Why do cocktail ice served in expensive restaurants are clear whereas the ice formed in your refrigerator is cloudy?

Transcription

1 Phase Diagrams

2 Why do cocktail ice served in expensive restaurants are clear whereas the ice formed in your refrigerator is cloudy? What is a solder alloy? What is the best composition for solder? How is ultrpure Si for computer chips produced? d

3 Melting point of an alloy Liquid, L 1453ºC L ºC Solid solution, Cu Wt% Ni Ni

4 Equilibrium i phase diagram or Equilibrium i diagram or Phase diagram A diagram in the space of relevant thermodynamic variables (e.g., T and x) indicating phases in equilibrium is called a phase diagram.

5 Components The independent chemical species (element or compound) in terms of which the composition of the system is described are called components. System Water Water +ice shikanji components H 2 O H 2 O nimbu, chini and pani phases liquid Liquid+solid liquid solution Mild steel Fe + C + Fe 3 C

6 A single component phase diagram: Unary diagram A two-component phase diagram: Binary diagram A three component phase diagram: A three-component phase diagram: Ternary diagram

7 Cu-Ni binary phase diagram Any given point (x,t) on the phase diagram represents an alloy of composition x held at equilibrium at temperature T Point A: 60 wt% Ni at 1100ºC Point B: 35 wt% Ni at 1250ºC Callister, Fig. 9.2

8 Phase Diagrams For any given point (x,t) the phase diagram can answer the following: 1. What phases are present? 2. What are the phase compositions? ons? 3. What are the relative amounts of the phases (phase proportions or phase fractions)?

9 Point A: 60 wt% Ni at 1100ºC Q: Phase present? Ans: Q: Phase composition? Ans: 60 wt%ni Q: Phase amount? Ans: 100%

10 Point B: 35 wt% Ni at 1250ºC Q: Phases present? Ans: + L Q: Phase compositions? Tie Line Rule Q: Phase amounts? Lever Rule

11 Composition of phases in the two-phase region L Tie Line Rule Tie Line C L = 31.5 wt% Ni C = 42.5 wt% Ni

12 Amount of phases in the two-phase region L Tie-Line: A lever Alloy composition C 0 : Fulcrum f L : weight at liquidus point f : weight at solidus point The lever is balanced f f L f L 1 ( C0 CL) f ( C C0) f L f 1 Tie Lever Rule f L C C C C 0 L opposite lever arm total lever arm

13 The Lever Rule: A Mass balance Proof Prob. 7.6 L Wt of alloy = W Wt of in alloy = f W Wt of L in alloy = f L W Wt of Ni in alloy = W C/100 0 Wt of Ni in = f WC /100 Wt of Ni in L = f L WC L /100 Wt of Ni in alloy = Wt of Ni in + Wt of Ni in L C f + C L f L = C 0 f + f L = 1 f L C C C C 0 L opposite lever arm total lever arm

14 Development of Microstructure during solidification f f L f Single phase polycrystalline

15 Solder alloy? An alloy of Pb and Sn What is best composition of the solder alloy? Requirements: 1. should melt easily 2. should give a strong joint

16 Solder alloy rule Eutectic diagram 327 L 183 L Eutectic horizontal L 232 Eutectic point Pb Eutectic temperature 62 Wt % Sn Sn Eutectic composition

17 Pb: monatomic fcc Sn: monatomic bct : Pb rich substitutional solid solution of Pb and Sn crystal structure: monatomic FCC : Sn rich substitutional st tut solid solution of Pb and Sn crystal structure: t monatomic BCT

18 Woods metal tea party Bi wt% Pb 25.0 wt% Cd 12.5 wt% Sn 12.5 wt% An eutectic alloy with m.p. of 70ºC 100 g US$ 181 Anti-Fire Sprinklers

19 Eutectic reaction Invariant reaction cool L 183ºC wt%sn wt%sn wt%sn Eutectic mixture Callister Figs. 9.11, X

20 L (40 wt% Sn) L L 18 wt% Sn L+ L (62 wt% Sn) Eutectic mixture + Primary 18 wt% Sn Eutectic (18 wt % Sn) Eutectic (97 wt % Sn) Hypoeutectic alloy Microstructure of hypoeutectic alloy

21 Eutectc mixture Proeutectic or Primary f total Amount of total and total at a temperature just below 183ºC Tie line just below 183ºC (red) f total

22 Eutectc mixture Proeutectic or Primary f eut Amount of proeutectic at a temperature just below 183ºC = Amount of at a temperature just above 183ºC Tie line just above 183ºC (green) f pro

