Structure of Metals
All materials are made of atoms, which are made of a nucleus of protons and neutrons surrounded by electrons. An element has a specific number of protons and electrons.
Pure metals are a series of elements found naturally. They are grouped together because of the type of bonds these atoms typically form. The so-called precious metals (Platinum, Palladium, Gold, and Silver) fill Group 8 and 1B of the periodic table, as they share similar characteristics.
The way that atoms interact and arrange themselves affects their physical and chemical properties. The properties and behavior of any metal can be related back to its structure and composition.
Nearly all metals and alloys are crystalline. In a crystal, atoms are arranged in a regular structure that repeats itself in all three dimensions. This can be described using a lattice. Different atoms will arrange themselves in different ways; there are different types of crystal lattice. Metals with different lattice structures behave differently.
A material's properties are controlled by the structure of the material. The structure of a material depends on its chemical composition and how the material is processed. They are all interrelated!
The structure of a material can be split into three main length scales:
Atomic structure – How the atoms arrange themselves. Can be seen in only the most advanced electron microscopes
Microstructure – How small crystals (grains – see below) arrange themselves. Metallurgists regularly study this using optical and electron microscopes, and it dramatically affects their properties.
Macroscopic shape – The shape of an object we see without eyes and what a jeweler works with.
Atomic Structure
As discussed, most metals are crystalline. A crystal is a solid in which atoms are arranged in a regular order that repeats itself in all three dimensions.
The three main types of lattices are hexagonal close-packed (HCP), face-centered cubic (FCC), and body-centred cubic (BCC).
Gold, Silver, Palladium, and Platinum are all fcc, owing in part to their similar size and chemical characteristics (determined by the structure of electrons in the atom). FCC metals are extremely ductile (can be easily drawn into wires), and atoms are densely packed together.
Microstructure
Nearly all macroscopic metal objects are made up of many small crystals. It is polycrystalline. It is extremely difficult to form one large perfect crystal – on cooling; many crystals will want to nucleate from the liquid and grow.
In a pure metal, these crystals will have the same structure and composition but are oriented differently. They are known as grains. Because of their different orientation, the grains do not pack perfectly, and the regions that separate them are known as grain boundaries.
The size and shape of grains and features at the grain boundaries (such grain boundaries often attract impurities and are much weaker than the grains themselves) form part of the microstructure. Changes to the microstructure during manufacture and processing dramatically affect the mechanical properties of a metal.
General properties of metals
Pure metals typically have the following properties:
Good thermal and electrical conductivity
High surface reflectivity (polish or lustre)
Malleable (good ductility, capable of being heavily deformed)
Heavy, i.e., high density
Toughness
Reasonable strength and hardness
Remember, these properties do not define metals. Metals are a group of elements that typically form "metallic" bonds. Some electrons in a metal are "delocalized," which gives rise to many of these properties.
What is an alloy?
The four precious metals are far too soft when pure to be used for jewelry. Instead, metallic elements are mixed together to form alloys.
Alloying is typically used in jewelry materials to:
Improve strength and hardness (i.e., scratch resistance)
Change the color
Improve castability (how easy it is to form an object from the liquid metal)
Reduce cost
The extent of alloying is carefully controlled. Hallmarking is used to define the weight fraction of a particular precious metal or "purity". "purity" is misleading because many of the alloying elements are deliberately added!
Microstructure of alloys
In pure metals, the crystal structure, composition, and properties of the system are the same everywhere. The system has a single phase. The fact there are grains with different orientations is not important to defining the phase. It forms a separate part of the description of the microstructure.
It may be energetically favorable for an alloy to have two or more phases. That is, regions with a different crystal structure and/or composition.
Key Points
The key points to remember:
Metals are a group of elements defined by the typical nature of the bonding between atoms.
Most metals are crystalline, where atoms are arranged in a regular, repeating structure that repeats.
Almost all metal objects are polycrystalline; they are made of lots of small crystals with different orientations called grains. They do not pack perfectly. The regions between them are called grain boundaries.
Regions of the material with the same composition, crystal structure, and properties are described as a phase. A material may have multiple phases co-existing.
The chemical composition, crystal structure, and grain structure control the properties of an alloy. They can be changed by processing (deforming or heating) an alloy; they are all interrelated!