Steels, Polymers and Phase Diagrams Flashcards
Decribe free energy change during nucleation, to include:
- Initial assumptions
- Contributions to free energy change
- Description of graphical representation of cluster growth
- Assume that the temperature is such that the solid phase is more stable (has a lower free energy) than the liquid phase
- ΔG = ΔGs + ΔGv * (4πr^3)/3 where ΔGv is the change in volumetric free energy and ΔGs is the the change in surface free energy. ΔGv is always negative as liquid is changing to solid, and solid is more stable than the liquid at that temp.
- ΔGs = 4π(r^2)γ, is always positive as surface area is growing so less stable.
- ΔGs(r) looks like is a positive quadratic through the origin, ΔGv(r) is a negative cubic through the origin, therefore at first the superposition i.e. overall free energy, is positive and increasing as ΔGs dominates, but past the critical radius (at a maximum free energy) ΔGv is more negative than ΔGs is positive, so ΔG starts to decrease and eventually become more and more negative as the radius increases and the particle gets bigger.
- The maximum ΔG, or activation free energy, is the free energy required for the formation of a stable nucleus.
Why is processing steel important?
Processing e.g. work hardening (roller applying strain to metal), temp. over time shapes the microstructure, i.e. the size and distribution of grains and phases, which controls the physical properties such as strength and ductility.
Describe the solidification of a pure metal.
- Well above melting point, all liquid
- Temp starts to lower, still all liquid until temp reaches melting point
- At melting point, solid crystals start to form. If pure metal, temp remains constant until all metal converted from liquid to solid (thermal arrest).
Describe the solidification of single phase alloys.
- v high temp, all liquid
- as soon as you cool to the melting point of one of the phases, solid crystals start to form.
- as cooling continues, more grains formed, so liquid to solid transformation occurs over a range of temperatures.
- liquidus line is the temp above which all the phases are liquid, solidus line is temp below which all phases are solid
Describe the formation of Pearlite
a) Initial cementite (Fe3C) nucleus forms at gamma grain boundary
b) Cementite plate grows and α now nucleates
c) α plate grows, forming a layered structure of cementite and ferrite.
d) Cementite nucleates but new cementite plate grows with a different orientation
Overall structure is called pearlite.
What effect does adding carbon to iron have on microstructure?
Mix of ferrite and pearlite phase
As carbon content increases, pearlite content increases
As pearlite structure is stronger than ferrite structure, adding carbon leads to higher tensile strength and hardness, and lower ductility.
After 0.8 wt% carbon added, pure cementite phase starts to form instead of eutectic microstructure, so pearlite content decreases.
What is undercooling?
Difference between current temp and eutectic temp.
What is hardenability?
Ability of a steel to form martensite (based on carbon content)
Measure of critical cooling rate to produce martensite from austenite.
If you increase hardenability, you decrease the critical cooling rate to still get martensite so can form thicker sections and lower spec. heat cap. coolant e.g. oil not water or air not oil (cools slower so less risk of cracking).
How can you increase hardenability?
- Add alloying elements:
- low alloy steels w/ few % of Cr, Ni and Mb
- Alloy element slows down the rate of diffusion of carbon, favours the the production of non-equilibirum structure
What are the general physical properties of ceramics?
- Low density compared to the pure metals from which they are made.
- High Young’s modulus
- Low thermal expansion
- High melting point
- Generally brittle
- Usually high strength but dependent on microstructure and defects
- Sometimes ferrimagnetic (spins on electrons not aligned in a particular direction like in ferromagnetic materials so can cancel each othr out, leading to weaker magnetic properties than ferromagnetic materials)
- Generally high corrosion resistance but many important exceptions
- Thermal conductivity usually low
How are ceramics processed?
- ceramic powder synthesised
- powder shaped into desired form
- sintering used to consolidate powder into dense product
- finishing (secondary processing) to arrive at finished sintered ceramic product
What are the three classes of high perfomance ceramics?
Oxides, carbides and nitrides
Main uses of oxide-based ceramics?
Silica used in glasses etc. good thermal insulator, used as abrasive, used in optical fibres can be used in paits or reinforcements for tyres.
Alumina used as high temp. tubes, applications where high strength and high temp resistance required.
Zirconia used to manufacture oxygen sensors, can be used in jewlery
Main uses of carbide based ceramics?
High temp applications e.g. brake discs made from SiC
Tungsten carbide (WC) v. stiff and v. dense
Main uses of nitride based ceramics?
Si3N4 used in bearings, less dense but can handle v. high temps.
TiN used to harden and protect surfaces (5 micron thick coatings)
