MATS_15301: Microstructures Flashcards
What phase transformation occurs upon cooling at
Eutectic point?
One liquid phase transforms to two different solid phases, α and β
Where α and β are 2 diff solid solutions
Transformation is spontaneous
What phase transformation occurs upon cooling at
Eutectoid point?
One solid phase turns into two other solid phases
Transformation is spontaneous
What phase transformation occurs upon cooling at
Peritectic point?
A liquid and a solid phase, α, transform to another solid phase, β
Transformation is spontaneous
What phase transformation occurs upon cooling at
Peritectoid point?
Two solid phases, transform to another solid phase
What phase transformation occurs upon cooling at
Monotectic point?
The first liquid phase L1 transforms into another liquid phase L2 and a solid phase
Note the difference between these and the pairs we see in other flashcards
What is the equation for a ternary eutectic reaction?
L → α+β+γ
where L is the liquid phase and α,β,γ are the solid phases
True or False:
The melting temperature Tm, of a eutectic system point is lower than the Tm of the metals that form it?
True
What is the product of a eutectic reaxn called?
Eutectic mixture
What is te degree
What are the phases present in the microstructure of a cooled hypoeutectic alloy?
α and β
What are the microconstituents of the microstructure of a cooled hypoeutectic alloy?
Proeutectic α and eutectic micture (α+β)
What is another name for proeutectic α?
Primary α
In binary eutectic phase diagrams:
What is primary α?
aka ‘proeutectic α’
The alpha phase that is formed during cooling of a hypoeutectic alloy before the eutectic isotherm Te is crossed.
When we say ‘before’, it should be visualised as ‘up until that point’.
What is a solid solution?
A homogenous mixture of two or more elements with a specific composition of each
Note the keyword
What is the mathematical representation of Fick’s first law of Diffusion
J=-D * (dC/dx)
Where J is the diffusion flux, D is the diffusion coefficient dC is the difference in conc btw the two positions, and dx is the distance btw the two positions
What are intrinsic lattice defects?
They exist in semi-equilibrium with the rest of the lattic structure and* their concentration varies with temperature*
What is an example of an intrinsic lattice defect?
Vacany in lattice structure
True or False:
Alloy elements and dislocations are deliberate lattice defects
True
What are the two main models of atomic diffusion within a crystal lattice
Interstitial and Vacancy Diffusion
True or False:
At elevated temperatures there is a higher concentration of vacany in a crystal structure
True
What is the driving force for nucleation?
Gibbs free energy difference between solid and liquid phases
What is the driving force for nucleation?
Energy required for surface to form, surface energy of cluster formed at nucleation site.
In relation to critical size, r*, when does nucleation occur?
When the radius of the cluster is greater than the critical size.
True or False:
Critical volumes are the same for homogenous and heterogenous nucleation
False
The critical volumes for homogenous nucleation is higher than that for heterogenous nucleation
True or False:
Critical radii are the same for homogenous and heterogenous nucleation
True
True or False:
Diffusion occurs much easier over shorter distances than longer ones.
True
What is the most widely used engineering material?
Steel
What is a phase?
A phase is a physically separable region in a system whose thermodynamic properties (e.g. enthalpy, entropy…) are distinguishable from other regions in the system.
What is a phase diagram?
This is a grsaphical representation of the most stable phase at equilibrium under a given set of thermodynamic conditions.
A phase is often a part of a thermodynamic system, however, the entire system can also be one phase
True or False:
Enthalpy is mainly determined by bonding energy in materials
True
The higher the bond energy, the lower the enthalpy
What is metastable equilibrium?
A long-lived (chemical) state that is not the equilibrium state but will require a lot of energy to be converted to the equilibrium state
For a binary isomorphous system:
The weight fraction of what phase increases upon continued heating
Heating from a solid
The Liquid phase
How is a binary isomorphous system different from a binary eutectic system?
In a binary isomorphous system, the two components, A and B are fully soluble in each other in both solid and liquid phases at all temperatures
What is the consequence of non-equilibrium cooling in a binary isomorphous system?
Coring
A binary isomorphous sytem of components A and B has been subject to non-equilibrium cooling conditions. B has a higher melting point than A. As cooling occurs, which of the two would you expect to have a higher concentration in the centre of the grain?
Component B
The element with the higher melting point has a higher concentration in the centre of the grain. This reduces as the grain grows outwards (as coring progresses)
What is the resultant structure of a eutectic reaction on cooling
A lamellar structure of alternating layers of solid solutions α and β
α and β are solid solutions (mixtures) rich in a particular element. They are not the pure form of that element!
For a hyper/hypo-eutectic alloy:
What is the difference between primary α/β and eutectic α/β
Primary α/β is formed on cooling before the liquid reaches eutectic % composition.
If the alloy was always at eutectic % composition, the eutectic reaction occurs with no primary α/β phase formed upon cooling
Which type of diffusion occurs faster in metal alloys?
Vacancy diffusion or Interstitial Diffusion
Interstitial Diffusion
The atoms are smaller and there are more interstitial positions than vacancies so this occurs faster than the other
