Eindhoven University of Technology MASTER In-situ electron backscatter diffraction at high temperature development of a heating and a tensile stage van Stratum, R.J.F.J.

MASTER

In-situ electron backscatter diffraction at high temperature development of a heating and a tensile stage

van Stratum, R.J.F.J.

Award date:

2019

Link to publication

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TU/e Mechanical Engineering

In-situ electron backscatter diffraction at high temperature: development of a

heating and a tensile stage.

2018-2019

Coordinators:

Committee:

dr.ir. J.P.M. Hoefnagels M.P.F.H.L. van Maris

dr.ir. J.A.W. van Dommelen dr. N.J. Dam

dr.ir. J.P.M. Hoefnagels

Author:

R.J.F.J. van Stratum 0818547

May 24

, 2019

Page 1

Page 2

Preface

The goal of this master project is the development of a set of tools that can be used for the in-situ observation and characterization of microstructures and their crystallographic evolution at extremely high temperatures of up to 1500°C. To this end, two devices have been conceived to be used in combination with the electron backscatter diffraction technique inside a scanning electron microscope.

The main part of this thesis consists of the design process, creation and proposed validation of a precision heating stage that is capable of reaching 1500°C inside the vacuum chamber of a scanning electron microscope while performing EBSD experiments. As the choice has been made to write the conception of the heating stage as a journal article, the first chapter is dedicated to the heating stage while various elements of the process and additional research can be found in the appendices.

In addition to the static heating stage, a module is designed that can be retrofitted to an existing, EBSD dedicated tensile stage. This module enables the possibility to perform in-situ tensile EBSD experiments at temperatures of up to 1500°C. Where the dedicated heating stage is designed to be a precision tool that minimally influences acquired data with respect to an unheated experiment, the tensile heating module is not able to reach similar accuracy due to the dynamic nature of tensile experiments. This justifies the need for two separate devices for two different applications in the same field. Since the tensile heating module has been designed but not yet built, and therefore not validated, its design is separately placed in Chapter 2 as it could not yet figure in a journal article.

For both devices, two systems must be in place before the devices can be used. First, as the detector heats up due to thermal radiation resulting in thermal noise, a cooling system for the EBSD detector is designed and proposed in Appendix A. Second, a simple vacuum feedthrough is created to allow the insertion of thermocouples and optical fibres inside the vacuum chamber of the SEM. The use of this new feedthrough was necessary as no feedthroughs were available that met this purpose and conventional flange feedthroughs are quite expensive. This feedthrough can be found in Appendix B.

As both the dedicated heating stage and tensile heating module use similar concepts and techniques in their designs, the underlying reasoning behind these elements is grouped together for both designs and is elaborated upon in Appendices C,D,E. As the heating technique used in both stages is quite important to the functioning of the devices, a significant investigation has been conducted to select the adequate heating source. This can be found in Appendix C. As the conception of a new type of heat shield was necessary, research was carried out in order to determine whether the new design could be incorporated in the dedicated heating stage and tensile heating module. The reasoning and experiments validating the use of this design can be found in Appendix D. In addition, thermal simulations have been performed to validate the designs and can be found in Appendix E. In addition to the incorporated design elements, sample preparation is described Appendix F. Finally, the detailed drawings of both the heating stage, tensile stage, flange feedthrough, and screen cooling can be found in Appendix G.

Page 3

Contents

Preface ... 2

Chapter 1: ... 6

Abstract ... 6

1. Introduction ... 7

2. Design ... 10

2.1 Selection of the heating method ... 10

2.2 Protection of the SEM ... 11

2.3 Minimization of thermal effects on data acquisition ... 12

2.4 Coarse Design ... 13

2.4.1 Design Components ... 13

2.4.2 Thermal Simulations ... 15

2.5 Optimized Design ... 16

2.5.1 Manufacturing ... 19

3. Experimental validation ... 19

3.1 Experimental results ... 21

4. Proof of Principle ... 24

4.1 Recrystallization of tungsten ... 24

4.2 IF-Steel Phase Transformation ... 24

5. Conclusion and Recommendations ... 26

Chapter 2: A retrofittable heating module for use in in-situ EBSD tensile experiments ... 28

