Lecture 10-11 Flashcards
Where is the eutectic point on a phase diagram?
The V that is created where the composition of the material has a lower melting point than originally
Eutectoid reaction is
where one solid phase forming two other solid phases
Solvus lines separate the
single phase alpha and beta regions with the two phase alpha + beta region
Lamellae are formed when what soldifies
Eutectic alloys
A lamellae is
grains made up of alternating layers of alpha and beta
Where on the phase diagrams are hypoeutectic alloys found?
to the lower end from the eutectic point
Where on the phase diagrams are hypereutectic alloys found?
to the upper end from the eutectic point
3 step and result of microstructure evolution for a hypoeutectic alloy
- Primary alpha phase with state beginning in liquidus
- Primary alpha phase begins to solidify
- Eutectic microstructure/lamellae layers form around the primary alpha phase
Result; alpha + beta layers surrounding the alpha solids
Similarly to a pure substance, an alloy with eutectic composition solidifies…
at one temperature until it has changed state fully
Similarly to a standard alloy, an alloy with hypereutectic or hypoeutectic composition solidifies…
What happens at the eutectic temp?
over a temperature range depending on composition.
at the eutectic temp the remaining liquid solidifies
Strengthening means to increase the
yield and tensile strength of a metal
Four main methods of strengthening for alloys:
- Increased number of dislocations
- Presence of grain boundaries
- Presence of solute atoms
- Presence of second phase particles
Two main methods of strengthening for pure metals:
- Increased number of dislocations
- Presence of grain boundaries
Work hardening is aka
cold working
Work hardening is
plastically deforming a metal
Work hardening pro and con
Increases strength, BUT reduces ductility
Work hardening takes place during which processes, name 5
- Rolling
- Forging
- Drawing
- Extrusion
- Forming
An elongated grain structure is called
anisotropic
If a metal is stressed into the plastic region and the load is removed the —- —– is recovered, but permanent —— has occurred
elastic strain, deformation
As the number of dislocations in a metal increased, the dislocations become
harder to slip due to neighbouring grains impeding the slip
σ y = σ o + k y X d -1/2
What does each letter signify?
- σ o and k y are constants for the material
- σ y is the yield stress
- d is the avg grain diameter
In Solid Solution Strengthening what is preferred about the property of the solute atoms? Why?
A large difference in the size of the atoms to disrupt the uniform layers
Four pros and two cons for solid solution strengthening?
Pros:
- increase in yield strength
- increase in tensile strength
- increase in hardness
- increase in creep resistance
Cons:
- reduced ductility
- reduced electrical conductivity
Dispersion strengthening to strengthen a material is when
there are second phase particles or regions in amongst the matrix which reduces the ease of slippage of dislocations
What is an improved form of dispersion hardening?
Age hardening (precipitation)
Pro about age hardening?
second phase particles are more highly dispersed
Two preferred properties of the precipitate particles used in dispersion strengthening. Why for each?
- Precipitate particles should be small and numerous -> maximise interference with slip
- Precipitate particles should be round rather than needles -> avoid stress raising
What is not used in dispersion strengthening to make age hardening an enhanced process?
Heat treatment
What is over ageing and how can it come about
Particles grow too large and become less effective at preventing dislocation slip
Higher ageing temps and and longer times
Why can’t precipitation hardened alloys be used in high temp environments?
The second phase particles tend to re-dissolve or grow in size and the strengthening mechanism is lost
Annealing is
the process used to increase the ductility of a metal, typically after cold working.
basically heating
Three stages of annealing are
- Recovery
- Recrystallisation
- Grain growth
Two cons of annealing on metals:
- if the metal is to operate in high temperatures, the metal will anneal slowly becoming weaker and more prone to yielding
- certain metal joining processes heat the local area which can cause local recrystallisation and grain growth so strength in this area is reduced drastically
Recovery in annealing is
(aka stress relief anneal) the process where the original electrical and thermal properties are returned by low temp heating. It does not change the mechanical properties of the metal though
Annealing is not a —– process for metals.
strengthening
Recovery in annealing helps the dislocations in the metal to do what?
Helps them arrange themselves to minimise the interactions they have other dislocations
Annealing does not distort the
grain structure
Recrystallisation is
the process by which a higher temperature is applied for a longer time
Recrystallisation allows the distorted grains in the alloy to be replaced by a set of
strain free, smaller grains
Recrystallisation reduces the number of —– and the metal’s —– .
dislocations, strength
4 things the recrystallisation temperature depends on:
- Amount of cold work already completed
- Original grain size
- Alloy composition and melting temp
- Annealing time
The opposite of grain size strengthening is
grain growth
Annealing to a higher temperature after crystallisation is the process of
grain growth
Grain growth reduces
the number of grain boundaries there are in the metal
As the grain size of the metal —-, the strength of the metal —–.
increases, decreases
