Question

1-Identify three advantages of age hardening process in aluminium alloys.

2- Chromium in stainless steel reacts with oxygen to form a protective layer called chromium oxide; identify the

protective layer formed on aluminium to protect it from corrosion. Explain how this protective layer on

aluminium behaves differently as compared to rust formed on the surface of iron?

3-Explain the terms substitutional alloys and interstitial alloys. Provide two examples for each type of alloys.

4- Explain the terms nucleation and grain growth in metals, and how does it differ in annealing and quenching

heat treatment processes?

Answer 1

1.Advantages of age hardening in aluminum alloys

• Increses the tensile and yield strength of the alloy.
• Increases the resistance to wear of the alloy.
• Age hardening also produces alloys that are easier to machine.

2.Aluminum Oxide (Al_{​​​​​2}​​O_​​​3) is the protective layer on aluminum that protects it from corrosion.

This proctective layer on aluminum is practically an impervious layer that prevents any further oxidation or any other reaction of the aluminum beneath it; whereas the iron oxide (rust) does not offer this protection and all the iron gets rusted eventually(rust accelerates the further rusting in most cases).

3.In substitutional alloys  some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. Eg: Bronze, Brass.

In interstitial alloys the atoms of the smaller constituent gets into the voids in the lattice of the larger constituent.Eg: Steel,FeN

4.Nucleation is the process which starts the crystallization of a metal from its liquid form.It is the process of formation of the first set of atoms that get arranged in a specific crystal strucuture and the crystallization proceeds outwards from these nuclei.

Grain growth is the process of crystal growth according to the crystal orientation to obtain a solid object.If the growth is uniform from a single nucleus,we get a perfect crystalline solid.But in most cases the growth is from multiple nuclei and hence we get a solid with grains.

In annealing the grain growth is at a slower rate to produce a tough and ductile metal whereas in quenching the grain growth is very rapid and from a larger number of nuclei which results in a brittle and hard metal.