Phase diagrams. R.D.Makwana,IT,NU. R.D.Makwana,IT,NU

Transcription

1 Phase diagrams

2 Phase?? Phase is region throughout which all properties of material are essentially uniform Uniform region of a system which has uniform physical and chemical characteristics

3 Phase diagram? Diagram showing phases of material with temperature, pressure, composition as variables Phase diagram of water

4 Solubility limit

5 Phase diagram- sugar water syrup

6 Phase equilibrium free energy is a function of the internal energy of a system, and also the randomness or disorder of the atoms or molecules A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition. In a macroscopic sense, this means that the characteristics of the system do not change with time but persist indefinitely; that is, the system is stable. A change in temperature, pressure, and/or composition for a system in equilibrium will result in an increase in the free energy and in a possible spontaneous change to another state whereby the free energy is lowered.

7 One component diagrams (Unary system) Binary diagrams

8 Why to study phase diagrams? Wires made of Cu-Ni Vessels made of Cu-Ni to reduce corrosion Different composition-different properties

9 Cooling curves

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11 Interpretation of phase diagrams

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15 Composition- weight percentage Phase Melting point Solidus line Liquidus line

16 Composition Weight percentage (%wt) Atomic percentage(%at)

17 Derive equation for conversion of wt% to at% Determine the composition in atom percent of an alloy that consists of 97 wt% aluminum and 3 wt% copper, Atomic weight of Cu and Al is gm/mol and gm/mol respectively

18 Gibbs phase rule P+F = C+N P=no.of phases F=Degree of freedom(independent variables which must be specified to completely define state of the system) C=no. of components N=no.of non compositional variables(pressure and temperature)

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21 Gibbs phase rule: Cu-Ni system 1.Single phase F = 2 2. Two phase F = 1

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23 What to get from phase diagram? Phases Phase composition Microstructure Amount of phases

24 Microstructure Many times, the physical properties and, in particular, the mechanical behavior of a material depend on the microstructure In metal alloys, microstructure is characterized by the number of phases present, their proportions, and the manner in which they are distributed or arranged. The microstructure of an alloy depends on such variables as the alloying elements present, their concentrations, and the heat treatment of the alloy

25 Phases amount determination The tie line Lever rule

26 1. The tie line is constructed across the two-phase region at the temperature of the alloy. 2. The overall alloy composition is located on the tie line. 3. The fraction of one phase is computed by taking the length of tie line from the overall alloy composition to the phase boundary for the other phase, and dividing by the total tie line length. 4. The fraction of the other phase is determined in the same manner. 5. If phase percentages are desired, each phase fraction is multiplied by 100. When the composition axis is scaled in weight percent, the phase fractions computed using the lever rule are mass fractions the mass (or weight) of a specific phase divided by the total alloy mass (or weight). The mass of each phase is computed from the product of each phase fraction and the total alloy mass.

27 Derivation of lever rule Exercise Find out amount of solid and liquid for the given diagram at the given point. C L =31.5 wt% Ni, C α =42.5 wt% Ni, C 0 =35 wt% Ni

28 Non Equilibrium cooling Equilibrium is diffusion dependent Diffusion is time dependent With change in temperature composition of solid-liquid changes System requires time to achieve equilibrium

29 Microstructures in Cu-Ni phase diagram an isomorphous system Equilibrium cooling

30 Microstructures in Cu-Ni phase diagram an isomorphous system NonEquilibrium cooling

31 Mechanical properties of isomorphous system

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33 Eutectic reaction Eutectic- < Greek eútēkt (os) easily melted, dissolved ( eu- eu- + tēktósmelted)

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35 Copper-Silver system

36 Lead-tin system

37 Solubility limit Liquidus line Solidus line Two solids-two phases Hypoeutectic hypereutectic

38 At 40 wt% Sn and 150 C determine phases present, composition of phases, amount of phases.

39 Microstructure in eutectic diagram

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46 Finding amount of primary eutectic, alpha and beta. P Q R

47 Take C4 =25, find amount of, β, total and amount of eutectic