浙江大学材料热力学与动力学.ppt

Chapter 1 : Thermodynamics and Phase Diagrams,1-1,Chapter 1 : Thermodynamics and Phase Diagrams,1-2,1.0.1 Thermodynamics Systems,Chapter 1 : Thermodynamics and Phase Diagrams,1-3,Open System can exchange mass, heat and work with its surroundings Closed System no mass exchange, heat and work exchange possible Isolated System no mass, no heat, no work exchange,Chapter 1 : Thermodynamics and Phase Diagrams,1-4,A. Energy, Heat and Work,The energy of an isolated system is constant The work done on a thermally isolated system is independent of the type of work and the route,1.0.2 The First Law of Thermodynamics,Chapter 1 : Thermodynamics and Phase Diagrams,1-5,Chapter 1 : Thermodynamics and Phase Diagrams,1-6,Chapter 1 : Thermodynamics and Phase Diagrams,1-7,Chapter 1 : Thermodynamics and Phase Diagrams,1-8,Chapter 1 : Thermodynamics and Phase Diagrams,1-9,1.0.3 The Second Law of Thermodynamics,Chapter 1 : Thermodynamics and Phase Diagrams,1-10,Chapter 1 : Thermodynamics and Phase Diagrams,1-11,2 parts of DS (inner and external),DSinner : from inner processes DSexner : due to heat exchange,DSexter,reversible process : DS i = 0, DS = DS e irreversible process : DS i 0, DS DS e adiabatic process : DS e = 0, DS = DS i irr. adiabatic process : DS 0 rev. adiabatic process : DS = 0,Chapter 1 : Thermodynamics and Phase Diagrams,1-12,The Second Law of Thermodynamics,The entropy of a closed system can not decrease.,Clausius: Heat can not flow automatically from cold side to hot side.,perpetual machines,type I,Planck: Such a process is impossible if its only result were to exchange heat to work.,type II,Chapter 1 : Thermodynamics and Phase Diagrams,1-13,Mr. Tompkins in Paperback G. Gamow Cambridge University Press, 1965,Heat can not flow automatically from cold side to hot side !,Chapter 1 : Thermodynamics and Phase Diagrams,1-14,1.1 Equilibrium in a Closed System,Chapter 1 : Thermodynamics and Phase Diagrams,1-15,How is system stability measured ? by its Gibbs free energy (at const. T and P),G = H - TS,(1.1),H : a measure of the heat content of the system ( H = U + PV ) S : a measure of the randomness of the system,low T : TS small, solids are most stable (strongest atomic binding, low H) high T : TS dominates, liquids or gases are stable (atoms more free, high S),Chapter 1 : Thermodynamics and Phase Diagrams,1-16,Stable, Metastable and Unstable,G,dG = 0,B,A,C,dG = 0,dG = 0,an arbitrary state parameter,B,A,C,Stable: graphite, single crystal silicon,Metastable: diamond, amorphous,Unstable: super-cooling liquid (nucleation),B,A,C,Chapter 1 : Thermodynamics and Phase Diagrams,1-17,Possibility and Realizability : Thermodynamics and Kinetics,G,B,A,C,G1,G2,Energy Hump,DG = G2 G1 0 : only possible for the transformation from B to A Amorphous Alloy (short-lived), Diamond (long-lived) Temperature : kinetic key (vibration frequency and amplitude),Chapter 1 : Thermodynamics and Phase Diagrams,1-18,Chapter 1 : Thermodynamics and Phase Diagrams,1-19,1.2 Single Component Systems,Chapter 1 : Thermodynamics and Phase Diagrams,1-20,Chapter 1 : Thermodynamics and Phase Diagrams,1-21,Chapter 1 : Thermodynamics and Phase Diagrams,1-22,Hsolid,Hliquid,Gsolid,Gliquid,T (K),H, G,298,at all temperature: H liquid H solid since G = H - TS G liquid G solid at low T G liquid G solid at high T,Tm,DHm,at Tm : H liquid - H solid = DHm G liquid = G solid,solid stable,liquid stable,H,G,Chapter 1 : Thermodynamics and Phase Diagrams,1-23,Chapter 1 : Thermodynamics and Phase Diagrams,1-24,Tm,T,DG,G,GS,GL,Free energies,at Tm,1.2.3 The Driving Force for Solidification,G L = H L - TS L,G S = H S - TS S,DG = DH - TDS = 0,for most metals L R (8.3 J mol-1K-1),at T with small DT , ( Tm - T = DT ),can be ignored, DH & DS independent on T,difference on,Chapter 1 : Thermodynamics and Phase Diagrams,1-25,1.3 Binary Solutions,1.3.1 The Gibbs Free Energy of Binary Solutions,XA mole A,XB mole B,XA + XB = 1,MIX,1 mole A+B,G1 = GAXA + GBXB,G2 = G1 + DGmix,DGmix : mixing free energy,DGmix = DHmix - TDSmix,Chapter 1 : Thermodynamics and Phase Diagrams,1-26,1.3.2 Ideal Solutions,DHmix = 0 DSmix = -R ( XAlnXA + XBlnXB ) DGmix = RT ( XAlnXA + XBlnXB ),Note: