# Near-Equilibrium Transformation Sequence of NiCrBSi Self-Fluxing Nickel Alloy

Anthony Ty, Yannick Balcaen, Morgane Mokhtari, Andrew Fitch, Olivier Dalverny, Joël Alexis

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Anthony Ty, Yannick Balcaen, Morgane Mokhtari, Andrew Fitch, Olivier Dalverny, et al. Near-Equilibrium Transformation Sequence of NiCrBSi Self-Fluxing Nickel Alloy. Metallurgical and Materials Transactions A, 2025, 56, pp.1690 - 1705. 10.1007/s11661-025-07732-5. hal-05457992

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# Near-Equilibrium Transformation Sequence of NiCrBSi Self-Fluxing Nickel Alloy

ANTHONY TY, YANNICK BALCAEN, MORGANE MOKHTARI, ANDREW FITCH, OLIVIER DALVERNY, and JOËL ALEXIS

NiCrBSi alloys are widely used as wear-resistant deposits. However, despite several studies, there are still some remaining questions concerning their microstructure and the transformation sequence during solidification. This study investigates a gas-atomized powder's non-equilibrium microstructure and a cast sample's near-equilibrium microstructure by synchrotron XRD coupled with SEM observations. NiCrBSi alloys consist of a nickel-rich matrix with CrB, Cr<sub>7</sub>C<sub>3</sub>, Ni<sub>3</sub>B, Ni<sub>3</sub>Si<sub>12</sub> crystals and γ' Ni<sub>3</sub>Si β<sub>1</sub> precipitates. The transformation sequence is studied by *in situ* XRD analyses using a synchrotron source. Melting and solidification of Ni<sub>3</sub>Si<sub>12</sub> and Ni<sub>3</sub>B and solid-state transformations of Ni<sub>3</sub>Si<sub>12</sub> and Ni<sub>3</sub>Si β<sub>1</sub> are depicted. Finally, the nickel-rich γ (Ni) matrix, CrB and Cr<sub>7</sub>C<sub>3</sub>, are likely to remain solid at high temperatures up to 1200 °C.

https://doi.org/10.1007/s11661-025-07732-5
© The Author(s) 2025

## I. INTRODUCTION

SELF-FLUXING alloys (SFA) developed during the '50s are commonly used as coatings for applications requiring wear and corrosion resistance at high temperatures (power generation, motorization, mining, glass molds, cement works, etc.).[1-6] NiCrBSi alloys are the most widely used and studied.[3] According to several papers, NiCrBSi alloys are composed of a nickel-rich matrix saturated with iron, chromium and silicon with further addition of boron and carbon, generating chromium borides, chromium carbides, nickel borides and nickel silicide hardening crystals.[7-42] However, other phases are mentioned in some works, such as B<sub>4</sub>C,[14] Ni<sub>3</sub>Fe,[14] Cr<sub>5</sub>Si,[14] Cr<sub>5</sub>Si<sub>3</sub>,[14] Fe<sub>2</sub>Si,[15] Fe<sub>1.1</sub>Cr<sub>0.9</sub>B<sub>0.9</sub>[16] or Fe<sub>2</sub>B.[17] Furthermore, according to several publications, among chromium borides, chromium carbides, nickel borides and nickel silicide, CrB, Cr<sub>7</sub>C<sub>3</sub>, Ni<sub>3</sub>B and Ni<sub>3</sub>Si are, respectively, the most common.[10-43] However, other phases such as Cr<sub>2</sub>B,[11,22] Cr<sub>5</sub>B<sub>3</sub>,[16,23-25] Cr<sub>3</sub>B<sub>4</sub>,[16,26] CrB<sub>4</sub>[16] Cr<sub>3</sub>C<sub>2</sub>,[10,21,26] Cr<sub>23</sub>C<sub>6</sub>,[14,15] Ni<sub>2</sub>B,[22-24,27] Ni<sub>4</sub>B<sub>3</sub>,[23,24] Ni<sub>3</sub>Si<sub>2</sub>,[23,24] and Ni<sub>5</sub>Si<sub>2</sub>[28] are also mentioned. Those differences in the literature can be explained by different chemical compositions or by phase identification difficulties due to the complexity of the microstructure. Indeed, the broad range of phases with low symmetry

makes conventional X-ray diffraction (XRD) unreliable due to peak overlap.[23] However, Hemmati *et al.*[23] mentioned that synchrotron XRD might solve some difficulties. Such experiments have not been published yet.

