Materials- Metals: Non-ferrous Alloys Flashcards
Limitations of ferrous alloys
Relatively high density
Comparatively low electrical conductivity
An inherent susceptibility to corrosion in some common environments
What are cast alloys?
Alloys that are so brittle that forming or shaping by appreciable deformation is not possible and are therefore generally cast
What are wrought alloys?
Alloys that are amenable to mechanical deformation
What are heat-treatable alloys?
Alloys whose mechanical strength is improved by precipitation hardening or a martensitic transformation both of which involve specific heat-treating procedures
General properties and uses of aluminium
Low melting temperature (660). Lower density than steel. High electrical and thermal conductivities, resistance to corrosion in common environments, easily formed, high ductility even at very low temperatures (due to FCC).
Aircraft structural parts, beverage cans, automotive parts.
Which is more abundant in earth’s crust, iron or aluminium?
Aluminium
Aluminium extraction process
Extracted from bauxite (mixed oxide ore containing about 50% aluminium). Separated from other oxides by dissolution and reprecipitation by reaction with NaOH (Bayer process). Then dissolved in molten salt mixture of cryolite (Na3AlF6) and electrolytically reduced to molten aluminium (Hall-Herodotus process). 14000kWh power consumed per tonne of aluminium produced.
On the Al-Cu phase diagram, what is formed to the right of the right vertical line?
An intermetallic of Al2Cu
What happens during equilibrium cooling of aluminium alloy with 4% wt copper?
A few nuclei start to form as the liquidus line is crossed and have a small wt% copper. These grow as the temperature decreases and become higher in wt% copper and so does liquid phase. Solidification is complete when the solidus line is crossed to form α phase where the wt% copper has increased to 4% throughout.
What happens during non-equilibrium cooling of aluminium alloy with 4wt% copper?
Nuclei begin to form when liquidus line crossed with small wt% copper. Atoms diffuse quicker in the liquid phase than in solid so when the nuclei grow, the outer parts of the grains have a higher wt% copper than the central parts. Forms a core-shell structure in the grains. The liquid phase is still increasing in wt% copper. The solidus line has effectively been made steeper so melting temperature lowered. The outer layers have more than 4wt% copper by the end.
Describe homogenisation
After non-equilibrium solidification m the alloy is heated to a temperature below the eutectic temperature and held for a long time to remove the eutectic structure and homogenise the solute content. This is all through atomic diffusion and produces compositionally homogeneous grains.
