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PHASE DIAGRAMS

PHASE DIAGRAMS. STUDY OF PHASE RELATIONSHIPS IMPORTANT IN KNOWING PROPERTIES OF MATERIALS MAP OF TEMPERATURE, PRESSURE AND COMPOSITION BETWEEN PHASES IN EQUILIBRIUM IN A SYSTEM GIBBS PHASE RULE P + F = C + 2 Eg: states of matter- gas, liquid and solid – single phase

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PHASE DIAGRAMS

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  1. PHASE DIAGRAMS • STUDY OF PHASE RELATIONSHIPS IMPORTANT IN KNOWING PROPERTIES OF MATERIALS • MAP OF TEMPERATURE, PRESSURE AND COMPOSITION BETWEEN PHASES IN EQUILIBRIUM IN A SYSTEM GIBBS PHASE RULE P + F = C + 2 Eg: states of matter- gas, liquid and solid – single phase Liquid mixture- oil and water- 2 phases In solid , several phases depending on crystal structure • STUDY IMPORTANT IN ALLOYS • ALLOY- SUBSTANCE COMPOSED OF 2 OR MORE CHEMICAL ELEMENTS • MAIN CONSTITUENT- BASE METAL AND OTHERS ALLOYING ELEMENTS

  2. FUSION LINE ALMOST VERTICAL- VARIATION IN PRESSURE –NO EFFECT ON M.P. OF ICE Phase Rule FUSION WATER ICE Pressure 76cm B VAPORISATION 30 cm WATER VAPOUR SUBLIMATION T A 0 100 50 Temperature o C

  3. At ‘A’ , water vapour - 1 phase • At ‘B’ , water and water vapour co exist -2 phases • At ‘T’ , ice, water and water vapour exist – 3 phases • At ‘A’ • 1 + F = chemical compound H2O + 2 • F = 2 …. BIVARIANT • At ‘B’ • 2+ F = 1 +2,F = 1… UNIVARIANT • At ‘T’ • All three phasesP = 3, 3 + F = 1 + 2; F = 0 INVARIANT

  4. Equilibrium Diagram Case 1: Binary Alloy with COMPLETE SOLUBILITY IN BOTH LIQUID AND SOLID PHASES in all compositions Eg: Ag-Au Cu-Ni Ge-Si Al2O3-Cr2O3 Sb-Bi Silver-Palladium Co-Ni Cu-Pt Fe-Pt Ni-Pt Ta-Ti HUME ROTHERY’S RULE- FICK’S LAWS OF DIFFUSION FIRST LAW SECOND LAW

  5. Elements A and B in a Binary Alloy Cooling Curves & Phase Diagram

  6. Phase (Equilibrium) Diagram Liquidus curve L + α Solidus curve Composition, C (% wt of B)

  7. LEVER RULE • With Fulcrum atP, weightsWAandWBat the end of a lever, for equilibrium, the lever rule states: WA / WB= b/a WB WA P a b

  8. Liquid P1 P Y X Liquid + Solid Solid 47.5 16 37 58 For P: SS/LS = (37-16)/(58-37)= 1/1 For P1: SS/LS = 31.5/ 10.5= 3/1

  9. Liquid P1 P Y X Liquid + Solid Solid 31.5/ 10.5= 3 47.5 16 37 58 = (37-16)/(58-37)

  10. The structures shown are at NON EQUILIBRIUM CONDITIONS

  11. There areThree variables,one of these can be chosen as independent If fl and fs are the liquid and solid fractions,

  12. Case 2: Binary Alloy with COMPLETE SOLUBILITY IN LIQUID STATE in all compositions, but COMPLETELY INSOLUBLE IN THE SOLID STATE • A very doubtful situation in practice, since most solid metals appear to dissolve small quantities of other metals • In Bismuth-Cadmium, mutal solid solubility is negligible. • Bi- heavy, brittle- positioned near to non metals in periodic table- Rhombic type structure-covalent bond • Cadmium- HCP-

  13. Bismuth- Cadmium Equilibrium Diagram

  14. When two metals show complete solubility in liquid state, and complete insolubility in the solid state,they do so by crystallising out as alternate layers of the two pure metals.This laminated structure termed as EUTECTIC Te 40Cd/60Bi

  15. When two metals show complete solubility in liquid state, and complete insolubility in the solid state, they do so by crystallising out as alternate layers of the two pure metals.This laminated structure termed as EUTECTIC INVARIANT REACTION Te (EUTECTIC Temperature) 40Cd/60Bi

  16. At E, solid Cadmium (40%) and solid Bismuth(60%)co-exist EUTECTIC A: Molten homogeneous alloy – 1 phase with 2 components, Bi and Cd 1+F = 2 +1 (only temperature is the variable, not pressure) , F=2 • B: 2 + F = 2 + 1, F= 1 • C: 3 + F = 2 + 1, F=0

  17. Eutectic is considered as an intimate mixture of two metals Phase Rule applied, P+F = C+ 1 3 + F = 2 +1, F = 0

  18. Cooling curve for Eutectic (similar to pure metal)

  19. For compositions to left /right of Eutectic Temperature o C Time

  20. HUME ROTHERY’S RULE

  21. Gold- Silver, Copper- Nickel, Germanium- Silicon, Antimony- Bismuth, Aluminium Oxide- Chromium Oxide etc. are examples

  22. FICK’S LAWS OF DIFFUSION Mass Flow Process by which atoms (molecules) change their positions relative to their neighbours in a given phase under the influence of thermal energy and gradient :

  23. FICK’S LAWS OF DIFFUSION

  24. FIRST LAW dn/dt = no. of moles of B atoms crossing per unit time D= Diffusion coefficient A= Planar area dc/dx= concentration gradient If J = flux flow / unit area per unit time,

  25. SECOND LAW If D is independent of concentration,

  26. INVARIANT REACTIONS

  27. Case 3: Two metals completely soluble in all proportions in liquid state, but partially soluble in solid state • Melting Point of Lead:3270C • Melting Point of Tin: 2320C • Eutectic Temperature: 1830C • Eutectic Composition: 62% Sn, 38%Pb • Max. solid solubility tin in lead at 1830C: 19.5% tin • Max. Solid solubility of lead in tin at 1830C: 2.6% lead • Eutectic of two solid solutions α and β (instead of two metals) form

  28. Melting Point of Tin (Pb) : 2320C Melting Point of Lead (Sn) :3270C Eutectic Temperature: 1830C Eutectic Composition: 38%Pb, 62% Sn Max. solid solubility tin in lead at 1830C: 19.5% tin Max. Solid solubility of lead in tin at 1830C: 2.6% lead

  29. Liquid solubility of salt in water & partial solid solubility of one metal in another- ( (similarity schematically represented)

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