Lecture - Chapter 9

Lecture - Chapter 9 - Chapter 9 Phase Diagrams Phase...

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Unformatted text preview: Chapter 9 Phase Diagrams Phase Diagrams Binary Isomorphous Systems Binary Eutectic Systems Why study phase diagrams? Metal alloys are heat treated to obtain optimum properties. Phase diagrams are road maps for heat treatment. By knowing what phase (a soft or a hard phase) is present at a particular temperature you can control the heat treatment procedure to obtain desirable properties. Learning objectives Define isomorphous, eutectic, eutectoid and peritectic phase diagrams and eutectic, eutectoid and peretectic transformations. Given the composition and temperature of a binary alloy, determine the phases present, and composition and mass fraction of each phase. Basic concepts Components and phases Example: Syrup Components are sugar and water. Phases are syrup (liquid solution) and solid sugar. Phase Diagram Figure 9.1 is a phase diagram of sugarwater system. The x-axis represents composition of sugar from left to right and y-axis represents temperature. Only one phase is present on the left side of the solubility limit. Two phases coexist on the right side of the solubility limit. Fig 9.1 The solubility of sugar (C12H22O11) in a sugar-water syrup. Binary Isomorphous System Binary system means a two-component system. Isomorphous means a complete solubility. Example: Cu-Ni system is a binary isomorphous system since Cu and Ni are completely soluble in each other. Fig 9.2 (a) The copper-nickel phase diagram. Binary Isomorphous system Fig 9.2 (a) is the phase diagram of a binary isomorphous system (Cu-Ni alloy) x-axis represents wt% Ni and y-axis represents temperature. Above the liquidus line there is only one single phase - liquid phase. Below the solidus line one single solid phase. In between liquidus and solidus line is the two phase region liquid and solid phase. Binary Isomorphous system Phases Present In fig 9.2(b) consider a Cu-Ni alloy of composition Co, 35 wt% Ni, heated to a temperature of 1250oc (the point B in the figure). The phases present in the alloy at that temperature are and liquid. Fig 9.2 (b) A portion of the copper-nickel phase diagram for which compositions and phase amounts are determined at point B. 1250 31.5 42.5 Binary Isomorphous system Determination of Phase Compositions Draw a horizontal line (tie line). The intersection of the tie line with the liquidus line gives the composition of the liquid phase: Cl = 31.5 wt% Ni The intersection of tie line with the solidus line gives the composition of the solid phase: C = 42.5 wt % Ni. Binary Isomorphous system Determination of Mass Fractions The relative amount or mass fraction of each phase present is determined in the following way: The relative amount of liquid phase present (WL): WL = opposite side of tie line/total tie line length The relative amount of solid phase present (W): W = 1 - WL Where: WL = relative amount of the liquid phase (mass fraction) W = relative amount of the solid phase (mass fraction) Fig 9.2 (b) A portion of the copper-nickel phase diagram for which compositions and phase amounts are determined at point B. 1250 31.5 42.5 Development of Microstructure The microstructure of an alloy is determined by several factors. However, the primary factor is the phases that an alloy passes through as it cools through the phase diagram, which determines microstructure. In Figures 9.3 and 9.4, there are two examples of how alloys of different compositions end up with different microstructures upon cooling to lower temperatures. Fig 9.3 Schematic representation of the development of microstructure during the equilibrium solidification of a 35 wt % Ni 65 wt % Cu alloy. Fig 9.4 Schematic representation of the development of microstructure during the non equilibrium solidification of a 35 wt% Ni-65 wt% Cu alloy. Strength and Ductility in Binary Isomorphous Systems For binary isomorphous systems there exists some maximum strength and minimum ductility associated with a certain percentage of one element to the other. For example, copper can be strengthened by the addition of nickel, but only to a certain extent, after which further additions of nickel with actually weaken the alloy. Reference Figure 9.5. Fig 9.5 For the copper-nickel system, (a) tensile strength versus composition, (b) ductility vs composition at room