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Metal Alloys

What are metal alloys?

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Alloys are metallic compounds composed of one metal and one or more metal, or non-metal, elements.

Examples of common alloys include:

  • Steel, a combination of iron (metal) and carbon (non-metal)
  • Bronze, a combination of copper (metal) and tin (metal) and
  • Brass, a mixture of copper (metal) and zinc (metal)

Properties:
Individual pure metals may possess useful properties, such as good electrical conductivity, high strength and hardness, or heat and corrosion resistance. Commercial metal alloys attempt to combine these beneficial properties in order to create a metal that is more useful for a particular application than any of its component elements.

The development of steel, for example, required finding the right combination of carbon and iron (about 99% iron and 1% carbon, as it turns out) in order to produce a metal that is stronger, lighter and more workable metal than pure iron.

The precise properties of new alloys are difficult to calculate because elements do not just combine to become a sum of parts, but form through chemical interactions that depend on their component parts as well as the production method. As a result, much testing is required in the development of new metal alloys.

One thing that is for certain is that when metals are alloyed, the melting temperature is always affected. Galinstan®, a low-melt alloy containing gallium, tin and indium, is liquid at temperatures above 2.2°F (-19°C), meaning that its melting point is 122°F (50°C) lower than pure gallium and more than 212°F (100°C) below indium and tin.

Galinstan® and Wood's Metal are examples of eutectic alloys. Eutectic alloys have the lowest melting point of any alloy combination containing the same elements.

Composition:
Thousands of alloy compositions are in regular production, while new compositions are developed regularly.

Accepted standard compositions include the purity levels of constituent elements (based on weight content). The make-up, as well as mechanical and physical properties for common alloys are monitored by international organizations such ISO, SAE International and ASTM International.

Production:
Some metal alloys are naturally occurring and require little processing to be converted into industrial grade materials. Ferro-alloys such as ferro-chromium and ferro-silicon, for instance, are produced by smelting mixed ores and are used in the production of various steels.

Commercial and trade alloys, however, generally require greater processing and are most often formed by mixing molten metals in a controlled environment. Yet, one would be mistaken in thinking that alloying metals is a simple process.

If, for example, one were to simply mix molten aluminum with molten lead, we would find that they would separate into layers, much like oil and water. The procedure for combining molten metals, or mixing metals with non-metals, varies greatly depending on the properties of the elements required.

Metal elements have a great variance in their tolerance of heat and gases. While elements like the refractory metals are stable at high temperatures, others begin to interact with their environment, which can affect purity levels and, ultimately, the alloy quality.

Important considerations when alloying metals include the melting temperatures of component metals, impurity levels, the mixing environment and the alloying procedure.

In some cases, intermediate alloys must be prepared in order to persuade elements to combine.

An alloy of 95.5% aluminum and 4.5% copper is made by first preparing a 50% mixture of the two elements. This mixture has a lower melting point than either pure aluminum or pure copper and acts as a 'hardener alloy'. This is then introduced to molten aluminum at a rate that creates the right alloy mix.

Sources:
Street, Arthur. & Alexander, W. O. 1944. Metals in the Service of Man. 11th Edition (1998).

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