# High temperature EBSD experiment versus Crystallographic reconstruction to study phase transformation induced microstructures - Application examples for steels

Maissa Fekih, T. Martinez Ostormujof, L Germain, Michel Piette, Denis Sornin, Nathalie Gey

► To cite this version:

Maissa Fekih, T. Martinez Ostormujof, L Germain, Michel Piette, Denis Sornin, et al. High temperature EBSD experiment versus Crystallographic reconstruction to study phase transformation induced microstructures - Application examples for steels. ICOTOM 2024 - 20th international conference on textures of materials, Jun 2024, Metz, France. cea-05516020

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Submitted on 17 Feb 2026

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# High temperature EBSD experiment versus Crystallographic reconstruction to study phase transformation induced microstructures - Application examples for steels

M. Fekih¹, T. Martinez Ostormujof¹,², L. Germain¹,², M. Piette⁴, D. Sornin³, N. Gey¹,²

¹Université de Lorraine, CNRS, Arts et Métiers Paris Tech, LEM3, F-57000 Metz, France

²Laboratory of Excellence on Design of Alloy Metals for Low-mAss Structures (DAMAS), Université de Lorraine, France

³Université Paris-Saclay, CEA, Service de Recherches en Matériaux et procédés Avancés, 91191, Gif-sur-Yvette, France

"PIETTE Michel" <michel.piette@vallourec.com>

Corresponding authors:....

**Abstract:** (à faire en dernier)

In this study, we investigate the microstructural evolution and phase transformations in steels using in-situ high-temperature electron backscatter diffraction (HT-EBSD) in a scanning electron microscope (SEM). A CMOS EBSD detector enabled real-time observations of austenite nucleation and transformation events up to 1050°C, with rapid cooling capabilities. The in-situ approach was compared to indirect crystallographic reconstruction methods, highlighting its ability to directly capture fine microstructural details such as twins and small grains, which may be overlooked in reconstructions. Austenite grain size evolution was analyzed in two case studies: a medium-carbon steel subjected to quenching and tempering cycles and a 10Cr-ODS steel where standard reconstruction methods fail due to ultrafine grains. Additionally, dynamic martensitic transformations were monitored, allowing a direct evaluation of the Kurdjumov-Sachs orientation relationship. While HT-EBSD provides unique insights into transformation mechanisms, challenges remain, including temperature measurement uncertainties and surface-bulk discrepancies. This study emphasizes the complementarity of in-situ and reconstruction methods for a comprehensive understanding of phase transformations in steels.

**Keywords:** In-situ High temperature EBSD; phase transformation; crystallographic reconstruction; steels; microstructure; in-situ observation

## Table des matières

<table><tr><td>1.</td><td>Introduction</td><td>3</td></tr><tr><td>2.</td><td>Materials and Experimental Procedure</td><td>3</td></tr><tr><td>2.1</td><td>Materials</td><td>4</td></tr><tr><td>2.2</td><td>Heating set-up for in-situ EBSD experiment</td><td>5</td></tr><tr><td>2.3</td><td>Temperature measurement within the SEM chamber</td><td>6</td></tr><tr><td>3.</td><td>Results and discussion</td><td>6</td></tr></table>
<table><tr><td>3.1 High temperature parent phase analysis: potential and limitation</td><td>6</td></tr><tr><td>3.2 Complementarities between in-Situ measurements and indirect reconstruction</td><td>8</td></tr><tr><td>3.3 In-Situ EBSD addressing challenges beyond traditional reconstruction methods : Dynamic martensitic transformations and reconstruction limits in ODS steels</td><td>10</td></tr><tr><td>3.4 Some limitations in performing in-situ HT EBSD</td><td>12</td></tr><tr><td>4. Conclusion</td><td>13</td></tr><tr><td>References</td><td>14</td></tr></table>
# 1. Introduction

The mechanical properties of steels are fundamentally governed by their microstructural evolution, particularly the high-temperature austenite phase and its subsequent transformation during cooling. Understanding these transformation mechanisms, both during heating and cooling, is crucial for controlling and optimizing the final microstructure and properties [1-3].

