Since the first scanning electron microscope(SEM) was created in 1935 by Knoll [1], the technique has been developed further to research materials at smaller and smaller scales. Where previously electron microscopy was limited to high vacuum and post mortem studies, efforts have been made to overcome these restrictions. For example, the ability of performing experiments under low-vacuum (ESEM), and the possibility to conduct in-situ tensile tests have been developed. An imaging technique that has become frequently used in the SEM since its conception in 1984 is electron backscatter diffraction (EBSD). This method is used to provide quantitative information on the microstructure of crystalline materials [2]. It can provide information on grain size, grain orientation and phases in the material and, with recent developments of high resolution EBSD, research into relative stresses and elastic strain has been performed [3, 4].
In EBSD, a flat, crystalline sample is tilted to an angle around 70 degrees with respect to an EBSD detector and 20 degrees to the electron beam. The entering electrons coming from the beam interact with the material and will backscatter. These electrons scatter towards the EBSD detector, which is equipped with a phosphor screen where incident electrons excite the phosphor layer to such an extent that it starts to emit photons. As the electrons exit the material at the Bragg condition, they interact and forms so-called Kikuchi patterns on the detector screen, which can then be indexed. These patterns contain information on the crystal structure at the location where the electrons exited the sample material. An example of such a Kikuchi pattern and a global EBSD set-up can be seen in Figure 1 below.
Where EBSD is used to characterise the microstructure of materials, the technique could historically only be applied in post-mortem studies, as in-situ heating could not yet be performed.
Figure 1: EBSD pattern of Nb at 15kV (a), Geometric set-up of EBSD detection (b) [5]
In the past 20 years, efforts have been made to perform in-situ heating experiments on various materials to be able to characterize their behaviour and structural evolution at elevated temperature [6]. These efforts have resulted in the ability to perform EBSD studies at temperatures of up to 1000°C. This temperature range makes it possible to perform in-situ characterization experiments [8-22].
By being able to characterize the microstructure of materials and combining this with in-situ heating, materials with high temperature applications can be analysed in ways that were not possible before.
For example, where crystalline material have been cold worked, upon heating to the recrystallization
Page 8 temperature, the microstructure change and can now be imaged in-situ. Looking at recrystallization in commonly used materials, it is found that the recrystallization temperature, defined as the temperature where recrystallization reaches completion in one hour, lies between a third and a half of the melting temperature of the crystalline material [7]. As the most commonly used metals do not have a melting temperature exceeding 2000°C, a maximum temperature a 1000°C for EBSD analysis would seem sufficient. However there are materials that form important exceptions, for instance, recent plans for the International Thermonuclear Experimental Reactor (ITER) call for material research at high temperature. In the fusion reactor, the divertors (where heat produced by the fusion reaction is extracted) are subjected to a heat flux of more than 10 MW/m2 [8]. The material chosen to armour these components is tungsten; because of tungsten’s high melting temperature [9]. Since the tungsten does not only endure the significant heat flux but is also subject to crystal damage due to neutrons, helium and hydrogen collisions originating in the fusion reaction [10], it is necessary to characterize its behaviour at elevated temperatures, replicating the fusion reactor. As tungsten has such a high melting temperature point (3.400°C), the temperature at which recrystallization will occur (and therefore microstructural changes in general) lies between 1200°C and 1500°C [11]. This makes it interesting to perform EBSD at these elevated temperatures.
While the use of EBSD at elevated temperature for recrystallization experiments opens up a new spectrum of research possibilities, other applications are also novel and useful. An example of such an application, is the monitoring of the processes occurring during the transformation of a molten, to a solid material. Where, for example, low carbon steel has a melting temperature that lies around 1600°C, high temperature EBSD would enable the in-situ observation of the phase transformation processes from temperatures close to melting, through the entire temperature range down to room temperature. As this can yield new insights in how a material behaves at elevated temperatures, it then becomes possible to determine which parameters to change during the manufacturing process to get a better material. In addition, an additional use of EBSD at elevated temperatures can be that of the analysis of heat-protective tiles used in ovens or on the exterior of space craft, which upon atmospheric re-entry reach temperature of 1650°C [12]. Analysing their structure behaviour may lead to novel insights as how to better the material resulting in, for example, weight reduction of space craft.
Typically, high temperature EBSD (HT-EBSD) is performed by heating the sample up to a certain temperature, rapidly cooling it and performing the EBSD scan at a lower temperature varying between room temperature and 550°C [13, 14, 15, 16, 17]. This way, the delicate parts of the SEM are not exposed to any extreme temperatures. With this technique, sample temperatures of up to 1180°C have been reached [14, 17]. The primary reason behind this type of heating sequence, is to protect the EBSD detector from the heat radiating from the sample and heater. Another benefit is the fact that scan time is not limited by thermal noise to the phosphor screen of the detector. If the scan would be done while the sample is at an elevated temperature, the detector would heat up over time. This increase in temperature decreases the signal to noise ratio to the point that the Kikuchi patterns cannot be recognized [18].