While the complex microstructure of NiCrBSi alloys is not deeply studied, the transformation sequence generating this microstructure needs also to be investigated more. DTA (differential thermal analysis)[8,25] and thermal simulations[44,45] show some contradictions. Indeed, according to DTA,[8,25] solidification begins with the formation of chromium borides CrB, followed by Cr<sub>7</sub>C<sub>3</sub> chromium carbides and a nickel-rich matrix. Finally, different eutectics, such as γ (Ni)/Ni<sub>3</sub>B/Ni<sub>5</sub>Si<sub>2</sub>, appear at the last stage of the solidification. This could be linked with thermal simulations from Chun *et al*.,[44] showing that a boride and a carbide subsist at high temperatures. However, according to thermodynamical simulations by Pereira *et al*.,[45] the solidification begins with the formation of chromium carbide Cr<sub>7</sub>C<sub>3</sub>, followed by the nickel-rich matrix and Cr<sub>3</sub>C<sub>2</sub>. Those contradictions can be again explained by the experimental difficulties in determining phase transformations and by analysing NiCrBSi alloys with various chemical compositions. Recent work has been published using *in situ* synchrotron XRD during cooling on a ternary Ni-Cr-B alloy, showing that this technique is relevant for phase transformation determination.[46] However, given that NiCrBSi alloys are mainly studied from a tribological point of view,[47] such advanced techniques have yet to be used.

Ni<sub>3</sub>Si precipitates have been observed in rare cases in NiCrBSi alloys.[8,27,48] They are comparable to γ' (Ni<sub>3</sub>Al/Ti) found in precipitates hardened nickel-based

<footnote>ANTHONY TY, YANNICK BALCAEN, MORGANE MOKHTARI, OLIVIER DALVERNY, and JOËL ALEXIS are with the Laboratoire Génie de Production, Université de Toulouse, UTTOP, LGP, 47 Avenue d'Azereix, 65016, Tarbes, France. ANDREW FITCH is with the ESRF, the European Synchrotron, 71 Avenue des Martyrs, CS40220, 38043, Grenoble Cedex 9, France. Contact e-mail: yannick.balcaen@uttop.fr</footnote>
superalloy.[49] Therefore, they might be generated by solid-state transformation. However, to the best of the author's knowledge, information concerning the precipitation of γ' Ni₃Si in the case of NiCrBSi alloys are almost non-existent.

Thus, in this paper, the microstructure is deeply analysed *via* synchrotron XRD coupled with SEM imaging. The transformation sequence is determined by *in situ* synchrotron XRD and correlated with DTA. Special attention is taken to determine solid-state transformation generating NiₓSiᵧ precipitates.

## II. MATERIALS AND METHODS

### A. Material

The material used is a powder custom-made by Aubert & Duval *via* nitrogen gas atomization. The granulometry of the powder is $D_{10} = 17$ µm, $D_{50} = 31$ µm and $D_{90} = 47$ µm. This granulometry is chosen to be suited with laser-powder bed fusion (L-PBF) for further applications.[50] The chemical composition determined by inductively coupled plasma (ICP) and by LECO (infrared absorption and thermal conductivity of combustion gases) is exposed in Table I. Near-equilibrium samples considered cast samples are the melted powder either in the synchrotron XRD capillary or the DTA crucible (described underneath). In both cases, the powder is heated to 1200 °C at a heating and cooling rate of 15 K·min⁻¹. The sample obtained in the XRD synchrotron capillary is used for diffraction analyses. However, given the small size of the cast sample contained in the capillary or in the DTA crucible, we used a larger cast sample for microstructural observations. This sample has been obtained by melting the powder at 1200 °C in a vacuum furnace. The heating rate is set at 5 K·min⁻¹ and the cooling rate is at 3 K·min⁻¹ maximum (furnace cooling).

