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1、NucleationnNucleation is the very initial stage of phase transformation during which small embryos of the new phase(or phases)become large enough to grow by themselves.nNucleation has both a thermodynamic driving force and a thermodynamic barrier.MT321:Principles of Materials ProcessingLecture 4:Nuc
2、leation of Solidification1nIn this lecture,we learn the classical theory of nucleation of single element solid from single element liquid.MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification2Thermodynamic Driving Force of NucleationnWhen T=Tm,GS=GL,both the liquid and the s
3、olid are stable phases,and there is no thermodynamic driving force for solidification of the liquid or melting of the solid.(Tm is the melting point of the solid,and GS and GL are the free energies of the solid and the liquid respectively.)MT321:Principles of Materials ProcessingLecture 4:Nucleation
4、 of Solidification3nWhen T Tm,GS Tm,GS GL,the liquid is a stable phase,but the solid is not,so there will be a tendency for the solid to melt.nSo the thermodynamic driving force of nucleation of solidification is the free energy difference of solid and liquid,DG=GS GL.nDG needs to be negative to hav
5、e the thermodynamic driving force.MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification4nBased on the learning on thermodynamics,we know:GS=HS TSS(1a)GL=HL TSL(1b)(HS and HL are the enthalpies of the solid and the liquid respectively,SS and SL are the entropies of the solid
6、and the liquid respectively.)nFrom equations(1a)and(1b),we can get:DGS=DHS TDSS(2)DGS=GS GL is the change of free energy due to solidification DHS=HS-HL is the change of enthalpy due to solidification(DHS is also called latent heat of solidification)DSS=SS SL is the change of enthalpy due to solidif
7、ication.MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification5MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification6MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification7The Thermodynamic Barrier of Nucleation of Solidificati
8、onNucleation of a solid crystal from a liquid requires rearrangement of atoms into ordered crystal structure,so introduces new solid/liquid interfaces.The introduction of the new solid/liquid interface which has interfacial energy,AsSL,adds to the free energy of the solid/liquid system,and makes it
9、higher than the original free energy of the starting liquid.(A is the surface area of the crystal.)This free energy increase caused by the interfacial energy of the new solid/liquid interface is the thermodynamic barrier of nucleation of solidification.MT321:Principles of Materials ProcessingLecture
10、 4:Nucleation of Solidification8MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification9The structure of a liquid close to or below the melting pointInstantaneous embryoTypes of NucleationHomogeneous nucleation:A solid phase nucleates within a liquid without any assistance of
11、foreign solid surfaces.Heterogeneous nucleation:A solid phase nucleates on the surface of a foreign solid that is in contact with the liquid.MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification10CrystalsLiquidHomogeneousNucleationCrystalsInclusionsLiquidHeterogeneousNucleat
12、ionHomogeneous NucleationMT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification11(6)(7)MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification12The free energy change as a function of the embryo radius r during homogeneous nucleationqDifferentiating DG
13、with respect to r,we have:(9)qFrom equations(8)and(9)and the graph,we can make the following observation:When r is very small,DG0,which means that the free energy of the solid embryo is higher than that of the liquid.When r is very small,d(DG)/dr0,which means that DG increases with increasing the si
14、ze of the embryo.MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification13MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification14MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification15nBy substituting equation(11)into equation(
15、8),we have:nFrom Equation(12),we can make the following observation:qThe barrier of nucleation decreases with decreasing temperature,once T 0.15wt%,the active grain refiner is Al3Ti which is formed through peritectic transformation during cooling of the liquid.When Ti 0.15wt%,the Ti addition can als
16、o refine aluminium grains.This is attributed to(a)the effect of Ti solute on crystal growth of Al and (b)formation of TiC by reacting Ti in the liquid and CO2 in air.Very often TiB2 particles as well as Ti are added together to aluminium melt to maximise the grain refinement effect.The grain refinem
17、ent effect of some grain refiners diminishes with time when the melt is held at high temperature for a long time.This is called Fading of grain refiners.MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification31MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification32MT321:Principles of Materials ProcessingLecture 4:Nucleation of Solidification33Effects of pouring temperature and the content of Ti on the grain sizes of aluminium cast ingot.
金属材料热处理原理：Solidification and Casting-2016-lecture 3
