MME292 Metallic Materials Sessional

Transcription

1 Department of Materials and Metallurgical Engineering angladesh University of Engineering and Technology, Dhaka MME292 Metallic Materials Sessional July 2016 Term Experiment 2 Construction and Interpretation of Phase Diagrams Theoretical ackground 1. Introduction to the Phase Diagram binary phase diagram of a system is a temperature - composition map which indicates the phases present at a given temperature and composition at its equilibrium condition. lthough the data for the construction of equilibrium diagrams can be determined experimentally by a variety of methods, thermal analysis method is by far the most widely used. In this method, cooling rates are recorded experimentally over a range of compositions. We will use these diagrams to understand and predict the alloy microstructure obtained at a given composition. binary phase diagram shows the phases formed in differing mixtures of two elements over a range of temperatures. Compositions run from 100% Element on the left of the diagram, through all possible mixtures, to 100% Element on the right. The composition of an alloy is given in the form - x%. For example, Cu - 20%l is 80% copper and 20% aluminium. Weight percentages are often used to specify the proportions of the alloying elements, but atomic percent may also be used. The type of percentage is specified e.g. Cu - 20wt%l for weight percentages and Cu - 20at%l for atomic percentages. We shall use weight percentages throughout this experiment. lloys tend to solidify over a temperature range, rather than at a specific temperature like pure elements. t each end of the phase diagram only one of the elements is present (100% or 100% ) and therefore a specific melting point exists. Sometimes there is a mixture of the constituent elements which produces solidification at a single temperature like a pure element. This is called the eutectic point. The eutectic point can be found experimentally by plotting cooling rates over ranges of alloy composition. The phase diagrams for some very simple binary alloys, however, do not have eutectic points. 1

2 Temperature Temperature melting point of pure melting point of pure eutectic point y cooling alloys from the state and recording their cooling rates, the temperature at which they start to solidify can be determined and then plotted on the phase diagram. If enough experiments are performed over a range of compositions, a start of solidification curve can be plotted onto the phase diagram. This curve will join the three single solidification points and is called the us line. In the same way that sugar dissolves into hot tea (a solution) it is possible for one element to dissolve in another, whilst both remain in the solid state. This is called solid solubility and is characteristically up to a few percent by weight. This solubility limit will normally change with temperature. The extent of the solid solubility region can be plotted onto the phase diagram and labeled appropriately. solid solution of in (i.e. mostly ) is traditionally called alpha and a solid solution of in (i.e. mostly ) is called beta. It is worthwhile to note that some elements that are alloyed may have zero solid solubility; a good example is l - Si alloys, where aluminium has zero solid solubility in silicon. If an alloy s composition does not place it within the small solid solution regions at either side of the phase diagram, the alloy will become fully solid at the eutectic temperature, shown as the eutectic line on the phase diagram. t alloy compositions and temperatures between the start of solidification and the point at which it becomes fully solid (the eutectic temperature) a mushy mix of either alpha or beta will exist as solid lumps with a mixture of and. These partially solid regions are marked on the phase diagram. The region below the eutectic line, and outside the solid solution region, will be a solid mixture of alpha and beta, and is labeled to reflect this. 2

3 2. Determination of Phase Diagram using Thermal nalysis inary phase diagrams are usually plotted with temperature (T), in degrees centigrade or Fahrenheit, as the ordinate and the alloy composition (X), in weight percentage, as the abscissa. In the thermal analysis method of constructing phase diagrams, a plot is made of temperature (T) vs. time (s) of various alloys of the given alloy system at constant composition and pressure. Such plots, commonly known as the cooling curves, show a change in slope when a phase change occurs because of the evolution of heat by the phase change. The temperature at which such change in slope is observed is known as the arrest point. Depending on the alloy systems, four different types of cooling curves are resulted. These are shown in below. (a) (b) (c) (d) 3

4 In case of pure metals and eutectic alloys and most compounds, s solidify isothermally into solids and, thus, the cooling curves show a horizontal line at the melting or freezing point, Fig. (a). Most solid solutions solidify over a range in temperature (through which and solid phases coexist) and show two arrest points in the cooling curve, Fig. (b). The first and second breaks indicate, respectively, the beginning and end of solidification. Some alloys show a range of solidification and then complete solidification by forming eutectic, Fig. (c). For solid solutions having a limited solid solubility, the cooling curves show three arrest points, the first two for the start and end of solidification and the third for the precipitation of second phase (i.e., supersaturation), Fig. (d). 3. Determination of Isomorphous Phase Diagram s an example, we shall determine a hypothetical - binary isomorphous phase diagram using the cooling curve method. To do this, we have selected the following six alloy composition. lloy Composition Pure Pure Since the two metals and are completely soluble in both the and solid states, the solid phase formed in any - alloy will be a solid solution and the cooling curve such as Fig. (b) will be resulted. The result of determining a series of cooling curves of the following alloy compositions is shown in the figure below. In order to see the relationship between the cooling curves, they have been plotted on a single set of axes Percent Percent T 1 T 2 Time The cooling curves for pure metals and show only a horizontal line because the beginning and end of solidification take place at a constant temperature. However, since intermediate compositions form a solid solution, these cooling curves will be similar and show two breaks or changes in slope. The sense of the phase diagram, or some idea of its form, may be obtained by drawing a line connecting all the points that show the beginning of solidification, the upper dotted line in the above figure, and another line connecting all the points that show the end of solidification, which is the lower dotted line in the figure. The actual phase diagram for the hypothetical - system can now be plotted in a temperature vs. composition map. The appropriate arrest point temperatures of the respective alloys are taken from the cooling curves above cooling curves and plotted on the new diagram shown below. In this figure, the left axis represents the pure metal and the melting temperature T is marked along this line. 4

5 Similarly, T is marked. For the alloy 2 (80-20), a vertical line is drawn, and T 1 (the beginning of solidification) and T 2 (the end of solidification) are marked along this line. The same procedure is used for all other intermediate alloy compositions. L T T 1 Liquidus T 2 L + T Solidus Composition, wt.% The phase diagram consists of two points, two lines, and three areas. The two points T and T represents the freezing points of the two pure metals. The upper line, obtained by connecting the points showing the beginning of solidification, is called the us line. The lower line, obtained by connecting the points showing the end of solidification, is called the solidus line. The area above the us line is a single-phase region and any alloy in that region will consists of a homogeneous solution. Similarly, the area below the solidus line is a single-phase region and any alloy in that region will consists of a homogeneous solid solution. It is a common practice to write solution by capital letter L and the solid solution be Greek letter. In this case, let us label the solid solution as. Uppercase letters such as and are used to represent pure metals. etween the us and solidus lines, there exists a two-phase region. ny alloy in this region will consist of a mixture of a solution L and a solid solution. 5