Two complementary approaches have emerged to study these transformations. The first, indirect parent-phase reconstruction, calculates austenite grain orientations from the orientation data of the transformed phase, based on established orientation relationships. Since the early seminal work [4-6], there has been an impressive development of complementary approaches for reconstructing crystallographic orientation maps of the parent phase with varying confidence levels [7-15]. Notably, Cayron et al. pioneered the application to the same Prior Austenite Grains in steels [7]. Today, these advanced techniques are now integrated into mainstream EBSD data post-processing software (MTEX [16], EDAX OIM, Oxford – juste les ref). While powerful, these reconstruction methods have limitations - they are primarily only applicable to displacive transformations, and the accuracy of some reconstructed grain boundaries remains uncertain. Although these methods can refine the average orientation relationship between parent and daughter phases [17,18], direct experimental validation remains essential for a deeper understanding of microstructure formation mechanisms.

The second approach leverages recent technological advances in compact hot stages and high-speed EBSD cameras, enabling direct in-situ observations of microstructural evolution at elevated temperatures. Early attempts at high-temperature EBSD were hampered by limited camera sensitivity and indexing speeds [19-29]. However, modern high-speed EBSD systems have overcome these limitations, allowing real-time tracking of phase transformations. Notable achievements include Taylor et al.'s development of a fully automated system capable of measurements at 1000°C [30,31], and Taniyama et al.'s pioneering observation of the austenite-to-martensite transformation in low carbon steel using a specially designed heating stage [32].

Despite these advances, a critical evaluation of in-situ high-temperature EBSD for studying phase transformations - including both its capabilities and limitations - remains lacking. Furthermore, few studies have explored the synergistic potential of combining direct measurements with indirect reconstruction methods.

This paper addresses these gaps by providing a comprehensive state-of-the-art analysis of the latest generation of high-temperature EBSD experiments in comparison with crystallographic reconstruction methods for studying phase transformation induced microstructures in steels. The analysis was conducted through two representative case studies: The first examines austenite grain size evolution and its potential refinement through thermal cycling in the austenitic range. The second investigates a nuclear steel grade, with and without ODS strengthening, to demonstrate the acquisition of high-temperature data that was previously inaccessible through indirect methods. Our aim is to highlight the strengths of high-temperature EBSD measurements while acknowledging its limitations, demonstrating how these measurements can be effectively combined with reconstruction approaches to provide a more complete understanding of phase transformations in steels. This analysis serves as a valuable reference for materials scientists selecting appropriate methodologies for their specific research needs in steel phase transformation studies.
## 2. Materials and Experimental Procedure

### 2.1 Materials

In-situ HT EBSD experiments were conducted on different steels as part of two cases of studies.

The first case concerns a medium-carbon steel used for Oil Country Tubular Goods applications (referenced here as OT- steel). Its composition and transformation temperatures are detailed in Table 1. Our purpose was to characterize the high temperature austenite grain size distribution by direct in-situ HT EBSD analysis. Further, our point was to investigate the effect of austempering cycling on austenite grain size refinement. The general framework aims to improve the sulfide stress cracking (SSC) performance with microstructure refinement [33,34]. Thus two metallurgical states of OT-steel were considered: state A results from a single austempering treatment (called Q/T) and state B from two successive Q/Ts (each heating cycle was carried out at AC3+30°C for 10min followed by quenching to obtain martensite).

<table><caption>Table 1. Nominal composition (weight %) and transformation temperatures (°C) of the OT-Steel.

**Question à Michel Piette : AC1 et AC3 (quelle Vitesse et mesurer par quelle technique ? dilato ?)**</caption><thead><tr><th colspan="4">Nominal composition</th><th colspan="4">Transformation temperatures (°C)</th></tr><tr><th>C</th><th>Cr</th><th>Mo</th><th>V</th><th>Ac1</th><th>Ac3</th><th>Ms</th><th>Mf</th></tr></thead><tbody><tr><td>0.4</td><td>1</td><td>1.3</td><td>0.2</td><td>775</td><td>850</td><td>310</td><td>120</td></tr></tbody></table>

The second study investigates a 10% wt chromium oxide dispersion-strengthened (ODS) steel, denoted as "10Cr-ODS", fabricated via powder metallurgy and hot extrusion at 1050°C. This material represents a promising candidate for next-generation nuclear reactors, demonstrating enhanced creep resistance and neutron irradiation resilience. For comparative analysis, a non-ODS variant of the same steel, labeled 10Cr, was also examined. The chemical composition and transformation temperatures are detailed in Table 2.

