Material Science Chapter 8 PDF

CHAPTER 8 Phase Diagrams 1  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Phase Change in Materials and Industrial Relevance In any application or industry in which the structure of a material changes from...

CHAPTER 8 Phase Diagrams 1  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Phase Change in Materials and Industrial Relevance In any application or industry in which the structure of a material changes from one form to another, either by design or accidentally, there are engineering consequences that must be considered. Examples of such industries include: Ø Metal casting industries in which molten metal is solidified in die Ø Welding and soldering industries in which solid feeder material is melted to join two components and solidified again. Ø Heat treatment industry in which phase changes occur in the solid state of the material. Ø Energy industries use phase change in materials to store and release energy. Ø Power plant industry use phase change from liquid to vapor to power turbines. Therefore, it is crucial that all engineers understand the concept 2 of phase change and its influence on materials behavior. Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Phase Equilibrium What is a phase? A region in a material that has a uniform structure, properties, and composition and maintains a distinct boundary with other unlike phases. Ø Ice, water, and water vapor are three distinct phases of H2O due to differences in the state (solid, liquid and vapor). Ø Glass fiber and epoxy matrix (fiberglass) are both solid but are distinct phases due differences in composition Ø Iron at room temperature (BCC) and above 9120C (FCC) are both solid and have the same composition but are distinct phases due to differences in structure (BCC versus FCC). What is equilibrium? The state of a system when all forces and energies are balanced resulting in a stable system with no tendency to change with time. When more than one phase is present in the system with influential variables such as pressure, temperature, and composition, phase analysis is needed. 3 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Phase Diagrams Phase diagrams: Ø Graphical representations of phases present in a material system at different temperatures, pressures and compositions. Ø Are developed based on assumption of equilibrium conditions resulting from slow cooling (equilibrium is approached but never fully maintained). Ø Indicate the parameters under which phases change and coexist. Class Discussion Topic: Discuss what happens to ice melting temperature with increasing pressure. What is a triple point? Pressure-Temperature phase 4 diagram for pure water. Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gibbs Phase Rule Using thermodynamics considerations, Gibbs developed a relationship that determines the number of phases that can co-exist in equilibrium in a given system. P = number of phases that coexist in a system P+F = C+2 C = Number of components F = Degrees of freedom For example, for pure water, at triple point, 3 phases coexist, P=3. There is one component (water) in the system, C=1. Therefore, the degree of freedom F is 3+F=1+2; F=0 F indicates the number of variables that can be changed without changing number of phases at that point. 5 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cooling Curves Temperature of molten metal is recorded versus time as it cools to room temperature – called a cooling curve. Ø There is distinct change in slope very time there is phase change. Iron Ø For a pure metal, the cooling curve shows a flat region at a specific temperature where liquid transforms to solid (liquid to solid phase change). Ø For pure metals, the flat region or plateau indicates thermal arrest : heat Pure metals solidify at a specific lost = heat supplied by solidifying temperature with some required metal. undercooling (dotted line) but alloys solidify over a range of Ø In the plateau region, there is a mixture temperatures. of two phases in equilibrium. 6 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cooling Curves Cooling curves detect all phases changes including liquid to solid and solid to solid phase transformations. Information gathered from cooling curves are used to develop phase diagrams Class Discussion Topic: Discuss the key features of the pure iron cooling curve. What does each Pure Iron plateau indicate? Pure iron cooling curve 7 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Phase Diagram from Cooling Curves To construct a phase diagram for a binary system, for instance Cu- Ni, a series of cooling curves at different metal compositions are first constructed, see figure below left for various Cu-Ni compositions. Points of change of slope in the cooling curves are identified. Note pure Cu and Ni solidify at specific temperatures. However, other alloys start solidification at one temperature and finish solidification at a lower temperature. If you connect all the “start” points and all the “finish” points at the given compositions, you generate a phase diagram. Above the “start” line, the alloy is all liquid. In the region between start Below the “finish” line, the alloy is all solid. and finish, a mixture of solid 8 and liquid exists. Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Binary Isomorphous Alloy Systems In the Cu-Ni binary alloy system, the phase diagram constructed from the cooling curves, is given below. Notice the liquid region (L), the solid region (α), and the mixture region (L + α). The line that indicates start of the solidification process is liquidus. The line that indicates completion of the solidification process is called solidus. This particular alloy system is called a binary isomorphous system because the two components are completely soluble in each other in both liquid and solid states (they do not from a third phase). 