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 (a and b). Aluminum- (lighter Copper phase) Alloy (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 Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.2 © 2011 Cengage Learning Engineering. All Rights Reserved. 10 - 4 Phase Equilibria: Solubility Limit • Solution – solid, liquid, or gas solutions, single phase • Mixture – more than one phase Sugar/Water Phase Diagram • Solubility Limit: Maximum concentration for 100 Solubility which only a single phase ) L C 80 Limit ° solution exists. ( (liquid) e r 60 u + t L Question: What is the a r S solubility limit for sugar in e 40 (liquid solution p i.e., syrup) m (solid water at 20°C? e T 20 sugar) Answer: 65 wt% sugar. At 20°C, if C < 65 wt% sugar: syrup 0 20 40 6065 80 100 r r At 20°C, if C > 65 wt% sugar: syrup + sugar e C = Composition (wt% sugar) t a a g u W S 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. 10 - 8 © 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.3 10 - 10 © 2011 Cengage Learning Engineering. All Rights Reserved. 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. 10 - 11 © 2011 Cengage Learning Engineering. All Rights Reserved. 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. 10 - 12 © 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.8 10 - 13 © 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 10: Solid Solutions and Phase Equilibrium Isomorphous Phase Diagrams . Phase diagram . Indicate phases as a function of Temp., Comp. and Pressure (under equilibrium condition) . 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. 10 - 14 © 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.9 10 - 15 © 2011 Cengage Learning Engineering. All Rights Reserved. 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) 1600 • Examples: 1500 L (liquid) A(1100, 60): s ) du 5 ui 1 phase: 3 q 1400 , li us 0 id 5 ol Cu-Ni B(1250, 35): 2 s 1 phase ( 1300 + 2 phases: L + B L (FCC solid diagram 1200 solution) 1100 A(1100,60) Melting points: Cu = 1085°C, Ni = 1453 °C 1000 0 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. Cu-Ni system • Examples: T(°C) A T tie line A dus At TA = 1320°C: qui 1300 L (liquid) li Only Liquid (L) present + L s C = C ( = 35 wt% Ni) B lidu L 0 TB so At TD = 1190°C: + Only Solid (a) present 1200 L D (solid) T Ca = C0 ( = 35 wt% Ni) D 20 30 40 50 At TB = 1250°C: 3235 43 C C C Both and L present L o wt% Ni a CL = C liquidus ( = 32 wt% Ni) Ca = C solidus ( = 43 wt% Ni) 17 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: Co = 35wt%Ni T(°C) Cu-Ni system At T A: Only Liquid (L) A TA tie line s WL = 100wt%, W = 0 idu iqu At T D: Only Solid ( ) 1300 L (liquid) l + L s WL = 0, W = 100wt% B lidu TB so At T B: Both and L R S + 1200 L S D (solid) W CS Co 43S 35 TD R WWL 73wt % LL R S 43 32 CRSCL R S 20 3032R35 S4043 50 CLCo C R wt% Ni C Co S W CRo CL R W W R S = 27wt % L R S R S C CL C CL R S 18 Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.15 10 - 19 © 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.16 10 - 20 © 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 10: Solid Solutions and Phase Equilibrium Figure 10.17 10 - 21 © 2011 Cengage Learning Engineering. All Rights Reserved. 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 2 components with a min. melting T. T(°C) Cu-Ag system 1200 • 3 single phase regions L (liquid) (L, a, b) 1000 • Limited solubility: a L + a T 800 779°C L+b b a: mostly Cu E 8.0 71.9 91.2 b: mostly Ag 600 • TE : No liquid below TE a + b 400 • CE : Composition at temperature TE 200 0 20 40 60 C 80 100 Eutectic reaction E • C , wt% Ag L(CE) a(CaE) + b(CbE) cooling L(71.9 wt% Ag) (8.0 wt% Ag) (91.2 wt% Ag) 26 heating Pb-Sn Phasec10f08 Diagram 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 Pb-rich another. Formation of eutectic Sn-rich 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. 28 Pb-Sn Microstructures The dark layers are Pb-rich α phase, the light layers are the Sn- rich 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 (a iron) has a BCC crystal structure.