The Mechanical And Chemical Properties Of Different...

1. Introduction

Within Part A, I aim to explain the mechanical and chemical properties of different materials, explain an alloy system and how its grain structure can affect its material properties and the effect temperature can have on semi-crystalline and amorphous polymers. Part B will cover material selections for the given applications and go into explaining why I made these choices.

By carrying out this report, I hope to achieve a good understanding of material properties across all of these different applications and how effective correct material property selection can be.

2. Part A

3.1. Bonding and Crystal/Molecular Structure

3.2.1. Metals

These have a metallic bond, which is a weak covalent bond but a strong atomic structure. The fewer electrons contained in the metal the higher the electrical/thermal conductivity because the valance molecules are loosely packed, this will also mean a lower hardness/melting point and higher ductility. Softer metals such as Tungsten have a BCC structure, which is a weak bond, which means it has lower hardness, is more reactive and has the higher electrical/thermal conductivity. FCC structures, such as Gold, have higher ductility and are less brittle. HCP structures, such as Magnesium, are less ductile and are more brittle. FCC and HCP structures are the most common structures found in metals.

3.2.2. Polymers

A polymer is composed of many simple molecules that are repeating structural units called monomers in a HCP structure. A single polymer molecule may consist of hundreds to a million monomers and may have a linear, branched, or network structure, which affect the strength and density of the material, linear being the weakest and network structure the strongest. Covalent bonds hold the atoms in the polymer molecules together; each of these valence electrons can form a covalent bond to another carbon atom or to a foreign atom. Secondary bonds then hold groups of polymer chains together to form the polymeric material.

3.2.3. Engineering Ceramics

Engineering Ceramics have an Ionic/Covalent bonding with a HCP structure. This means it is strong in compression, hard, brittle, weak in shearing and torsion, chemical resistant, can withstand high temperatures and creep at high temperatures. The yield strength of Engineering Ceramics increases when the grain size decreases up to a point, this being, when the grain size is reduced too much, the material becomes increasingly brittle.

3.2. Alloy System

3.3.4. Equilibrium Diagram for Aluminium/Copper Alloy

Green arrow represents the melting point of Copper (1083°C)
Blue arrow represents the melting point of Aluminium (660°C)
Red line represents the 50/50 point of the alloy

3.3.5. Alloy cooling

The above diagrams show the cooling process of an alloy from the melt. These diagrams show how the nuclei begin to form on cooling and then go onto form dendrites. These dendrites then form the secondary arms and continue to grow, as they grow out the arms thicken eventually becoming completely solid and the grain boundaries become visible.
As shown below, the pure metal is the largest part of the grain and the impurities create the edges which are more noticeable.

When the alloy solidifies, the crystals are created from the mixture of the two metals, because the metals will generally have different melting points they will solidify at different rates. This difference in solidification can create “coring”, which is when the metal with the lower melting point will solidify on the outside edges and the metal with the higher melting point will be contained within the centre of the cast as can be similarly seen from the diagram below.

If an alloy is allowed to cool too quickly, the grain structure will not form correctly and create a weaker bond. This can have a detrimental effect on the material properties which could potentially

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