Phase Diagrams Binary Eutectoid Systems Iron-Iron-Carbide Phase Diagram Steels and Cast Iron 1

What is Phase? The term ‘phase’ refers to a separate and identifiable state of matter in which a given substance may exist. Applicable to both crystalline and non-crystalline materials An important refractory oxide silica is able to exist as three crystalline phases, quartz, tridymite and cristobalite, as well as a non-crystalline phase, silica glass, and as molten silica Every pure material is considered to be a phase, so also is every solid, liquid, and gaseous solution For example, the sugar–water syrup solution is one phase, and solid sugar is another 2

Introduction to Phase Diagram There is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram It is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium Provides valuable information about melting, casting, crystallization, and other phenomena 3

4 When we combine two elements... what equilibrium state do we get? In particular, if we specify... --a composition (e.g., wt% Cu - wt% Ni), and --a temperature (T ) then... How many phases do we get? What is the composition of each phase? How much of each phase do we get? ISSUES TO ADDRESS... Phase B Phase A Nickel atom Copper atom

Solubility Limit At some specific temperature, there is a maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution, which is called as Solubility Limit The addition of solute in excess of this solubility limit results in the formation of another compound that has a distinctly different composition This solubility limit depends on the temperature 5

Solubility Limit Sugar-Water 6

Microstructure the structure of a prepared surface of material as revealed by a microscope above 25× magnification The microstructure of a material can strongly influence properties such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature behavior, wear resistance, etc 7

8 Components: The elements or compounds which are present in the mixture (e.g., Al and Cu) Phases: The physically and chemically distinct material regions that result (e.g.,  and  ). Aluminum- Copper Alloy Components and Phases  (darker phase)  (lighter phase)

9 Effect of T & Composition (C o ) Changing T can change # of phases: D (100°C,90) 2 phases B (100°C,70) 1 phase path A to B. Changing C o can change # of phases: path B to D. A (20°C,70) 2 phases Temperature (°C) CoCo =Composition (wt% sugar) L ( liquid solution i.e., syrup) L (liquid) + S (solid sugar) water- sugar system

PHASE EQUILIBRIA Free Energy -> a function of the internal energy of a system, and also the disorder of the atoms or molecules (or entropy) A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition 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 10

Unary Phase Diagram Three externally controllable parameters that will affect phase structure: temperature, pressure, and composition The simplest type of phase diagram to understand is that for a one-component system, in which composition is held constant Pure water exists in three phases: solid, liquid and vapor 11

Pressure-Temperature Diagram (Water) Each of the phases will exist under equilibrium conditions over the temperature–pressure ranges of its corresponding area The three curves (aO, bO, and cO) are phase boundaries; at any point on one of these curves, the two phases on either side of the curve are in equilibrium with one another Point on a P–T phase diagram where three phases are in equilibrium, is called a triple point 12

Binary Phase Diagrams A phase diagram in which temperature and composition are variable parameters, and pressure is held constant—normally 1atm Binary phase diagrams are maps that represent the relationships between temperature and the compositions and quantities of phases at equilibrium, which influence the microstructure of an alloy. Many microstructures develop from phase transformations, the changes that occur when the temperature is altered 13

14 Phase Equilibria Crystal Structure electroneg r (nm) NiFCC CuFCC Both have the same crystal structure (FCC) and have similar electronegativities and atomic radii (W. Hume – Rothery rules) suggesting high mutual solubility. Simple solution system (e.g., Ni-Cu solution) Ni and Cu are totally miscible in all proportions.

15 Phase Diagrams Indicate phases as function of T, C ompos, and Press. For this course: -binary systems: just 2 components. -independent variables: T and C o (P = 1 atm is almost always used). Phase Diagram for Cu-Ni system 2 phases: L (liquid)  (FCC solid solution) 3 phase fields: L L +   wt% Ni T(°C) L (liquid)  (FCC solid solution) L +  liquidus solidus

16 wt% Ni T(°C) L (liquid)  (FCC solid solution) L +  liquidus solidus Cu-Ni phase diagram Phase Diagrams : # and types of phases Rule 1: If we know T and C o, then we know: --the number and types of phases present. Examples: A(1100°C, 60): 1 phase:  B(1250°C, 35): 2 phases: L +  B (1250°C,35) A(1100°C,60)

17 wt% Ni T(°C) L (liquid)  (solid) L +  liquidus solidus L +  Cu-Ni system Phase Diagrams : composition of phases Rule 2: If we know T and C o, then we know: --the composition of each phase. Examples: T A A 35 C o 32 C L At T A = 1320°C: Only Liquid (L) C L = C o ( = 35 wt% Ni) At T B = 1250°C: Both  and L C L = C liquidus ( = 32 wt% Ni here) C  = C solidus ( = 43 wt% Ni here) At T D = 1190°C: Only Solid (  ) C  = C o ( = 35 wt% Ni) C o = 35 wt% Ni B T B D T D tie line 4 C  3

18 Rule 3: If we know T and C o, then we know: --the amount of each phase (given in wt%). Examples: At T A : Only Liquid (L) W L = 100 wt%, W  = 0 At T D : Only Solid (  ) W L = 0, W  = 100 wt% C o = 35 wt% Ni Phase Diagrams : weight fractions of phases wt% Ni T(°C) L (liquid)  (solid) L +  liquidus solidus L +  Cu-Ni system T A A 35 C o 32 C L B T B D T D tie line 4 C  3 R S At T B : Both  and L = 27 wt% WLWL  S R+S WW  R R+S

