The ability of performing in situ high temperature electron backscatter diffraction analysis has shown to enable the study of important material processes. A heating stage has been developed, dedicated for this purpose. This heating stage is capable of reaching temperatures of over 1500°C using laser heating independent of sample material and without damaging the scanning electron microscope.
The heating stage minimizes the thermal drift of the sample and is compliant to the thermal expansion of its components. A novel type of heatshield leads to a decrease in radiation towards the EBSD detector and its active cooling keeps the exterior of the heating stage at room temperature.
Various heating techniques have been investigated and have shown laser heating to be the most suitable as no external influences are introduced inside the vacuum chamber. A coarse design has been conceived taking into account various design principles such as thermal centres and thermal isolation. Through thermal simulations, this design has been optimized for thermal stability, manufacturing and meeting the set requirements. This optimized design has been manufactured and tested.
While the faulty use of a thermocouple caused the system to overheat, the system has shown its capabilities in reaching the goal temperature while remaining thermally stable and not damaging any components of the SEM. By comparing the a post-mortem EBSD scan of the heated sample and comparing it to a non-heated counterpart, the stage’s potential has been shown. In addition, the use of a novel heatshield has been validated for a range of 69° to 75° sample tilt angles. Combining this heatshield with the use of the developed EBSD detector screen cooling, the thermal influence on EBSD data acquisition is minimized. Finally, the use of a proposed vacuum feedthrough for the optic fibre has shown to work adequately as the vacuum has reached similar values to regular operation.
In order to validate the set-up’s potential, proof of principle experiments should still be conducted;
two possible examples have been mentioned in the previous chapter. However, before being able to perform research with the dedicated heating stage, the thermocouple and its location should be changed. A type R,S or B thermocouple should be used for their temperature range, and it should be located touching both the bottom on of the alumina blocks in the nest of springs and the SiC disk.
With this, the maximum temperature of the stage can be monitored to avoid overheating. In addition, a thermocouple should be placed at the sample surface to accurately measure the temperature at which material processes occur. To calibrate the thermocouple a phase transformation experiment can be conducted on pure iron. As the phase transformation temperatures are known, they can be used to verify the validity of the thermocouple read-outs.
While the aforementioned method of section 3 for determining drift is adequate, using digital image correlation can reveal the exact location and movement of the thermal centre, making it possible to quantify occurring drift.
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Chapter 2:
A retrofittable heating module for use in in-situ EBSD tensile experiments
Abstract
Continuing on the design of a dedicated heating stage for in-situ high temperature electron backscatter diffraction (EBSD) analysis, a heating module is proposed to be retrofitted in a EBSD specific tensile stage to perform heated tensile EBSD experiments. With this tensile heating module, material characterization may be performed under tensile conditions at a temperature of up to 1500°C. For the same reasons as mentioned in Chapter 1, laser heating is chosen to heat the sample.
The proposed design consists of a heater assembly and two hooks that accommodate a dog bone tensile specimen and can be clamped in the jaws of the existing tensile stage. These hooks support and pull on the sample in such a way that its middle line is thermally centred and the sample is free to thermally expand. By making use of components and materials with specific thermal properties, the tensile heating module is isolated from the tensile stage so that operation of the tensile stage is not influenced by the heating. In addition, a water-cooled heatshield is placed around the heated assembly to minimize thermal radiation into the SEM vacuum chamber. The thermal behaviour of the design has been validated using thermal simulations, leaving only the manufacturing of the tensile heating module before in-situ high temperature tensile tests can be observed and analysed using electron backscatter diffraction.
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1. Introduction
Most dynamic material processes such as phase transformations and recrystallization are not only initiated by thermal energy, but also by mechanical energy. Therefore, in addition to simply heating a sample, it is interesting to mechanically deform samples and observe the microstructural changes that occur. Even more interesting is the combination of high temperatures and mechanical testing. In addition to simply analysing materials under mechanical and thermal loads, heated tensile tests also form a useful tool in the analysis of formability and workability for material use in the industry. Also, in the aerospace and automotive industries, high temperature mechanical behaviour plays an important role with respect to the failure of materials. The capacity of looking at these processes on the microscopic scale enables better understating of material behaviour and opportunities to better them [35]. The observations made with high temperature deformation, may not only offer insights into material behaviour, but also offer validation for numerical models [36].
For heated tensile tests, the same limitations apply to heated tensile experiments as to the “simply”
heated experiments described in Chapter 1. First, the thermal radiation from sample and heater may potentially damage components situated within the vacuum chamber. This radiation will also heat up the detector screen and create thermal noise that is detrimental to the quality of the acquired data.
Second, infrared(IR) radiation and thermal electron emission of the sample are also caused by elevated temperatures. These difficulties do not differ for the tensile situation with respect to the heating assembly, as they are only material and temperature dependent.
