Properties and Uses of Metals and Alloys for the ESAT

The Relationship Between Properties and Uses

Metals are selected for specific tasks based on how their properties match the requirements of the application. These properties include physical traits like electrical conductivity, thermal conductivity, density, and malleability, as well as chemical traits like reactivity and resistance to corrosion. For the ESAT, you must understand why specific metals are chosen for their roles and how alloying can modify these properties.

Aluminium ($Al$)

Aluminium is a highly versatile metal due to its low density and its ability to resist corrosion. While it is technically a reactive metal, it quickly reacts with oxygen in the air to form a very thin, tough, and transparent layer of aluminium oxide ($Al_{2}O_{3}$). This layer sticks firmly to the surface, protecting the metal underneath from further reaction.

1. Aerospace and Transport: Its low density makes it ideal for aircraft bodies and engine parts, where reducing weight is critical for fuel efficiency.
2. Packaging: Its malleability allows it to be rolled into thin foils, and its corrosion resistance ensures it does not react with food.
3. Electrical Cables: Although copper is a better conductor, aluminium is often used for long distance overhead power lines because it is much lighter and cheaper, reducing the stress on support towers.

Iron ($Fe$)

Pure iron is relatively soft because its atoms are arranged in regular layers that slide over each other easily. However, iron is almost never used in its pure form. It is typically converted into alloys called steels.

1. Construction: Iron is used in the form of steel for the frameworks of buildings and bridges because of its high tensile strength.
2. Magnetism: Iron is one of the few magnetic metals, making it essential for the cores of electromagnets and transformers.
3. Cautions on Iron: A significant drawback of iron is that it reacts with oxygen and water to form rust ($Fe_{2}O_{3} \cdot nH_{2}O$), which flakes off and eventually destroys the structure. This is why iron must be painted, galvanised, or alloyed with chromium to form stainless steel.

Copper ($Cu$)

Copper is a transition metal prized for its exceptional electrical and thermal conductivity. It is also very ductile, meaning it can be drawn into thin wires without breaking.

1. Electrical Wiring: Most household and industrial wiring is made of copper because it allows electricity to flow with minimal energy loss as heat.
2. Plumbing: Copper is used for water pipes because it is easy to bend, does not react with water, and has antimicrobial properties that help keep water safe.

Silver ($Ag$) and Gold ($Au$)

These are known as noble metals because they are at the bottom of the reactivity series and are very resistant to chemical attack.

1. Silver ($Ag$): Silver is the best electrical and thermal conductor of all metals. It is used in specialised electronics and high quality mirrors, though its high cost and tendency to tarnish in the presence of atmospheric sulfur limit its use in general wiring.
2. Gold ($Au$): Gold is almost completely unreactive, meaning it stays shiny and does not corrode over thousands of years. It is extremely malleable and is used in jewellery and for reliable electrical connections in computers and smartphones where corrosion would lead to circuit failure.

Titanium ($Ti$)

Titanium is often described as a 'super metal' because it combines the strength of steel with a density closer to that of aluminium. It is also highly resistant to corrosion by seawater and chlorine.

1. Medical Implants: Titanium is bio-compatible, meaning the body does not reject it. It is used for hip replacements, bone pins, and dental implants.
2. High Performance Engineering: Its high melting point and strength to weight ratio make it essential for jet engine components and spacecraft.

The Science of Alloys

An alloy is a mixture of two or more elements, where at least one of the elements is a metal. Alloys are created to produce materials with specific properties that pure metals lack.

In a pure metal, the atoms are all the same size and are arranged in regular layers. When a force is applied, these layers can slide over one another, which makes pure metals relatively soft and easy to shape.

In an alloy, atoms of different elements have different sizes. When these different sized atoms are mixed into the lattice, they disrupt the regular arrangement of the layers. This disruption makes it much more difficult for the layers to slide over one another. As a result, alloys are significantly harder and stronger than the pure metals from which they are made.

Example: Steel Types

• Low carbon steel: Contains about 0.25 percent carbon. It is easily shaped and used for car bodies.
• High carbon steel: Contains up to 2.5 percent carbon. It is very hard but brittle, used for cutting tools.
• Stainless steel: Contains iron, chromium, and nickel. The chromium creates a protective oxide layer, making it resistant to rusting.