Then the number of degrees of freedom (F) of the system is related to the number of components (C) and the number of phases (P) as follows:

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2 PHASE RULE Let the equilibrium between a number of phases be not influenced by gravity, electrical or magnetic forces, but only by pressure, temperature and concentration. Then the number of degrees of freedom (F) of the system is related to the number of components (C) and the number of phases (P) as follows: F = C - P + 2 2

3 Phase : Air a mixture of N 2, O 2, CO 2 etc. But it forms a single gaseous phase Two immiscible liquids form two liquid phases and a vapour phase e.g. Benzene-water Two completely miscible liquids form one liquid phase and one vapour phase. e.g. alcohol-water Every solid forms a separate single phase 3

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5 Component Component is the minimum number of independently variable constituents by means of which the composition of each phase can be expressed in the form of a chemical equation Let us consider the equilibrium : ice (s) water (l) vapour (g) There are three phases. All have the same chemical composition namely H 2 O. The component of the system is one 5

6 Thermal decomposition of CaCO 3 : It is a two component system CaCO 3(s) CaO (s) + CO 2(g) The composition of each phase can be expressed in the form of a chemical equation using two constituents. When CaCO 3 and CO 2 are components, Phase Component CaCO 3 = CaCO 3 +0 CO 2 CaO = CaCO 3 - CO 2 CO 2 = 0 CaCO 3 + CO 2 PCl 5(s) PCl 3(l) + Cl 2(g) is a two component system 6

7 Degree of Freedom It is the minimum number of independent variables such as pressure, temperature and concentration that must be fixed to define the system completely. Let us consider the equilibrium : ice (s) water (l) vapour (g) The degree of freedom of the system is zero. The system is non-variant. 7

8 Phase Diagram Phase diagram is the graph obtained by plotting one degree of freedom against another. When a phase diagram is plotted between temperature and pressure, it is called P-T diagram. It is used for one component system. When a phase diagram is plotted between temperature and composition, it is called T-C diagram. It is used for two component system. 8

9 Uses of Phase Diagram We can predict from phase diagram whether a eutectic mixture or solid solution or compound is formed on cooling a liquid mixture of two metals. We can understand the properties of materials in a heterogeneous equilibrium system. A study of low melting eutectic alloys used in soldering can be made using phase diagram. 9

10 Application of Phase rule to Water system Water system is a one component system. There are three equilibria: ice (s) water (l) water (l) vapour (g) ice (s) vapour (g) The phase diagram contains curves, areas and a triple point. 10

11 Pressure Critical pressure (218 atm) C A O B Temperature (374 o C) Critical Temperature 11

12 Curves Along the vapourisation curve OA, water and vapour are in equilibrium. water (l) vapour (g) Along the sublimation curve OB, ice and vapour are in equilibrium. ice (s) vapour (g) Along the melting point curve OC, ice and water are in equilibrium. ice (s) water (l) 12

13 Along all the curves, two phases are in equilibrium. Applying phase rule, F = C - P + 2 = = 1 The degree of freedom is one or univariant. To define any point along the curve, it is enough to mention either pressure or temperature. 13

14 Areas Areas AOC, BOC, AOB represent water, ice and vapour respectively. In any area, only one phase is present. Applying phase rule, F = C - P + 2 = = 2 To define any point in an area, we have to mention both pressure and temperature. 14

15 Triple Point The three curves OA, OB and OC meet at the tripe point O. At O point ice, water and vapour are in equilibrium, ice (s) water (l) vapour (g ) Applying phase rule, F = C P+2 = = 0 The degree of freedom is zero. The triple point is self-defined. Temperature and pressure at O are C and 4.58mm respectively. 15

16 Reduced or Condensed phase rule A solid-liquid equilibrium of an alloy has practically no vapour phase and the effect of pressure is negligible. The experiment is regarded as being conducted at constant atmospheric pressure. Since the vapour phase is neglected, the system is called a condensed system. As pressure is kept constant, the phase rule becomes F' = C - P

17 Classification of a two component system Two component system is classified into three types based on solubility and reactivity Simple eutectic formation. Compound formation with congruent melting point. Compound formation with incongruent melting point Formation of solid solution 17

18 Differences Melting point : It is the temperature at which a solid and its liquid phase, having the same composition are at equilibrium. Solid A liquid A Eutectic point : It is the temperature at which two solids A and B and a liquid phase are at equilibrium. Solid A + Solid B Liquid 18