Abstract ... 28

1. Introduction ... 29

1.1 Problem Definition ... 29

2. Design ... 30

2.1 Application of the heating technique ... 30

2.2 Thermal protection of the SEM and the tensile stage ... 31

2.3 Minimization of thermal effects on data acquisition ... 31

2.4 Coarse Design ... 32

2.4.1 Design Components ... 32

2.4.2 Thermal Simulations ... 34

2.5 Optimized Design ... 36

3. Conclusion ... 40

Appendices ... 42

Page 4

A. Detector Screen Cooling ... 42

A.1 Design ... 42

A.2 Validation ... 44

A.3 Conclusion ... 45

B. Flange Design ... 46

C. Heating System ... 48

C.1 Possible choices ... 48

C.2 Validation design ... 49

C.3 Experimental Validation ... 51

D. Heat Shielding Design ... 53

D.1 Heatshield design possibilities ... 53

D.2 Validation ... 54

D.3 Manufacturing... 56

E. Thermal simulations ... 57

E.1 Material ... 57

E.2 Heat transfer ... 58

E.3 Heating Stage ... 59

E.4 Tensile Stage ... 62

E.5 Detector Screen Cooling ... 66

F. Sample preparation ... 67

G. Detailed Drawings ... 68

G.1 Heating Stage ... 68

G.2 Tensile Heating Module ... 76

G.3 Screen Cooling ... 81

H. Code of Conduct... 82

Bibliography ... 83

Page 5

Page 6

Chapter 1:

A Laser powered heating stage

dedicated to in-situ EBSD: reaching extreme temperatures

Abstract

Electron backscatter diffraction has become a useful tool to explore morphological features of crystalline materials. Where previous research has shown for it to be possible to conduct in-situ high temperature EBSD studies up to a 1000°C, developments in the field of material technology call for even higher temperature EBSD studies. As no prior research has shown to be able to attain these temperatures, this work proposes a newly developed, EBSD dedicated heating stage for microstructural characterization at temperatures of up to 1500°C. In order to minimize influence of electrical and magnetic fields, a laser heating method is chosen. In combination with this heating system, a stage is developed that supports the sample in a thermally centred manner that allows for thermal expansion. Finally, the hot assembly is shielded by a water-cooled heatshield that protects the vital parts of the SEM. Thermal simulations have been performed to validate the design and the manufactured design has been experimentally validated. Finally, a set of research proposals is illustrated to demonstrate the capabilities of the heating stage and to show its added value to the field of experimental mechanics.

Page 7

1. Introduction

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 Sustaining 1 day

 Electron Backscatter Diffraction o Tilt range of 69 to 75 degrees o Minimum working distance of 25mm o Minimal thermal radiation

 Thermal stability

o Negligible thermal drift through thermal expansion

o Compliant to the thermal expansion of the stage and sample

2. Design

2.1 Selection of the heating method

Various heating techniques have been explored both by literature study and by performing experiments. During the search for an adequate heating method, it has been found that every technique using an electrical current inside the vacuum chamber of the SEM brings along some disturbance related to this electrical current. The majority of these disturbances are caused by the formation of magnetic and electric fields [29]. For example the formation of a ground loop causes vibration due to a constantly varying magnetic field and, applying a current to the heating system causes the image to shift. The heating of a sample occurs at different current magnitudes for different temperatures and the current is continuously varied in a control loop to keep the temperature constant. Combining this with a dependency on material properties, it is difficult to accurately predict the exact behaviour of these effects. While these types of problems may not be insurmountable, a simpler alternative exists: laser heating. Instead of using a resistive heater with a sample on top, a material absorbs the electromagnetic energy of a laser and transforms it into thermal energy [30]. The only input into the SEM vacuum chamber is an optical fibre carrying light and thus, this technique does not have any magnetic or electric influence inside the vacuum chamber. As the aim is to create a system that operates at a same level of quality as a non-heated system, the use of a heating technique that doesn’t influence the data acquisition is preferable. For this reason, the choice has been made to use laser heating. A more extensive description of the

Page 11 possible choices with respect to heating and the motivation for choosing laser heating, can be found in Appendix C. Heating System

While laser heating forms a good alternative to conventional heaters, it does not satisfy the requirement of material-independent heating, i.e. while some materials absorb light very well, others do not. To solve this problem the choice is made to add a heater material in between the incoming laser light and sample material. This material serves the purpose of absorbing the laser light, transforming it into thermal energy and conducting it to the sample. This automatically means that the heater material must be equally able to withstand high temperatures as the sample itself at the goal temperature.

Where different materials absorb different wavelengths of light with a certain efficiency, 3C-SiC (cubic silicon-carbide) does an excellent job when combined with a laser of wavelength 974nm. This combination results in an absorption of almost 99.8% of the light [31]. As lasers are expensive pieces of equipment for which the price increases with increased power, a high efficiency is preferable to keep the cost low. Besides efficiency of the material-laser combination and the thermal suitability of the material, the usage of lasers with the specific wavelength of 974nm is common in commercial applications such as laser welding. This makes the laser system relatively cheap with respect to an option that would use a laser that is not commercially available.

