DOBSON+CH10 - PHASE DIAGRAMS When we combine two...

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Unformatted text preview: PHASE DIAGRAMS When we combine two elements, what is the resul3ng equilibrium state? • In par3cular, if we specify... - - the composi3on (e.g., wt% Cu - wt% Ni), and - - the temperature (T ) then... How many phases form? What is the composi3on of each phase? What is the amount of each phase? Components, Systems & Phases •  Components: The elements or compounds which are present in the alloy (e.g., Al and Cu) •  System: a series of alloys, varying in specific composi3on, but not components; a specific body of material under considera3on •  Phases: The physically and chemically dis3nct material regions that form (e.g., α and β). Aluminum- Copper Alloy β (lighter phase) α (darker phase) Components, Systems & Phases Consider the following phase diagram: What is the system? What are the components of the system? What phases are present? Components, Systems & Phases •  What is the equilibrium phase composi3on of the system at 70 wt % sugar and 80 deg. C? •  What is the solubility limit of sugar in water at 20 deg. C? liquid Controlling parameters There are 3 externally controlled parameters that will affect phase structure: Temperature, Composi7on, and Pressure Altering Temperature can change # of phases: path A to B. Altering Composi3on can change # of phases: path B to D. water- sugar system Temperature (ºC) 100 B (100˚C: C = 70%) 1 phase 80 60 L (liquid solu3on 40 i.e., syrup) 20 0 D (100˚C: C = 90%) 2 phases L (liquid) + S (solid sugar) A (20˚C: C = 70%) 2 phases 0 20 40 60 70 80 C = Composi3on (wt% sugar) 100 One- component (Unary) Phase Diagrams •  Simplest Form – one component stays constant (pure substance) •  In phase diagrams, solid lines (phase boundaries) represent the separa3on between two phases. •  As you cross boundary, the phases transform to another •  For condi3ons corresponding to a point on a line, the two (or more) phases are said to be in equilibrium) •  “O” is the triple point, where all three phases are in equilibrium One- component (Unary) Phase Diagrams What is the mel3ng point of ice at 100 atm? At what pressure will water boil at 65 deg. ? - 5° Binary Phase Diagrams • Phase diagrams indicate phases as a func3on of T, C, and P. • For this course: - binary systems: 2 components - independent variables: T and C (P = 1 atm is almost always used) T(˚C) 1600 Phase Diagram for Cu- Ni system 1500 L (liquid) Mel3ng temp of Ni 1400 3 different phase fields: L L + α α 1300 α 1200 Mel3ng temp of Cu (FCC solid solu<on) 1100 1000 0 20 40 60 80 100 wt% Ni Nomenclature Note: For metallic alloys solid solu3on are designated by greek lemers α, β, and γ etc. Isomorphous Binary Phase Diagrams • Phase diagram: Cu- Ni system T(ºC) 1600 • System is: 1500 - - Binary i.e., 2 components: Cu and Ni. - - Isomorphous i.e., complete solubility of one component in another; α phase field extends from 0 to 100 wt% Ni. L (liquid) Cu- Ni phase diagram 1400 1300 α 1200 (FCC solid 1100 1000 solu3on) 0 20 40 60 80 100 wt% Ni Phase Diagrams Three kinds of informa3on are available from Phase Diagram 1.  Which phases are present 2.  The composi3on of the these phases (how much of each component) 3.  