Metals and Alloys Flashcards
Types of metal cubic structures
Cubic - corners have atoms
Face centred cubic - corners and faces have atoms
Body centred cubic- corners have atoms and connected one atom in the centre of the cube
Definition of a Metal
an aggregate of atoms in a crystalline structure
Definition of an alloy
combination of metal atoms in a crystalline structure (metals are the building blocks of these)
Describe the parts of the cooling curve
upper descending curve- molten metal
Flat line - liquid > solid
Lower descending curve - Cooling
What are metal nuclei
aggregates of metal atoms in little spheres that form during cooling
What do nuclei turn into and then what do they turn into after that
Dendrites
Grains
What is the area where grains touch called
Grain boundary
Name the three types of grains and what defines them
Equi-axed grains- grains of equal size (gradual cooling)
Radial grains- formed from rapid cooling (quenching)
Fibrous- formed when grains undergo cold working
How to alter grains
Quenching (rapid cooling)
Slow cooling (large nuclei, coarse grains)
Nucleating agents - act as a foci for crystal growth
What are the pros and cons of small grains
pros - high elastic limit, increased ultimate tensile strength and hardness
cons- decreased ductility
What is slip (clue dislocation of columns)
- defect in metal grain
- force applied
- defect moves along an axis parallel to that force
- reaches grain boundary
- crystal structure changes with no defect
Describe Cold working
- done at temperatures below recrystallisation (altering existing grains)
- causes slip
- Increases = elastic limit, UTS and hardness
- Decreases = ductility, corrosion resistance, impact resistance
Residual stress (as a result of cold working)
- Cold working helps some properties but results in imperfection in the lattice
- results in undesirable distortion
- relieved by annealing
What terms are used to describe the metallic components of alloy grains
Phase- physically distinct homogenous structure
- One phase = grains of metal A only
- Two phase = Individual grains of A + B in lattice network
Solution- homogenous mixture at atomic level
- Solid solution = One phase but Metal A + B in homogenous mixture
What is Liquidus
temperature the alloy begins to crystallise at
What is solidus
temperature where it solidifies (completely crystallised)
What is the difference between metals and alloys in terms of crystallisation
metals crystallise at one temperature
alloys crystallise over a temperature range
What is Coring and how is it rectified
- It happens when an alloy is cooled rapidly
- results in a concentration being formed from the centre of an alloy to the periphery
- rectified by using a homogenising anneal
Why is slip more difficult in alloys than metals
- because the metal atoms have to climb over and through atoms of different sizes instead of ones the same size as it
- the slip plane in alloys is much less easier to roll over than in metals
What is a Eutectic Alloy and what makes it unusual
An alloy that melts at a temperature higher than that of its individual metals
i.e together stronger
Therefore it breaks the rule of alloys having a temperature melting range, it instead has one melting temperature