### B. Differential Thermal Analyses

Transformation's temperature is determined by DTA using a Setsys Evolution S60/58341 from Setaram instrumentation with a 4 mm diameter alumina crucible loaded with 80 mg of powder. All the transformations are explored from 20 °C to 1200 °C (melting temperature announced at around 1000 °C) under a 40 mL·min⁻¹ argon flow at a heating and cooling rate of 5, 15 and 30 K·min⁻¹. Finally, transformation temperatures are determined according to recommendations from Boettiger *et al.*[51]

### C. Synchrotron X-Ray Diffraction

Following the proposal MA-5643, Synchrotron XRD analyses are carried out at the ID22 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The diffractometer utilises a 1 × 1 mm² X-ray beam with a wavelength of 0.35431 Å. The powder is set in a quartz capillary rotating at 251 rpm to reduce texture effects (Figure 1(a)). Data are acquired with a

<table><caption>**Table I.** Chemical Composition of the NiCrBSi Powder (Wt Pct)</caption><thead><tr><th>Elements</th><th>Ni</th><th>Cr</th><th>B</th><th>Si</th><th>Fe</th><th>C</th><th>O</th></tr></thead><tbody><tr><td></td><td>bal.</td><td>14.0</td><td>3.5</td><td>3.9</td><td>3.1</td><td>0.63</td><td>0.012</td></tr></tbody></table>

Dectris Eiger2 X 2M-W CdTe detector equipped with thirteen Si (111) crystals.[52–54] XRD analyses are performed with a scan time of 16 minutes and rebinned with a 0.001 deg step to obtain well-defined diffraction patterns. *In situ* XRD analyses are conveyed with a scan time of 20 seconds and rebinned with a 0.003 deg step to obtain different diffraction patterns quickly. A purpose-built furnace is used to heat and cool the sample at 15 K·min⁻¹. The furnace structure consists in a graphite core hollowed to contain both a thermocouple and the capillary in two symmetrical chambers. This graphite block is heated by an inductor and heating power is controlled by a conventional PID regulation system based on the thermocouple signal (Figure 1(b)). The *in situ* XRD analysis frequency permits a diffraction pattern to be obtained every nine Kelvin.

### D. Diffraction Pattern Analyses

According to different databases such as the International Centre for Diffraction Data (ICDD),[55] The Material Project (TMP)[56] and Crystallography Open Database (COD),[57] chromium borides, chromium carbides, nickel borides and nickel silicide present very different space groups and lattice parameters. Furthermore, given the need for more consensus regarding those alloys' microstructure, traditional software methods comparing experimental diffraction patterns with a single database are insufficient. Therefore, all chromium borides, chromium carbides, nickel borides, nickel silicide and ternary nickel-chromium silicon phases from different databases are stored in the CrystalDiffract® software. Lattice parameters of best-fitting phases are tweaked to fit the experimental diffraction pattern better. Finally, Rietveld refinement is conducted with Maud software to quantify phase fraction and better define lattice parameters. Errors in phase fraction and lattice parameters due to peak broadening are considered and a special care is taken to calibrate the experimental peak broadening using a diffraction pattern obtained with a LaB₆ standard. Continuous background is quantified using, first, a constant value and, as recommended by Lutterotti,[58] by inserting a phase (Ni₃B in our case) with a very small crystallite's size setting (20 nm). A diffraction pattern is analysed every 50 K for *in situ* XRD analyses to have a global picture of the transformation sequence. Furthermore, a diffraction pattern is analysed every 9 K for temperature ranges exhibiting transformations for a better resolution.

### E. Thermodynamical Computing

Phases transformations are performed with ThermoCalc® using the database TCNI12.1. Scheil-Gulliver
<figure><img src="image_2.png" /><figcaption>Fig. 1—Overview of the synchrotron XRD setup (a) without the furnace and (b) with the furnace.</figcaption></figure>

solidification is calculated assuming that carbon is a fast
diffuser.

