There are three states being gas, liquid and solid. The state of an element will depend on the temperature and pressure. Here is an example of the state of metal:
- As a metallic gas, particles exist as single atoms otherwise known as ‘monatomic’.
- As the temperature falls, the metal condenses to a liquid metal (at the boiling point) which has weak forces between the atoms which allows the atoms to flow around each other.
- When the metal cools even further, the metal turns to a solid where the atoms align themselves in a lattice structure (or a crystalline structure).
It is the release of heat energy that causes the phases change. As well as this, it is important to know that not all materials change phase at the same rate. The rapidity at which the phase changes for a material will affect the properties of it.
An amorphous solid is a liquid or gas which has cooled extremely fast (otherwise known as supercooled) to the point that the atoms cannot arrange themselves into a lattice structure. This means that the atoms and molecules are arranged ‘randomly’. Here is how an amorphous solid works:
- An example of an amorphous solid is Quartz which has a lattice structure.
- However, when melted, with silicon dioxide to make glass and supercooled, the crystalline structure cannot be formed. Instead, the atoms arrange themselves randomly and an amorphous solid is formed.
- When reheated, the solid does not change phase from a solid to liquid but gradually softens instead (phase doesn’t change sharply).
- For amorphous materials, the properties of it are the same in every direction which is also known as having an isotropic property.
- Cannot control dislocations/cracks direction in amorphous solids. Take glass – the crack expands randomly.
Below is an image of an amorphous structure (left) and a crystalline structure (right).
Crystalline structures make up over 90% of all naturally occurring solids. Examples of these include diamond, limestone, sand, clay and many minerals.
All metals have a crystalline structure too: a repeated arrangement of atoms/molecules.
- The energy required is reduced when atoms are packed closer together to do something on an atomic or microscopic level (i.e. shift some atoms).
- The amount of stabilization achieved by anchoring reactions between particles is greater. A crystalline lattice is formed.
The crystalline lattice contributes to the properties of the material because it enables atoms to slide past each other more easily on close packed planes (ductility).
The simplest kind of crystal structure is the cubic structure which consists of a box shape with an atom in each corner. However, this is not the most common type of cubic structure. The body-centred structure (BCC) and the face-centred structure (FCC) are more common. The symmetry of these structures provide closely packed planes in several controllable directions.
- FCC are more ductile.
- BCC are typically stronger.
Above shows the difference between BBC and FCC. With BCC, there are atoms in the centre of the cube while FCC has the atoms on the face of the cube.
FCC has a close packing factor due to the atoms being closer together. FCC materials include aluminium, copper, gold nickel and silver. The fact that there are no atoms in the middle of a FCC means that the atoms can slide more easily over each other making the properties of the materials ductile.
On the other hand, BCC is looser packing (has a lower packing factor) and the atoms cannot move as easily over each other making the properties of BCC materials hard such as lithium, sodium, potassium, chromium and tungsten.
It is important to note out that some metals BCC structure will only occur when the metal is ‘softened’ as the temperatures rises closer to the metal’s melting point.
Hexagonal Packed Structures
Hexagonal packed structures have a similar packing factor to face-centred structures – the only fundamental difference is the stacking sequence.
The above graphs show a material solidifying over time. You will see the dip in the left graph – this is the critical mass of the material and once the graph levels out after that, crystallisation states.
A useful tip to remember is that it is difficult to cast iron with the same size crystals because different parts of iron will cool at different rates and, therefore, produce different size crystals.
A more stressed material becomes a harder material. Defects obstructs the force applied so are good in hardening materials.
Dendrites occur from a small nuclei in a liquid material which acts as the centre of crystal growth. Dentrites are basically a crystalline mass with a branching, tree-like structure. The tree-like structure occurs from this nuclei (which has solidified into a crystal around the liquid) and grows/branches off it creating a tree-like shape. These tree-like crystals grow bigger and bigger until solidification is complete and at this moment, there is little evidence of the dentrite structure remaining.
Shrinkage is when a material contracts or shrinks during solidification or cooling and is a result of:
- Contraction of a liquid as it cools prior to its solidification.
- Contraction during phase change from a liquid to solid.
- Contraction of the solid as it continues to cool to ambient temperature.
The coolest part of a liquid is where it is in contact with a mould or die, you will find that solidification usually occurs here first. As the crystals continue to move inwards, the material shrinks. If the solid surface is too rigid to deform and accommodate for the internal shrinking, the stresses can be so high that it causes a crack to form.
Due to the way the crystals form first on the outside of the material, the smallest crystals are on the outside, and the get larger the further into the material. However, towards the centre, the crystals decrease in size as they are cooled down in little time.