The percentages or frac3ons of the phases Determining phases present Rule 1: If we know T and Co, then we know - - which phase(s) is (are) present. A(1100˚C, 60 wt% Ni): 1 phase: α B (1250˚C, 35 wt% Ni): 2 phases: L + α T(˚C) 1600 B (1250˚C 35% Ni) B • Examples: 1500 1400 1300 L (liquid) A(1100˚C, 60% Ni) 1100 1000 α (FCC solid solu3on) 1200 Simply determining which phase is present at a given temperature and composi3on Cu- Ni phase diagram 0 20 40 60 80 100 wt% Ni Determining phases present Rule 2: If we know T and C0, then we can determine - - the composi7on of each phase. Cu- Ni system T(ºC) • Examples: Consider C0 = 35 wt% Ni At TA = 1320˚C: Only Liquid (L) present CL = C0 ( = 35 wt% Ni) At TD = 1190˚C: Only Solid (α) present Cα = C0 = 35 wt% Ni) ( At TB = 1250˚C: Both α and L present CL = C liquidus = 32 wt% Ni) ( Cα = C solidus ( = 43 wt% Ni) TA 1300 TB 1200 TD 20 A L (liquid) Tie Line B D α (solid) 3032 35 40 4 3 CL C0 50 Cα wt% Ni E L 1 Determining phases present Rule 3: If we know T and C0, then we can determine - - the weight frac7on of each phase. • Examples: Consider C0 = 35 wt% Ni At TA : Only Liquid (L) present W L = 1.00, Wα = 0 At TD : Only Solid ( α ) present WL = 0, W α = 1.00 Cu- Ni system T(˚C) A TA 1300 L (liquid) B TB 1200 TD 20 D 3 032 35 CL C0 α (solid) 4 0 4 3 5 0 Cα wt% Ni The Lever Rule Tie line – Overall alloy composi3on - connects the phases in equilibrium with each other – also some3mes called an isotherm. T(ºC) 1300 L (liquid) B T B 1200 20 What fraction of each phase? Think of the tie line as a lever (teeter-totter) 3e line R 3 CL 0 S C0 4 0 Cα 5 0 R wt% Ni WL = C − C0 S =α R + S Cα − C L Mα ML α (solid) Wα = S C − CL R =0 R + S Cα − C L The Lever Rule Rule 3: con7nued… • Examples: Consider C0 = 35 wt% Ni At TB : Both α and L present WL = Wα = S R + S R R + S 43 − 35 = = 0.73 43 − 32 = 0.27 Cu- Ni system T(ºC) A TA 1300 3e line L (liquid) B R S TB 1200 TD 20 D 3 032 35 CL C0 α (solid) 4 0 4 3 5 0 Cα wt% Ni Example: Cooling of a Cu- Ni alloy Phase diagram: Cu- Ni system. T(ºC) L (liquid) Consider the microstuctural changes that accompany the cooling of: C0 = 35 wt% Ni alloy 130 0 L: 35 wt% Ni α: 46 wt% Ni L: 35wt%Ni A 32 35 B C 43 46 D 24 36 120 0 *This represents extremely slow cooling, such that each stage is at equilibrium. Cu- Ni system L: 32 wt% Ni α: 43 wt% Ni E L: 24 wt% Ni α: 36 wt% Ni α (solid) 110 0 20 3 0 Adapted from Fig. 9.4, Callister & Rethwisch 8e. α: 35 wt% Ni 35 C0 4 0 5 0 wt% Ni Cored vs. Equilibrium structures • Cα changes as we solidify. • Cu- Ni case: First α to solidify has Cα = 46 wt% Ni. Last α to solidify has Cα = 35 wt% Ni. Slow rate of cooling: Equilibrium structure Uniform Cα: 35 wt% Ni Fast rate of cooling: Cored structure First α to solidify: 46 wt% Ni Last α to solidify: < 35 wt% Ni Mechanical proper3es: Cu- Ni system Effect of solid solu7on strengthening on: - - Duc3lity (%EL) 60 400 TS for pure Ni 300 TS for pure Cu 200 0 20 40 60 80 100 Cu Ni Composi3on, wt% Ni Elonga3on (%EL) Tensile Strength (MPa) - - Tensile strength (TS) 50 %EL for pure Cu %EL for pure Ni 40 30 20 0 20 40 60 80 100 Cu Ni Composi3on, wt% Ni Binary eutec3c systems CuAg eutec3c system exhibits three single phase regions L, α, β Three two- phase regions α  α+ L, β + L, α+ β •  solvus, liquidus, solidus •  The lowest temperature at which a pure liquid will exist is called the invariant (or eutec3c) point (E) •  This point corresponds to the eutec7c composi7on (CE) and the eutec7c temperature (TE) The CuAg alloy systems exhibits a eutec3c phase diagram. The eutec3c composi3on is observed to be 71.9% Ag. • Eutectic reaction L(CE) α(CαE) + β(CβE) Pb- Sn Eutec3c System: Example For a 40 wt% Sn - 60 wt% Pb alloy at 150˚C, determine: A) The phases present Answer: α + β B) The phase composi3ons Answer: Cα = 11 wt% Sn Cβ = 99 wt% Sn 300 C) The rela3ve amount of each phase 200 Answer: 150 Cβ - C0 S = W α = R+S C - C = β Pb- Sn system T(˚C) α 99 - 40 59 = = 0.67 99 - 11 88 C - C W β = R = 0 α Cβ - Cα R+S 40 - 11 29 = = 0.33 = 99 - 11 88 100 L (liquid) α L + α 18.3 183˚C R 0 11 20 Cα 61.9 L + β β 97.8 S α + β 40 C0 60 80 C, wt% Sn 99 100 Cβ Pb- Sn Eutec3c System: Example 