It can be argued that the previously described sequence is not truly in-situ as the actual EBSD scan is performed at lower temperatures. In order to qualify as in-situ HT-EBSD, it could be said that the EBSD scan should also be performed at high temperatures. Various experiments where the scan is performed during the heating have been conducted in conventional SEMs at elevated temperatures
Page 9 of up to 880°C [19, 20, 21, 22, 23]. Since HT-EBSD has proven its use and value in these studies, a dedicated SEM (CamScan X500 CrystalProbe) has been developed to overcome the difficulties encountered while performing HT-EBSD. This dedicated SEM has been used to conduct experiments at temperatures of up to 1000°C and has shown to be capable of maintaining these temperatures while simultaneously scanning for several hours [24, 25, 26, 18, 27].
As mentioned before, conducting EBSD experiments at high temperatures brings along several difficulties that influence the quality of the acquired data. Regarding the material, the elevated temperature gives rise to the emission of infrared (IR) radiation, thermal electron emission and causes thermal vibration of the sample [22]. Also, due to the high temperature and the tilt angle of the sample, image drift can play a significant role depending on the heating rate and fixation of the sample in the sample holder. As the sample rises in temperature, it starts to thermally expand. This expansion can cause the sample to move around and with that cause the previously mentioned drift.
By performing the experiments at constant temperature, the sample does not expand and drift can be minimized [23].
In addition, due to thermal radiation, delicate detectors in the vacuum chamber of the SEM may become damaged. To avoid this, detectors can be protected by shielding them or even removing them. Of course, as the EBSD detector is used, it cannot be covered or removed and thus, should be protected in a different manner. This can be achieved by placing a heat shield over the specimen and/or monitoring the phosphor screen’s temperature and pausing the experiment when its temperature gets too high [23]. Besides damaging the system, thermal radiation also causes thermal noise at the EBSD detector. To reduce this thermal noise, IR-filters have been fitted to the EBSD detector. To reduce noise from thermally emitted electrons (which have less energy than backscattered electrons), the scintillator layer (i.e. the phosphor screen) can be coated with an aluminium layer which stops the low-energy electrons [18]. Besides using filters to avoid noise, bias voltages have been applied to the heating stage, heat shields and grid electrodes to reduce noise as they capture thermally emitted electrons (or thermions) before they reach the phosphor screen [28].
Using these solutions in combination with background noise subtraction can even result in an increased band contrast for an increasing temperature [26].
The heat shields that are commonly used for in-situ HT-EBSD are generally open over the entire sample, and rather shield the heating system beneath the sample, and the sides of the sample. This leaves the entire sample surface free to radiate towards the delicate detectors and vacuum chamber.
As the heat shield will also heat up and the specimen can radiate freely towards the EBSD detector, scan time may be limited to avoid damage to the detector originating from this radiation [22].
The previously mentioned, HT-EBSD dedicated, CamScan X500 CrystalProbe SEM is able to perform experiments at high temperatures for several hours by overcoming some of the previously mentioned difficulties; it has a water-cooled heatshield, a tilted gun enabling a horizontal sample orientation and it uses an aluminium layer on its phosphor screen as IR- and thermion filter. A downside of this dedicated SEM is the fact one would have to purchase a very expensive system just to perform one type of experiment, while a maximum temperature of 1000°C still limits the range of applications, as explained above. Therefore, a new device is created in order to be able to perform EBSD studies at temperatures up to 1500°C. Moreover, as one of the major limitations of HT-EBSD is
Page 10 the phosphor detector screen, it is interesting to investigate the possibility of actively cooling the screen to simplify the achievement of these high temperatures.
Therefore the objective of this research is to perform electron backscatter diffraction at temperatures of up to 1500°C. To achieve this, a dedicated heating stage will be conceived together with a cooling system for the EBSD-detector screen to create a stand-alone high temperature EBSD system.
The newly designed heating stage should be able to achieve sample temperatures of up to 1500°C without damaging the imaging system. While the sample is being heated, the heating and operation of the heating system should minimally, and preferably not at all, influence the quality and the acquisition of data.
In order to achieve the design goal, the following requirements are formulated:
Temperature range of ambient to 1500°C
o Negligible damaging thermal radiation to the system o Material-independent heating
o Negligible thermal drift through thermal expansion
o Compliant to the thermal expansion of the stage and sample