Lab 7 Phase Diagrams. Objectives Concepts Background Experimental Procedure Report Requirements Discussion. Objectives. Concepts

Transcription

1 Lab 7 Phase Diagrams Objectives Concepts Background Experimental Procedure Report Requirements Discussion By Mutlu Ozer (Instructor of ENGR ) Objectives The objective of this experiment is to obtain the cooling curves for several lead-tin alloys and use this information in conjunction with the lead-thin phase diagram to determine the chemical composition of each alloy. Experimentally determine the lead-tin (Pb-Sn) equilibrium phase diagram to demonstrate phase equilibrium in a binary system. Show the effect of changes in composition on microstructure. Concepts A metal sample of lead and tin shift from a molten phase to a solid phase in a consistent manner such that its behavior can be plotted on a diagram, which could illustrate various amounts of solid elements as to the percentages of concentration within a given sample. An example of such an illustration is the lead-tin equilibrium phase diagram. Using this diagram, and tools provided by the Engineering Department at SFSU, you could successfully determine the composition of the sample and gain insight of the solidification process of metallic alloys.

2 Background Eutectic Systems If the elements in a two-component system have limited solid-state solubility, a singlephase solid solution can exist for only a limited range in composition. In this terminal solid solution, the solubility limit is a function of temperature. This is analogous to adding salt to water. When the water is saturated with salt, adding additional salt results in two phases, salt water and solid salt at the bottom of the glass. Increasing the water temperature will raise the solubility limit and the remaining solid can be incorporated into the liquid to make a single phase. The phase boundaries in the binary equilibrium phase diagram with terminal solid solutions represent this change in solubility limit with temperature. In a solid-state binary system where the solubility limit is exceeded, two solid phases will exist in equilibrium. A eutectic system can occur when terminal solid solutions exist on both ends of the binary equilibrium phase diagram. An example of a binary eutectic system is lead (Pb) - tin (Sn). Although the atomic size difference is less than 10%, Pb has an FCC crystal structure while Sn is an unusual metal with a non-cubic tetragonal structure. This results in limited solid state solubility with the maximum solubility of Sn in the FCC Pb equal to 19.2 wt%sn while only 2.5wt% of Pb is soluble in the tetragonal Sn structure. At compositions and temperatures, which exceed these solubility limits, two solid phases will exist in equilibrium. The phase is the FCC Pb with some substitutional Sn atoms and the phase is tetragonal Sn with only a few substitutional Pb atoms. Figure 7.1. Eutectic Pb-Sn Phase Diagram

3 These maximum solid-state solubility both occur at 183 which is referred to as the eutectic temperature. At this temperature, there exists a point on the phase diagram (a single combination of composition and temperature) where three phases (the two solids and a liquid) can exist simultaneously in equilibrium. This combination of temperature and composition is an invariant point on the binary diagram like the freezing point of water on the single component system the eutectic reaction where upon cooling represents the isothermal transformation of liquid into two different solids. Depending upon the overall bulk composition of the system, a variety of different equilibrium microstructures are possible. However, as mentioned above, equilibrium requires sufficient time for the system to find the minimum in free energy. In real systems, this is not always possible and non-equilibrium microstructures are common. When this same type of reaction occurs in the solid state where one solid decomposes into two new solid phases isothermally, this is called a eutectoid reaction. Experimental Phase Diagram Determination Cooling curves with a constant cooling rate provide a method to identify the temperatures where phase transformations begin and end. For a pure material (single component system) or a binary composition with an invariant transformation (e.g., eutectic composition), the cooling curve from liquid to solid shows a horizontal thermal arrest until the transformation is completed. In a single component system, this corresponds to the extraction of the heat of fusion at the melting point of the pure material. For binary materials, which experience a two-phase region (e.g., liquid and solid) upon cooling from the liquid, the cooling curves show changes in slope at the beginning and end of the transformations. By using a number of different bulk compositions, the temperatures of the transformations can be located and the phase boundaries experimentally determined. To accurately identify the phase boundaries requires many samples.

4 Figure 7.2. Cooling Curve

5 Experimental Procedure In this experiment you will find the chemical composition of an unknown sample mixture of lead and tin. To accomplish this; use two thermocouples, a crucible, a Bunsen burner, a recorder, and a lead-tin equilibrium phase diagram to plot the cooling curve of our sample mixture against known Proeutectic and eutectic points. The mixture was heated until it liquefied. As the mixture cooled, the recorder measured the change in temperature and its variation in slope provided information useful for calculating the proeutectic point and subsequently the chemical composition of the mixture the experimentation procedure as follows: Figure 7.3 Experimental set up 1) Ignited Bunsen burner. (Pb/Sn mixture already in crucible) (Thermocouple A already lodged in solid mixture) 2) Placed thermocouple B in ice filled thermos. 3) Turn on chart recorder: Set paper advance rate to 2cm per minute, Zeroed chart recorder at convenient x/y axis on graph paper. Turn off chart recorder. 4) Once mixture liquefied: Removed thermocouple A from mixture, Placed thermocouple A next to thermocouple B in ice filled thermos. Wait 30 seconds

6 5) Turn off Bunsen burner 6) Place thermocouple A back in molten mixture. 7) Immediately turn on chart recorder (stylus will be jumping to the right). 8) Over the next 10 minutes you will be observing the graph from the chart recorder. 9) When the mixture had completely solidified you will be proceeding to interpolate mv to C using the chromel-alumel thermocouples chart. With the Proeutectic point clearly identified determine % error then readjust your data to coincide with a known theoretical Proeutectic point and thus calculate the percentages of each metal in the mixture. Each section divides into four groups. This experiment uses four Pb-Sn compositions. The lab instructor will demonstrate the technique for determining the temperatures for phase transitions. Each group is responsible for phase transition temperatures from one of the other six compositions. By sharing the data and referring to the phase diagram in, connect the points of the experimental data to construct the entire diagram. Report Requirements 1) Draw the experimental Pb-Sn phase diagram. 2) Determine the composition of the alloy from the cooling curve and the phase diagram. 3) Describe the successive changes in structure that took place during solidification of the alloys. 4) What is supercooling? Was it observed in your experiment? 5) What is the eutectic point? Discussion 1.What conditions determine equilibrium in binary systems? 2.Why does the PbSn system exhibit a eutectic binary equilibrium phase diagram? 3.How do cooling rates influence the experimental phase equilibrium in the Pb-Sn system? 4.How can phase diagrams be utilized for engineering applications? Chapter X equilibrium phase liquidus solidus eutectic reaction eutectic temperature eutectic composition eutectic point Definitions (*) Chapter X invariant hypoeutectic hypereutectic austenite ferrite cementite pearlite eutectoid

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