19 Triple point : It is the temperature at which three phases namely, solid, liquid and vapour are in equilibrium. Solid A liquid A vapour A Eutectic point is a melting point. But melting point need not be eutectic point. 19

20 Pb Ag Pb Simple eutectic formation Ag Mg Compound formation 20 Mg

21 ii) a) Compound formation with congruent melting point Here the two component system of two solids form one or more compounds with definite composition. If the compound melts at a constant temperature, with the same composition in the solid and molten state, the compound has a congruent melting point. b) Compound formation with incongruent melting point If the compound formed in a two component solid system, decomposes at a temperature below its melting point, the compound has incongruent melting point. Also there is formation of a new solid phase with a different composition from that of the original. 21

22 Use of Cooling curves From cooling curves, melting points and eutectic points are obtained. Behaviour of compounds is understood. Percentage purity of compounds is determined. The composition corresponding to the freezing point gives the composition of an alloy. 22

23 Simple eutectic system or lead-silver system A B O 23

24 i) Curve AO: A is the freezing point of pure silver. As lead is gradually added to silver, melting point of silver decreases along AO. Solid Ag and liquid melt are in equilibrium along AO. Solid Ag melt Applying reduced phase rule, F' = C - P + 1 = F' = 1 Along AO the system is univariant. 24

25 ii) Curve BO: B is the freezing point of pure lead. As silver is gradually added to lead, the melting point of lead decreases along BO. Solid lead and liquid melt are in equilibrium along BO. Applying reduced phase rule, Solid Pb melt F' = C - P + 1 = ; F' = 1 Along BO, the system is univariant. 25

26 iii) Eutectic point O: The curves AO and BO meet at the point O. The temperature at O is 303 C. At O, solid Pb Solid Ag and their liquid are at equilibrium. Applying reduced phase rule, At O, the system is non-variant. F' = C - P + 1 = = 0 The eutectic point O is the lowest temperature at which any mixture of silver and lead will melt. Below this temperature, liquid phase cannot exist. The eutectic composition is 2.6% Ag and 97.4% Pb. 26

27 iv) Areas : In the area below curve AO, solid Ag and liquid melt are present. In the area below BO solid Pb and liquid melt are present. Applying reduced phase rule, F' = C - P + 1 = = 1 At any point in these areas, the system is univariant. In the area above curve AOB only a molten liquid of Pb and Ag exists. Applying reduced phase rule, F' = C - P + 1 = = 2 At any point above AOB, the system is bivariant. 27

28 v) Applying Pattinson s process in the desilverisation of argentiferrous lead Argentiferrous lead contains about 0.1% silver. It is melted to liquid state. Let us consider the melt at the point p as in the figure. If the melt is allowed to cool, Pb begins to crystallize at q. The solution becomes richer and richer in silver. If cooled further, more lead separates along BO. The melt becomes richer in Ag. When the eutectic point is reached the percentage of Ag rises to 2.6%. The process of raising the percentage of lead in the alloy is Pattinson s process. 28

29 Uses of eutectic systems Study of eutectic system helps to predict suitable alloy composition. Eutectic system is useful to prepare solders, used to join two metal pieces. 29

30 Merits of phase rule The rule is applied to physical as well as chemical equilibrium. It shows that different systems with the same degree of freedom have similar behavior. We can decide whether the given number of substances can remain in equilibrium or not. We can classify equilibrium states in terms of phases, components and degrees of freedom. 30

31 Limitations of phase rule The rule is applied to only systems in equilibrium. All the phases must be at the same temperature and pressure. Solid and liquid states must not be in finely divided state. Otherwise there may be deviations. External forces like electrical, magnetic and gravitational forces should be absent. Only P, T, C variables are to be considered. 31

32 Temperature ( o C) Zn-Mg Alloy system 600 Solution C Mg 500 Zn A 420 o C Zn Solution 380 o C 300 MgZn 2 + Solution B 590 o C MgZn 2 + Solution E 1 E 2 Mg + Solution 650 o C 347 o C 200 Zn+ MgZn 2 MgZn 2 + Mg 100% Zn % Mg Weight % Mg (Composition) 32 32

33 ALLOYS Alloy Alloy is a homogeneous solid solution of two or more elements, one of which is a metal. Alloy with mercury as a constituent is called amalgam. 33

34 Properties of alloys Alloys are harder and less malleable. They have lower melting point than the component metals. Alloys have low electrical conductivity. They resist corrosion and action of acids. 34

