Structure of Metals and Alloys

Atoms- are tiny building blocks of matter. Electron (-), Protons (+) and Neutrons (no charge).

Ions- is an atom that has unequal number of electrons and protons. An ion has an overall positive or negative charge.

Atomic Structure- is how atoms are arranged and bind to each other. This is largely determined by the chemical bands between the atoms.

Element- is a material that is made up of only one type of atom

Metals- are the largest sub group of all the elements.

Alloys- is a combination of metals/other elements where Components are mixed at the atomic level.

Chemical Bonds

Ionic Bonds-One atom donates one or more electrons to another atom. This makes one atom negative (anion) and the other positive (cation). This forms a very strong bond.

Covalent Bonds-One atom shares one or more electrons with another atom

Metallic Bonds

Metallic Bonds- The atoms donate one or more electron to a sea of electron, which are shared by atoms. This sea of electrons is free to move and has a negative charge. The positive metal ions (cations) are held together by the sea of negative electrons.

The ions arrange themselves into crystal lattice

Metallic Bonds explain the primary properties of Metal

Metals have high tensile strength melting point/boiling points because it takes a lot of energy and force to break metallic bonds.

Metals are good conductors of electricity because electrons are free to move around in the 'sea' of electrons. Electricity is the movement of electrons.

Metals are good conductors of heat because the electron transmit heat vibrations throughout the metal quickly. The metal ions are packed closely together so they can transmit heat vibrations from, one ion to the next.

Metals are malleable and ductile because an external face can cause metal ions to move and slip into new positions within the crystal lattice without breaking the metallic bond. Slip allows metals to deform plastically. Higher forces can break the metallic bands causing fracture.

Crystalline Structures

Crystalline Solids- In a crystalline solid atoms are organised and bonded in repeated patterns - lattice structure. Metals, Diamond

Amorphous Solids- In an amorphous solid, there is no pattern to the arrangement of atoms. Glass, Most plastics

Metal Crystal Structure

Body Centered Cubic (BCC)

• Iron (ferrous) 

• Chromium

Atoms are positioned at the 8 corners of a cube, plus one atom in the middle of the cube

Properties of BCC metals 

More Brittle

The atoms are not closely packed, and they are not 'in line' with each other.

A larger force is required to cause the atom to slip over each other- there is more of a 'hill' to climb.

Because the BCC structure blocks slip, BCC metals are stronger, less ductile and more brittle. 

Face Centred Cubic (FCC)

• Aluminium, copper, lead, silver, gold
• Iron (austenite)

Atoms are positioned at the 8 corners of a cube plus one atom  in the middle of each face of the cube

Properties of FCC Metals

More Ductile

The atom are more closely packed, and they are more ' in line' with each other.

A smaller force is required to cause the atoms to slip over each other- there is less of a 'hill' to climb

Because the FCC structure allows easier slips, FCC metals are more ductile and malleable and less strong.

Dendritic Growth of Crystals 

Crystal structures are formed when metals cool from liquid to solid

Lots of tiny crystals start to form (nucleate) as different parts of the liquid cool at different rates 

These tiny crystals grow by dendritic growth, in a tree like pattern, similar to a snowflake.

The dendrite branches meet up to form grains which are single individual crystals.

So, metals are in fact polycrystalline structures composed of multiple crystals called grains.

Metal Grains

Metal and alloys are made up of grains.

Each grain is a distinct crystal which has its own(random) orientation. 

Where the grains meet is called a grain boundary. 

Eg: Steel may have hundreds of grains in a mm2 but you can see grains with the naked eye on zinc.

Grain Sizes + Metal Properties

 Smaller grains are stronger and less ductile. 

This is because grain boundaries act as barriers to slip fractures, because the crystal structure are not lined up

It takes more force to cause slip or fracture to cross or change direction along a grain boundary

Even if a slip or crack crosses or moves along a grain boundary, with many grains, it wont be long before it meets another grain boundary barrier