i. What is the basic microstructure of metals, polymers and ceramics?

a. Metals

Metallic materials are normally combination of metallic elements. A crystalline material, metal has a repeating, long ranged atomic arrangement. A closer look at the microstructure shows that metals consist of positive metallic ions (cations) held together in a sea of delocalized valence electrons by electrostatic attraction. This is otherwise known as metallic bonding.

As for crystal structures, different metals have different structures. Steel, which is made up of iron has body centered cubic structure (BCC), where atoms are located at all eight corners and a single atom at the cube center. As for aluminum, it has a face centered cubic structure (FCC), where the atoms are located at each of the corners and the centers of all the cube faces.

b. Polymers

Polymers are mostly organic compounds that are chemically based on carbon, hydrogen and other non-metallic elements. They are often only partially crystalline as a slight chain disorder or misalignment will result into an amorphous region. Polymeric materials are also made up of layers of chained repeating molecules which alternate between crystalline and amorphous.

A more in-depth exploration of the microstructure of polymers shows that the atoms are held together by covalent bonds that are the sharing of electrons between adjacent atoms. However, much weaker secondary bonds known as Van der Waals’ forces hold the chains of molecules together.

c. Ceramics

Ceramics are compounds between metallic and nonmetallic elements. The atomic bonding in these materials ranges from purely ionic to totally covalent, but for ceramics, they exhibit a combination of the two bonding types. For ionic bonding, metallic cations, that are positively charged, and nonmetallic anions, that are negatively charged, are held in place in a lattice by electrostatic attraction between the oppositely charged ions.

Ceramics can also either be crystalline or amorphous, depending on the bonding that is predominant. For crystalline ceramics, the predominant bonding is ionic and vice versa. Silica is an example for crystalline ceramic while glass is the representative of amorphous ceramic.

ii. How does the microstructure determine the strength and ductility of metals, polymers and ceramics?

a. Metals

Metals usually have high strength due to the fact that it has closed packed crystal structure. Metallic materials also have high ductility as the mobility of the electron cloud enables it to readily accommodate any distortion in the lattice. This can be seen from the diagram below.

From the above diagram, it can be seen that when a force is applied, metallic material does not undergo distortion in lattice.

b. Polymers

For polymers, they have low strength and high ductility. Due to the weak secondary Van der Waals’ forces, the chains of molecules are easily separated, leading to low strength. As for the high ductility, many polymers contain the spherulitic structure which consists of numerous chain-folded ribbons (lamellae) that radiate outward from the center. Separating these lamellae are areas of amorphous material. Below is a diagram of 2 adjacent chain folded lamellae and the interlamellar amorphous material before any deformation has occurred.

When deformation starts, the amorphous material merely is stretched while the chains get oriented. This can be seen in the next diagram and hence explains why polymers have high ductility.

c. Ceramics

For ceramics, they have high strength, yet are brittle, i.e. low ductility. This can be attributed to the strong bonding within the structure.

Should the predominant bonding be ionic, due to the electrically charged nature of the ions, there is only few slip systems along which the dislocations caused during deformation can move, resulting in the high strength of ceramic. However it is also due to the ionic bonding that ceramics are brittle as when a large force is applied, the ions of like charge are bought within close proximity to one another. This leads to electrostatic repulsion in between atoms and thus causing cracks.

Should the predominant bonding be covalent, the ceramic is also brittle due to the relatively strong covalent bonds which limit the number of slip systems and the complex dislocation structures.

iii. Why are steels stronger than aluminum alloys?

Before answering to the question, one should be aware that melting point is a measure of relative strength of bonding within lattice. Therefore a higher melting point indicates that more energy is needed to break the bonds, leading to the conclusion that the bonds are stronger. With regards to that, as the melting point of steel is 1538ºC while the melting point of aluminum is 660ºC, steel is stronger than aluminum. This is due to the fact that steel is an alloy, in which impurity atoms (in this case, carbon) have been added to increase mechanical strength: The addition of carbon introduces some lattice strains on the adjacent host atoms. Furthermore, as aluminum have a FCC structure, the layers of cations can slide across each other more easily in comparison to steel, which have a BCC structure, causing more resistance between the atoms and the inability of cations to slide across each other. This means higher strength for steel.

iv. Account for the difference in the strength of solution-treated and aged aluminum and annealed aluminum alloys?

For solution-treated and aged aluminum alloy, the alloy is heated until is consists of only phase α, i.e. at temperature T1. This is followed by rapid quenching to temperature T3 such that formation of phase β is prevented. With aging occurring at T2, precipitation is formed from the quenching and thus increasing the contact area in between molecules. This result in stronger alloys and it can be seen from the results where solution-treated and aged aluminum has a tensile strength of 337.82 MPa in comparison to annealed aluminum which has a tensile strength of 149.78 MPa.

Annealed aluminum alloy however, is only subjected to heating and is cooled slowly to room temperature, causing less precipitation and thus less contact area between the molecules. This leads to a weaker alloy.

v. Is PS (Polystyrene) or PE (Polyethylene) stronger, and why?

PS is stronger than PE as it has a higher molecular mass and is a cross linked polymer. Below are the atomic structures of the repeating units for PE and PS:

PE (Polyethylene)

PS (Polystyrene)

As the mass of is larger then that of a hydrogen atom, the Van der Waals’ forces holding the long chain of molecules together for PE is weaker than that of PS. This is due to the fact that Van der Waals’ forces depended on the molecular mass of the molecule.

However, the strength of PS also depends on the structure of the molecular chains. For PS, as it is cross linked, it is harder to disentangle and rearrange in comparison to PE which has linear arrangement of its molecular chains. The molecular structure for both polymers can be seen as shown below.

PE (Polyethylene)

PS (Polystyrene)

vi. Comment on the testing technique and strength of glass.

Glass is a ceramic. Therefore glass has high strength but is brittle. Due to these mechanical properties, glass is not tested using the tensile test; it is instead tested by using the 3 point bend test. This is firstly due to the fact that it is difficult to prepare and test specimens having the required geometry. Secondly, it is difficult to grip the glass without fracturing it and lastly, as ceramics fail after only about 0.1% strain, glass must be perfectly aligned so as to prevent the presence of bending stresses, which is not easily calculated. Hence, the 3 point bend test is used.