Our purpose was again to characterize the high temperature austenite grains of 10Cr ODS steel in a challenging configuration where crystallographic reconstruction fails, as observed previously by Durand et al [35,36]. In addition, we tackle in-situ analysis of martensite transformation on cooling in the reference grade 10Cr and evaluate the austenite/martensite orientation relationship by direct measurement in comparison to that deduced by indirect reconstruction.

<table><thead><tr><th></th><th>C</th><th>Cr</th><th>W</th><th>Mn</th><th>Mo</th><th>Ni</th><th>Ti</th><th>O</th><th>Al</th><th>Si</th><th>N</th><th>Y</th></tr></thead><tbody><tr><th>10Cr</th><td>0.104</td><td>9.97</td><td>0.49</td><td>0.26</td><td>0.3</td><td>0.3</td><td>0.1</td><td>0.03</td><td>0.005</td><td>0.019</td><td>0.006</td><td>-</td></tr><tr><th>10Cr ODS</th><td>0.11</td><td>9.7</td><td>0.88</td><td>0.26</td><td>0.45</td><td>0.31</td><td>0.22</td><td>0.12</td><td>0.01</td><td>0.06</td><td>0.004</td><td>0.21</td></tr></tbody></table>

(a)

<table><caption>Transformation temperatures (°C)<br>Measured by dilatometry (heating at 1°C/s and cooling at 10°C/s)</caption><thead><tr><th></th><th>Ac1</th><th>Ac3</th><th>Ms</th><th>Mf</th></tr></thead><tbody><tr><th>10Cr</th><td>852</td><td>883</td><td>385</td><td>260</td></tr><tr><th>10Cr ODS</th><td>946,8</td><td>993,8</td><td>328,8</td><td>219,0</td></tr></tbody></table>

(b)

<caption>Table 2. : a. Chemical composition, b. Phase transformation temperatures (°C) of 10Cr and 10Cr ODS [36]</caption>
## 2.2 Heating set-up for in-situ EBSD experiment

High-temperature EBSD experiments were conducted on a ZEISS Auriga 40 FIB-SEM microscope equipped with the latest generation of Oxford Instruments CMOS Symmetry camera and a heating stage. This EBSD detector allows for high-resolution data acquisition at elevated temperatures while maintaining a favorable signal-to-noise ratio and minimal distortion [29]. An infrared (IR) filter was installed to protect the camera from IR radiation emitted during heating cycles. The CMOS Symmetry camera can operate at temperatures up to 1000°C with a 156 × 128 pixel resolution at indexing speeds of 3000 patterns per second. Furthermore, an angular resolution better than 0.2° can still be achieved at high speeds. It is also possible to acquire SEM images during the experiments.

The heating module (Fig. 1), developed by Kammrath & Weiss, features a compact design with optimized geometry for full integration in the SEM chamber and ideal EBSD conditions with a working distance of 15 mm. It is capable of reaching a maximum temperature of 1050°C. The heating system operates by applying a current to a ceramic heating element, indicated by number 2 in Fig. 1 (a), which is covered by a molybdenum (Mo) cap where the sample is fixed (number 3 in Fig. 1 (b)). To enhance thermal conductivity, a thin layer of graphite is positioned between the heating element and the Mo cap. A drilled hole is located in the bottom right of this Mo cap to allow for the insertion of a thermocouple (indicated by number 1 in Fig. 1 (a)). Two protective shields are mounted over the device to minimize radiation and prevent interference with the diffraction signal (indicated by number 4 and 5 in Fig. 1 (b)). The module is positioned on a pre-tilted support at 70°.

Temperature is regulated with a PID controller offering two different modes and the option to program heating cycles. The first mode, called "PID control," allows the heating rate to reach 3°C/s under standard conditions, from room temperature up to 900°C. The second mode, "Rapid heating," operates at maximum power with a heating rate of 50°C/s, though this can result in an overshoot of 20°C and temperature oscillations. The fastest way to cool the sample is to cut the power. The heating module is water-cooled, enabling a minimum cooling speed of 7°C/s in both modes. For a ΔT of 600°C, the cooling rate is 3.7°C/s. The allowed sample size is 10 mm × 10 mm and 1 mm thick. To minimize the temperature gradient between the heated surface and the analyzed top face, it is crucial to keep the sample thickness under 1 mm.

