Strengthening a metal by adding a small amount of another element is achieved by introducing obstacles to dislocation movement within the metal's crystal lattice. Dislocations are linear defects in the crystal structure that allow the metal to deform under stress. When a metal is deformed, dislocations move through the lattice. The presence of an alloying element disrupts this movement.

A common example is steel, which is iron strengthened by adding carbon. Carbon atoms are much smaller than iron atoms. When carbon atoms substitute for iron atoms in the iron lattice, they create distortions and strain fields around themselves. These distortions make it more difficult for dislocations to move, thus increasing the strength and hardness of the steel. The more carbon added (up to a certain limit, typically around 2%), the stronger and harder the steel becomes. However, excessive carbon can reduce ductility and make the steel brittle.

Therefore, the addition of a small amount of a different element (the alloying element) creates obstacles to dislocation movement, effectively strengthening the metal. This is a fundamental principle behind the strengthening of many alloys.

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.

Here are three examples of alloys and their applications:

1. Steel (Iron and Carbon): Steel is an alloy of iron and carbon. The addition of carbon increases the hardness and strength of iron, making it suitable for construction (bridges, buildings) and manufacturing machinery (engine blocks, gears). The increased strength allows these structures to withstand greater loads and stresses.
2. Brass (Copper and Zinc): Brass is an alloy of copper and zinc. It is known for its corrosion resistance and machinability. This makes it ideal for musical instruments (trumpets, trombones), where durability and a bright sound are important. It's also used in decorative items and plumbing fittings due to its resistance to corrosion.
3. Bronze (Copper and Tin): Bronze is an alloy of copper and tin. It is harder and more durable than copper alone, and is also more resistant to corrosion. This makes it suitable for statues and sculptures, where longevity is crucial. Bronze is also used in bearings and springs due to its strength and wear resistance.