23 Eutectc mixture EXPERIMENT 5 Proeutectic or Primary C 51 0 wt% Sn Let the fraction of proeutectic in micrograph f pro = 0.25 Let the composition (wt% Sn) of the alloy be C 0 Tie line just above 183ºC (green) f pro 62 C

24 Optimum composition for solders For electronic application Eutectic solder 62 wt% Sn Minimum i heating For general application Cheaper Hypoeutectic solder Allows adjustment of joint during j j g solidification in the L range

25 Modern Trend Lead-free solders Phase diagrams can help in identification of such solders Sn-Ag-Cu

26 Please collect your Minor I answer books from Lab in the afternoon Those who can, do. Those who can t teach GB G.B. Shaw

27 Gibbs Phase Rule Thermodynamic variables: P, T, Phase Compositions (overall composition is not considered) If there are C components then C-1 compositions have to be specified for each phase Therefore total number of composition variables: P (C-1) With Pressure and Temperature total number of With Pressure and Temperature, total number of variables = P (C-1) + 2

28 Gibbs phase rule states that one cannot specify all of the above P (C-1) + 2 variables independently in a system at equilibrium Degrees of Freedom F: No. of thermodynamic variables that can be specified independently

29 Gibbs Phase Rule F = Degrees of freedom C = No. of components in the system P = No. of phases in equlibrium F = C P + 2 If pressure and temp both are variables F = C P + 1 If pressure is held constant

30 F = C P + 1 C=2 F =2 F=3-P At eutectic reaction P=3 (L,, ) F =1 F=0 Invariant reaction

31 The Iron-carbon system Mild steel wt% C Bicycle frame Ship hull Car body Medium C steel wt% C 910 steel Cast iron 725 Rail wheel rail axle rails High C steel wt% C R bl d Razor blades scissors, knives

32 Phases in Fe-C system Phase Symbol Description Liquid L Liquid solution of Fe and C -Ferrite Austenite Interstitial solid solution of C in -Fe (high temperature bcc phase) Interstitial solid solution of C in -Fe (FCC phase of Fe) Ferrite Interstitial solid solution of C in -Fe (room temperature bcc phase) Soft and Ductile Cementite Fe 3 C Intermetallic compound of Fe and C (orthorhombic h system) Hard and Brittle

33 Ferrite Austenite

34 Invariant Reactions in Fe-C system A horizontal line always indicates an invariant i reaction in binary phase diagrams Peritectic Reaction 1493 C (0.1 wt % C ) L (0.5 wt % C ) (0.18 wt % C ) Eutectic Reaction o L o 1150 C ( 4.3 wt% C) (2.1 wt% C) Fe3C (6.67 wt% C) Eutectoid Reaction o 725 C (0 0.8 wt % C ) ( wt % C ) Fe3 C ( wt % C )

35 Eutectoid t Reaction o Fe 725 C cool Pearlite C

36 Eutectoid Reaction o Fe 725 C cool Pearlite C Ammount of Fe 3 C in Pearlite Red Tie Line below eutectoid temp f F C pearlite

37 Development of Microstructure in a hypoeutectoid steel

38 Proeutectoid Ferrite EXPERIMENT 5 Pearlite Microsructure of a hypoeutectoid steel, 0.38 wt% C

39 f pearlite f pearlite below T E = f austenite above T E Tie-Line above the eutectoid temperature T E f pearlite

40 Development of Microstructure in a hypereutectoid steel

41 Pearlite Proeutectoid cementite on prior austenite grain boundaries Microsructure of a hypereutectoid steel, 1.4 wt% C

42 F proeutectoid cementite =f cementite above T E f proeutectoid cementite

43 Phase vs. microconstituents A phase or a mixture of phases which has a distinct identity in a microstructure is called a microconstituent Pearlite is not a phase. It is microconstituent which is a mixture of two phases and Fe 3 C.

44 Eutectoid steel +Fe 3 C Hypoutectoid steel +Fe 3 C Pearlite Pearlite + proeutectoid ferrite Hypereutectoid steel +Fe 3 C Pearlite + proeutectoid cementite

45 T T m A L Principle of Zone Refining T C < CL +LL A C C 0 C L Wt % B

46 SemiconductorTransistor was invented by Bardeen, Brattain and Shockley At AT&T Bell Labs One needs ultrapure Si (impurity level few ppm) Zone Refining was invented by Pfann at Bell Labs as a process to obtain ultrapure Si Basis for modern Si technology

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