F. *Microstructural Observation*

Metallography sample preparation is performed with a series of abrasive grinding (SiC) paper with decreasing coarseness to 4000 grit, followed by polishing with a solution containing 3 and 1 µm diamond particles and a final polishing with a colloidal solution containing Al₂O₃ particles of 0.25 µm (OPA). Samples are then etched by a 30-second exposition under the Marble reagent (20 mL of HCl, 4 g of CuSO₄ and 20 mL of H₂O). Crystallographic data obtained *via* an EBSD Symmetry S2 camera and chemical analyses obtained *via* an EDS Ultim Max 100 detector are coupled to determine the spatial phase dispersion. Those analyses are conducted in a JEOL JSM-7100F SEM (scanning electron microscope) with a Field Emission Gun. An accelerating tension of 20 kV and a working distance of 11 mm is used. Crystallographic data determined *via* Synchrotron XRD for each phase are inserted into the Aztec software. EBSD maps are corrected with ten iterations at six neighbours' pixels *via* the software AztecCrystal. Finally, for analyses that do not require EBSD, a ZEISS Sigma 300 VP with SE, SE *in-lens*, BSE and EDS (X-MaxN 50 XMX1003) detectors is used. An accelerating tension of 15 kV and a working distance of 11 mm are used for chemical analyses with BSE and EDS detectors. Furthermore, for high-resolution analyses with the SE *in-lens* detector, the accelerating tension and the working distance are reduced to 5 kV and 5 mm, respectively.

III. RESULTS

A. Powder Microstructure

First, this NiCrBSi powder's microstructure should be
known to study phase transformations during the

heating of the powder. Powder's microstructure in BSE contrast exhibits dark, grey and light areas (Figure 2(a)). According to the literature, it seems well established that dark blocky phases are attributed to chromium borides, grey dendritic phases to chromium carbides and light areas to the matrix. This is confirmed by the EDS profile exhibiting a pronounced spike concerning the chromium content of those dark and grey phases (Figure 2(b)). Furthermore, small peaks related to the boron and carbon content confirm that dark areas are chromium borides and grey areas are chromium carbides. According to the literature,[10–43] it is most likely that those phases are CrB and Cr<sub>7</sub>C<sub>3</sub>, respectively. However, given the difficulties in quantifying carbon and boron, the nature of those phases is further studied from a crystallographic point of view *via* synchrotron XRD. It should be mentioned that other phases as γ (Ni), Ni<sub>3</sub>B or Ni<sub>3</sub>Si<sub>12</sub> are only revealed after etching. Therefore, they are not visible on non-etched BSE micrographs presented in Figure 2(a). Furthermore, the microstructure of a single powder particle shown in Figure 2(a) is not representative of the whole material. The diffraction pattern reveals that the powder is composed of the nickel-rich matrix γ (Ni) (space-group *Fm*–*3*m) with CrB chromium boride (space-group *Cmcm*), Cr<sub>7</sub>C<sub>3</sub> chromium carbides (space-group *Pnma*), Ni<sub>3</sub>B nickel borides (space-group *Pnma*) and Ni<sub>3</sub>Si<sub>12</sub> nickel silicide (space-group *P321*) (Figure 3). Further analyses results obtained with the Rietveld method are presented in Table II and reveal a significant amount of continuous background. This last can be explained by the small size of the grains coupled with the peak density. On the other hand, the amount of CrB is negligible. Finally, experimentally refined lattice parameters are given in Table II.

B. *Transformations of the Powder During Heating*

The transformation's nature during heating is deter-
mined by *in situ* XRD synchrotron analyses and the
<figure><img src="image_3.png" /><figcaption>Fig. 2—Microstructure of powder: (a) high magnification micrograph (BSE contrast), non-etched, and (b) EDS profile along specified line.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3—Diffraction pattern of the powder (a) general view, and (b) between 8 and 12 deg $2\theta$.</figcaption></figure>

transformation's temperatures are determined via DTA. Firstly, a diffractogram every 50 K is studied to have a general view of fraction phases vs the temperature (Figure 4(a)). Secondly, a diffractogram every 9 K is used for temperature ranges exhibiting transformations for a more accurate result (Figures 4(b) and (c)). Nickel silicides (Ni₃Si₁₂ and Ni₃Si β1) present a complex sequence of transformations. Indeed, between 600 °C and 700 °C (Figure 4(b)), there is a gradual decrease in Ni₃Si₁₂ content and a gradual increase in Ni₃Si β1 content. Inversely, between 990 °C and 1000 °C (Figure 4 (c)), a gradual decrease in Ni₃Si β1 content and an increase in Ni₃Si₁₂ content is observed. The melting of Ni₃Si₁₂ quickly follows this transformation at around 1010 °C. On the other hand, γ (Ni) and Ni₃B exhibit a slightly decreasing content from room temperature to approx. 1000 °C following the slight increase in continuous background. However, it rapidly decreases between 1000 °C and 1040 °C as the continuous background increases abruptly, confirming that those phases are melting (Figure 4(c)). Indeed, γ (Ni) and Ni₃B, γ (Ni) and Ni₃Si₁₂ are known to form eutectics