2 For a 40 wt% Sn- 60 wt% Pb alloy at 220˚C, determine: A) The phases present: Answer: α + L B) The phase composi3ons Answer: Cα = 17 wt% Sn CL = 46 wt% Sn Pb- Sn system T(˚C) 300 220 200 α L + α C) The rela3ve amount of each phase Answer: CL - C0 46 - 40 = W α = CL - Cα 46 - 17 6 = = 0.21 29 C0 - Cα 23 = WL = = 0.79 CL - Cα 29 100 0 17 20 Cα R L (liquid) S 183˚C L + β β α + β 80 40 46 60 C0 CL C, wt% Sn 100 Microstructure in Eutec3c Systems Consider an alloy for which: C0 < 2 wt% Sn @ 350˚C •  As we cool through the liquidus, α grains start to form in the liquid. •  As we cool through the solidus, the polycrystalline α alloy solidifies and the composi3on remains the same upon further cooling to room temperature. Microstructure in Eutec3c Systems Consider an alloy for which: 2 wt% Sn < C0 < 18.3 st% Sn @ 350˚C •  As we cool through the liquidus, α grains start to form in the liquid. •  As we cool through the solidus, the polycrystalline α alloy solidifies •  Upon further cooling through the solvus line a solid, polycrystalline α & β phase alloy forms and is stable to room temperature. Microstructure in Eutec3c Systems For alloy of composi3on C0 = CE = 61.9 wt% Sn •  Composi3on remains the same as we cool towards the eutec3c point. •  As we pass through the eutec3c point, alterna3ng layers (lamellae) of α and β phases form. Micrograph of Pb- Sn Eutec3c microstructure 160 µm Lamellar Eutec3c Structure Microstructure in Eutec3c Systems Consider an alloy for which 18.3 wt% Sn < C0 < 61.9 wt% Sn Intermediate phases or compounds Eutec3c, Eutectoid & Peritec3c Eutec3c - liquid transforms to two solid phases cool L α + β heat Eutectoid – one solid phase transforms to two other solid phases S2 S1+S3 γ cool α + Fe3C heat Peritec3c - liquid and one solid phase transform to a second solid phase S1 + L S2 cool δ + L eat γ h Eutectoid & Peritec3c Peritectic transformation γ + L Cu- Zn Phase diagram Eutectoid transformation δ γ+ε δ The Iron- Iron Carbide (Fe- Fe3C) Phase Diagram The Iron- Iron Carbide (Fe- Fe3C) Phase Diagram Defining characteris3cs: Three phase of pure iron- α - Ferrite (BCC) γ- Austenite (FCC) δ- Ferrite (BCC) Rela3vely low concentra3on of soluble carbon Intermediate compound - Fe3C (iron carbide, cemen<te) Greatest solubility of carbon in austenite The Iron- Iron Carbide (Fe- Fe3C) Phase Diagram T(˚C) 2 important points 1600 δ L ⇒ γ + Fe3C - Eutectoid (B): γ ⇒ α + Fe3C L 1400 γ +L γ 1200 (austenite) 1000 800 α 1148˚C γ γ γ γ 120 µm Result: Pearlite = alterna3ng layers of α and Fe3C phases L+Fe3C γ +Fe3C 727˚C = T eutectoid B 600 400 0 (Fe) A Fe3C (cemen3te) - Eutec3c (A): α 3C +Fe 1 0.76 2 3 4 4.30 5 6 6.7 C, wt% C Fe3C (cemen3te- hard) α (ferrite- sot) The Iron- Iron Carbide (Fe- Fe3C) Phase Diagram Phase transforma3ons occurring for %C below the eutectoid invariant point are said to be hypoeutectoid. The Iron- Iron Carbide (Fe- Fe3C) Phase Diagram Phase transforma3ons occurring for %C above the eutectoid invariant point are said to be hypereutectoid. Summary • Phase diagrams are useful tools to determine: - - the number and types of phases present, - - the composi3on of each phase, - - and the weight frac3on of each phase given the temperature and composi3on of the system. • The microstructure of an alloy depends on - - its composi3on, and - - whether or not cooling rate allows for maintenance of equilibrium. • Important phase diagram phase transforma3ons include eutec3c, eutectoid, and peritec3c. ...
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This note was uploaded on 02/28/2012 for the course EMA 3010 taught by Professor Unknown during the Fall '08 term at University of Florida.

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