Phase Diagram

Write a short summary of your lab activity – much the same as you would for a lab activity summary report. RESULTS: (fill in this table with your calculation results) System Bulk composition & Temperature Number of phases Composition of each phase Relative quantities of each phase Figure number Phase Diagram Microstructure Unknown sample from SEM Image: (insert your calculation results here) • Total intersections: • Al2Cu intersections: • Wt% of Al2Cu: • Wt% of α Al • Bulk composition: DATA AND CALCULATIONS: (Insert your raw data. Insert your microstructure images. Insert the relevant phase diagrams. Organize the images/figures in a logical way. Include figure numbers / captions.) Objectives: 1. To study the effect of alloy composition on microstructure. References: Callister, W.D., Materials Science and Engineering: An Introduction. Chapter 9 Introduction: Phase Diagrams A Phase Diagram is a graphical representation of what phases are present in a material system at various temperatures, pressures, and compositions. They are used for many applications in materials and engineering including: 1. Predict what phases are in equilibrium for selected alloy compositions at desired temperatures. 2. Determine the chemical composition of each phase. 3. Calculate the quantity of each phase that is present. Definitions Phase (P) - a macroscopically homogenous body of matter. This can be solid, liquid or gas. In a solid it can also be a volume of material that differs in structure and/or composition from another region in the same material. Component (C) - an element, compound or solution in the system. Degree of Freedom (F) - The number of possible variables such as pressure, temperature, and composition. Gibbs Phase Rule The Gibbs Phase Rule describes how many phases can be present under given circumstances at equilibrium. The general equation is: P + F = C + 2 However, for most engineering Phase Diagrams, the pressure is assumed to be constant at 1 Atmosphere (1 atm). Therefore, one degree of freedom is removed from the system and the Gibbs Phase rule must be modified. Condensed Gibbs Phase Rule (assumes constant pressure) P + F = C + 1 Phase diagrams typically are divided into categories by the number of components in the system. They are: Unary – 1 Component (pure) Binary – 2 Components Ternary – 3 Components Quaternary – 4 Components Pure Components A pure component is solid until it reaches the melting point temperature. At the melting point temperature some of the material begins to melt causing a partially solid and partially liquid material. As energy is added, more and more solid turns to liquid until eventually all the solid has become liquid. At temperatures above the melting point, the material is entirely liquid. (Think of the melting of an ice cube.) Binary Alloys: There are two types of binary alloys: 1. Alloys in which both components are 100% soluble in each other. This is analogous to having two different colored containers of water; one red the other blue. When mixed, the blue and red water become completely mixed and the result is purple water. Cu-Ni and Nb-Ta are examples of metals that are 100% soluble in each other. In the case of Cu-Ni, both atoms are approximately the same size and have the same crystal structure (FCC). So they mix easily. The Phase Diagram for this alloy is very simple. (Figure 1) Figure 1: Cu-Ni phase diagram. (Figure 9.03 from Callister, 8th Ed). It shows 3 regions: Solid  (alpha) – a 2-component material of Cu-Ni Liquid (L) Liquid +  The line separating the  and the  + Liquid is called the Solidus Line. (Solidus – the locus of temperatures below which all compositions are solid.) The line separating the L from the + Liquid is called the Liquidus Line. MATLENG 201 Lab Manual – Spring 2018 46 (Liquidus – the locus of temperatures above which all compositions are liquid.) Every phase diagram of 2 or more components must show a liquidus and a solidus and an intermediate freezing range. At the left axis is 100% Cu. This is the pure component. At this point the solidus and liquidus lines meet. As discussed above this is because the pure component (100% Cu in this case) has a well-defined melting point. The same is true at the right axis of the diagram, 100% pure Ni. However, in the region between the pure components, there are the alloys of Cu and Ni. These alloys do not have a well-defined melting point. They melt over a range of temperatures defined by the solidus and liquidus lines. Additionally, at each temperature, the composition (i.e. the ratio of Cu to Ni) also varies. In this case, as the alloy begins to melt the liquid will be rich in copper and the solid will be rich in nickel. Eventually as the Solidus line is hit, and the material is all liquid the ratio of Cu and Ni must return to the original composition of the alloy. 2. Alloys in which each Component is NOT 100% soluble in the other. This is similar to having blue water and red oil. Since oil and water do not mix, the materials will always be separate with possibly a very thin line of purple between them. In materials this can be because the sizes of the atoms are very different (e.g. Fe-C), have different crystal structures (e.g. Fe-C), or form chemical compounds (e.g. CuAl2). In this case as the material freezes or melts it will form not only liquid, but will also form different phases of solids. This means that, although the material is 100% solid, some of the grains will be rich in one component while other grains will be rich in the other component. A good example of this is shown in the Al-Cu Phase Diagram (Figure 2). The composition of each phase (i.e. the ratio of Al to Cu in the Copper rich phase) can be determined directly from the phase diagram. Lever Rule It is important to determine the relative quantity of each phase that is present at a given temperature. The tool used to do this is called the lever rule. In order to use the lever rule, a tie-line is constructed. This tie-line is a horizontal line made at the given temperature extending across the two-phase region which connects the phased boundary lines on either side. 1. A tie-line is constructed across the two-phase region at the temperature of the alloy. MATLENG 201 Lab Manual – Spring 2018 47 2. The intersections of the tie-line and the phase boundaries on either side are noted. 3. Perpendiculars are dropped from these intersections to the horizontal composition axis from which the composition of each of the respective phases is read. Given a material of a given composition, CO, that Phase A is on the left side of the boundary and Phase B is on the right side of the boundary, we have the equations: Weight Fraction of A =Wt%A=(CB-CO)/(CB-CA) x 100 Weight Fraction of B =Wt%B=(CO-CA)/(CB-CA) x 100 CA+CB=1 Procedure: 1. Metallographic samples of various alloys have been prepared. a. Cu – 30wt% Zn (a – Brass) – Cold rolled and recrystallized b. Al – 12wt% Cu – as cast c. Al – 33wt% Cu – as cast d. Sn – 25wt% Cu – slow cooled from liquid 2. Take optical micrographs of each of the samples at magnifications that best represent the scale of each structure. Sometimes several images at different magnifications are best. You decide. 3. Observe the microstructure of the unknown sample and obtain a microstructure image. The alloy contains aluminum and copper. Work with your classmates and TA to figure out a way to estimate the elemental composition of the alloy (what is the wt%Cu and wt%Al of the entire material)? Results (for steps 1 and 2): 1. Image of each alloy microstructure 2. Discuss with your TA whether or not each sample is "equilibrium" and how to account for that when analyzing the structure. 3. Accompanying phase diagram showing original composition and tie-line at the lowest temperature shown on the diagram. (You can use the “SnapShot” tool in Adobe Reader to copy the phase diagrams from this lab notebook into your report. Be sure to include a reference in your report if you do this.) 4. Determine the composition of each phase from the phase diagram. 5. Determine the relative quantities of each phase using the lever rule, if necessary. Results (for step 3): 1. Image of the unknown sample microstructure 2. Estimation/calculation of the elemental (bulk) composition (show your work / method used for this estimation) MATLENG 201 Lab Manual – Spring 2018 48 Figure 2: Partial Al-Cu Phase Diagram. (Figure taken from “Teach Yourself Phase Diagrams” by Granta Design) MATLENG 201 Lab Manual – Spring 2018 49 Figure 3: Cu-Zn Phase Diagram (Figure taken from “Teach Yourself Phase Diagrams” by Granta Design) MATLENG 201 Lab Manual – Spring 2018 50 Figure 4: Partial Sn-Cu Phase Diagram