temperature. Binary Eutectic Systems Most alloy systems are not isomorphous, and therefore have more than two single phases. In fact often four or more single phases may be present in the phase diagram. One of the simpler binary eutectic systems to analyze however, is the copper-silver phase diagram which only has three distinct phases. Binary eutectic system CuAg phase diagram In this phase diagram there are three singlephase regions, with two different solid phases. Two solid phase regions are present because Cu and Ag are not completely soluble to each other like Cu and Ni. They are: -solid phase rich in Cu. -solid phase rich in Ag. Two phase regions are: -solid + Liq -solid + Liq -solid + solid Binary eutectic system CuAg phase diagram The alloy of composition CE as it is cooled down from the liquid phase goes through the eutectic reaction in which the liquid changes to two solid phases. The eutectic reaction can be expressed as follows: Liquid (CE) -solid (CE) + -solid (C E) Binary eutectic system CuAg phase diagram Eutectic reaction in Cu-Ag system is: Liq (71,9 wt% Ag) (8 wt% Ag) + (91.2 wt% Ag) Fig 9.6 The copper-silver phase diagram. Eutectic point 8.0 71.9 91.2 Binary eutectic system SnPb phase diagram In the lead-tin phase diagram there are two different solid phases present and one liquid phase. Two solid phase regions are present because as with Cu and Ag, lead and tin are not completely soluble in each other. They are: -solid phase rich in Pb. -solid phase rich in Sn. Two phase regions are: -solid + Liq -solid + Liq -solid + solid Binary eutectic system SnPb phase diagram Eutectic reaction in Sn-Pb system is: Liq (61.9 wt% Sn) (18.3 wt% Pb) + (97.8 wt% Sn) Eutectic point Fig 9.7 The lead-tin phase diagram. Fig 9.8 The lead-tin phase diagram. For a 40 wt% Sn-60 wt% Pb alloy at 150 C (point B). Fig 9.9 Schematic representations of the equilibrium microstructures for a lead-tin alloy of composition C1 as it is cooled from the liquid-phase region. Fig 9.10 Schematic representations of the equilibrium microstructures for a lead-tin alloy of composition C2 as it is cooled from the liquidphase region. Fig 9.11 Schematic representations of the equilibrium microstructures for a lead-tin alloy of eutectic composition C3 above and below the eutectic temperature. Fig 9.12 Photomicrograph showing the microstructure of a lead-tin alloy of eutectic composition. Fig 9.13 Schematic representation of the eutectic structure for the lead-tin system. Fig 9.14 Schematic representations of the equilibrium microstructures for a lead-tin alloy of composition C4 as it is cooled from the liquid phase region. Fig 9.15 Photomicrograph showing the microstructure of a lead-tin alloy of composition 50 wt% Sn-50 wt% Pb. Eutectoid Reaction Eutectoid reaction is present in Cu-Zn system phase diagram: solid phase (72 wt% Zn) cooling heating solid phase(68 wt% Zn) + solid phase(79 wt% Zn) Eutectoid Reaction Fig 9.17 The copper-Zinc phase diagram. Peritectic Reaction Peritectic reaction is shown in Fig 9.19: (76 wt% Zn) + Liquid (89 wt% Zn) cooling heating solid phase(78.6 wt% Zn) 74 79 Fig 9.19 A region of the copper-zinc phase diagram that has been enlarged to show eutectoid and peritectic invariant points, labeled E and P respectively. The IronCarbon system The Iron-Iron Carbide (Fe-Fe3C) phase diagram. The portion of the Fe-Fe3C phase diagram for commercial importance is shown in Fig 9.21. Fig 9.21 The iron-iron carbide phase diagram. 0.022 2.14 4.3 The IronIron Carbide (FeFe3C) phase diagram The Eutectic reaction occurs at 1147C and 4.30 wt% C, and is: Liq (4.3 wt% Zn) solid phase (2.14 wt% C) + Fe3C solid phase (6.70 wt %C) The IronCarbon system The Eutectoid reaction occurs at 727C and 0.76 wt% C, and is: , Austenite (0.76 wt% C) , ferrite solid phase (0.22 wt%C) + Fe3C, cementite (6.7 wt% C) The IronIron Carbide (FeFe3C) phase diagram cont. and are the solid solution phases of iron and carbon, and Fe3C, cementite is a compound phase of iron and carbon. Fig 9.22 Photomicrographs of (a) ferrite and (b) austenite. Development of Microstructures in FeC alloys Consider a Fe-C alloy of eutectoid composition (0.76 wt% C), which is cooled down from the point X in Fig 9.23. At the point a in the , austenite singlephase region if you put the alloy under an optical microscope, you will see a single solid phase (Fig 9.22 (b)). Fig 9.23 Schematic representations of the microstructures