The alloys, also called metal alloys, are substances that combine a metal with other elements, both metallic and nonmetallic, and having different characteristics to the separate elements.
By controlling the quantities of each element of the alloy we can design materials according to the desired characteristics, which is why alloys are of such interest and occupy more than 95% of the metallurgical industry worldwide.
What is an alloy?
An alloy is defined as a homogeneous mixture of a metal with one or more different elements, both metallic and non-metallic.
Alloys are often described as a “mixture of metals”, but it is clear that there is always a base metal to which at least one other element is added, which may or may not be metallic.
For example, cast iron is an alloy of iron (metal) and carbon (not metal), while bronze is an alloy of copper and tin, both metals.
The elements of the alloy are usually separated into two categories, the base metal and the alloying elements:
- Base or primary metal: It is the majority element of the alloy and often gives the name to the type of alloy. For example, ferrous alloys are alloys in which iron is the base metal.
- Alloy elements: These are the elements mixed with the base metal.
If the alloy consists of only the base metal and an alloying element, the alloy is a binary alloy. If it is made up of a base metal and two alloying elements, it will be a ternary alloy, and so on.
There are also some lead-free alloy which are generally using in medical sector and sometime using in jewelry industry.
In general, an alloy can be described as an impure metal, but in alloys the impurities (alloying elements) are known and their concentration controlled to achieve specific characteristics.
Each metal in its pure state can have useful properties, such as good electrical conductivity, good resistance to temperature and chemical corrosion, high hardness and mechanical resistance, even combining several of these properties in the same metal.
The alloys are made with the intention of combining or modifying these properties and creating suitable materials for specific applications.
For example, the correct combination of iron and carbon forms the alloy known as steel, stronger, lighter and more malleable than pure iron.
Furthermore, if chromium is added to the steel, high resistance to oxidation and chemical corrosion characteristic of stainless steel is achieved.
The specific properties that an alloy will have are very difficult to predict. It is not simply the addition or subtraction of elements, so the design of new alloys requires a lot of testing and research.
How are alloys made?
Traditionally, alloys have been made by mixing the different molten components and allowing them to cool in what is known as a solid solution.
At present, casting is still widely used, together with other techniques that allow obtaining alloys with special characteristics, even when the components are not soluble in each other or in a liquid state.
These techniques include powder metallurgy. It consists of mixing the different materials in pulverized solid state and then fusing them using high pressure and temperature, but high pressure allows using temperatures lower than their melting points.
Another technique for making alloys is called ion implantation. This technique is performed by placing thin sheets of the base metal in vacuum chambers and bombarding them with ions of the desired alloys. This creates an alloy layer on the base metal.
Ion implantation is used primarily in the manufacture of electronic circuits, microchips, and semiconductor materials.
Interstitial and substitution alloys
The alloys can be classified into two types according to the arrangement of the alloying elements in the matrix of the base metal:
Alloying by substitution: Atoms of the alloying element replace atoms of the base metal. This occurs when the size of the atoms in both elements are similar, so they are usually close in the periodic table. Example: brass is an alloy of copper with 10-35% of its atoms replaced by zinc.
Interstitial alloy: When the atoms of the alloying element are much smaller than those of the base metal, the alloying element is located in the spaces or “interstices” left by the atoms of the base metal. Example: in steel, carbon is located in the interstices left by the iron atoms.
Mixed alloy: Substitution and interstitial alloy takes place.
Examples of alloys
Among the most important industrially and most common alloys in everyday life, we can quote the following examples:
- Steel: Iron and carbon alloy up to a maximum of 2% carbon and, in general, smaller amounts of other elements.
- Stainless steel: It is a type of steel to which a minimum of 10.5% of chromium is added, and generally also nickel and other elements to increase the resistance to chemical corrosion.
- Bronze: Copper alloy as base metal and between 3 and 20% tin as alloying.
- Cupronickel (CuNi): Copper-nickel alloy.
- Cuproaluminum: Copper and aluminum alloy.
- Amalgams: Mercury alloys with other metals. These alloys are usually pasty in consistency.
- Alpaca: Alloy of copper (45-70%), zinc (8-45%) and nickel (15-26%).
- Brass: Copper-zinc alloy.
- Petre: Tin alloy with copper, lead and antimony.
- Zamak: zamak alloys is the combination of zinc, copper, aluminum and magnasium.
- 18k gold: An alloy of gold (75%) and other metals such as copper, nickel or zinc.
- White gold: An alloy of gold and a white metal such as silver, palladin, or nickel.
- Silver: Silver alloy with another metal, usually copper.