UNIT 10 – PROPERTIES AND APPLICATIONS OF ENGINEERING MATERIALS
ASSIGNMENT 1: Structures and Properties of Engineering Materials.
Special attention should be paid to the Grading Criteria, as this will influence the way you answer each of the tasks. For each question all tasks need to be answered correctly to achieve the relevant criteria.
Question 1: (P1)
With the aid of clear, annotated diagrams, describe the structure associated with the following engineering materials: -
a) Iron at room temperature.
To meet the criteria you will need to describe the following: -
• The type of atomic lattice structure it has.
• The way the grain structure develops as the metal cools from a molten state.
BODY CENTERED CUBIC STUCTURE
Iron has a BCC crystal lattice structure at room temperature. The open-packed planes of atoms make the structure rigid, and hard. When external forces are applied the atoms don’t easily move over each other.
When iron is heated to around 800 º C its crystal lattice structure changes to FCC. The closely packed atoms move more readily over each other when external forces are applied.
FACED CENTERED CUBIC
When the molten iron solidifies, latent heat is given off and crystals or grains start to form at different points. These embryo grains are known as dendrites, which multiply and grow until the material is solidified. The size of the grains depends on the cooling rate.
METALLIC DENDRITE GROWTH
FORMATION OF GRAINS
To meet the criteria you will need to consider the following: -
• Whether the material is a thermoplastic or thermosetting polymer.
• The atomic structure associated with this class of polymer, NOT the chemical constituents.
Polycarbonate is a thermoplastic.
Thermoplastics have a linear or branched molecular structure which determines their strength, solid