explaining the concomitant diminishing in phase fraction. However, a small amount of γ (Ni) matrix remains between 1000 °C and 1200 °C. Finally, the chromium carbide and boride content are stable from room temperature up to 1200 °C.

To study those transformations further, the lattice parameters of each phase are plotted with the temperature (Figure 5). Lattice parameters of each phase increase with the temperature (Figures 5(a), (c), (d), (f)). However, lattice parameters of γ' Ni₃Si β1 precipitates decrease quickly after their detection before increasing (Figure 5(b)). According to several descriptions of γ' Ni₃Al/Ti precipitates found in more traditionally studied nickel-based superalloys, this can be attributed to the evolution of the precipitate's shape.[59–64] Finally, the a axis of Ni₃B exhibits a decrease between approx. 700 °C and 800 °C (Figure 5(e)). It should be mentioned that due to the high continuous background and the relative small phase fraction, lattice parameters of the γ (Ni) matrix cannot be determined properly above 1040 °C. DTA signal during heating exhibits overlapped peaks,
<table><caption>Table II. Phase Fraction and Lattice Parameters of the Powder</caption><thead><tr><th>Phase</th><th>Fraction (Vol Pct)</th><th>a (Å)</th><th>b (Å)</th><th>c (Å)</th></tr></thead><tbody><tr><td>Continuous Background</td><td>19±1</td><td>—</td><td>—</td><td>—</td></tr><tr><td>γ (Ni) (Fm-3m)</td><td>21.1±0.8</td><td>3.5383±0.00005</td><td>—</td><td>—</td></tr><tr><td>CrB (Cmcm)</td><td>0.7±0.2</td><td>2.973±0.002</td><td>7.795±0.008</td><td>2.931±0.002</td></tr><tr><td>Cr<sub>7</sub>C<sub>3</sub> (Pnma)</td><td>6.2±0.5</td><td>4.4672±0.0009</td><td>7.049±0.004</td><td>12.200±0.007</td></tr><tr><td>Ni<sub>3</sub>B (Pnma)</td><td>41±2</td><td>5.3026±0.0002</td><td>6.6084±0.0002</td><td>4.4076±0.0001</td></tr><tr><td>Ni<sub>31</sub>Si<sub>12</sub> (P321)</td><td>11.6±0.5</td><td>6.6673±0.0003</td><td>—</td><td>12.313±0.001</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Fig. 4—Phase fraction vs temperature of the powder during heating (a) general view, (b) zoom between 600 °C and 700 °C for Ni₃₁Si₁₂ and Ni₃Si β₁ and (c) zoom between 950 °C and 1050 °C for the continuous background, γ (Ni), Ni₃B, Ni₃₁Si₁₂ and Ni₃Si β₁ (CrB and Cr₇C₃ are hidden for the sake of readability).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5—Lattice parameters vs temperature of the powder during heating of (a) γ (Ni), (b) γ' Ni₃Si, (c) CrB, (d) Cr₇C₃, (e) Ni₃B and (f) Ni₃₁Si₁₂. A diffractogram every 50 K is used to draw the general view, but a diffractogram every 9 K is used to picture the transformations between 600 °C and 700 °C and between 950 °C and 1050 °C.</figcaption></figure>

making transformation temperatures difficult to determine (Figure 6).

