Thermal Analysis of heating and cooling curves for phase diagram determination Practical 1P6 - v

Transcription

1 Thermal Analysis of heating and cooling curves for phase diagram determination Practical 1P6 - v Safety Hazards High temperature molten metal Control measures Safety glasses and lab coat must be worn throughout entire time when in the lab. Lab coat, safety glasses and visor, thermal resistant gloves, aprons and shoe protector are worn when working in the furnace room. Follow instructions, training and supervision. Incorrect operation of Follow instructions, training and supervision. furnaces Safety Rules: Demonstrators must be present when you are working with molten metal. Operate furnaces and equipment only as instructed. Do not play around with the settings. The furnaces and temperature inside the sample tube must not be allowed to go above 300C. Listen and follow instructions carefully. If you do not understand instructions, then ask before you perform the tasks. 1 1P6 Thermal Analysis

2 Contents 1- What you should learn from this practical 2- Overview of the practical 3- Experimental details 4- Data analysis 5- What should be in the report 2 1P6 Thermal Analysis

3 1. What you should learn from this practical Science This practical will help you to understand what phase diagrams mean, why they are useful and how they are determined experimentally. The practical ties in with the lecture courses on thermodynamics and microstructures. In this practical you will determine the phase diagram for a given alloy by thermal analysis. It will show that whereas pure metals freeze at one temperature, alloys usually freeze across a range of temperature. Eutectic alloys freeze at a single temperature, but produce two solid phases. This is all predicted by the Phase Rule. Practical Skills The practical involves the measurement and interpretation of cooling curves, and making an alloy of uniform composition. You will: Operate a simple tube furnace; Learn to handle molten metals safely; Learn how to set up equipment to carry on simple measurements. Understand the basic working principles of thermocouples and use them to measure the temperature of a sample as it is first melted and then solidified; Computing skills Use LabView to control hardware equipment and record temperatures; Use Matlab to import and analyse datasets. 3 1P6 Thermal Analysis

4 2. Overview of the practical There are seven pre-prepared samples: one of pure element X, one of pure element Y (not Yttrium) and the others of alloys with various compositions, as given in the table 1. On Day 1 you will test six of these samples and be given the data for the seventh alloy to analyse while your experiment is running. Sample A B C D E F G Composition (wt%) X X-10Y X-18Y X-22Y Y-43X Y-20X Y Table 1: summary of all sample compositions. Working as a group, you will record the heating and cooling curves of one of these pre-prepared samples and share the data with the other groups. You will then analyse all the datasets with Matlab and use these results to construct the phase diagram for the X-Y system. Your group will also choose a composition to make your own sample (labelled H n, where n is your group number) and obtain heating and cooling curves for it. You will again share your dataset (but not the compositions this time) for your alloy H n with the other groups, and using the phase diagram you have 4 1P6 Thermal Analysis

5 constructed you will determine the composition of all the other groups alloy H n. Finally, you will compare the obtained phase diagram with a given one from the literature and find which elements are X and Y. Rough Timetable Day 1: 1- Set up furnaces and measuring equipment; 2- Measurement of heating and cooling curves for pre-prepared sample; 3- Preparation of alloy H n of composition %X and %Y; 4- Preliminary data analysis of given curve for Alloy E (Y-43X). Day 2: 1- Measurement of heating and cooling curves for your alloy H n ; 2- Analysis of datasets from Day 1 samples, including those from other groups; 3- Determination of phase diagram; 4- Analysis of data for your alloy H n and another groups alloy H n ; 5- Determination of other groups alloy H n composition. 3. Experimental Details 3.1. Collecting heating and cooling curves using a tube furnace For this practical, each group is provided with one manually controlled tube furnace, a shared laptop running LabView for data acquisition and shared data logging hardware. There are three data logging units each of them connected to a laptop and recording the temperature of two experiments. The data logging is carried out using National Instrument hardware and software: a NI 9211 thermocouple module, which read a voltage and convert it to temperature, is 5 1P6 Thermal Analysis