2.2 Protection of the SEM

While self-evident, the subgoal of minimizing influence and damage to the operation of the SEM or the SEM itself, is nevertheless important. One way of minimizing the potential for damage, is to minimize the amount of energy that is fed into the SEM. While losses due to heat transfer will inevitably occur, the losses are smaller than the energy fed into the system. Inherently, a system that needs 100W to reach the target temperature can inflict less damage to the SEM than a system that needs 200W. Looking at the simple equation for a temperature difference originating from a heat load, a solution is to minimize mass:

∆𝑇 = 𝑄

𝑚 ∗ 𝑐 ^{Eq. 1}

Here, Q is the amount of added energy, m is the mass and c is the specific heat. While negating the thermal losses, if the mass is halved, the needed amount of energy to reach the same temperature change is also halved. For this reason, not only is the complete stage designed to be as small as possible, it has also been decided to restrict the sample dimensions to be a disk of 3mm in diameter and a maximum of 1mm in thickness. The small dimensions do not only benefit the reduction in input energy, but also reduce the previously negated thermal losses, i.e. when the dimensions are kept to as small as possible, not only is the weight kept small, but also the area. As radiative energy loss is a function of area, this minimization of area leads to a minimization of thermal radiation.

Inevitably, not only the sample heats up, as some elements will have to touch the sample to support and fixate it. These elements should be designed in such a way that they thermally isolate the hot sample from its environment and the rest of the stage. This is done by using materials that have a low thermal conductivity and small cross-sectional area with a relatively large length. The reasoning behind this can be understood by looking at the temperature difference between two points as a function of heat flux, thermal conductivity and the distance between those two points.

Page 12

𝛥𝑇 = ^𝑘∗𝑙_𝑃 ^{Eq. 2}

From equation 2, It can be seen that for a given heat flux 𝑃, increasing the length 𝑙 between the two points results in an increase in temperature difference. As one of these points represents the sample and therefore has a constant temperature of 1500°C, increasing 𝑙 results in a decrease of the temperature at the other point, for example at a connection between sample and SEM. Together, minimizing mass and thermally isolating the sample takes care of keeping the thermal energy to a minimum.

Besides thermal radiation, light also has the potential to damage or influence the SEM. Since the laser needs to use a large amount of power to reach the goal temperatures, no light may escape from the heating stage into the SEM. For this reason, the room where the laser enters the heating assembly, is closed off so no light leaks out of the heating stage. This box will be designed to also serve as a heat shield that blocks thermal radiation from the sample towards the SEM chamber.

2.3 Minimization of thermal effects on data acquisition

Since the goal of any EBSD experiment is to gather data as accurately as possible, it follows that any system added to a regular ESBD set-up should not or minimally influence data acquisition and data quality. To minimize effects, first, the detrimental agents through which degradation occurs must be identified. In the case of heating experiments these are: thermal drift, thermal radiation and thermal expansion [22, 23, 18].

As mentioned in the previous section, heat shields are an effective way of blocking thermal radiation.

However, since the path of the incident electron and diffraction cone must be free of obstacles, a heatshield that completely covers the sample is not possible. The choice is therefore made to construct a partial heat shield. Here, instead of making a hole in the heatshield over the entire sample, only the areas where the electrons enter and exit the material are laid bare.

Figure 2: Schematic representation of a simply supported (a) and thermally centred expansion(b).

To minimize drift, the sample should be supported in all directions instead of being held in place by a clamp under which it is susceptible to slipping. In addition, if manufactured correctly, such a support should also allow for thermal expansion with minimum displacement of the measurement location.

By constructing a thermal centre (the point around which a body thermally expands) and placing this

Page 13 point in the path of the electron beam, the point itself will not move at all and the points around it will move as little as possible. This is depicted in Figure 2 above. For both cases, the field of view (the area where the data acquisition occurs which is depicted by the vertical grey lines) has a length l and is located in the middle of the sample with length L0 when at room temperature, i.e. the left two beams. When heated the rod expands from its original length L0 to its thermally expanded length LT , the thermal strain can be described as: λ = LT / L0 .

For the simply supported beam (a), the midpoint of the beam which first was in the centre of the field of view, is now located at LT/2 to the right. On top of this, the point at the edge of the field of view has moved lT/2 to the right, giving a total displacement of LT/2 + lT/2. The maximum displacement in the field of view for the simply supported case is thus given by: xa = λ*( L0/2 + l/2).

For the thermally centred support (b), while the thermal expansion is the same as for (a), its moves around the midpoint of the beam which remains centred within the field of view. Therefore the maximum displacement is simply given by the displacement of the edge of the field of view: lT/2, or xb = λ*(l/2). The ratio between the two cases xa / xb can be simplified to l/(L0 + l). Therefore, for a typical area of measurement of 100μm in diameter and a specimen of 3000μm in diameter, the thermally centred support reduces thermal drift by a factor of 30. In the case of tungsten (for which the coefficient of linear thermal expansion equals 4.5 μm/(m/K)) the difference in maximal thermal drift at 1500°C is reduced from 11μm, which can significantly distort EBSD maps, to an acceptable value of 360nm.

By combining a heatshield with a partially covered sample and a thermally centred support in the design, the quality of the data acquisition will be as close as possible to that of a regular, unheated EBSD experiment.

2.4 Coarse Design

2.4.1 Design Components

Following the previously described design elements, the heating stage is designed rotationally symmetric around the mutual centreline of the sample and the supporting 3C-SiC disk, see Figure 3a.

