You’re no doubt familiar with the term alloy, but maybe not some of the intricacies of their characteristics. We rarely use pure metals, other than in decorative or catalytic applications - they’re just not very strong compared to alloys.
Alloys are metals made up of two or more elemental metallic constituents, often with non-metal additions. The addition of various elements to a pure metal’s lattice structure enables metals to have properties that they do not have in their pure forms. Typically, alloys are stronger, harder, more durable and in many cases, more corrosion-resistant than their pure metal counterparts.
Alloys have a lot of different compositions. You see properties of some of the modifying additions ‘adopted’ by the mixture. The primary element in the alloy is typically a material that can accept dissolution of other metals to a degree, to avoid regionalization and disuniformity.
Examples of alloys include steel, brass and aluminum alloys, such as aluminum 6061, which is one of the most common alloys used by Xometry customers for CNC machined parts. Alloys are used in a wide range of applications, from infrastructure and vehicles to consumer goods and medical equipment. In this article, we will explain what an alloy is and review the different types, compositions, and applications of alloys.
What Is an Alloy?
An alloy is a material composed of a metallic base, usually the large majority component, and additional metal or non-metal components that are added as property modifiers. Alloys are manufactured and carefully tuned by experiment to deliver desirable properties that are not present in the primary material.
Many alloys are made purely of metals, but non-metal additions such as Silicon, Sulfur, Carbon, Nitrogen, and other light elements are commonly used as property adjusters.
The History of Alloys
Alloys have been used since as early as 3000 BCE. The first known alloys were brass (Copper and Zinc) and bronze (Copper and Tin). Both of these likely originated from early metallurgical learning, where two ores of different compositions were smelted together. We will never know if the first alloys were the result of brilliance or mistake, but what followed is the entire history of metallurgy and our technological society.
Notably, the element Nickel was not isolated until the 19th Century, but its presence was felt in some Copper deposits, which, when smelted, formed cupro-nickel. These ores were often described as ‘having the devil in them’ as the Nickel content made the Copper very hard to work. Nick is an old word for the devil.
Brass and the much more serviceable bronze both alter the soft and ductile nature of Copper to deliver harder, tougher, and more resilient materials - and in the case of bronze, the ability to hold an edge. Bronze was the first weapons-grade metal, and it destroyed empires. The best bronze in ancient Europe came from Cyprus, and the Mycenaeans, Greeks, and Romans built their empires on it. Today, brass and bronze are still frequently used to create parts and components; you can find them as auto-quotable options within Xometry’s instant quoting engine.
In about 1,600 BCE, wrought iron and cast iron began to be produced. Iron is much harder to extract from the ore, and it is unlikely that it was refined by accident. We have experimental metallurgists from 4,000 years ago to thank for it. Pure iron is soft, ductile and malleable and really not of much utility. The big step comes in smelting and working the Iron with a pretty high Carbon content, to alter its structure. The first Carbon is used as a reducing agent in the smelt, so it became an alloying agent by stealth. Cast and then hot-worked (wrought) Iron displaced bronze, and empires fell.
Iron and the alloy family then remained mostly unchanged for 3,000 years, other than a few highlights:
- Adding/controlling the Carbon content became an art form. Sword makers in Toledo (which is an awesome city in Spain with great views) and Jōmon period Japan (up to 1,000 BCE) both learned to make steel, either by adding Carbon by hand (Japan) or by using wooden anvils (Toledo).
- Later on, in various regions, a kind of case hardening by quenching in Nitrogen-rich water added an extra bite to the blade.
It really wasn’t until the Industrial Revolution in the 18-19th centuries that metallurgy became a formal science, delivering many of the alloys commonly used today. Advances in chemistry allowed the isolation of metallic elements such as Manganese, Nickel, and Chromium, Aluminum, Titanium, Magnesium and other elements used in alloys today. The Industrial Revolution is one of my favorite periods of history and continues to shape our lives today.
The Composition of Alloys
Alloys are merged materials composed of a primary base element combined with various secondary elements. The base element provides the fundamental structure and typically the solubility medium that disperses the other components uniformly, while the secondary elements are added in specific proportions to adjust and bequeath desirable properties of the final material. The resulting alloy inherits a summary of the characteristics of all its constituents and in many cases, unexpected cooperative gains that none of the individual constituents display, leading to selectively improved performance.
How Alloys Are Made
Alloys are made by smelting and blending the base metal and additional elements (metals and/or non-metals) and allowing them to cool. The admixing is often performed in the melt, but many non-metallic additives can be worked in after initial solidification, by various methods. Two primary types of alloys are used; substitutional and interstitial alloys.
- In substitutional alloys, like brass and bronze, the atoms of all of the alloying elements are similar in size. The atoms of the alloying elements substitute for the same sites the atoms of the base material would occupy in its lattice structure. This lends distributed property adjustments to the lattice that are intrinsic to the metals involved. In most cases the substitution disrupts and stresses the lattice, reducing planar slip potential by blocking.
- In interstitial alloys such as steel, the atoms of the alloying elements (Carbon, Silicon, Nitrogen) are smaller and fit in between the atoms of the base metal. This placement also acts to disrupt slippage and fracture. However, some non-metallic elements such as Silicon act as crystal growth triggers, altering the typical crystal size to add strength and resilience as more and smaller crystals deliver a tougher material.
