Phase Diagrams, Solid Solutions, Phase Strengthening, Phase Transformations

Components and Phases Components: The elements or compounds that are mixed initially (Al and Cu). Phases: A phase is a homogenous, physically distinct and mechanically separable portion of the material with a given chemical composition and structure ( and ). AluminumCopper Alloy (lighter phase) (darker phase) 2

Solid solution A solution that contains two or more types of atoms or ions that are dispersed uniformly throughout the material Solute The impurities that may occupy regular lattice sites in the crystal or interstitial sites Alloys A material made from multiple elements that exhibits properties of a metallic material Stainless steels Alloys that usually contain iron (Fe), carbon (C), chromium (Cr), nickel (Ni), and some other elements Single-phase alloy An alloy consisting of one phase Multiple-phase alloy An alloy that consists of two or more phases

Phase Equilibria: Solubility Limit Solution solid, liquid, or gas solutions, single phase Mixture more than one phase Sugar/Water Phase Diagram Solubility Limit: Question: What is the solubility limit for sugar in water at 20 C? 100 Temperature ( C) Maximum concentration for which only a single phase solution exists. Solubility Limit 80 60 L 40 (liquid solution i.e., syrup) 20 L (liquid) + S (solid sugar) Sugar Water Answer: 65 wt% sugar. At 20 C, if C < 65 wt% sugar: syrup 0 20 40 6065 80 100 At 20 C, if C > 65 wt% sugar: syrup + sugar C = Composition (wt% sugar)

Equilibrium A system is at equilibrium if its free energy is at a minimum, given a specified combination of temperature, pressure and composition. The (macroscopic) characteristics of the system do not change with time the system is stable. A change in T, P or C for the system will result in an increase in the free energy and possible changes to another state whereby the free energy is lowered. 6

Chapter 10: Solid Solutions and Phase Equilibrium Conditions for Unlimited Solid Solubility Hume-Rothery rules Size factor The atoms or ions must be of similar size, with no more than a 15% difference in atomic radius, in order to minimize the lattice strain. Crystal structure The materials must have the same crystal structure. Valence The ions must have the same valence. Electronegativity The atoms must have approximately the same electronegativity. 2011 Cengage Learning Engineering. All Rights Reserved. 10-8

Chapter 10: Solid Solutions and Phase Equilibrium Solubility and Solid Solutions Polymeric systems Copolymer: A polymer that is formed by combining two or more different types of monomers, usually with the idea of blending the properties affiliated with individual polymers. For example, acrylonitrile (A), butadiene (B), and styrene (S) monomers can be made to react to form a copolymer known as ABS. 2011 Cengage Learning Engineering. All Rights Reserved. 10-11

Chapter 10: Solid Solutions and Phase Equilibrium Solid-Solution Strengthening Solid-solution strengthening Increasing the strength of a metallic material via the formation of a solid solution. 2011 Cengage Learning Engineering. All Rights Reserved. 10-12

Chapter 10: Solid Solutions and Phase Equilibrium Isomorphous Phase Diagrams Phase diagram Indicate phases as a function of Temp., Comp. and (under equilibrium condition) Pressure Binary phase diagram A phase diagram for a system with two components. Isomorphous phase diagram A phase diagram in which the components display unlimited solid solubility. 2011 Cengage Learning Engineering. All Rights Reserved. 10-14

Determination of phase(s) present Rule 1: If we know T and Co, then we know: --how many phases and which phases are present. T( C) A(1100, 60): 1 phase: B(1250, 35): 2 phases: L + 150 0 L (liquid) B (1250,35) Examples: 160 0 140 0 130 0 + L 120 0 Melting points: Cu = 1085 C, Ni = 1453 C 110 0 100 0 0 s u d i il qu us l id o s (FCC solid solution) Cu-Ni phase diagram A(1100,60) 20 40 60 80 100 wt% Ni Solidus - Temperature where alloy is completely solid. Above this line, liquefaction begins. Liquidus - Temperature where alloy is completely liquid. Below this line, solidification begins. 16

Phase Diagrams: composition of phases Rule 2: If we know T and Co, then we know: --the composition of each phase. Examples: At TA = 1320 C: Only Liquid (L) present CL = C0 ( = 35 wt% Ni) At TD = 1190 C: Only Solid ( ) present C = C0 ( = 35 wt% Ni) At TB = 1250 C: Both and L present CL = C liquidus ( = 32 wt% Ni) C = C solidus ( = 43 wt% Ni) Cu-Ni system T( C) TA 130 0 A L (liquid) TB 120 0 TD 20 L + tie line dus i liq u B + L s D (solid) 3032 35 C LC o id sol 40 43 u 50 C wt% Ni 17