While the same difficulties are present with respect to the dedicated heating stage (see the previous chapter), additional challenges arise when performing tensile experiments. Due to the dynamic nature of tensile experiments, the thermal stability cannot be ensured in the same way as for the heating stage. Where, previously, the sample was supported all around, the tensile sample is clamped at both extremities forming a double clamped beam. This results in a greater unpredictability and controllability of the thermal drift, thermal expansion and thermal vibration as the sample is, to a larger degree, free to move. Either these problems must be overcome, which possibly may not be feasible, or, these issues must be minimized in order to obtain the best result possible. While a relatively high precision module may be developed, it will be less precise than a stage developed solely for heating. With this, the stage both justifies its own existence with regard to deformation experiments, and the heating stage’s existence with regard to high precision.
1.1 Problem Definition
As the interest to characterize material processes under dynamic conditions and at elevated temperatures grows, it has become worthwhile to investigate options with respect to a heated tensile system dedicated to EBSD. As no system is known that can achieve temperatures upwards of 1000°C, a new device must be created to meet this need. In order to keep the cost low and increase the versatility of existing equipment, a heating module must be designed that can be retrofitted to an existing EBSD dedicated tensile stage.
Therefore the objective of this research is to perform high temperature deformation electron backscatter diffraction analysis at temperatures of up to 1500°C. To meet this objective, a retrofittable heating module will be designed for an existing tensile stage.
Page 30 To meet the previously described objective, a design goal is formulated. The main design goal is formulated as follows:
The retrofittable module should be able to achieve temperatures of up to 1500°C without damaging the imaging system or tensile stage. While in use, the operation of the heating system should minimally, and preferably not at all, influence the quality and the acquisition of data.
To achieve this design goal, the following requirements are formulated:
Temperature range of ambient to 1500°C
o No damaging thermal radiation to the system o Material independent heating
Comparing this list of requirements to that of the dedicated heating stage shows that both designs have the same criteria. The only difference lies in thermal drift and thermal isolation. In addition, as the tensile heating module is placed inside an existing tensile stage, it may not influence this stage, i.e. if the tensile heating module would have a thermal effect on the tensile stage, this would influence the workings and precision of the stage. Therefore, the module must be isolated from the stage.
2. Design
2.1 Application of the heating technique
For the tensile heating module, the heating method remains the same as for the dedicated heating stage as the same limitations in terms of external influences apply to this design. In addition, using the previously purchased laser, keeps the cost of the project down. However, the application of laser heating is less straight forward in the tensile heating module than in the dedicated heating stage.
Where in the dedicated heating stage the sample and SiC disk were pressed together against the alumina hood, creating a closed box and good thermal contact, this is not possible for the tensile heating module as the sample will be moving and the stage’s clamps do not permit a closed box.
Since a closed box is necessary for the laser light not to enter the vacuum chamber, another solution for the contact area problem and closed box issue must be found.
In addition, as the sample geometry is now a dog bone tensile specimen instead of a small disk, the amount of energy added to the sample and its distribution through the sample changes. The power needed to heat the disk shape sample of 3mm in diameter, is not sufficient to heat a dog bone specimen that has a larger volume. Therefore the first choice that is made, is to heat the specimen only in the middle and to observe only the heated area. This choice is not only beneficial to the
Page 31 necessary amount of power, but it also effects the heating of the clamps and tensile stage in a positive way, i.e. it won’t heat up as easily as for an entirely heated specimen.
2.2 Thermal protection of the SEM and the tensile stage
Similarly to the way in which the SEM is protected from the dedicated heating stage, the same principles can be applied for the tensile heating module. By limiting thermal conductance through narrow parts, minimizing contact area and using materials with low thermal conductivity, both the amount of hot mass and the energy needed to heat up this mass are kept to a minimum. In addition to the minimization of thermal energy in the system, the SEM can be protected from the system by using a heat shield. This heat shield can be designed in a similar fashion as that of the dedicated heating stage and could be modified from a hole to a slit if wider scan areas would be preferred over a small square.
Besides the SEM, the tensile stage must also be protected from thermal energy for it not to heat up.
This can be achieved by increasing the thermal resistance at the contact point between sample and stage, for example in the clamps. Additionally, besides the sample, all hot parts should be actively cooled by circulating water.
2.3 Minimization of thermal effects on data acquisition
As the factors that are detrimental to the image quality and acquisition remain the same as for the dedicated heating stage: thermal radiation, thermal expansion, and thermal drift, similar solutions must be found. Due to the dynamic nature of the tensile experiment, these solutions must differ from those created for the dedicated heating stage.
As previously mentioned, the sample cannot be pressed against a hood as it deforms during the experiment. This holds that, in the tensile case, the sample’s radiation may not only radiate freely from its top surface, but also from its sides and, depending on the heating system, also from its lower surface. To solve this problem, the heat shield must also shield the sides and possibly the bottom.
Contrary to the shielding, the accommodation of thermal expansion of the sample, is more easily achieved in the tensile case. In force controlled experiments and displacement controlled experiments, the thermal expansion in the length direction of the sample will simply result in, respectively, a larger displacement and a lower force. Lateral and vertical expansion aren’t a problem either as the expanding sample has no constraint due to its double clamped nature. The only precaution needed, is to minimize the amount of thermal displacement away from the scan area, which depends on the location of the thermal centre. Assuming that the scan area lies in the middle of the sample, the displacement is minimal if the sample’s thermal centre also lies in the middle of the tensile stage. If, for example the sample would be supported on one of the its sides, the lateral displacement would be significantly larger. This case is schematically shown in Figure 16 below.