<figure><img src="image_2.png" /></figure>
Fig. 1 Kammrath & Weiss heating module. (a) Shows (1) the thermocouple and (2) the ceramic heating element. (b) Shows (3) the sample, (4 and 5) the radiation shields and the 70°C pre-tilted stage

## 2.3 Temperature measurement within the SEM chamber

To ensure safe conditions of EBSD analysis, preliminary heating experiments to 1050°C have been carried out to measure the temperatures of the surrounding components of the SEM chamber and the EBSD camera. Thermocouples have been attached at different locations: (1) on the SEM's pole piece, (2) at the top of EBSD screen where the infrared radiation is maximal, (3) on the right side of the EBSD screen and finally (4) on the top of the heating module. For these experiments, the EBSD camera was positioned at 195 mm, with a detector distance of 0,63 mm and an elevation of -4,7mm.

None of the four thermocouples recorded a temperature above 78°C (Fig. 2 (b)), confirming that the operating conditions inside the SEM are completely safe and secure.

<figure><img src="image_3.png" /><figcaption>Fig. 2 (a) Thermocouples positioned on various components within the SEM chamber prior to the installation of the heating module, (b) temperature curves recorded for the components relative to the heating temperature with the heating module installed.</figcaption></figure>

# 3. Results and discussion

## 3.1 High temperature parent phase analysis: potential and limitation

Fig. 3 shows the result of a large field of view EBSD acquisition at 840°C on sample OT-steel-B in austenite phase (see section 2). The map was acquired with a 0.5 µm step size over a 290 × 210 µm area at 1000 Hz, completing the entire acquisition in less than 4 minutes. The average indexing rate exceeded 88%.

This result demonstrates the capabilities of the latest EBSD technology in characterizing parent phase microstructures at high temperature. High-speed acquisition with reliable indexing is clearly evidenced and opens up the possibility of capturing dynamic changes in the microstructure. In this case, we observed limited grain growth at the plateau temperature.

Interestingly, this large EBSD acquisition revealed austenite grain size heterogeneities organized in vertical bands approximately 10-20 µm thick, spaced about 100 µm apart. These
bands contain very small austenitic grains, likely induced by localized variations in chemical composition.

<figure><img src="image_4.png" /><figcaption>Fig. 3 High-temperature austenite map acquired at 840°C on sample OT-steel-B : (a) Pattern Quality map, (b) Grain boundary map (misorientation angles between 5–15° in white, >15° in black, >45° in blue, and >58° in red)</figcaption></figure>

The main drawback of such high temperature acquisition remains the inaccurate control of the temperature of the sample. On our hot stage, the temperature is controlled with a thermocouple inserted in a hole drilled in the bottom right of the Mo cap. Temperature variations between the thermocouple and sample surface can exceed several tens of degrees as observed by analyzing the temperatures start/finish of the transformation (Ac1/Ac3). Of course, an alternative is to weld an additional thermocouple on the sample to better control the temperature. This is technically possible but with the risk of unexcepted local changes in the sample composition and uncontrolled in-situ thermal cycle. (faire un commentaire sur le décalage observe pour vos manip en fonction du retour de M. Piette – essai in-situ chauffage à 3°C/s)

To overcome these limitations in the analysis of microstructures inherited by phase transformation, we have conducted our high temperature experiments by step wise heating to detect precisely the transformation start and finish (Ac1/Ac3) temperature and set the desired temperature from these reference temperatures (instead of considering the absolute temperature measured by the thermocouple).

Using this strategy, we were able to evaluate the grain size refinement with austempering cycles. Fig. 4 (a-b) shows the EBSD maps acquired for sample OT-steel-A and sample OT-steel-B at Ac3+30°C after fifteen minutes (t₁₅) holding time (the EBSD analysis has been performed outside of the vertical band seen in Fig.3). The average grain size value and the grain size distribution for each sample is indicated respectively in Fig. 4c. The grain size is given by the equivalent diameter of the detected grains weighted by their surface fraction (grain detection was obtained with a threshold angle of 5° ignoring twin boundaries). Notice that the EBSD data have been acquired with a step size of 0.3µm, a map size of 180×130µm and a speed of 970Hz resulting in a total acquisition time per map of 4min. The average indexing rate of the maps
was above 91%. The grain size analysis was performed on cleaned and extrapolated data up to
100%.

The result shows that the additional austempering cycle seen by sample B reduced the austenite grain size from 7.1µm to 5.1µm, confirming austenite grain refinement through austempering. Similar results were obtained by indirect parent map reconstruction [37].