The sample assembly is placed inside a nest of horizontal springs (k1 in Figure 3) that form a thermally centred support by axisymmetrically exerting a radial, compressive force on the sample and SiC disk.

Simultaneously, three springs exert a vertical force on the SiC from behind (k2 in Figure 3). The three legs of the nest of springs have a V-groove at the bottom that rests on top of three recessed barrels that are part of the base plate., see Figure 3. This is done in order to accommodate for the radial thermal expansion of the nest of springs, while keeping the thermal centre in position, as shown in Figure 3b andFigure 4a. Both the nest of springs and the base plate are manufactured out of tantalum, see Figure 4b, due to its low thermal conductivity and high melting temperature [9].

The vertical force exerted by the springs, k2, is countered by a hood with three pawls that is placed over the nest of springs and is attached to the base plate by fixing each pawl in a groove with alumina bolts. With respect to of all the heating stage components, the hood has the largest contact area with the sample, and is thus made out of aluminium-oxide (Al2O3) for its low thermal conductivity [9]. The working of the entire assembly is depicted in Figure 3c in the form of a free body diagram, where the depicted situation occurs three times rotated around the central axis of symmetry. Since this assembly makes use of the thickness of the sample to fixate the sample, this

Page 14 means that the sample should always be 1mm thick. However, as the possibility of thinner samples is also interesting, a tantalum ring may be placed below the SiC-block. With such a ring, the laser light can still reach the SiC through the orifice of the tantalum ring and the positioning of the sample continues to work as intended. In order for the laser to be positioned close enough to the SiC-disk for it to shine all of its light on it, the base plate has a concave shape, as can be seen in Figure 3c and Figure 4b.

Figure 3: (a)The nest of springs holder, shown in grey, around the sample and 3C-SiC block, shown in turquoise and black respectively (b) cross-section of the 3D-model of the assembly with the holder on the base plate and covered with the alumina hood, where the sample is depicted in turquoise and the SiC disk in black. The labels correspond to those used in

the free body diagram of (c). (c) Free body diagram of the cross section of the heating assembly along one of three rotational symmetric plane (c),

Figure 4: (a) Nest of springs, with the specimen shown in turquoise, on the base plate, (b) bottom view of the copper cooling heatshield, shown in brown, with the baseplate assembly covered by the hood, shown in white, (c)heatshield with

two tapered conical openings for the electrons to pass through.

Figure 5: Front (a) and rear (b) view of the heatshield together with the heating assembly mounted on the steel frame. On the right, the ferrule clamp(black) and ferrule can be seen.

In Figure 4a, the nest of springs and base plate are shown without the hood. Since the centre assembly of the heating stage, consisting of sample holder, hood and base plate, becomes very hot during operation of the stage, this assembly must be isolated from the rest of the stage. To this end,

Page 15 the base plate is connected to the rest of the stage via three thin legs that are part of the base plate.

Due to their limited cross-sectional area, they forms thermal resistors. In addition, they are connected to the actively cooled heatshield, made of copper and shown as the brown cover in Figure 4. Therefore, the heat dissipating from the sample through the legs cannot reach the rest of the stage or the SEM as the cooling circuit acts as a barrier between them and the stage. The heatshield is actively cooled by a coil that is attached to the back of the heatshield and through which water flows. Besides shielding the sides of the heated assembly, this coil also extracts the dissipated energy from the heating stage. For it to cool the heating stage and shield the heating assembly as well as possible, the heatshield and coil are created from one piece.

Shown in Figure 4c, are the conical openings in the heatshield for the electrons to pass through. The two openings in the heatshield are tapered so that the outgoing electron cone isn’t hindered, and the stage sample tilt can be varied from 69 to 75 degrees. In previous high-temperature EBSD methods, heatshields were typically designed to be open over the entire surface of a sample [16-26], therefore, the benefit of this heatshield has been validated while taking into account a limited manufacturing precision, see in Appendix D. It was found that this heatshield with tapered conical openings yields no significant decrease in data quality while it drastically decreases the amount of exiting radiation.

With respect to a heatshield that is open over the entire surface of the sample, this novel heatshield reduces the emitted thermal radiation by 35%, for which the validation is found in Appendix D. Heat Shielding Design

In order to connect the heatshield and base plate to the moveable stage of the SEM, the heatshield is attached to a steel frame as can be seen in Figure 5. In order to guide the laser to the sample, a ferrule clamp is attached to the frame so the termination of the optical fibre is not in direct contact with the hot components. This is important because the optical fibre termination of commercially available lasers are typically made of rubber.