Sometimes under shrinking, the material solidifies inwards and there is not enough atoms present to fill the available space and what you are left with is a void.
Defects contribute to the mechanical properties of metals. Although defects make it seem as if they are weakening the structure of the metal, it can be used to the metal’s benefit too. For example, by adding an alloy will produce a crystal defect that is beneficial to the properties of the metal. There are three main types of defects:
- Point defects which an atom is missing or placed irregularly in a lattice structure.
- Linear defects which are groups of atoms that are in irregular positions. This is commonly more known as dislocations.
- Planar defects which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.
Dislocation movement produces additional dislocations, and when dislocations run into each other it often impedes movement of dislocations. This drives up the force needed to move the dislocation or, in other words, strengthens the material.
Here are some examples of point defects:
- Self interstitial atom which is an atom that has crowded its way into a interstitial void in the crystalline structure.
- Substitutional impurity atom which is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. They are usually close in size (within around 15%) to the bulk atom.
- Interstitial impurity atoms are much smaller than the atoms in the bulk matrix and fit into the open space of the bulk atoms of the lattice structure. An example of this is the carbon atoms that are added to iron to make steel.
Linear Defects and Dislocations
Dislocations are areas were the atoms are out of position in the crystal structure. Dislocations are generated and move when a stress is applied. The motion of dislocations allow slip – plastic deformation to occur.
There are two basic types of dislocations: the edge dislocation and the screw dislocation.
The edge defect can be easily visualized as an extreme half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. The line runs along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line.
Like a caterpillar creates a small hump to move its whole body a bit forward, and edge dislocation only needs a small force to correct it. Only a small fraction of bonds are broken at any time and moved along until thee dislocation has moved along the whole plane and make a lattice structure. An edge dislocation does not require all the bonds along the plane to be broken at once. Only a small number of bonds are broken at any one time and parallel to the direction of stress.
The motion of a screw dislocation is also a result of a shear stress, but the defect line movement is perpendicular to direction of the stress and the atom displacement, rather than parallel like it is with edge dislocation.
Imagine a block of metal with a shear stress applied across one end so that the metal begins to rip. The atoms just above the rip have three stages:
- Ones that have not yet moved from their original position.
- Ones that have moved to a new position and have re-established metallic bonds.
- Ones that are in the process of moving.
Only a small portion of the bonds are broken at any given time so it requires a much smaller force than breaking all the bonds across the middle place simultaneously. Dislocations will always move along the densest plane of atoms. FCC and BCC metals have many dense planes, so dislocations move relatively easy and these materials have high ductility.
Metals are strengthened by making it more difficult for dislocations to move. This may involve the introduction of obstacles, such as interstitial atoms or grain boundaries, to ‘pin’ the dislocations.
Also, as a material plastically deforms, more dislocations are produced and they will get into each others way and impede movement. This is why strain or work hardening occurs. In ionically bonded materials, the ion must move past an areas with a repulsive charge in order to get to the next location of the same charge. Therefore, slip is more difficult and the materials are brittle. Screw dislocation is also a reaction to shear stress but the defect line is perpendicular. The bonds are significantly distorted in the immediate vicinity of the line only.
Grade Boundaries in Polycrystals
Another type of planar defect is the grain boundary. Grains can range in size from nanometres to millimetres across and their orientations are usually rotated with respect to neighbouring grains.
Where one grain stops and another begins is known as the grain boundary. Grain boundaries limit the lengths and motions of dislocations. Therefore, having smaller grains (more grain boundary surface area) strengthens a material.
The size of the grains can be controlled by the cooling rate when the material cast or heat treated. Generally, rapid cooling produces smaller grains whereas slow cooling result in larger grains.
Bulk defects occur on a much bigger scale than the rest of the crystal defects. Voids are regions where there are a large number of atoms missing from the lattice. Voids can occur for a number of reasons. When voids occur due to air bubbles being trapped when a material solidifies, it is commonly called a porosity. When a void occurs due to the shrinkage of a material as it solidifies, it is called cavitation.
Elastic and Plastic Deformation
When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation, is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other.
When the stress is sufficient to permanently deform the metal, it is called plastic deformation. Plastic deformation involves the breaking of a limited number of atomic bonds by the movement of dislocations. The movement of dislocations allow atoms in crystal planes to slip past one another at a much lower stress level. Since the energy required to move is lowest along the densest planes of atoms, dislocations have a preferred direction of travel within a grain of the material. This results in slip that occurs along parallel planes with the grain. These parallel slip planes group together to form slip bands, which can be seen with an optical microscope, but it is in fact made up of closely spaces parallel slip planes.