<figure><img src="image_5.png" /><figcaption>Fig. 4 High temperature misorientation map for austenite: EBSD data acquired at Ac3+30°C on sample OT-steel-A (a) and OT-steel-B (b) with the corresponding grain size distribution (c).</figcaption></figure>

**3.2 Complementarities between in-situ measurements and indirect reconstruction**

In the past decade, researchers have developed various indirect methods for reconstructing parent microstructures from orientation maps acquired at room temperature after displacive phase transformations. Whatever the reconstruction algorithm [10,16,38], the approach relies on some assumptions and the final result is often influenced by parameter selection, thereby raising significant questions about the reliability of the reconstructed parent contours. Nowadays in-situ measurements give access to the ground truth parent map and offer the possibility of validating the predictive capabilities of reconstruction algorithms.

Fig. 5 shows a comparison between in-situ measurement (Fig. 5 (a)) and reconstruction (Fig. 5 (c)) from EBSD data acquired at room temperature after fast cooling at 3°C/s to form a lath like microstructure (Fig.5a). The reconstruction was performed using Merengue, our homemade reconstruction tool [39]. A very good agreement was observed between both maps in terms of austenite orientations, grains size and shape. However, for specific configurations, the reconstruction encountered specific challenges: (1) Certain twins remained undetected in a limited number of grains (indicated by white stars), (2) some regions could not be reconstructed (appearing black in the map), (3) Small grains were occasionally not detected and covered by surrounding grains. These limitations stem mainly from an insufficient number of crystallographic variants locally to be able to determine a unique, high-confidence austenite orientation. Despite those minor discrepancies, the reconstruction yields reliable and realistic results that can be confidently interpreted. Similar comparisons between in-situ measurement and reconstruction are also available in [30].
<figure><img src="image_6.png" /><figcaption>Fig. 5 (a) IPF-Z map of the austenite acquired in-situ at 910°C, (b) Band Contrast map of the inherited microstructure at room temperature after in-situ cooling at 3°C/s and (c) IPF-Z map of reconstructed austenite. (Z: normal to the acquisition plane)</figcaption></figure>

However, on has to keep in mind that the quality of the reconstruction significantly depends on the existence of an orientation relationship between parent and daughter phases; the stricter this relationship, the better the reconstruction. As a consequence, for microstructure inherited from a more diffusive process, like upper bainite or ferrite, indirect reconstructions are at best limited or even out of purpose.

Notice also that reconstruction methods are also able to refine the Orientation Relation between parent and inherited phases. This point is discussed further in section 3.3 with a comparison between reconstruction and direct measurement.

Finally, combining reconstruction and in-situ measurement offers a powerful approach to understanding microstructure evolution during thermal cycling. Fig. 6 illustrates an original strategy: Starting with sample OT-steel-A, obtained after a single austempering treatment (Fig. 6 (a)), the prior austenite map was reconstructed (Fig. 6 (b)). Subsequent in-situ reheating of sample OT-steel-A to 870°C revealed critical microstructural dynamics on the high temperature austenite map ((Fig. 6 (c)). Some prior austenite grains are fully recovered during heating. Inherited variants within those grains transform by developing back their common parent orientation (examples are marked by stars in Fig. 6). Residual austenite may contribute to this "memory effect". Significant new austenite nucleation occurs also during heating, explaining grain refinement from the austempering cycle.

These key observations confirm grain refinement during austempering and provides insights to analyze austenite nucleation sites during thermal heating. This example demonstrates the complementary power of reconstruction and direct in-situ measurement in unveiling complex microstructural transformation mechanisms.
<figure><img src="image_7.png" /><figcaption>Fig. 6: (a) Martensite map of OT-steel-A obtained after a single austempering treatment, (b) Prior austenite map obtained by reconstruction, (c) austenitic map acquired in-situ at 870°C (all maps are plotted according to the IPF//Z color code) (insérer l'IPF//Z sur a. et mettre un petit schema du cycle thermique)-erreur b,c</figcaption></figure>

## 3.3 In-Situ EBSD addressing challenges beyond traditional reconstruction methods: Dynamic martensitic transformations and reconstruction limits in ODS steels

This section presents two studies highlighting the capabilities of in-situ EBSD to address challenges beyond traditional reconstruction methods. The first study offers a more advanced analysis, exemplifying the monitoring of the martensitic transformation during cooling, coupled with the analysis of orientation relationships. The second study demonstrates a case where it was impossible to reconstruct the high-temperature austenite, revealing the limitations of indirect reconstruction methods and the unique insights offered by in-situ analysis.