2.4.2 Thermal Simulations

In order to verify whether the design would yield a heating stage capable of meeting the set requirements, thermal simulations have been conducted. To accurately model the heating stage, various assumptions and simplifications have been made. First of all, the laser is modelled to be a simple heat flux incident on the backside of the SiC-disk for which the chosen value equals the energy put in to the system. As the experiments are conducted in vacuum, convection to the environment is neglected and black body radiation is assumed. For this radiation, all surfaces that can ‘see’ each other are coupled via radiation. The only location convection occurs is in the cooling circuit of the heatshield for which the convective heat transfer coefficient is chosen to be 500W/m²K [32]. All components that are in contact with each other are modelled to be in perfect contact. Also, the small bolts that hold various parts in place are neglected. Finally, the assumption is made that the heating stage does not thermally interact with the SEM as the model is refined to the point that the base of the heating stage frame remains at approximately the initial temperature. These choices are further motivated in Appendix E. Thermal simulations

From the simulations, of which the results are shown in Figure 6, it is found that the sample temperature reaches a maximum of 987°C in steady state at 100W laser power. This is more than 500°C less than the goal temperature of 1500°C. Moreover, the inside of the cooling circuit of the heatshield becomes too hot, namely 167°C. This is not allowed to happen as the water in the cooling

Page 16 circuit will start to boil. In addition, the rise in temperature reduces the effect of the actively cooled heatshield as a thermal barrier and leads to a rise in temperature up to 126°C at the bottom of the frame.

Figure 6: The base plate assembly with hood (left) and without hood (right) for an input power of 100W to the back-side of the SiC disk. The heatshield, cooling circuit and frame are not shown as they obstruct the view of the assembly.

This creates a significant temperature difference between SEM and heating stage and therefore, the assumption with regard to the absence of thermal interaction between SEM and heating stage becomes invalid. Since the amount of radiation coming from the sample is rather constant as it is a function of component temperature, the conductive losses should be investigated as those are simpler to optimize. Finally, it is found that due to the open backside of the heating stage, the radiative heat transfer to the optical fibre delivering the light leads to a significant temperature increase in the fibre ferrule as well. Where the maximum operational temperature of the fibre is 80°C, it becomes 189°C in the simulation.

To create a useful heating stage, both the issues regarding energy dissipation from the sample and the fibre temperature must solved.

2.5 Optimized Design

As it is found that the heating stage becomes too hot while the sample does not reach the goal temperature the conclusion is drawn that the energy loss from the sample is too large. To solve this issue, several solutions have been found and implemented. First of all, where previously the tantalum nest of springs made direct contact with the sample and SiC-disk, small Al2O3 blocks are inserted between the springs on one side and the sample and SiC-disk on the other, as shown in Figure 7a. These blocks form thermal resistors as Al2O3’s thermal conductivity is much smaller than that of tantalum.

The attachment points of the three base plate legs to the cooling shield, are thermally isolated by using zirconia (ZiO2) blocks. Zirconia has an extremely low thermal conductivity and therefore is useful as a thermal isolator. Note that the reason zirconia was not used in the hood or the nest of springs is that zirconia undergoes a phase transformation from monoclinic below 1170°C, to tetragonal between 1170 and 2370°C [33]. This transformation is accompanied by a 10% volume increase, which makes zirconia unsuited for the hood or the nest of springs. However, as the

Page 17 temperature remains below 1100°C at the end of the legs, zirconia is suited as a material for the endcaps of these legs.

Figure 7: (a) Nest of springs, with the specimen shown in turquoise and with the inserted alumina blocks shown in white, (b) base plate spider with the nest of springs and sapphire balls, shown in blue, and zirconia blocks, shown in white, (c) base

plate that is covered by the hood, shown in white, installed in the copper heat shield

Finally, to minimize the heat loss through conduction, the connection between the nest of springs and the base plate is altered. Instead of using recessed tantalum cylinders to centre the nest of springs, which form six line contacts, three sapphire balls (also recessed in the base plate) are used to form six point contacts. In addition, besides reducing the contact area, sapphire has a coefficient of thermal conduction that is twice as low as that of tantalum [9]. Additionally, the balls do not only thermally isolate the nest of springs but also increase the ease of manufacturing as no cylinders have to be milled out of the tantalum base plate, but three conical holes can be drilled to accommodate the sapphire balls, as shown in Figure 8.

Figure 8: Exploded view of the entire heating stage without the frame. The sample and SiC disk are placed inside the nest of springs which, in turn, is placed on top of the sapphire balls that are recessed in the base plate. The alumina hood is placed over the nest of springs and bolted to the base plate, which has zirconia isolators at the end of its legs. The base plate and isolators are bolted to the actively cooled heatshield, and the backside heatshield is bolted to the cooling to close of the

stage.

Page 18 In order to be able to use a standard optical fibre, it must be shielded from the baseplate. To this end, a heatshield is placed at the rear of the heating stage and is connected to the actively cooled copper tubing, as shown in Figure 8. With this heatshield and resulting increased distance of the fibre to the SiC-disk to keep the temperature of the fibre end connector low enough, a significant part of the light lands outside of the SiC-disk. This is solved by inserting an internally polished thin steel tube, which acts as an extension of the optical fibre to deliver all of the light to the SiC-disk. As this tube may reach temperatures well over 1000°C, it can be placed in the vicinity of the sample. This is illustrated in the exploded view of the entire heating stage shown in Figure 8. By aiming the laser beam in this tube, the light is delivered to the sample without the optical fibre reaching too high temperatures.