6 connected to a laptop via a cdaq-9171 USB module, which allows the communication between the thermocouple module and the computer. Each module can read 4 thermocouples simultaneously and it is controlled by LabView (Laboratory Virtual Instrument Engineering Workbench). The temperatures will be recorded using thermocouples. Thermocouples are electrical device consisting of two dissimilar conductors forming electrical junctions at differing temperatures. They produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. There are several types of thermocouples differing in the combination of the alloys employed; in this experiment, we will use K-type thermocouples (chromel-alumen), which are well suited for measuring temperatures of non-reactive molten metals in the range 200 C to C. On Day 1, the specimens will have been made up already in crucibles and loaded into the tube furnace, but you will have to set up the data logging equipment and modify the given LabView code in order to be able to start the experiment. As a first task connect the thermocouples to the module, remember that they have a positive and a negative end and that polarity is important so attach them the right way around! (Hint: thermocouples wire colors are standard, google knows the answer, but you have to ask the right question ). When all thermocouples are connected, move to the laptop, launch LabView and open the file Thermocouple Data Acquisition_v2.vi (see Fig.1), which is in the folder named DataAcquisition_Labview within the practical main folder. The next window popping up is the front panel of the code, which was written for this practical (Fig. 2). Press ctrl+e to open the underlying block diagram (Fig. 3). 6 1P6 Thermal Analysis

7 Fig. 1: main LabView window. Open the file 'Thermocouple Data Acquisition v1.vi. 7 1P6 Thermal Analysis

8 Fig. 2: front panel to generate the GUI to carry on the data logging. Press ctrl+e to see the block diagram. Press the run button on the top left of the window to launch the program. As a second task familiarize with the diagram and compare it with the front panel. Try to understand the functioning and discuss with your group the following questions: - Where does the code begin? - How many loops are there in the diagram? - What is their role? - How often do the loops run? 8 1P6 Thermal Analysis

9 Fig. 3: block diagram running behind the front panel. Once you think you understood the overall functioning of the diagram return to the GUI window and try to run it pressing the run button ( ) on the top left to check if the thermocouple are well connected. If there are no issues, you should be able to read the 4 temperatures in the central plot otherwise you need to find the problem and fix it. If you get the program working, you will notice that the acquisition rate is very low. This was intentional and it is your next task to change it to a suitable value. To do so you need first to find out how to reset it and then to work out which would be a suitable acquisition rate. Stop the program pressing the red button Stop Programme. Do not stop it in any other way as it can cause problem of communication between the computer and the NI modules, in which case you might have to restart LabView. 9 1P6 Thermal Analysis

10 Go to the block diagram and try to identify where the sampling rate is set. When you find it, try to reset it to another value and test if it works. Now that you know how to correct the acquisition rate, you will then need to choose an appropriate value for the parameter (think carefully!). (Hint: you might find some help in the NI 9211 module manual). You are now ready to begin the experiment! Start the datalog pressing Start Log? and give the file a meaningful name. REMEMBER that each computer is reading the temperature of 2 experiments and also think about which temperature reading you will need to use in order to create your heating and cooling curves (write down which channel correspond to your experiment in your lab-book). The furnace is controlled by a Eurotherm temperature controller, first set up the target temperature (300 C). DO NOT TOUCH THE FURNACE SETTING AFTER THE TARGET TEMPERATURE HAS BEEN SET. Then turn on the furnace (big central button on the furnace controller) and one group member must remain by the equipment to watch the temperatures inside the sample tube increase to 300C. Once the temperature reaches 300C, immediately turn off the furnace DO NOT ALLOW THE TEMPERATURE INSIDE THE TUBE TO GO ABOVE 300C. The rest of the group go prepare your own alloy, specimen X (see instructions below), ready for analysis on Day 2.!!! DO NOT TOUCH THE FURNACE AFTER IS TURNED OFF!!! Move to the computer room for the rest of the lab 10 1P6 Thermal Analysis