In-situ EBSD analysis was performed on 10Cr steel to capture the progression of phase transformations during cooling from 950°C to room temperature, with a cooling rate of 3°C/s. Martensite was detected at 390°C, which is 5°C higher than the martensite start temperature determined by dilatometry (385°C). Fig. 7 illustrates the microstructure observed at 380°C, acquired with a step size of 0.1 µm over a map area of 115×85 µm. The data were collected at a speed of 970 Hz with an indexing rate of 95%. At this temperature, the transformation into martensite is not yet complete, with residual austenite accounting for 25% of the microstructure in the form of blocky and interlaths. By tracking the same region of interest, the microstructural evolution was observed continuously, from the high-temperature state through a plateau in the martensitic domain, and up to the total transformation at room temperature. Fig. 8 displays the cooling microstructure evolution, focusing on a single grain extracted from the overall microstructure shown in Fig. 7, along with the corresponding reconstructed austenite morphology.

Fig. 8 (e-f) illustrates pole figures showing the orientation of the austenite phase at 390°C and 380°C. It can be observed that there is a gradient in orientation evolution within the austenite phase, corresponding to the progression of the phase transformation. This gradient is likely due to temperature-induced microstructural changes, such as strain accumulation and phase transformation dynamics. Furthermore, as the transformation progresses towards room temperature, the residual austenite completely transforms, the martensitic variants inherited from the austenite grain are represented on the pole figure (Fig. 8 (g)).

The transformation of austenite to martensite follows a distinct orientation relationship (OR), such as Kurdjumov-Sachs (KS) or Nishiyama-Wassermann (NW) [8]. Fig. 8 (h) presents the
angles between [1-11]bcc and [1-10]fcc of the OR as a function of angles between (110)bcc and (111)fcc, measured directly within the analyzed grain, along the entire microstructure at 380°C and the reconstructed one. The OR determined directly from the EBSD map at 380°C correlates well with that obtained through indirect reconstruction, exhibiting a maximum deviation of just 0.5° between (110)bcc and (111)fcc. The orientation relationship calculated inside a single grain is close to the OR of the entire map, which further confirms the validity of our calculations. It is important to note that the OR is determined after the orientations have been averaged. For this type of material, this study demonstrates that KS and NW orientation relationships are reasonable approximations for many grains, but they are not strictly followed in all cases. In practice, each grain can display a slightly different orientation relationship compared to its neighboring grains, reflecting local variations in the transformation process.

<figure><img src="image_8.png" /><figcaption>Fig. 7 Band contrast (BC) map with austenite phase in blue showing the microstructure of in-situ observation at 380°C, black pointers define the boundaries of the analyzed grain</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8 (a-c) Band contrast maps with IPF-Z colour showing the dynamic transformation (390°C to RT). (d) The reconstructed microstructure after heating to 950°C and cooling to RT. (e-f) Shows pole figures</figcaption></figure>
of (100) plane of the austenite orientation at 390°C and 380°C. (g) Shows pole figure of (100) plane of the martensitic orientation at RT. (h) angles between [1-11]$_{bcc}$ and [1-10]$_{fcc}$ of the orientation relationship as a function of angles between (110)$_{bcc}$ and (111)$_{fcc}$

In the second study, we build upon previous work [35] that demonstrated the challenges in reconstructing the parent phase from a quenched microstructure of 10Cr ODS Steel (oxide dispersion strengthened), reinforced with nanoparticles of yttrium oxide (Y₂O₃) and titanium hydride (TiH₂). Fig. 9 shows the EBSD maps of the austenite microstructure acquired at 1000°C with a step size of 0.075 µm over 48 x 36 µm, a speed of 970 Hz with an indexing rate of 63%. Acquiring an EBSD map with a resolution of 0.075 µm at such a high temperature is particularly challenging due to thermal effects, sample stability, and diffraction signal quality. The combination of relatively high indexing and fine resolution at this temperature represents a significant advancement in the in-situ analysis of ODS steels. This microstructure, featuring ultrafine austenitic grains, is reported for the first time using high-temperature EBSD.