### C. Transformations of the Liquid During Cooling

The plotting of each phase fraction vs the temperature exhibits a multitude of transformations during the solidification between 950 °C and 1050 °C (Figure 7). Ni₃B and Ni₃₁Si₁₂ are solidifying at around 1000 °C. Furthermore, at a solid state, Ni₃Si β1 precipitates at about 980 °C (Figure 7(b)). The content of γ (Ni), Ni₃B and Ni₃Si β1 gradually increases throughout the cooling. On the other hand, the content of CrB, Cr₇C₃ and

Ni₃₁Si₁₂ is stable. Given that the precipitation of Ni₃Si β1 does not engender a diminution in the Ni₃₁Si₁₂ content, it seems that Ni₃Si β1 precipitates from the supersaturated matrix. Possible transformations that do not engender a phase fraction modification are studied by plotting the lattice parameters of each phase with the temperature (Figure 8). As for the heating, each phase's lattice parameters decrease with a temperature decrease (Figures 8(a), (c) through (f)). However, lattice parameters of Ni₃Si precipitates increase quickly after their appearance before decreasing (Figure 8(b)). This can be attributed to the evolution of γ' Ni₃Si β1 precipitate's shape from spheric to cubic to dendritic.[59–64] It should
<figure><img src="image_7.png" /><figcaption>Fig. 6—DTA curve of the powder during heating between 900 °C and 1100 °C at different heating rates.</figcaption></figure>

be mentioned that due to the high continuous background and the small quantity of this phase, lattice parameters of the γ (Ni) matrix cannot be determined above 1040 °C. Despite different heating rates, DTA signal during cooling exhibits several overlapped peaks, making the determination of the transformation's temperature difficult to determine (Figure 9). Thermodynamic computation depicted in Figure 10 displays phase stability at equilibrium state (Figure 10(a)) and the near-equilibrium solidification path predicted by the Scheil–Gulliver model (Figure 10(b)). At equilibrium state, γ (Ni), CrB, CrC, Ni₃B and Ni₃Si β₁ are found to be the most stable phases in Figure 10(a). The absence of Ni₃₁Si₁₂ in thermodynamic computation confirms that this phase is metastable. However, CrC is found to be more stable than Cr₇C₃ which is experimentally found. Near-equilibrium solidification path (Figure 10(b)) described, as found in our experiments, that Ni₃B melts at around 1000 °C and CrB is still solid up to 1200 °C. However, the *solidus* of γ' Ni₃Si β₁ and γ (Ni) appears lower than our experiments. Finally, the Scheil–Gulliver model predicts B₄C to form instead of Cr₇C₃ which is experimentally found. The microstructure of the material extracted from the capillary or the DTA crucible is considered at a near-equilibrium state.

## D. Cast Microstructure

The diffraction pattern of this cast product reveals the nickel-rich γ (Ni) matrix (space-group *Fm*–*3*m) with further addition of CrB (space-group *Cmcm*), Cr₇C₃ (space-group *Pnma*), Ni₃B (space-group *Pnma*), Ni₃₁Si₁₂ (space-group *P*321) and Ni₃Si β₁ precipitates (space-group *Pm*–*3*m) (Figure 11). To study the spatial dispersion and the shape of those phases, a bigger cast sample obtained by melting the powder in a furnace is used. The EBSD phase map with associated EDS spectrum for each phase are shown in the Figure 12. This analysis confirms that blocky phases are CrB and elongated phases are Cr₇C₃. Furthermore, the nickel-rich matrix γ (Ni) and Ni₃B are forming a globular eutectic shape. On the other hand, Ni₃B and Ni₃₁Si₁₂ are forming a lamellar eutectic shape. Finally, this analysis reveals a

small amount of Cr₃C₂ (Tongbaite) crystals (space-group *Pnma*). This phase has not been determined by XRD supposedly due to its very low amount and its low crystallography symmetries. Furthermore, this phase was only determined after investigation of Kikuchi patterns and EDS spectrum was not recorded. However, γ' Ni₃Si β₁ precipitates are not observed with this technique. Indeed, they might have precipitated from the supersaturated matrix. Therefore, high-resolution micrographs on etched samples are proposed (Figure 13). The high-resolution micrograph shows γ' Ni₃Si β₁ precipitates with different shapes (Figure 13(b)). According to several descriptions of γ' Ni₃Al/Ti precipitates found in more traditionally studied nickel-based superalloys, a multimodal γ' dispersion is observed. Indeed, primary γ' with a dendritic shape, secondary γ' with a cubic shape and ternary γ' with a spheric shape are observed. Therefore, the cast sample's microstructure comprises the nickel-rich matrix γ (Ni) containing spheric, cubic and dendritic γ' Ni₃Si β₁ precipitates. Furthermore, Ni₃B and Ni₃₁Si₁₂ form globular and lamellar eutectic shapes, respectively. Finally, the near-equilibrium microstructure contains CrB, Cr₇C₃ and Cr₃C₂ crystals.