The applied changes have also been simulated, see Figure 9. The result is a sample temperature of 1650°C at a laser power of only 60W with a temperature distribution throughout the heating stage that is well within the limitations. The sapphire balls have been modelled as extremely small contact surface as the used software did not support point contact. Since a surface conducts heat better than a point, the sample temperature can only rise when using sapphire balls justifying this simplification.

Figure 9: (a) The temperature distribution of the heating assembly of the optimized design both with the alumina hood. (b) the temperature distribution inside the alumina hood. (c) The temperature distribution in the entire heating stage including

heat shield and cooling. The images above are for a laser input of 60W.

Page 19 In addition to the target temperature being met, the temperature in the cooling circuit rises to only 50°C which is amply sufficient to cool the heating stage. Additionally, the addition of the back heat shield lowered the fibre temperature to 65°C, enabling the use of a regular optical fibre. Finally, the exterior of the copper heatshield and cooling, and the frame remain at room temperature validating the assumption regarding the lack of thermal interaction between stage and SEM. This is shown in Figure 9c. Further results of the thermal simulations can be found in Appendix E. Thermal simulations 2.5.1 Manufacturing

Figure 10: Front and back of the optimized heating stage. Here, the heatshield and combined cooling circuit are brown, the frame is a blueish grey, the clamp that holds the optical fibre is depicted in black, and the zirconia isolators are depicted in

white.

As the coarse model has been constructed from a conceptual point of view, it is not yet fit for manufacturing. For this reason several adaptions have been made to (further) increase the ease of manufacturability. To be able to access all bolts that fix the base plate to the cooling circuit, the assembly is rotated 180° around its central axis, as can be seen by comparing Figure 7c to Figure 4bFigure 5. This is also necessary for the cooling water connections to provide the space they need.

Where first the cooling circuit has been modelled as two superimposed circular tubes, the design is adapted so that it forms one conduit and can be manufactured. Finally, the connection between frame and cooling is made by four bolts. The final design is found in Figure 10 and the detailed drawings for all the separate parts can be found in Appendix G. Detailed Drawings

3. Experimental validation

In order to determine whether the stage meets the set requirements , various experiments and tests regarding the applied principles and safe usage are proposed. While the main goal is to reach 1500°C, first, the other requirements should be tested to ensure the workings of the stage before pushing it to its limits. As the design has been split up into three requirement categories, the testing also follows these categories.

First, the influence of the geometry of the stage on the EBSD system should be tested. In the requirements it is formulated that a tilt range of 69 to 75 degrees must be achievable with a working distance of 25mm. These requirements are tested by inserting the stage and performing EBSD scans at one degree increments from 69 degrees to 75 degrees. This test is done at room temperature as there is no need for elevated temperatures to validate the working of the conical holes through

Page 20 which the electrons pass. These scans should be performed with flowing water in the cooling circuit to verify the influence of cooling on the stability of the system, especially with respect to vibrations.

To verify the thermal stability of the system, the remaining thermal drift of the thermal centre of the sample is investigated. Because the thermal centre of the sample is not a priori known, any displacement of a point can possibly be a combination of thermal drift and thermal expansion. To verify whether drift occurs, for three images taken at room temperature and two elevated temperatures, the thermal centre is constructed and tracked, the combination of drift and expansion can be monitored. The principle behind this monitoring is elaborated upon below.

Figure 11: Real and virtual thermal centres for an expanding sample. Where TC1, TC2, and TC3 represent the real thermal centres, the intersecting lines represent the virtual thermal centre, i.e. the point that appears to be stationary in the images. In the case of TC1 and no drift, as the real and virtual thermal centre coincide, it is concluded that no thermal drift

occurs.

In Figure 11 above, a schematic representation is given of how the movement of a real thermal centre and a virtual, constructed thermal centre behaves when a sample is heated. In the image, the green dots represent the unheated situation and TC1 is the location of the thermal centre of this sample. If no drift occurs, TC1 remains stationary when heated and the thermal expansion causes the indentations to move away from TC1 following uexpansion,ΔT1. The new location of the indentations is represented by the blue dots, lines drawn through the pairs of blue and green dots coincide at the location of the thermal centre TC1 as they should following the definition of a thermal centre [34].

Subsequently, if the temperature is increased, the indentations will travel further away from TC1

following uexpansion,ΔT2 as depicted in purple, but the lines drawn through their new locations and their initial location will still intersect at TC1. If this situation would be observed in experiments, it would prove that the sample is thermally centred.