for an iron-carbon alloy of eutectoid composition above and below the eutectoid temperature. Development of microstructures in FeC alloys When it is cooled down to the point b, the two phase region ( and Fe3C ) you will see alternate layers of , ferrite, and Fe3C, cementite phases within each grain Reference Figure 9.24. Fig 9.24 Photomicrograph of a eutectoid steel showing the pearlite microstructure consisting of alternating layers of ferrite and Fe3C. Fig 9.25 Schematic representation of the formation of pearlite from austenite; direction of carbon diffusion indicated by arrows. Hypo Eutectic Less Than Eutectoid Hypo Eutectoid Alloys (Between 0.022 Wt% C To 0.76 Wt% C ) Consider an alloy of composition Co cooled down from the point y in Fig 9.26. At the point c you can see , austenite, phase under optical microscope. At the point e in you will find two different phases , austenite, and , ferrite phases. Hypo Eutectoid Alloys Reference Figure 9.26 At the point, f, the remaining austenite, , phase changes to a pearlite microstructure which is the alternate layer of cementite and ferrite. Whatever ferrite phase was formed above 727C does not change. Fig 9.26 Schematic representations of the microstructures for an ironcarbon alloy of hypo eutectoid composition C0 as it is cooled from within the austenite phase region to below the eutectoid temperature. + Fig 9.27 Photomicrograph of a 0.38 wt% C steel having a microstructure consisting of pearlite and pro eutectoid ferrite. Hypo Eutectoid Alloys From Fig 9.28, the fraction of formed, Wp, is given as: Wp = T/(T+U) = (C o 0.022)/(0.76-0.022) = (C o 0.022)/ 0.74 pearlite Fig 9.28 A portion of the Fe-Fe3C phase diagram used in computations for relative amounts of pro eutectoid and pearlite micro constituents for hypo eutectoid and hyper eutectoid compositions. Hypo Eutectoid Alloys The fraction of pro eutectoid , W , is given as: W =U/(T+U) =(0.76- C o )/(0.76-0.022) = (0.76 - C o )/ 0.74 Hyper Eutectic More Than Eutectoid Hypereutectoid alloys (For Fe C alloys, between 0.76 to 2.14 wt % C) Consider a hypereutectoid alloy of composition C1 in figure 9.29: While cooling down this alloy from the point Z, one finds , austenite phase only. At point h the phases present are , austenite and Fe3C, cementite. At the point i whatever cementite phase was formed remains unaltered and , austenite, changes to pearlite micro-structure. Fig 9.29 Schematic representations of the microstructures for an ironcarbon alloy of hypereutectoid composition C1, as it is cooled from within the austenite phase region to below the eutectoid temperature + Fe3C Hypereutectoid alloys The relative amount or fraction of pearlite formed is given as using lever rule: Wp = X/(V+X) = (6.70 - C 1 )/(6.70 0.76) = (6.70 - C 1 )/ 5.94 Hypereutectoid alloys And the fraction of pro eutectoid, cementite, WFe3C is given as WFe3C=V/(V+X) = (C 1 0.76)/(6.70 0.76) = (C o 0.76)/ 5.94 Fig 9.30 Photomicrograph of a 1.4 wt% C steel having a microstructure consisting of a white pro eutectoid cementite network surrounding the pearlite colonies. Nonequilibrium Cooling For the discussion of microstructural development in this chapter, it has been assumed that upon heating and subsequent cooling that sufficient time has been allowed at each stage of phase change shown in the phase diagrams that conditions of metastable equilibrium have been obtained. Nonequilibrium Cooling cont. However, in most situations it is not practical or desirable to wait such a long time, and therefore generally speaking nonequilibrium states are usually present, especially at lower temperatures (i.e. room temp) and are the subject of the next chapter. The Influence of Other Alloying Elements on the Euctectoid Temperature and Composition When other alloying elements such as Titanium, Molybdenum, Tungsten, Nickel, Chromium, Manganese, Silicon, etc., are present in an iron alloy, they tend to lower the eutectoid composition and in some cases to also raise the eutectoid temperature as well. Reference Figures 9.31 and 9.32 Fig 9.31 The dependence of eutectoid temperature on alloy concentration for several alloying elements in steel. Fig 9.32 The dependence of eutectoid composition (wt% C) on alloy concentration for several alloying elements in steel. Homework HW #8 9.5, 9.6, 9.11, 9.22, 9.51, 9.59 Due 4/13 Next Class: Read Chapter 10 ...
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