The core issue with reconstructing the parent phase arises from the extremely fine grains detected at high temperatures, with an average grain size of approximately 0.59 µm. In such conditions, there are insufficient variants to accurately reconstruct the austenite grains. Furthermore, another complication stems from the presence of non-indexed zones around these fine grains, which significantly hinders the reconstruction process. It is also noteworthy that these issues are unique to ODS steels. To highlight this, unlike the ODS steel, the non-ODS steel studied previously (Study 1), analyzed via in-situ high-temperature EBSD, exhibited no non-indexed zones or irregular microstructure, despite sharing the same composition. This demonstrates that the presence of ODS particles plays a critical role in the formation of irregular topography and non-indexed zones, which cannot be captured using conventional reconstruction methods.

<figure><img src="image_10.png" /><figcaption>Fig. 9 (a) Band contrast with grain boundaries between 1° and 3° are plotted in yellow, boundaries angles between 3° and 62° in black, (b) IPF-Z map of the austenitic microstructure acquired during plateau at 1000°C</figcaption></figure>

**3.4 Some limitations in performing in-situ HT EBSD**

Performing in-situ EBSD at high temperatures has revealed certain limitations, highlighting the need for further development and advancements in methodology and technology of the system. Maintaining focus on a region of interest while avoiding image drift during continuous heating
or cooling is challenging. To effectively capture dynamic processes like grain nucleation and phase transformation, a strategy involving incremental temperature changes could be a solution to facilitate the acquisition of high-quality EBSD maps without encountering drift during short-term plateaus.

Furthermore, it is important to note that in-situ EBSD is a surface analysis technique, which often lacks detailed information about the bulk material, as the data acquired from the surface may not fully represent the behavior and transformations occurring within the entire sample. Fig. 10 shows the microstructure of the top surface after cooling and in the bulk. When comparing the two microstructures, the misorientation histogram for the surface map (represented by the blue peak on the histogram) reveals a higher proportion of low-angle misorientations, ranging between 1° and 3°, as well as high-angle misorientations compared to the bulk. These differences are potentially linked to the elastic strain energy, which is relieved more effectively at the surface than in the bulk. Moreover, the surface microstructure displays notable topography, likely due to its direct contact with the free space of the SEM and the absence of constraints at the surface. This explains the differences observed between the surface and bulk in terms of misorientations at grain boundaries.

<figure><img src="image_11.png" /><figcaption>Fig. 10 Band contrast with grain boundaries of (a) top surface, (b) bulk and (c) grain boundary histogram</figcaption></figure>

Additionally, the phase transformation temperature cannot be determined exactly with in-situ EBSD, first, due to a slight separation between the sample and the thermocouple, which introduces a temperature difference between the measured temperature and the actual temperature of the sample. Secondly, the diffusion from austenite to martensite is hindered by elastic strain energy. However, at the top surface, this energy is reduced, enabling faster diffusion compared to the bulk, which could have influenced the transformation temperatures. This discrepancy explains why there is a difference between the temperature detected by dilatometry and that observed during in-situ EBSD measurements.

Finally, local chemistry variations, such as surface decarburization, can also influence the results of high-temperature in-situ EBSD analysis. Performing double thermal cycles on the sample is not feasible due to the limitations of the experimental setup and the microstructural changes that occur during heat treatment.

## 4. Conclusion

This study demonstrates the advantages of in-situ HT-EBSD for characterizing phase transformations in steels, providing direct access to parent-phase microstructures and
transformation mechanisms that remain challenging to analyze through indirect reconstruction
methods. By combining high-speed EBSD with an advanced heating module, we successfully
monitored austenite nucleation, grain refinement, and dynamic martensitic transformations in
real time. Compared to indirect reconstruction, in-situ HT-EBSD offers a more accurate
depiction of austenite morphology, particularly in cases where orientation relationships are not
strictly followed or where ultrafine grains hinder reconstruction. However, limitations persist,
including challenges related to temperature control, surface-bulk differences, and image drift
during prolonged heating. Future developments should focus on improving temperature
measurement accuracy, integrating bulk-sensitive characterization techniques, and refining
reconstruction algorithms to enhance data interpretation. By leveraging both in-situ HT-EBSD
and crystallographic reconstruction, researchers can gain a more comprehensive understanding
of microstructural evolution in steels, ultimately aiding in the optimization of heat treatment
processes and material performance.

Acknowledgment

Data availability statement

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Remerciement : plateforme