Since XA and XB are 1, DSmix is positive, DGmix is negative.,XB,0,1,Molar free energy G,GA,GB,G 0,DGmix,At higher temperature,Low T,High T,mixing free energy DGmix,XB,0,1,the absolute free energy is not of interest!,Chapter 1 : Thermodynamics and Phase Diagrams,1-27,1.3.3 Chemical Potential,Multi-Component System,dnA,constant T and P,total free energy of the system : G G + dG if dnA small enough, dG proportional to dnA, or : dG = mAdnA,Definition: Chemical potential , or Partial molar free energy,Note: G: total free energy of the system G: molar free energy (one mol),Chapter 1 : Thermodynamics and Phase Diagrams,1-28,geometric meaning of chemical potential,A,B,XB,mA,mB,G,GB,GA,RT lnXA for ideal solution,RT lnXB for ideal solution,G,tangent line at XB,Chapter 1 : Thermodynamics and Phase Diagrams,1-29,Chapter 1 : Thermodynamics and Phase Diagrams,1-30,A-A,B-B,Before mixing,B,B,A,A,A-B,B,A,B,A,A-B,After mixing,eAA + eBB,2eAB,Energy change per A-B bond : e = eAB - (eAA + eBB) AB bonds per mol : PAB = Na z XAXB Therefore : DHmix = w XAXB , where w = Na ze,0,DHmix,XB,1,w,Chapter 1 : Thermodynamics and Phase Diagrams,1-31,Chapter 1 : Thermodynamics and Phase Diagrams,1-32,Chapter 1 : Thermodynamics and Phase Diagrams,1-33,Chapter 1 : Thermodynamics and Phase Diagrams,1-34,Chapter 1 : Thermodynamics and Phase Diagrams,1-35,1.4 Equilibrium in Heterogeneous Systems,Chapter 1 : Thermodynamics and Phase Diagrams,1-36,Chapter 1 : Thermodynamics and Phase Diagrams,1-37,Heterogeneous Equilibrium,Condition of Equilibrium in a Heterogeneous System continue : about the common tangent line,A,B,a,P,and :,Chemical potentials of component A and B in a phase with the composition of,and :,Chemical potentials of component A and B in b phase with the composition of,b,Q,Chapter 1 : Thermodynamics and Phase Diagrams,1-38,1.5 Binary phase diagrams,Chapter 1 : Thermodynamics and Phase Diagrams,1-39,Chapter 1 : Thermodynamics and Phase Diagrams,1-40,Chapter 1 : Thermodynamics and Phase Diagrams,1-41,1.5.4 Fe-Si System,Chapter 1 : Thermodynamics and Phase Diagrams,1-42,1.5.5 Fe-C System,Chapter 1 : Thermodynamics and Phase Diagrams,1-43,1.6.1 The solid solvus,Equilibrium T1 :,For dilute solution of B in a :,1.6 Thermodynamics and Phase Diagrams,Chapter 1 : Thermodynamics and Phase Diagrams,1-44,Gibbs-Helmhotz equation:,Enthalpy change when 1 mol b-form B atoms dissolve in a phase to make a dilute solution,DHB : enthalpy difference between the b-form B and a-form B W : energy change of a-form B dissolved into a phase,Therefore:,Chapter 1 : Thermodynamics and Phase Diagrams,1-45,Chapter 1 : Thermodynamics and Phase Diagrams,1-46,Chapter 1 : Thermodynamics and Phase Diagrams,1-47,Chapter 1 : Thermodynamics and Phase Diagrams,1-48,Chapter 1 : Thermodynamics and Phase Diagrams,1-49,1.7 Thermodynamics during Phase Transformations,Chapter 1 : Thermodynamics and Phase Diagrams,1-50,The driving force of solidification,Y0,Liquid,Solid a,L,S,P, T1,G,x,x,xL,xS,Q,Lever Rule,xS,xL,1 mol x, GP,initial,after equilibrium,xS mol xS, GS,xL mol xL, GL,DG = GP - GQ,DG : per mol solution not per mol solid nuclei,R,xS mol xS, GR,DG = 0,DG1 = GR - GS,DG1 : per mol solid nuclei,Free energy change for 1 mol solution: DG1xS = DG,Chapter 1 : Thermodynamics and Phase Diagrams,1-51,The driving force of nucleation,Y0,Liquid,Solid a,L,S,P, T1,G,x,x,xL,xS,Q,During nucleation, xL will not change. This means DG1 = GR - GS is not the nucleation driving force.,If we move point L to P, we see the free energy per mol nuclei will be at point T (insect point of the tangent line Y at P with xS).,R,T,Y,Chapter 1 : Thermodynamics and Phase Diagrams,1-52,The driving force of nucleation,Y0,Liquid,Solid a,L,S,P, T1,G,x,x,xL,xS,Q,R,T,Y,DG2 = GT - GS is the driving force per mol nuclei with the composition of xS.,But the composition of initial nuclei could differ from xS, if larger driving force available other than xS.,T,S,x,Example: nuclei with the composition of x have a driving force of GT - GS , which is larger than those at xS.,What is the maximal driving force ?,Chapter 1 : Thermodynamics and Phase Diagrams,1-53,The maximal driving force,Drawing a tangent line on the liquid