Metals Flashcards
What are the most used metals, and how much of them were used in 2018?
Steel: 1,808 million metric tonnes
Alumium: 21.9 million metric tonnes
copper: 14.9 million metric tonnes
gold: 2,600 metric tonnes
When was the first usage of cast iron during the structural revolution?
1779 - Iron bridge, coalbrookdale
What was the first major bridge built with wrought steel?
Forth Railbridge, scotland, 1890
What kind of bonding is present in metals?
Metallic bonds
What are the properties of metallic bonds
Free to move electrons make them malleable and good conductors, usually have high melting and boiling points
What kind of metals are most engineering metals?
Alloys
What do FCC, HCP, BCC dstand for
FCC: Face centered cubic
HCP: hexagonal close packed
BCC: body centered cubic
What metals have an FCC structure?
Aluminium, copper, gold. lead, nickel, platinum, silver
What metals have a BCC structure?
Chromium, Iron (alpha), Molbdenum
What metalls have an HCP structure?
Cobalt, magnesium
What type of imperfections are found in the chrystal structure?
- Substitutional Atoms (solutes)
- Interstital atoms (solutes)
- Dislocations
- Vacancies
- Grain boundaries
What is the definition of a grain boundary?
The interface between adjacent chrystalline regions that have different orientation
What is the definition of a dislocation?
An irregularity within a crystal strucutre that contains an abrupt change in the arrangmement of atoms
What is the difference between interstital and substitutional atoms?
interstital allow for the same number of metal atoms and is nessled into the microstructure,
Substiutional replace one of the metal atoms in the chrystal
What failure are grain boundaries resposible for?
Intergranular fracture
What failure are dislocations responsible for?
Plastic deformation
Why do dislocations cause failures?
Because the stress required to deform a crystal is mich less than the stress calculated from considering a defect-free structure
How do dislocations move through the structure?
- The row of bonds will break and reattatch itself to a diffferent row of bonds
- Much easier for 1 row to bend rather than an entire plane
What causes dislocations to occur?
When a plane of atoms is present that distorts the lattice structure
What is the effect of shear stress on dislocation?
Moves allong in the direction of the imposed stress
How do we hinder the movement of dislocations?
Creating obstacles to dislocation motion
What is the definition of an alloy?
Metal mixed with other elements
What are the properties of an alloy determined by?
The composition of the microstructure and the distribution of the phases
How much carbon and manganese is in steel?
<2% carbon and 1% manganese
Hoe many differrent grades of steel are there?
3,500
What are the benefits of steels?
They are stronger and lighter, more fit for purpouse
Modern cars are stronger and 25% lighter
What is the benefite of a grain boundary?
act as a barrier to dislocation motion
Increases yield stress
How do we increase the yield stress?
Reduce the grain size
What is the Hall-Petch equation?
sigma_y = sigma_o +k/sqrt(d)
sigma_y = yield stress
sigma_o, k = material constants
d = grain size
What does the Hall-Petch equation show?
That we can achieve theoretical max strength of materials by decreasing the grain size
What is work hardening?
- Dislocations have an associated strain field
- This strain field makes a barrier to dislocation motion
- plastic deformation gives increases dislocation density leading to increased interactions and higher strength
- Dislocations increase in density during plastic flow, and tange and pile up
What conclusion does work-hardening allow us to draw?
THere is an ever increasing shear stress required for deformation, this increases the yield stress
What is solid solution strengthening?
Solute atoms have an associated strain field
These interact with the strain field around dislocations and inhibit motion
What is precipitation strengthening/hardening?
- Fine distribution of second phase particles
- have an associated straing field that interacts with that of dislocations
- makes it harder for the dislocation move
- effectively pin and lock dislocations
When using the lever rule how do you find the phase you are interested in?
By using the longest line from the point to the ‘boundary’
