Question

1)Which of the following statements is true regarding aluminum / aluminum alloys ?

		Aluminum alloys are generally considered to be mechanically inferior compared to pure aluminum in terms of yield strength, because their microstructures contain precipitate phases that strain the lattice and therefore soften the alloy relative to pure aluminum.
		Aluminum is extremely rare in abundance within the earth's crust.
		Aluminum is very resistant to creep, because its ceramic (Al2O3) oxide layer thermally insulates the base metal and prevents grain sliding due to its high degree of adherence to the base metal.
		High-strength aluminum alloys that leverage secondary phase hardening are often less resistant to corrosion compared to pure aluminum.
		Aluminum and it alloys are extremely difficult to melt, so they are rarely used as materials for casted parts.

Answer 1

i) False: Pure aluminium is very soft and yield strength is about 11MPA where as yield strength of Aluminium alloys are much higher than that of pure aluminium. Strengthening takes place due to presence of secondary phase particles in case of alloys.

ii) false. Aluminium is one of the most abundant element in earth's crust.

iii) True. Due to its strength to weight ratio and creep resistance it is widely used in automobile and aerospace applications. As mentioned in question Al2O3 acts as thermal insulator and prevents the base metal from grain sliding.

iv) True. Secondary phase hardening leads to corrosion. So corrosion resistance is less in case of alloys than pure aluminium.

v) False.Melting point of aluminium is 660°C. It is often used for casting due to its low melting point and castability.