Where the blue lines depict the axis around which the sample laterally expands (in the middle of sample (a) and the left edge of sample (b)) the cross sections A-A and B-B are used to depict the lateral expansion of the sample. As all points on the red line in the middle of the sample move in the same way, the analogy of supported beam from the dedicated heating stage also applies in this situation. Here, the red dot in the middle of the beam, may be perceived as the cross section of the aforementioned red lines. Thus the ratio of thermal displacement of any point on the red lines is
Page 32 given by l/(L0 + l). For a typical area of measurement of 100μm in the lateral direction and a sample of 1000μm in width, the thermal drift at 1500°C is reduced from 3.38μm, which can significantly interfere with EBSD maps, to an acceptable value of 338nm.
Figure 16:Schematic representation of the thermal expansion for both support cases: (a) the thermal centreline (blue) coincides with the sample middle, (b) the thermal centreline (blue) lies on the left edge of the sample. For both cases, the
beam analogy is shown on the left, where the red dot depicts any point on the sample centreline. The red crosses depict the location of points that were initially the edge of the field of view.
2.4 Coarse Design
As the design must be retrofittable to an existing tensile stage, the design space is limited and the heating module must consist of various part in order to fit in the tensile stage. The module consists of two hooks that can be clamped in the tensile stage and thermally isolate the stage from the heated sample. From behind, the heating system is inserted into the tensile stage. This system presses a heated rod against the back of the sample in order to heat the sample.
2.4.1 Design Components
As a large contact area between the sample and a clamp is equivalent to having a large potential for energy conduction, the choice is made to create a hook instead of a clamp. This hook can simply be clamped in the tensile stage and the sample hooked in. This does not only decrease the potential for energy dissipation, but also allows for the thermal isolation of the sample with respect to the tensile stage by inserting a thermal resistor between the sample and hook. In addition, as the tensile heating module is an additional, intermediate component, the system can be actively water-cooled without having to modify the tensile stage itself. The proposed assembly of hooks and sample is shown in Figure 17.
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Figure 17: Both hooks that can be clamped in the tensile stage. The holes in the sides are used to cool the hooks while the white inserts are zirconia inserts to isolate the sample (which is depicted in turquoise) from the hooks.
In order to isolate the sample, the hooks are manufactured out of tantalum for its low thermal conductivity. To lessen the heat flux towards the tensile stage, a badly conducting ceramic insert is used between the sample and the hook. In the dedicated heating stage, alumina was chosen as a thermal isolator in proximity to the field of view for its high maximum operational temperature and thermal conductivity. However, as the sample only heats up to 1500°C in the middle and remains below 1000°C at the edges, zirconia can be used as inserts at the hooks. Zirconia conducts even less than alumina and therefore isolates the tantalum hooks from the sample even better. Finally, water ducts are created in the body of the hooks to accommodate for active cooling and limit the increase in temperature of the stage.
Figure 18: (a) Cross section of the heating assembly with the heatshield where the heated rod and SiC disk are suspended from a plate, which is depicted in grey. Also, the inside of the cooling of the heatshield is shown (b) Close-up of the aforementioned suspension where the vertical rods are connected to the plate to provide the suspension. The black disk is
the SiC with the heated rod inserted on top (right)
Similarly to the technique used in the dedicated heating stage, for the tensile heating module, the system makes use of a SiC disk upon which the laser shines and heats up making the heating process independent of the sample material. However, as the SiC must be pressed against the sample for the heat to travel to the sample and the SiC must be shielded for the laser not to pass by it, pressing the SiC directly against the sample is difficult to accomplish. Therefore a system is made where the heating assembly is shielded from the tensile stage and the rest of the SEM, and only a small heated rod sticks out which is pressed against the sample. This is depicted is Figure 18 on the right. Here, the SiC disk is supported by a suspended platform. The platform is suspended from a thin sheet of steel
Page 34 that is connected to the cooling circuit. Steel is chosen as the local temperature in the plate may rise significantly. As the heating assembly is actively water cooled and the SiC is shielded, the tensile stage and SEM are under no influence of either light or heat, other than emanating from the sample.
Finally, to shield the chamber and components in the chamber form thermal radiation, a heat shield is placed over and around (to shield the tensile stage) the sample. Where for the dedicated heating stage, the heatshield had a conical hole machined into it, the heatshield for the tensile heating module has this same hole only it is also machined along a line to allow for a broader scan area. For
Finally, to shield the chamber and components in the chamber form thermal radiation, a heat shield is placed over and around (to shield the tensile stage) the sample. Where for the dedicated heating stage, the heatshield had a conical hole machined into it, the heatshield for the tensile heating module has this same hole only it is also machined along a line to allow for a broader scan area. For