze, which is significantly stronger and harder than pure copper. The tin atoms disrupt the copper's crystal structure, making it more difficult for dislocations to move. This results in increased tensile strength and hardness. Bronze is also more resistant to corrosion than pure copper, although not as resistant as some other alloys.

In summary, alloying enhances strength by impeding dislocation movement and improves ductility by making fracture more difficult. The specific effect depends on the alloying element and its concentration.

Steel is an alloy of iron and carbon. The presence of carbon atoms within the iron crystal lattice significantly affects the metal's ability to deform.

Iron typically has a body-centered cubic (BCC) crystal structure. When carbon atoms are incorporated into this structure, they create lattice distortions. Carbon atoms are significantly smaller than iron atoms.

These smaller carbon atoms occupy positions within the iron lattice, causing the iron atoms to shift and distort. This disruption of the regular lattice structure creates stress points.

The stress points impede the movement of the iron crystal planes. The iron atoms have to work harder to slide past these distortions. This increased resistance to movement makes steel harder and stronger than pure iron.

The amount of carbon present in the iron lattice influences the extent of the distortion and therefore the hardness and strength of the steel. Higher carbon content generally leads to increased hardness and strength, but also reduced ductility. The carbon atoms effectively hinder the sliding of the iron layers, making the steel more resistant to deformation.