What is an alloy?

steel, a common alloy

Concept Introduction – Alloys

An alloy is a mixture of two or more metals. Many alloys exist that combine different metals and different ratios of metal. In this tutorial, learn about what an alloy is, the properties of alloys, and some of the most common alloys used today.

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What is an Alloy?

An alloy is a mixture of two elemental materials to make a metallic substance. The main component is a metal (often more than one metal) and metallic properties are retained in the new mixture. An alloy can be either a compound (solid) or solution (liquid). Most commonly alloys exist as a solid at room temperature.

Sterling silver ring (alloy of silver and copper)

Properties of Alloys

There are many beneficial and tunable properties of alloys including hardness, corrosion resistance, and ductility.

Ductility is the ability for a material to deform locally. Glass is not ductile and therefore breaks when pressure is applied. However, alloys can bend slightly to local pressure without breaking, meaning they are more ductile. This is an important property in building materials. Alloys tend to have higher ductility.

Corrosion resistance is also an important property of materials that will be exposed to the weather or harsh conditions. Corrosion occurs when a substance undergoes a chemical reaction such as oxidation, and slowly wears away the material. Rust is a common product of corrosion. Alloys commonly have higher corrosion resistance than elemental metals.

Hardness measures how easily a material is scratched. The Mohs hardness scale measures this property. Pure metals are much softer and more malleable to work with in many manufacturing scenarios. In construction, a very malleable material does not provide structural integrity well.

How is an alloy made?

Alloys are most commonly made by heating the required material until they are molten. Then the components mix together. The molten substance flows into a mold. The mold is allowed to cool, creating the new material in the desired shape.

molten bronze being poured into a mold to make a new alloy
Molten bronze being poured into a mold. Bronze contains the metals copper and tin.

It is important that the materials be soluble with each other. There are some metals, such as iron and copper, that do not mix together. Therefore these combinations make bad materials as an alloy.

Impurities in Alloys

An alloy is by definition an unpure substance. However, when referring to impurities in an alloy it references an element that should not supposed to be there. For example, steel commonly contains the impurity sulfur (which is supposed to just be carbon and iron). The most common impurity is dependent on the main metal in the alloy. In aluminum alloys, lithium, calcium, and sodium are the most common impurities. Oxygen causes wide spread impurities due to the formation of oxides with metals.

The problem with impurities is that the change the properties of an alloy. For example, the material may be weaker, and more liable to break, where impurities are present. Many steps during manufacturing occur to avoid impurities.

Common Alloys

  • Brass: Brass is a combination of copper and zinc.
  • Bronze: Copper and tin are the two components in bronze. The Bronze Age (3,000-1,200 B.C.) is named after this alloy because the discovery of the material changed that period of history so dramatically.
  • Cast Iron: Cast iron is a material commonly used in cooking pans. The material contains very high percentages of iron (>90%) and a small percentage of carbon.
  • Steel: Steel contains carbon (a nonmetal) and iron. Steel has improved strength and fracture resistance compared to pure iron.
  • Sterling Silver: A common component of jewelery is sterling silver. The standard for this alloy is 92.5% silver and 7.5% copper.
  • Solder: Lead and tin mixture. Used for connecting wires.
  • Fusible Alloy: A class of alloys that have low melting points. Soldering materials and fire sprinklers commonly use this material. They are great in fire sprinklers because they fire heats the alloy to the point that melts it. The melting alloy breaks a seal that allows water to release to quench the fire. These materials melt around 100oC.
Brass piece
Brass Piece. Brass is an alloy of copper and zinc

Further Reading