11 Allow the tube to cool to below 110C before stopping the data logging. Remember that a single data logger is recording the data for two furnaces so do not stop the logging until BOTH experiments are complete. Depending on the specimen you are looking at, and when you started the run, this may take the rest of the afternoon. You will be sent the data for all of the specimens once it has been collected Making sample H n In order to prepare specimen H n, you must first choose its composition. Next, weigh the appropriate quantities of pure X and pure Y using the raw materials provided to give a total weight of about 100g. You should take a note of the actual weights and the scales you use for this so that you can discuss your errors in your write-up. Put the weighed X and Y together in the supplied crucible, and let the Teaching Technician know you are ready for further instructions. NEVER GO IN THE FURNACE ROOM WITHOUT SUPERVISION from a member of staff. Under the supervision of the Demonstrators, put on protective clothing (visor, gloves and apron). With their guidance, place the crucible into the furnace preheated to 350C using the long-handled tongs. Secure a glass test-tube in a retort stand ready to receive the molten metal. After 30 minutes, remove the crucible containing the molten metal and stir with a graphite rod prior to pouring the molten metal into a glass test-tube. While still wearing gloves and while the metal is still molten insert the rubber bung so that the alumina thermocouple-housing goes into the centre of the melt. Allow the specimen to cool for 10 minutes. Specimen H n is now ready for analysis on Day P6 Thermal Analysis

12 *** Meanwhile, check the computer to see whether it is time to turn off the heaters and start cooling. *** 3.3. Sample E After preparing specimen H n and started the cooling curves, there will be time to start considering data interpretation. You will begin to analyse an example curves of alloy E, which you will find in a file inside the practical folder. Following the instructions below, start to prepare a plot of the heating and cooling curves for this alloy, determining the temperatures where solidification starts and finishes. Consider how this information will contribute to your construction of the phase diagram. 4. Data analysis The analysis of the collected heating and cooling curves will be carried out using Matlab to determine the temperatures where solidification starts and finishes and construct the phase diagram. At the end of this practical you will have the datasets for the 7 pre-prepared samples (one of which your group will have analysed on Day 1), your group s alloy X and all the other groups alloy X (which you will not know the composition of). Your group should analyse each data set as follows. First of all open Matlab and load the script named s_thermalanalysis_1p6.m. You can do that by either drag and drop the file in the Matlab Editor or simply double clicking on it. The script will guide you through the entire data analysis process and at the end you should be able to construct the phase diagram. 12 1P6 Thermal Analysis

13 5. What should be in the report 5.1. Structure The report should be written in a descriptive form (remember it is not a questionnaire or a list of actions!), reporting the experimental and analysis methods and presenting and discussing the results in a concise, complete and clear way. The report should include a description of the analysis of the cooling curves (including those from samples H n ) and of the methodology used for the determination of the phase diagram. It should also contain a discussion based on the question listed in section 5.2. The report should be divided in four main sections: - Introduction: briefly introduce the topic and explain aims and objectives of the experiment. - Experimental methods: include all the relevant information regarding the experimental set up (e.g. type of furnaces, model and type of modules used for the temperature measurements etc.), but not extensive descriptions of the functioning of the hardware or its background science. - Results and discussion: report only the relevant results for the interpretation of the cooling curves, the determination of the phase diagram and alloy system and the identification of the H n sample compositions. Also explain how you built the phase diagram, and discuss about the differences between yours and the given one from the literature. - Conclusion: briefly summarize the experimental aim, findings and relevance of the work. 13 1P6 Thermal Analysis

14 5.2. Questions you should try to answer Which alloy system did you analyse? How different is the measured phase diagram compared to the literature? Why? (Discuss about measurement accuracy) Was the thermal analysis sufficient to uniquely identify the composition of samples H n? What else could you do to confirm the thermal analysis results? Why do the heating and cooling curves have the form that they have? Why do samples of different compositions have different curves? (discuss about the different type of cooling curve) At some point in some of the cooling curves, the temperature goes up. Why does this happen? Where is the heat coming from? What is represented by each region of the phase diagram? 5.3. Length The report should be submitted electronically and should NOT exceed 2000 words of text with a maximum of 4 figures. The figures should be done in Matlab and could be divided in subfigures (or subplots). Compulsory figures are: your phase diagram, a figure containing all the cooling curves you used for the determination of the phase diagram, a figure which explain the data analysis method. References and figure captions do not contribute to the word count. 14 1P6 Thermal Analysis

15 5.4. Marking considerations Marking will be between 0% and 100% and the following criteria will be used to evaluate your report: Correctness and completeness of data analysis and interpretation (40%) Clarity of the description of the experimental and analysis methodologies (40%) Clarity and quality of the figure and Matlab plots (20%) 15 1P6 Thermal Analysis