35 Importance or need or purpose or use or advantages of making alloy i) Increasing the hardness of metals In general pure metals are soft, but their alloys are hard. Gold and silver are usually soft. When alloyed with copper they become hard. On addition of 0.5% Arsenic, lead becomes so hard that we can make bullets. 35

36 ii) Lowering the melting point The melting point of a metal decreases on alloying. (e.g.) Wood s metal (alloy of lead bismuth, tin and cadmium) melts at 60.5 C. This is below the melting point of any constituent metal. 36

37 iii) Increasing corrosion resistance Alloying a metal makes it less reactive and helps resist corrosion. (e.g.) Pure iron rusts. When alloyed with chromium, it opposes corrosion. iv) Change in chemical activity The chemical reactivity of a metal can be increased or decreased by alloying. 37

38 v) Change in colour Alloying improves the colour of metal. (e.g.) Brass with 90% Cu and 10% Zn is golden yellow in colour. vi) Casting property Alloying makes a soft, brittle metal into hard, fusible and easily castable. Alloy of lead with 5% tin and 2% antimony is used for casting printing type. 38

39 Function (or) Effect of alloying elements If we add small amounts of Ni, Cr, Mn, V, Mo, W etc., to steel, there is a drastic change in properties like tensile strength, resistance to corrosion, coefficient of expansion etc. The resulting products are called alloy steels or special steels. 39

40 S. Elem ent Effect on properties 1. Ni Fine grain formation, low coefficient of expansion, high corrosion resistance. 2. Cr High tensible strength, hardening, high corrosion resistance. Use Making balance wheels. Making surgical instruments, cutlery 3. Mn High abrasion resistance Making grinding wheel steering, spindles, rails 4. V Production of reversible stress, increase in tensile strength and abrasion resistance 5. Mb Cutting hardness at high temperature 6. W Increase in magnetic retention and cutting hardness Making axles, crank pin, piston and locomotive forgings Making high speed stools Making cutting tools, permanent magnets 40

41 Heat treatment of alloys (steel) Definition : It is the process of heating and cooling steel under carefully controlled conditions. 41

42 Purpose or objectives of heat treatment To improve corrosion resistance To remove internal stress and strain To improve electrical and magnetic properties To remove trapped gases To refine grain structure To reduce brittleness and increase toughness and ductility. 42

43 Various types (methods) of Heat treatment of alloys (steel). There are several types 1. Annealing 2. Hardening 3. Tempering 4. Normalizing 5. Carburizing 6. Nitriding 7. Cyaniding 43

44 1. Annealing The alloy or steel is heated to a high temperature and then slowly cooled in a furnace. Purpose To increase machinability To remove trapped gases 44

45 Types of annealing Low temperature annealing High temperature annealing. 45

46 Low temperature annealing The steel is heated to a temperature below the lower critical temperature and then cooled slowly. Purpose To increase machinability To reduce hardness To increase ductility and shock-resistance To remove internal stress and strain. 46

47 High temperature annealing The steel is heated to 30 to 50 C above the higher critical temperature. It is held at that temperature for sufficient time to allow for internal changes. Then it is cooled to room temperature. Purpose To make steel soft with increase in toughness. To increase ductility and machinability 47

48 2. Hardening (Quenching) The steel is heated beyond the critical temperature and then cooled suddenly in oil or brine-water. This process increases the hardness of steel. Purpose To increase abrasion resistance for making cutting tools. To increase resistance to wear. 48

49 3. Tempering Already hardened steel is heated to a temperature below its hardening temperature and cooled slowly. To retain strength and hardness, reheating temperature should not exceed 400 C. To increase ductility and toughness, reheating temperature must be in the range 400 to 600 C. Purpose To increase ductility and toughness and to reduce brittleness. To make cutting tools, blades etc. To remove internal stress and strain. 49

50 4. Normalising Steel is heated to a temperature above its higher critical temperature and cooled gradually in air. Purpose To make steel suitable for engineering works. To have homogeneity in structure with refined grains. To remove internal strain and stress To increase toughness. 50

51 5. Carburising Mild steel article is placed in a cast iron box containing pieces of charcoal. It is heated to C and kept at the temperature for sufficient time. Carbon is absorbed to required depth. The article is cooled slowly within the iron box. The outer part of the of the article becomes a high-carbon steel with 0.8 to 1.2% carbon. 51