Instead, if drift would have occurred for the same temperature increase, the thermal centre would no longer be positioned at TC1, but for example at TC2 depicted in orange following both uexpansion,ΔT1

and udrift,ΔT1. While the same thermal expansion occurred, the three indents have not only moved

Page 21 away the thermal centre, but have also drifted together with the thermal centre. The lines drawn through the pairs of orange and green dots still coincide but not in the location of the thermal centre TC2. While the thermal centre drifted to the left, the virtual thermal centre (constructed by the intersection of the lines through the new and initial location of the indents) has moved to the right.

Assuming this virtual centre to be correct and increasing the temperature results in the red situation following uexpansion,ΔT2 and udrift,ΔT2, where thermal expansion has increased and the real thermal centre travelled left to TC3 and the virtual thermal centre to the right. From the fact that the orange and red crossing of lines do not coincide, it can now be concluded that drift occurred.

Finally, if the various design requirements have been met, the temperature requirement of the stage should be tested. While monitoring the temperature of various components in the SEM, the sample and SiC-disk temperature is logged as a function of the power of the laser. The predicted temperature power relation can be found in Figure 12. As one of the possible applications of the heating stage is the research on tungsten at high temperatures and tungsten has a relatively low thermal conductivity [9], it is chosen to use a tungsten sample. With tungsten, it is shown that even for poorly conducting materials, the heating stage is able to reach the specified temperatures. While the power-temperature relation will be different for other materials, the test on tungsten validates the requirement of material independent heating.

Figure 12: Power temperature relation for a tungsten sample acquired by thermal simulations

In addition, to both validate the thermal simulations and ensure correct and safe working of the heating stage, the temperatures of various components of the heating stage should be monitored during heating by using thermocouples. As not all parts are sensitive to temperature, only those that may suffer from an increase in temperature are monitored. These are the heating stage frame, the front heatshield, the backside heatshield, and the tubing for the cooling circuit. If all parts remain at safe temperatures in steady state, it may be concluded that the heating stage is fit to conduct experiments of durations longer than 1 day.

3.1 Experimental results

The heating stage has been tested with a tungsten sample inside the vacuum chamber of the SEM at a power range of 0 to 30W. As the only available flange with a water feedthrough has the same diameter as the EBSD detector flange and only one feedthrough of this diameter is present on the vacuum chamber of the microscope, cooling and EBSD cannot be performed together at this point in time. No EBSD scans have been made using the heating stage since the water cooling must be active in order to safely reach elevated temperatures. However, the range of sample tilt angles is assumed

0 500 1000 1500 2000 2500

0 20 40 60 80 100

Temperature [°C]

Power [W]

Page 22 to work correctly as imaging by secondary electrons (SE) was possible over the entire tilt range where the entire sample surface remained visible. This is shown in Figure 13 below where the black oval is the sample location. At each tilt angle, the entire sample surface remains visible to the electron beam. As the imaging parameters are the same for all three images, the decrease in contrast is caused by the increased difficulty for the electrons to reach the SE detector. This is due to the position of the SE detector.

Figure 13: SE images of the sample at minimal magnification where the sample remained visible for the entire range of tilt angles. Shown above from left to right are the extreme values of 69° and 75°, and the middle value of 72°.

Using the tungsten sample, the heating capabilities of the stage has also been investigated using the SE detector. During the initial testing of the dedicated heating stage, it has been found that the temperature had risen to a range in the vicinity of 1500°C. This is justified by looking at post-mortem EBSD scans of the sample and comparing them to those of a sample that has been machined out of the same plate as the heated sample and has undergone the same polishing process. In Figure 14, EBSD scans of the unheated and the heated sample are shown and the difference in crystal structure can clearly be seen.

Figure 14: EBSD scan of a sample that has been heated and one that has been heated with the dedicated heating stage.

Where the grains on the left have a diameter of a few μm, the grains in (b) have significantly grown to diameters of tens of μm.

In Figure 14a, very fine grains of a few μm in diameter are observed while in Figure 14b, the grain structure is much more coarse. Since both samples have the same history and the grains of sample (b) are significantly larger than those of its unheated counterpart (a), it is concluded that significant

(a) (b)

Page 23 grain growth has occurred during the heating of sample (b). The combination of the duration of the heating, which was in the range of tens of minutes, and the change in microstructure leads to the conclusion that the temperature has reached values well above 1200°C, i.e. the recrystallization temperature of tungsten.

This conclusion is supported by the fact that the SiC disk had apparently reached the limit of its operational temperature of 1600°C. Where the SiC disk had been grinded to a flat surface to create optimal contact sample and disk, upon disassembly of the stage it was found that the surface was no longer flat but showed significant surface change, leading to the suspicion of extremely high temperatures. In addition, various components had a black layer deposited on them, which is possibly caused by vapour deposition of SiC. Finally, on the backside of the tungsten sample, a yellowish layer was found. This is suspected to be tungsten trioxide, a material that forms when tungsten oxidizes at temperature of above 1500°C . By verifying if the formed material is indeed tungsten trioxide, a conclusion on the attained temperature can be drawn.