19 Tie line – connects the phases in equilibrium with each other - essentially an isotherm The Lever Rule How much of each phase? Think of it as a lever (teeter-totter) MLML MM RS wt% Ni T(°C) L (liquid)  (solid) L +  liquidus solidus L +  B T B tie line C o C L C  S R

20 wt% Ni L (liquid)  (solid) L +  L +  T(°C) A 35 C o L: 35wt%Ni Cu-Ni system Phase diagram: Cu-Ni system. System is: --binary i.e., 2 components: Cu and Ni. --isomorphous i.e., complete solubility of one component in another;  phase field extends from 0 to 100 wt% Ni. Consider C o = 35 wt%Ni. Ex: Cooling in a Cu-Ni Binary  :43 wt% Ni L: 32 wt% Ni L: 24 wt% Ni  :36 wt% Ni B  : 46 wt% Ni L: 35 wt% Ni C D E 24 36

21 C  changes as we solidify. Cu-Ni case: Fast rate of cooling: Cored structure Slow rate of cooling: Equilibrium structure First  to solidify has C  = 46 wt% Ni. Last  to solidify has C  = 35 wt% Ni. Cored vs Equilibrium Phases First  to solidify: 46 wt% Ni Uniform C  : 35 wt% Ni Last  to solidify: < 35 wt% Ni

22 Mechanical Properties: Cu-Ni System Effect of solid solution strengthening on: --Tensile strength (TS)--Ductility (%EL,%AR) --Peak as a function of C o --Min. as a function of C o Tensile Strength (MPa) Composition, wt% Ni Cu Ni TS for pure Ni TS for pure Cu Elongation (%EL) Composition, wt% Ni Cu Ni %EL for pure Ni %EL for pure Cu

Eutectic System A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition 23

24 : Min. melting T E 2 components has a special composition with a min. melting T. Binary-Eutectic Systems Eutectic transition L(C E )  (C  E ) +  (C  E ) 3 single phase regions (L,  ) Limited solubility:  : mostly Cu  : mostly Ag T E : No liquid below T E C E composition Ex.: Cu-Ag system Cu-Ag system L (liquid)  L +  L+    CoCo,wt% Ag T(°C) CECE TETE °C

25 L+  L+   +  200 T(°C) 18.3 C, wt% Sn L (liquid)  183°C  For a 40 wt% Sn - 60 wt% Pb alloy at 150°C, find... --the phases present: Pb-Sn system EX: Pb-Sn Eutectic System (1)  +  --compositions of phases: C O = 40 wt% Sn --the relative amount of each phase: CoCo 11 CC 99 CC S R C  = 11 wt% Sn C  = 99 wt% Sn W  = C  - C O C  - C  = = = 67 wt% S R+SR+S = W  = C O - C  C  - C  = R R+SR+S = = 33 wt% =

26 L+   +  200 T(°C) C, wt% Sn L (liquid)   L+  183°C For a 40 wt% Sn - 60 wt% Pb alloy at 220°C, find... --the phases present: Pb-Sn system EX: Pb-Sn Eutectic System (2)  + L --compositions of phases: C O = 40 wt% Sn --the relative amount of each phase: W  = C L - C O C L - C  = = 6 29 = 21 wt% W L = C O - C  C L - C  = = 79 wt% 40 CoCo 46 CLCL 17 CC 220 S R C  = 17 wt% Sn C L = 46 wt% Sn

27 C o < 2 wt% Sn Result: --at extreme ends --polycrystal of  grains i.e., only one solid phase. Microstructures in Eutectic Systems: I 0 L +  200 T(°C) CoCo,wt% Sn CoCo L  30  +  400 (room T solubility limit) TETE (Pb-Sn System)  L L: C o wt% Sn  : C o wt% Sn

28 2 wt% Sn < C o < 18.3 wt% Sn Result:  Initially liquid +   then  alone   finally two phases   polycrystal  fine  -phase inclusions Microstructures in Eutectic Systems: II Pb-Sn system L +  200 T(°C) CoCo,wt% Sn CoCo L  30  +  400 (sol. limit at T E ) TETE 2 (sol. limit at T room ) L  L: C o wt% Sn    : C o wt% Sn

29 C o = C E Result: Eutectic microstructure (lamellar structure) --alternating layers (lamellae) of  and  crystals. Microstructures in Eutectic Systems: III 160  m Micrograph of Pb-Sn eutectic microstructure Pb-Sn system LL  200 T(°C) C, wt% Sn L   L+  183°C 40 TETE 18.3  : 18.3 wt%Sn 97.8  : 97.8 wt% Sn CECE 61.9 L: C o wt% Sn

30 Lamellar Eutectic Structure

wt% Sn < C o < 61.9 wt% Sn Result:  crystals and a eutectic microstructure Microstructures in Eutectic Systems (Pb-Sn): IV SR 97.8 S R primary  eutectic   WLWL = (1-W  ) = 50 wt% C  = 18.3 wt% Sn CLCL = 61.9 wt% Sn S R +S W  = = 50 wt% Just above T E : Just below T E : C  = 18.3 wt% Sn C  = 97.8 wt% Sn S R +S W  = = 73 wt% W  = 27 wt% Pb-Sn system L+  200 T(°C) C o, wt% Sn L   L+  40  +  TETE L: C o wt% Sn L  L 

32 L+  L+   +  200 C o, wt% Sn L   TETE 40 (Pb-Sn System) Hypoeutectic & Hypereutectic 160  m eutectic micro-constituent hypereutectic: (illustration only)       175  m       hypoeutectic: C o = 50 wt% Sn T(°C) 61.9 eutectic eutectic: C o = 61.9 wt% Sn