Unit 7: Properties and Applications of Engineering Materials
a) Element – elements are pure substances made up of atoms of the same type.
Atom – atoms are the smallest part of a chemical element that can exist, these are made up of a nucleus and electrons. The nucleus consists of positively charged neutrons and protons, these are called subatomic particles. Surrounding the nuclei are positively charged electrons.
b) A molecule is formed when two or more atoms of an element are chemically joined together. A compound is a type of molecule and is formed when the type of atoms joined together are different. However, not all molecules are compounds, some molecules only consist of one type of atom or element.
Carbon dioxide (CO2) has one carbon atom bonded to two oxygen atoms, making it a molecule.
Water (H2O) consists of two hydrogen atoms and one oxygen atom therefore it is a molecule and a compound.
Sodium Chloride (NaCI) or salt, has a mixture of sodium and chlorine elements, making it a compound.
c) Covalent bond – a covalent bond is the combining / sharing of electrons between two non-metal atoms, this creates a particle called a molecule. Some molecules contain thousands of individual atoms. An example of a covalent bond would be carbon dioxide.
Ionic bond – an ionic bond is the transfer of an electron from one atom to another, this creates a compound. An example of an ionic bond would be sodium chlorine. During this bond, the sodium atom loses an electron and becomes electropositive (positively charged); the chlorine atom gains this electron and becomes electronegative (negatively charged). Unlike charges attract creating sodium chlorine (salt).
Metallic bonding – In the process of metallic bonding, the outer shell of electrons leaves the metal atoms forming positive metal ions in a ‘sea’ of delocalised electrons. The positive ions repel each other but the negatively charged electrons hold them, creating equilibrium. This forms what is called a metallic lattice structure.
d) Lattice Structure – When elements join, they create what is called a lattice structure, these can be pictured as solid spheres packed together into layers. Many substances have a crystalline structure in the solid form, the most common in metal crystals are: –
BCC structure is relatively open packed, each cube shares its face with the adjacent ones; the open structure allows the adjacent layer of atoms to interlock deeply. Therefore, a BCC material will be more difficult to deform then.
On each face of the structure the atoms are tightly packed meaning the recesses between them only allows shallow interlocking of the adjacent row of atoms. Shallow interlocking means each layer can slide over one another easily, making the material easily deformed.
CPH structure resist slipping as there aren’t as many closely packed planes where slipping takes place. CPH metals tend to be less ductile then FCC materials but are more easily deformed than BCC materials.
Grain Structure – When a metal cools and solidifies, the crystal nuclei begin to form from microscopic impurities, creating a simple unit. As the atoms add to the crystal, it grows forming a dendrite.
Dendrite arms continue to grow and when they touch, solidification is complete; the dendrites have now become grains.
N.B. The rate at which the metal cools affects the size and shape of the grains.
Crystals and Crystal Growth – When molten metal begins to solidify, the atoms of a liquid begin to bond together at the nucleation point and start to form crystals. Nucleation is the moment a crystal begins to grow and the point where it occurs is the nucleation point; the final size of the crystals depends on the number of nucleation points. Crystals increase in size by the addition of atoms and keep growing until the start to touch the adjacent growing crystals.
Alloying interstitial – When two materials are mixed together when in molten state, interstitial alloy maybe be one of the results after cooling. An interstitial alloy is when the atoms of one material are very small compared to the other; the smaller atoms can occupy the spaces between the larger atoms. This effect strengthens and toughens the material as the small atoms make it less easy for the larger atoms to slip over each other.
Alloying substitutional – A substitutional alloy results when two or more materials with atoms of roughly the same size are mixed together. Different sizes of the atoms distort the structure, making it more difficult for the layers to slip over each other. This also creates a material that is tougher and stronger than its main component.
e) A phase equilibrium diagram represents the relationships between temperature, composites of the element based on carbon content and the phases at equilibrium. They show the different steps at which a material starts to melt or solidify and the range of solidification; they also indicate the equilibrium solid solubility of one compound in another. This is very beneficial to engineers because it allows them to establish which material is suited for a job.
N.B. The triple point is the pressure and temperature where liquid, gas and solid are all present. The critical point is where equilibrium has been achieved.