3.012 Fundamentals of Materials Science Fall 2005 Lecture 18: 11.09.05 Binary systems: miscibility gaps and eutectics Today: LAST TIME .........................................................................................................................................................................................2 Free energy and phase diagrams of ideal binary solutions ......................................................................................................2 BINARY SOLUTIONS WITH LIMITED MISCIBILITY IN THE SOLID STATE: MISCIBILITY GAPS ...........................................................4 The Regular Solution Model, part I ............................................................................................................................................4 Phase diagram of a regular solution ..........................................................................................................................................7 EUTECTIC SYSTEMS ..........................................................................................................................................................................8 .........................................................................................................11 Free energy diagrams of eutectic systems..................................................................................................................................8 Analyzing phase equilibria on eutectic phase diagrams Example eutectic systems ..........................................................................................................................................................12 REFERENCES ...................................................................................................................................................................................14 Reading: Lupis, Chemical Thermodynamics of Materials, Ch VII, ‘Binary Phase Diagrams,’ pp. 204-219 Supplementary Reading: Lecture 19 – Binary systems 1 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 Last time Free energy and phase diagrams of ideal binary solutions Key points: 1. Common tangents to free energy curves define composition regions where phase separation (two-phase equilibria) occurs. 2. Phase separation lowers the overall free energy by splitting the homogenous system into a weighted mix of two separate phases, which each have lower free energy than the starting homogeneous phase. 3. Compositions of the phases in two-phase regions are given by the tangent points, and the amount of each phase is determined by the lever rule. • Two-phase equilibrium introduces the phase fraction, determined by the lever rule: ! fS= mole fraction solid in whole system =nAS+ nBSnA+ nBfL= mole fraction liquid in whole system =nAL+ nBLnA+ nBfS+ fL= 1Lecture 19 – Binary systems 2 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 • Free energy diagrams directly relate to binary phase diagrams: Key points: 1. Free energy diagrams define the structure of the phase diagram. 2. Compositions of two-phase regions, which are tangent points on the free energy diagram, become phase boundaries on the phase diagram. 3. The lever rule is directly applied to tie lines on the phase diagram, to determine the amount of each phase present in two-phase regions. o The ideal solution is a special case-not all materials form binary solutions that are miscible at all compositions and all temperatures, particularly in the solid state. Materials that have incompatible crystal structures cannot form homogeneous solid solutions-they are forced to phase separate into an A-rich solid phase and a B-rich solid phase, to preserve their stable crystalline forms. Today we will examine the phase diagrams of these and other common binary materials systems. Lecture 19 – Binary systems 3 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 Binary solutions with limited miscibility in the solid state: Miscibility gaps The Regular Solution Model, part I • What happens if the molecules in the so! "H mix< 0lution interact with a finite energy? The entha! "H mix> 0lpy of mixing will now have a finite value, either favoring ( ) or disfavoring ( ) mixing of the two components. The simplest model of a solution with finite interactions is called the regular solution model: o Let the enthalpy of mixing take on a finite value given by: o We take the entropy of mixing to be the same as in the ideal solution. This gives a total free energy of mixing which is: Lecture 19 – Binary systems 4 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 o We can also use the equations above to determine the enthalpy, entropy, and Gibbs free energy of the regular solution: ! "G mix,rs= G rs# G unmixedG rs= "G mix,rs+ G unmixed= $XAXB+ RT XAln XA+ XBln XB[ ]+µAoXA+µBoXB! S rs= "#G rs#T$ % & ' ( ) P,n= "R XAln XA+ XBln XB[ ]! H rs= G rs+ TS rs= "XAXB+µAoXA+µBoXBo The regular solution model describes the liquid phase of many real systems such as Pb-Sn, Ga-Sb, and Tl-Sn, and some solid solutions. Today we will analyze the behavior of a system with this free energy function; in a few lectures we will show how the given forms of the enthalpy and entropy of mixing arise from consideration of molecular states (using statistical mechanics). o Let’s look at the behavior of regular solutions graphically. The plots below are all prepared for a binary system where the standard state chemical potentials are assumed to be approximately invariant with temperature (i.e., the endpoints of the free energy curves are not moving with temperature-approximately valid over certain temp. ranges for some solids): ! µAo= "2,000JmoleµBo= "1,000JmoleLecture 19 – Binary systems 5 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 • The free energy of the system varies both with the value of Ω and with temperature: o As a function of temperature at a fixed positive value of Ω: Lecture 19 – Binary systems 6 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 Phase diagram of a regular solution XB !TXB!Gmix,RSXB!Gmix,RSXB!Gmix,RSLecture 19 – Binary systems 7 of 14 11/9/053.012 Fundamentals of Materials Science Fall 2005 Eutectic systems • It is commonly found that many materials are highly miscible in the liquid state, but have very limited mutual miscibility in the solid state. Thus much of the phase diagram at
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