9 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Tie-Line and the Lever Rule In the mixture region (L + α), the composition of both liquid (L) and solid (α) phases at any temperature can be determined by drawing a tie line originating from the overall alloy composition, wo, and temperature (T) combination (the tie line is line LOS). The composition at point L is the composition of liquid, wl, in the mixture. The composition at point S is the composition of solid, ws, in the mixture The amount of each phase can be determined through application of the Lever Rule: 10 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Non Equilibrium Solidification of Alloys All phase diagrams are constructed based on very slow cooling rates to approach equilibrium. If the cooling rate is fast, non-equilibrium solidification of alloys will prevail which shifts the equilibrium solidus to the left. Rapid cooling results in a cored microstructure of solids with different compositions (see figure bottom right). Cored microstructures form often in castings but are not desirable. The homogenization heat treatment is given to cast alloys (below the melt temperature) to eliminate the cored structures and produce a more uniform microstructure. 11 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Binary Eutectic Alloy System Unlike the components in a binary isomorphous system (such as Cu- Ni), in some binary alloy systems, the two components have limited solid solubility in each other; for instance Pb-Sn; binary eutectic. Ø The regions of restricted solid solubility at each end of the diagram are designated as α and β, the terminal solid solutions. Ø The α phase is a lead rich solid solution and the β phase is a tin rich solid solution. Ø α can dissolve a maximum of 19.2 wt% tin in its structure while β can dissolve a maximum of 2.5 wt% lead in its structure. Ø The above maximum solubility occurs at 183oC, the location of eutectic isotherm. Ø In all binary eutectic systems, there is one specific alloy that has the lowest melt temperature called the eutectic alloy. Ø The composition of this alloy for the Pb-Sn system is 61.9 wt% Pb - 38.1 wt% Sn; the eutectic composition. 12 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Binary Eutectic Alloy System Alloys with Pb wt% greater than 19.2 and less than 61.9 are called hypoeutectic alloys. Alloys with Pb wt% greater than 61.9 and less than 97.5 are called hypereutectic alloys. Ø For alloy 1 on the figure, the eutectic alloy, note that upon cooling from a liquid state, the alloy directly transforms to a mixture of two solids (α + β). Ø The reaction is called the eutectic reaction: Ø The microstructure of the hypoeutectic alloy contains solids from the (α + L) region called primary or proeutectic α in addition to the eutectic phase. Ø The microstructure of the hypereutectic alloy contains solids from the (β + L) region called primary or proeutectic β in addition to the eutectic phase. Ø The eutectic alloy contains only the eutectic microstructure. 13 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Slow Cooling of 60% Pb – 40% Sn alloy Liquid at 300oC. At about 245oC, first solid forms – proeutectic solid. Slightly above 1830C composition of alpha follows solidus and wt% of Sn varies from 40% to 61.9%. At eutectic temperature, all the remaining liquid solidifies. Further cooling lowers the Sn content in α and the Pb content in β. 14 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Various Eutectic Microstructures 100% eutectic Microstructure consisting of layered α (dark) and β (light) phases (Pb-Sn alloy). Microstructure of hypereutectic cast iron consisting of primary β Iron carbide (white) and eutectic (layered) phases. Microstructure of hypoeutectic alloys consisting of primary α (white) and eutectic (dark) phases. 15 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Binary Peritectic Alloy System Peritectic reaction: Liquid phase reacts with a solid phase to form a new and different solid phase. Liquid + α β cooling Peritectic reaction occurs when a slowly cooled alloy of Fe-4.3 wt% Ni passes through Peritectic temperature of 15170C. Peritectic point is invariant which means it occurs at specific temperature and composition. cooling Liquid(5.4 wt% Ni) + δ (4.0 wt% Ni) γ 4.3 wt % Ni 16 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Peritectic Alloy System At 42.4 % Ag & 1400oC Phases present Liquid Alpha Composition 55% Ag 7%Ag Amount of Phases 42.4 –7 55-42.4 55 – 7 55 - 7 = 74% = 26% At 42.4% Ag and 1186oC – ΔT Phase Present Beta only Composition 42.4% Ag Amount of Phase 100% At 42.4% Ag and 1186oC + ΔT Phases present Liquid Alpha Composition 66.3% Ag 10.5%Ag Amount of Phases 42.4 –10.5 66.3-42.4 66.3 – 10.5 66.3–10.5 = 57% =43% 17 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Rapid Solidification in Peritectic System Surrounding or Encasement: During peritectic reaction, L+ α β , the beta phase created surrounds primary alpha. Beta creates diffusion barrier resulting in coring. 18 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Binary Monotectic Systems Monotectic Reaction: Liquid phase transforms into solid phase and another liquid. L1 Cooling α + L2 Two liquids are immiscible. Example:- Copper – Lead system at 9550C and 36% Pb. 19 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Intermediate Phases and Compounds Terminal phases: Phases that occur at the end of phase diagrams. Intermediate phases: Phases that occur in a composition range inside phase diagram. Examples: Cu-Zn diagram has both terminal and intermediate phases. Five invariant peritectic points and one eutectic point. 20 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Intermediate Phases in Ceramics In Al2O2 – SiO2 system, an intermediate phase called Mullite is formed, which includes the compound 3Al2O3.2SiO2. 21 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Intermediate Compounds In some phase diagrams, intermediate compound are formed – stoichiometric compounds. Percent Ionic/Covalent bond depends on electronegativy Example:- Mg-Ni phase diagram contains Ø Mg2Ni : Congruently melting compound Ø MgNi2 : Incongruently melting compound. 22 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ternary Phase Diagrams Three components Constructed by using a equilateral triangle as base. Pure components at each end of triangle. Binary alloy composition represented on edges. Temperature can be represented as uniform throughout the Whole Diagram Isothermal section. 23 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ternary Phase Diagram, Cont. Example:- Iron-Chromium-Nickel phase diagrams. Isothermal reaction at 6500C for this system Composition of any metal at any point on the phase diagram can be found by drawing perpendicular from pure metal corner to apposite side and calculating the % length of line at that point 24 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi