Analyse de la dilatation thermique par dilatométrie
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Overview
Source : J. Jacob Chavez, Ryan T. Davis, et Taylor D. Sparks, Department of Materials Science and Engineering, The University of Utah, Salt Lake City, UT
L’expansion thermique est extrêmement importante lorsqu’on considère quels matériaux seront utilisés dans les systèmes qui connaissent des fluctuations de température. Une expansion thermique élevée ou faible dans un matériau peut ou non être souhaitable, selon l’application. Par exemple, dans un thermomètre liquide commun, un matériau avec une expansion thermique élevée serait souhaitable en raison de sa sensibilité aux changements de température. D’autre part, un composant d’un système qui connaît des températures élevées, comme une navette spatiale qui rentrent dans l’atmosphère, aura besoin d’un matériau qui ne se dilatera pas et se contractera avec de grandes fluctuations de température afin d’éviter les contraintes thermiques et Fracture.
La dilatométrie est une technique utilisée pour mesurer les dimensions de la surface, de la forme, de la longueur ou des changements de volume d’un matériau en fonction de la température. L’une des principales utilisations d’un dilatomètre est le calcul de l’expansion thermique d’une substance. Les dimensions de la plupart des matériaux augmentent lorsqu’ils sont chauffés à une pression constante. L’expansion thermique est obtenue en enregistrant la contraction ou l’expansion en réponse aux changements de température.
Principles
Procedure
Results
The results of dilatometers generally include data of temperatures, expansion lengths, and time. Different softwares used together with dilatometers can return results in different ways. Some softwares only return data points, while others have plotting functions and other analysis features. The software used in the procedure above used WorkHorseTM. This program returns data in a .txt file that can then be plotted using a software such as- Matlab, Qtgrace or Excel. Figure 2 shows three different metals expanding and contracting as temperature is raised and lowered.
Figure 2: The change in length as function of temperature is plotted for stainless steel, cold worked steel, and aluminum. Samples are heated and then cooled with continuous length measurements to observe whether any hysteresis exists.
Thermal expansion is not always a linear function. This means that the coefficient of thermal expansion is not always constant. As seen in Figure 2, there is an unusual thermal expansion event that occurs in cold worked steel between 700oC and 900oC. In the case of stainless steel and aluminum the thermal expansion, as well as contraction, follow a linear slope. However, for the cold worked steel the expansion and contraction follows a non-linear change. This can be attributed to the dislocations in the cold worked steel. Locations where dislocation recovery occurs can experience different expansion/contraction as opposed to locations where normal expansion/contraction occur.
Applications and Summary
Dilatometry is a technique for measuring the dimensional thermal expansion of a material. Frequently this value is found by measuring the change in length as a material is heated and cooled. Thermal expansion is quantified by change in length divided by initial length. In addition to thermal expansion, the technique offers insights into vacancy formation, phase changes, and dislocation evolution in response to heat treatments.
While determining thermal expansion of materials is a very popular use for dilatometers, there are other applications for them. For example, these instruments can also be used as a method to monitor phase changes in certain alloys. Determining dislocation densities is another application of dilatometry.
Monitoring Phase Changes: The application of dilatometry in phase change research is due to the change of the specific volume of a sample during a phase transformation. Lattice structure changes when a material undergoes a phase change. By recording the transformations taking place over a range of conditions, it is possible to present the results in a graphical form. This shows the formation temperatures of microstructural constituents that may be obtained for a given cooling or heating condition. This technique is widely used to study the transformation behavior of steels during continuous heating, cooling, and isothermal holding. This has immense value in metallurgical applications. It is important in engineering industries where steels are used for construction.
Dislocation Densities: Dislocations occupy a volume and therefore, as dislocation density changes in response to heat treatments, dilatometry can be used to observe and quantify dislocations. High resolution dilatometry has extended the technique to the study of texture changes and rearrangement and annihilation of dislocations related to the recovery and recrystallization process. High-resolution dilatometry, together with a model of isotropic dilatation and atomic volumes can be used estimate the dislocation density introduced in microstructures due to isothermal decomposition of austenite.
Transcript
The thermal expansion of a material is extremely important when considering its use in a system with fluctuating temperature. Dilatometry is a technique used to measure the area, shape, length, or volume changes of a material as it experiences fluctuations in temperature. Thereby enabling the calculation of thermal expansion. In this video we will introduce the dilatometer, and demonstrate how to measure the thermal expansion of a metal sample in the laboratory.
Dilatometry is first performed by measuring the initial length of the sample using calipers. Then the sample is placed in a furnace, and in the case of this experiment, connected to a vertical push bar. A purge gas flows through the furnace to provide consistent conditions and prevent oxidation of the sample during heating. The sample then is heated to a predetermined temperature at a specified rate. The thermal expansion of the sample is transferred to the push bar, which is then transferred to the displacement sensor. Most materials expand with increased temperature and then contract upon cooling. Since the rod is also exposed to the high temperature in the furnace, it too experiences thermal expansion and contraction. Thus the measurement must be corrected to account for this.
The thermal expansion experienced by the sample is calculated by dividing the change in length by the initial length of the sample. This yields the average linear thermal expansion of the material. We can calculate the linear thermal expansion coefficient, αL, by dividing the average linear expansion by the change in temperature experienced. The volumetric expansion coefficient, αV, is then 3-times the linear expansion coefficient for isotropic materials. Some anisotropic materials, meaning a materials whose properties are direction dependent, may exhibit different linear expansion coefficients in different directions. Now that you’ve learned the basics of thermal expansion using a dilatometer, let’s take a look at the technique in the laboratory.
To begin, power up the dilatometer operating system and allow the sample to sit at room temperature to equilibrate. Make sure that the cooling system for the instrument is running, and that nitrogen gas is connected to the furnace. Do not turn the gas flow on yet, the gas will be turned on when the furnace turned is turned on. Now check that the calibration run has been performed on the system prior to testing your sample and select the most recent calibration that meets or exceeds your maximum temperature range and preferably is run at the same temperature ramp rate. Here we will use a previously conducted calibration run of the standard crystal locks. Next, accurately measure the length of the sample using high quality calibers.
Take several measurements along the length in order to establish the measurement error. Ensure that the sample is long enough to allow the push rod to exert some force on top of the sample. If it is not tall enough, use a spacer of a material with known thermal expansion and measure its height so that the spacer can be subtracted from the results. If a spacer is used, it must be parallel to the sample within the 1º. Then power the system on and ensure that the furnace is close to room temperature. Now raise the tube-chamber out of the furnace by pulling the knob on the side to release the tube. Raise up the tube and clean the bottom surface of the chamber with isopropanol and a wipe to ensure that the sample has a flat place to stand. Then, place the sample in the furnace with the flat surfaces towards the bottom of the chamber and the push rod and lower the push rod until it contacts the top of the sample. Lower the tube chamber containing the sample back into the furnace and ensure that the sample did not shift by checking the displacement gauge. Now, input the heating parameters into the dilatometer operating system.
Here the metal sample will be heated 20º-1000ºc at a constant rate of 5º/minute. To cool the furnace, just allow the temperature to equilibrate with room temperature. Before starting the test, double check that all systems are all on and functioning. Turn on the nitrogen purge gas and ensure that it is flowing to the system. Then initiate the test and check back periodically to make sure that it is running appropriately. When the run is complete and the system has cooled back to room temperature, export and save the data. Then repeat the scan another 2 times to account for any exaggerated expansion on the first run. After all runs have been completed and all data saved, ensure that the furnace is cool. Then raise the tube out of the furnace and remove the sample. To raise the tube out of the furnace, pull the black knob on the side of the furnace to release the tube. Finally, shut down the furnace, cooling system, and purge gas.
Now let’s take a look at the results. The program returns the values for: 1. Time, 2. Sample Temperature, 3. Gauge Reading, 4. Corrected Expansion, 5. Time in seconds, 6. Dimensionless Gauge Reading, 7. System Correction. First, calculate the change in length of the sample for each temperature point using a spreadsheet program, and then divide each value by the original length to obtain values of ΔL/L. Then plot ΔL/L vs Temperature. As you can see from the plot here, 3 metals were heated to a preset temperature and then cooled back down to room temperature. Though it was heated to a lower temperature, aluminum exhibited a more significant thermal expansion than stainless steel or cold worked steel.
In the case of aluminum and stainless steel, thermal expansion and contraction both follow a linear slope; meaning that thermal expansion was linear. And the linear expansion coefficient was constant. However, the thermal expansion is not always linear, meaning that the linear expansion is not always constant, as we see for cold worked steel. The cold worked steel sample exhibited a non-linear change between 700º and 900º, which can be attributed to defects in the lattice structure of the material called dislocations.
It is important to understand the thermal expansion and contraction of materials for a wide range of applications. For example, it is essential to account for the thermal expansion for materials when designing structures such as railroads and bridges. The thermal expansion of sections of railroad tracks is the main cause of rail buckling, which caused almost 200 train derailments in the US over a period of just 10 years. The measurement of thermal expansion using dilatometry can also be used to examine defects in crystals. Dislocations are defects in a materials lattice structure, and can take many different forms such as a point dislocation where one atom is missing, or an edge dislocation where an extra half plane of atoms is introduced in the lattice. Since dislocations occupy volume, density changes in response to heat treatment. Thus, high resolution dilatometry has extended the technique to study rearrangement of dislocations. Essential to understanding strength and possible areas of failure.
You have just watched Joves introduction to the analysis of thermal expansion via dilatometry. You should now understand the fundamentals of thermal expansion, the dilatometry technique, and some areas where thermal expansion is analyzed in structural and materials engineering. Thanks for watching.