52 6. Nitriding Steel or alloy is heated to about 550 C in pressure of ammonia. Ammonia decomposes to give N 2 which combines with the surface of alloy to form hard nitride. Purpose To form super hard surface. 7. Cyaniding: Steel is immersed in molten salt bath containing KCN at 1120K and then quenched in oil or water. Purpose to get hard surface. 52

53 Classification (or) types of alloys Based on the type of base metal, alloys are classified into ferrous and non-ferrous alloys. Properties of Ferrous alloys They have high strength and yield point. The possess ductility and weldability. They are corrosion and abrasion resistant. They have less cracks and distortion. 53

54 Nichrome Important ferrous alloys It is an alloy with 60% nickel, 12% chromium, 25% iron and 2% Manganese. Properties It has high resistance to oxidation and heat. It has high melting point. It possesses high electrical resistance. It withstands heat up to 1100 C. Use To make heating elements and resistance coils. To make machineries exposed to high temperature. Used in electric irons other household electrical equipments. To manufacture boiler parts, steam lines stills, retort, aeroengine valves, gas-turbines. 54

55 Stainless steel It is an alloy steel containing chromium with other elements like Nickel, Molybdenum. It is corrosion resistant due to the formation of a tough film of chromium oxide at the surface. There are two types of stainless steels. a. Heat treatable stainless steel b. Non-heat treatable stainless steel 55

56 a. Heat treatable stainless steel This steel contains up to 1.2% carbon, less than 12-16% of chromium. Properties It is magnetic, tough and resistant to weather and water. It can be heated up to 800 C. Use Making surgical instruments, scissors, blades etc. 56

57 b. Non-heat treatable stainless steel It has less strength at high temperature. Corrosion resistance is more. There are two types. i) Magnetic type Composition 12-20% chromium and 0.35% carbon. Properties It has very high corrosion resistance. It can be forged and rolled. Use To make chemical equipments and automobile parts. 57

58 ii) Non-magnetic type Composition 0.15% carbon, 18-25% chromium, 8 to 20% of nickel. Total percentage of nickel and chromium is above 23%. 18/8 stainless steel It has 18% chromium and 8% nickel and 0.15% carbon. Properties It has extreme resistance to corrosion. The resistance is increased by the addition of a small amount of molybdenum. Use To make household utensils, sinks and surgical instruments. 58

59 Non-ferrous alloys These do not have iron as a constituent. The main constituents of non-ferrous alloys are copper, zinc, lead, tin etc. They have low melting points compared to ferrous alloys. Properties Non-ferrous alloys are characterized by Softness and ductility Attractive colours Low density and coefficient of friction Corrosion resistance 59

60 Important Non-ferrous alloys Copper alloys Brass and Bronze are alloys of copper. Brass Brass is copper alloy with copper and zinc as main constituents. 60

61 Type of Brass Composition Property Use Commercial brass Cu = 90% Zn = 10% Dutch metal Cu = 80% Zn = 20% Cartridge brass Cu = 70% Zn = 30% German Silver Cu = 25 to 50% Zn = 10 to 35% Sn = 5 to 30% i. Golden colour ii. Stronger and harder than Cu i. Golden colour ii. Suitable for drawing and forming operation Soft and ductile cold, - deformed Silver colour, malleable, corrosion resistance Forging, rivet hardwires jeweler Tube, flexible hoses jeweler Condenser tubes, sheets household articles Ornament, bolt nut, utensil, coins Jobin brass Cu = 59 to 62% Zn = 35 to 38% Sn = 0.5 to 1.5% High corrosion and abrasion resistance Propeller and marine works 61

62 Bronze Bronze is a copper alloy with copper and tin as main constituents. Bronze has lower melting point than steel. It has better heat and electricity conducting property than steel. It has good corrosion resistance. 62

63 Type of Bronze Coinage Bronze Composition Property Use Cu = 89-92% Sn = 8-11% Gun metal Cu = 85% Zn = 4% Sn = 8% Pb = 3% Aluminium bronze Phosphorus Bronze Cu = 90-93% Al = 7-10% Cu = 87-90% Sn = 10-13% P = 0.4-1% Soft, ductile Hard and tough, resistant to explosive forces. Yellow colour, fusible, resistant to corrosion and abrasion. Hard, brittle, low coefficient of friction Pump, valve, coin, statue wire, utensils Foundry work, heavy load bearing, steam plant Jewellery, utensil, coin bushes Bearing, gear, tap, spring turbine, blade. Nickel bronze Cu = 90% Ni = 9% Fe = 1% Hard, good tensile strength, corrosion resistant Valves, semi-hard bearings. 63