Unfortunately, besides the heating stage components, also the optic fibre ferrule was found to have a black deposition on its surface. This deposition has also taken place inside the ferrule, which is a steel cylinder with a hole along its centre that is approximately 100μm in diameter. Through this hole the laser light coming out of the fibre is guided to its goal and therefore, the hole must be clean. As some particle have found their way into the ferrule, the light path has been obstructed causing the light beam to shoot back into itself. This created several hotspots where the light burned through the different layers of coating rendering the optical fibre useless. As the optic fibre is coupled to the laser diode, the laser system must be repaired before any new experiments can be conducted.

The use of a thermocouple of 0.2mm in diameter has shown to be inadequate as the temperature shown by the thermocouple was around 950°C when the system became too hot. Where the hole in one of the alumina blocks has been designed to accommodate a thermocouple of 0.5mm in diameter, the delay of this thermocouple wire’s delivery led to the use of the thinner thermocouple.

As the thin thermocouple was not correctly fitted in the block, the temperature read-outs have shown to be too low. This led to the belief that the sample and SiC disk were at significantly lower temperatures than they were in reality. In addition, according to thermal simulations, operating at 40W should not have led to such elevated temperatures. However, the assumption of perfect contact between sample and SiC disk may have led to a larger heat flow away from the SiC in the simulations than occurred in reality. This means that in reality, the SiC disk has been more isolated than in the simulations, explaining a much higher temperature in reality than in the simulations.

While the heating assembly has become too hot, during the operation of the system before the fatal heating, it has been found that the SE image quality did not deteriorate and that the sample was stable. Upon heating, various points at the edge of the alumina hood have been observed to leave the field of view in a near to radial direction. This validates the design of the nest of springs as the thermal centre is located inside the field of view. As no images have been taken during the experiments, the previously mentioned sequence for the determination of the thermal centre has not been performed.

In addition to this thermal stability, no component in the SEM heated up to temperatures that may damage them. Also, the frame did not heat up during the operation of the heating stage. This leads to the conclusion that the cooling circuit and heatshield work as intended. In addition, as the imaging

Page 24 was stable, the use of laser heating over conventional heaters has been justified. Also, the use of the proposed simple vacuum feedthrough showed to work adequately resulting in vacuum levels comparable to that of conventional flanges. Together, these results lead to the conclusion that the majority of the dedicated heating stage and the supporting systems work as they were intended to.

4. Proof of Principle

Where the previous section has demonstrated that the heating stage meets the pre-specified requirements with respect to the thermal stability, sample temperature and, long duration experiments, this section shows possible experiments to demonstrate the stage’s capabilities by performing two in-situ EBSD experiments. The recrystallization of a cold worked tungsten sample is described first, and, subsequently the phase transformation of interstitial free (IF) steel is elaborated.

4.1 Recrystallization of tungsten

To illustrate the applicability of the heating stage on high temperature processes, a recrystallization experiment is proposed on a tungsten sample. The sample, a 3mm in diameter disk of 1mm in thickness is machined out of a cold worked plate via wire-EDM, and is polished to a sufficient quality for EBSD application, this sequence can be found in Appendix F. Sample preparationAs recrystallization is the process of reaching a low energy strain energy state coming from a high energy strain state, the tungsten must have some degree of strain energy (or cold work), in its microstructure. From literature it is found that the recrystallization temperature of pure tungsten is equal to 1200°C, which is the temperature at which the material recrystallizes within one hour [7]. By increasing the temperature, the recrystallization time decreases. Having the possibility of reaching 1500°C gives large range of test durations. Through this, the sample temperature and test duration can be chosen so that the EBSD scans have enough time to be completed by not recrystallizing too quickly.

From the scans obtained during heating, the microstructural evolution can be found. Through this evolution, recrystallization time and temperature can be determined, proving that the heating stage is effective in its heating and that the quality of the images is sufficient to perform qualitative experiments at elevated temperatures on tungsten.

4.2 IF-Steel Phase Transformation

Another example of a possible demonstration is that of phase characterisation from melt to room temperature of a metal. In order to depicts the capabilities of the system, a material is chosen that displays a phase transformation in the vicinity of the maximum achievable temperature of the stage.

For this reason, interstitial free steel is chosen; it displays multiple phase transformations over the entire range of temperatures that can be achieved with the heating stage. The phase transformations are given by the following evolution and is depicted in Figure 15.

With this material, the stage can be proven to be useful over the entire range of temperatures and is not limited to solely high or low temperatures. As the sample material for this experiment comes in a 100μm sheet, a ring is must be placed between the nest of springs and the SiC disk for the assembly to be pre-loaded and work properly.

Page 25 α-Fe → γ-Fe → δ-Fe

Figure 15: Phase diagram for interstitial free steel, where the red line depicts the heating pattern for the phase transformation experiment.

Page 26

5. Conclusion and Recommendations

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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Page 28

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.