S R S Phase Diagrams: weight fractions of phases Rule 3: If we know T and Co, then we know: --the amount of each phase (given in wt%). Examples: C o = 35wt%Ni At T A : Only Liquid (L) W L = 100wt%, W = 0 At T D : Only Solid ( ) W L = 0, W = 100wt% At T B : Both and L SS C C 43S 35 W o 73wt % W WLLL R S R SCL R43 S32 C R C CL R W R o W R S = 27wt % RC S CL R S Cu-Ni system T( C) TA 130 0 A L (liquid) TB 120 0 TD 20 L+ R B tie line dus i li q u + L s S D id sol (solid) R S40 43 3032R35 C LC o u C wt% Ni 50 18

Importance of Phase Diagrams There is a strong correlation between microstructure and mechanical properties, and the development of alloy microstructure is related to the characteristics of its phase diagram. Phase diagrams provide valuable information about melting, casting, crystallization and other phenomena. 22

Microstructure In metal alloys, microstructure is characterized by the number of phases, their proportions, and the way they are arranged. The microstructure depends on: Alloying elements Concentration Heat treatment (temperature, time, rate of cooling) 23

Eutectic A eutectic or eutectic mixture is a mixture of two or more phases at a composition that has the lowest melting point. It is where the phases simultaneously crystallize from molten solution. The proper ratios of phases to obtain a eutectic is identified by the eutectic point on a binary phase diagram. The term comes from the Greek 'eutektos', meaning 'easily melted. 24

The phase diagram displays a simple binary system composed of two components, A and B, which has a eutectic point. The phase diagram plots relative concentrations of A and B along the X-axis, and temperature along the Y-axis. The eutectic point is the point where the liquid phase borders directly on the solid α + β phase; it represents the minimum melting temperature of any possible A B alloy. The temperature that corresponds to this point is known as the eutectic temperature. Not all binary system alloys have a eutectic point: those that form a solid solution at all concentrations, such as the gold-silver system, have no eutectic. An alloy system that has a eutectic is often referred to as a eutectic system, or eutectic alloy. Solid products of a eutectic transformation can often be identified by their lamellar structure, as opposed to the dendritic structures commonly seen in non-eutectic solidification. 25

Binary-Eutectic Systems has a special composition with a min. melting T. 2 components T( C) 1200 Cu-Ag system L (liquid) 3 single phase regions 1000 (L,, ) L+ Limited solubility: 779 C 800 T E : mostly Cu 8.0 : mostly Ag 600 TE : No liquid below TE 400 CE : Composition at temperature TE 200 0 Eutectic reaction L(CE) L(71.9 wt% Ag) (C E) + (C E) cooling heating 20 40 L+ 71.9 91.2 100 60 CE 80 C, wt% Ag (8.0 wt% Ag) (91.2 wt% Ag) 26

Pb-Sn Phase c10f08diagram Liquidus Solidus Solidus Solidus Solvus Solvus

Lamellar Eutectic Structure A 2-phase microstructure resulting from the solidification of a liquid having the eutectic composition where the phases exist as a lamellae that alternate with one another. Formation of eutectic layered microstructure in the Pb-Sn system during solidification at the eutectic composition. Compositions of α and β phases are very different. Solidification involves redistribution of Pb and Sn atoms by atomic diffusion. Pb-rich Sn-rich 28

Pb-Sn Microstructures The dark layers are Pb-rich α phase, the light layers are the Snrich β phase. 29

Iron-Carbon System Pure iron when heated experiences 2 changes in crystal structure before it melts. At room temperature the stable form, ferrite ( iron) has a BCC crystal structure. Ferrite experiences a polymorphic transformation to FCC austenite ( iron) at 912 C (1674 F). At 1394 C (2541 F) austenite reverts back to BCC phase ferrite and melts at 1538 C (2800 F). Iron carbide (cementite or Fe3C) an intermediate compound is formed at 6.7 wt% C. Typically, all steels and cast irons have carbon contents less than 6.7 wt% C. Carbon is an interstitial impurity in iron and forms a solid solution with the phases 30

Iron-Carbon System c10f28

c10f29ab Though carbon is present in relatively low concentrations, it significantly influences the mechanical properties of ferrite: (a) α ferrite, (b) austenite.

4 Solid Phases

Iron-Carbon (Fe-C) Phase Diagram - Eutectic (A): L + Fe3C - Eutectoid (B): + Fe3C 1600 L 1400 1200 +L (austenite) + 800 1148 C 1000 120 m Result: Pearlite = alternating layers of and Fe3C phases, not a separate phase. L+Fe3C +Fe3C B 727 C = T eutectoid 600 400 0 (Fe) A Fe3C (cementite) 2 important points T( C) +Fe3C 1 0.76 2 3 4 4.30 5 6 6.7 C, wt% C Fe3C (cementite-hard) (ferrite-soft) 34

Eutectoid reaction: + Fe3C Pearlite c10f30 Redistribution of carbon by diffusion Austenite 0.76 wt% C Ferrite - 0.022 wt% C Cementite - 6.70 wt% C