Metals Flashcards
Density =
Mass/ Volume (kgm^-3)
Why are metals so dense?
They have a closely packed atomic stucture
What does the stiffness of a metal indicate?
It’s resistance to elastic deformation; indicated by the young modulus.
Are metals, ceramics and polymers high or low YM?
Metals and ceramics tend to be high; whereas polymers tend to be low.
What does the strength of a material indicate?
The resistance to plastic/ permanent deformation.
Indicated by the yield strength.
The yield strength of metals, ceramics and polymers:
Ceramics have high yield strength; metals vary and polymers are low.
What does the toughness of a material refer to?
The resistance to cracking/ fracture.
Indicated by the fracture energy.
Would low toughness materials not be suitable for tension or compression?
Tension (maybe compression)
What does the conductivity of a material refer to?
How easily heat can pass through a material
Materials with higher conductivity tend to be…
Metals.
Benefits of metals…
They tend to be low cost materials
What is the mechanical behaviour of metals?
They are strong; do not deform easily and can take high loads.
When they do deform it is progressive.
Elastic deformation of metals:
When a load is applied the atomic bonds will stretch; when removed they do not return (not permanent)
Plastic deformation of metals:
When load is increased; atoms slip past each other and when the load is removed they remain in that position.
Dislocations of atoms in a metal:
Atoms will not slip a layer at a time but singularly. This happens because it requires a lot of energy to slip a whole layer.
What happens with dislocations and how it deforms?
For permanent deformation it requires a dislocation to move.
Slip allows for…
Ductility and toughness
Strengthening mechanisms for dislocations?
- Atoms in solution (alloys metal)
- Add another phase
- Other dislocations get tangled with each other
- Structure changes at grain boundary/ orientation.
What does ferrous mean and what metals have ferrous?
Indicates the presences of iron
Iron (Fe)
Steel (Fe +C)
Cast iron
High levels of carbon. Carbon phases lead to low strength and toughness in tension.
What would be the problem with cast iron joints in a structure?
Lead to collapse as there would be a stress concentration
What is steel?
Alloy of carbon and iron
How is steel processed?
- Heating an iron ore with coke and limestone in a blast furnace.
- Liquid metal and slag is removed every few hours.
- Adding oxygen reduces carbon content
Rolling metals =
This can squeeze out porosity and refine the microstructure
Forging =
Pressing the metal into shape
Extrusion =
Taking larger pieces of metal and thinning it out into a long rod.
Phases in steel:
- Austetine
- Ferrite
- Cementite
- Pearlite
- Martensite
Transformation from steel to pearlite:
Mixture of ferrite and cementite Fe3c; these grow alternating plates giving a strong structure and resistant to crack so tough.
Transformation to martensite:
Cooling quickly means that ferrite and cementite cannot form properly so the structure becomes distorted as carbon wont fit into bcc structure in ferrite. This causes a hard and brittle metal that doesn’t allow dislocations to get through.
Heat treatment of steel:
- Annealing; high temp
- slow cooling gives pearlite
- Quenching = fast cooling gives martensite
- Tempering = low temp allows carbides to form
Wrought steel =
Shaped by deformation; carbon is kept for weldability and higher strength. High strength low alloy = reduction in grain size. Produces carbide and nitride also strengthens.
Cast steel =
Casting steel directly into shape; cheaper and easier but poorer mechanical properties.
Phase changes cause martensite to be produced at surface.
Stainless steel =
Produces protective oxide surface, used for aesthetic reason as long-life.
Non-ferrous metals =
All the other metals; more expensive than iron based.
Aluminium =
Lightweight; alloyed for strength, susceptible to degradation by heat.
Titanium =
High strength; used in aerostructures
Ti-6Al-4V most common
Copper =
Easy to form; oxidation gives colour. Not used for primary load.
