PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering1How to calculate the total amount of  phase (both eutectic and primary)?Fraction of  phase determined by application of the lever rule across the entire  +  phase field:W = (Q+R) / (P+Q+R) ( phase) W = P / (P+Q+R) ( phase)Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering2Intermediate PhasesSo far only two solid phases (andTerminal solid solutionsSome binary systems have intermediatesolid solution phases. Cu-Zn: and are terminal solid solutions,’, are intermediate solid solutions.Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering3Intermetallic CompoundsIntermetallic compounds precise chemical compositions exist in some systems.Using the lever rules, intermetallic compounds are treated like any other phase, They appear as a vertical line. Two eutectic diagrams: Mg-Mg2Pb and Mg2Pb-Pb. Intermetallic compound Mg2Pb is considered a component. intermetallic compoundIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering4Eutectoid Reactions (I)Eutectoid (eutectic-like in Greek) reaction similar to eutectic reaction One solid phase to two new solid phases Invariant point (the eutectoid) Three solid phases in equilibriumUpon cooling, a solid phase transforms into two other solid phases (   +  below)EutectoidCu-ZnIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering5Eutectoid Reactions (II)Contains an eutectic reaction and an eutectoid reactionIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering6Peritectic ReactionsPeritectic  solid phase + liquid phase will together form a second solid phase at a particular temperature and composition upon cooling L +   Reactions are slow as product phase will form at boundary between two reacting phases separating themPeritectics are not as common as eutectics and eutectiods. There is one in Fe-C systemIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering7Congruent Phase TransformationsCongruent transformation  no change in composition (e.g, allotropic transformation such as -Fe to -Fe or melting transitions in pure solids)Incongruent transformation, at least one phase changes composition (eutectic, eutectoid, peritectic).Congruentmelting of Ni-TiIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering8The Iron–Iron Carbide (Fe–Fe3C) Phase DiagramSteels: alloys of Iron (Fe) and Carbon (C). Fe-C phase diagram is complex. Will only consider the steel part of the diagram, up to around 7% Carbon.Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering9Phases in Fe–Fe3C Phase Diagram-ferrite - solid solution of C in BCC Fe•Stable form of iron at room temperature. •The maximum solubility of C is 0.022 wt%•Transforms to FCC -austenite at 912 C-austenite - solid solution of C in FCC Fe•The maximum solubility of C is 2.14 wt %. •Transforms to BCC -ferrite at 1395 C •Is not stable below the eutectic temperature (727  C) unless cooled rapidly (Chapter 10)-ferrite solid solution of C in BCC Fe•The same structure as -ferrite•Stable only at high T, above 1394 C•Melts at 1538 CFe3C (iron carbide or cementite) •This intermetallic compound is metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into -Fe and C (graphite) at 650 - 700 C Fe-C liquid solutionIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering10Comments on Fe–Fe3C systemC is an interstitial impurity in Fe. It forms a solid solution with phases of ironMaximum solubility in BCC -ferrite is 0.022 wt% at727 C. BCC:relatively small interstitial positionsMaximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger interstitial positionsMechanical properties: Cementite (Fe3C is hard and brittle: strengthens steels. Mechanical properties also depend on microstructure: how ferrite and cementite are mixed.Magnetic properties:  -ferrite is magnetic below 768 C, austenite is non-magneticClassification. Three types of ferrous alloys: Iron: < 0.008 wt % C in ferriteat room T Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ) -ferrite +Fe3C at room T (Chapter 12) Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering11Eutectic and eutectoid reactions in Fe–Fe3CEutectoid: 0.76 wt%C, 727 C(0.76 wt% C)   (0.022 wt% C) + Fe3CEutectic: 4.30 wt% C, 1147 C L   + Fe3CEutectic and Eutectoid reactions are important in heat treatment of steelsIntroduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering12Microstructure in Iron - Carbon alloysMicrostructure depends on composition (carbon content) and heat treatment. Assume slow cooling  equilibrium maintained Microstructure of eutectoid steel (I)Introduction to Materials Science, Chapter 9, Phase DiagramsUniversity of Virginia, Dept. of Materials Science and Engineering13Pearlite, layered structure of two phases: -ferrite and cementite (Fe3C) Alloy of eutectoid composition (0.76 wt % C) Layers formed for same reason as in eutectic: Atomic diffusion of C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%)