# IV. DISCUSSION

## A. Phase Identification

This section discusses different aspects of phase identification, such as phase nature, crystal structure, lattice parameters and phase fraction.

### 1. Phase nature

According to the literature,[10–43] NiCrBSi alloys comprise a nickel-rich γ (Ni) matrix with CrB, Cr₇C₃, Ni₃B and Ni₃Si. Furthermore, some papers concerning those alloys attribute the eutectic shape to Ni₃Si/Ni₃B, while some attribute the Ni₃Si phase to small cubic precipitates. This paper demonstrated that the microstructure of a high-alloyed NiCrBSi obtained by casting or atomization is composed of γ (Ni), CrB, Cr₇C₃ and Ni₃B which are the most commonly observed phases for those alloys. Furthermore, two types of nickel silicide, Ni₃₁Si₁₂ and γ' Ni₃Si β₁, can be present depending on the cooling rate (Tables II and III). Indeed, the precipitation of Ni₃₁Si₁₂ seems to be favoured by the fast-cooling rate of the atomization process. This observation corresponds to the descriptions from Wu *et al.*[65] and Cui *et al.*[66] On the other hand, the near-equilibrium microstructure exhibits Ni₃₁Si₁₂ as a lamellar eutectic and Ni₃Si β₁ as small γ' precipitates within the γ (Ni) nick-rich matrix. It should be mentioned that Ni₃₁Si₁₂ is very rarely mentioned throughout the literature. However, depending on the reference, Ni₅Si₂ and Ni₃₁Si₁₂ belong to the same compound on the binary equilibrium diagram.[67,68] Therefore, those observations are in line with Lebaili *et al.*[8] describing the eutectic structure Ni₃B/Ni₅Si₂. Given that for many references concerning high-alloyed NiCrBSi alloys, γ (Ni), CrB, Cr₇C₃, Ni₃B and Ni₃Si are
<figure><img src="image_8.png" /><figcaption>Fig. 7—Phase fraction vs temperature of the powder during cooling (a) general view, and (b) zoom between 950 °C and 1050 °C (CrB and Cr<sub>7</sub>C<sub>3</sub> are hidden for the sake of readability).</figcaption></figure>

found,[10–43] our work demonstrated reliably that for high-alloyed NiCrBSi alloys, the phases γ (Ni), CrB, Cr<sub>7</sub>C<sub>3</sub>, Ni<sub>3</sub>B, Ni<sub>31</sub>Si<sub>12</sub> and γ' Ni<sub>3</sub>Si β1 are to be found (Ni<sub>31</sub>Si<sub>12</sub> or Ni<sub>3</sub>Si β1 depending on the cooling rate). Finally, this study demonstrated that the near-equilibrium microstructure contains a small amount of Cr<sub>3</sub>C<sub>2</sub>. Given that no Cr<sub>23</sub>C<sub>6</sub> formed by the decomposition of Cr<sub>3</sub>C<sub>2</sub> into Cr<sub>7</sub>C<sub>3</sub> and Cr<sub>23</sub>C<sub>6</sub> are found,[69,70] Cr<sub>3</sub>C<sub>2</sub> are most likely to be created by the transformation of Cr<sub>7</sub>C<sub>3</sub> into Cr<sub>3</sub>C<sub>2</sub> at high temperature.[71,72]

## 2. Crystal structure

According to Li *et al.*, [27] Cr<sub>7</sub>C<sub>3</sub> presents an orthorhombic crystal structure with a high cooling rate

and a hexagonal crystal structure with a low cooling rate. However, in our case, only the orthorhombic crystal structure is found after solidification at approx. 15 K·min<sup>-1</sup>, which can be considered as a low rate compared to atomized powder. According to The Material Project,[56] CrB, Cr<sub>7</sub>C<sub>3</sub> and Ni<sub>3</sub>Si β1 can present different space groups (Table III). In this study, synchrotron XRD allows the determination of the space group for each phase. The space group is unaffected by the cooling rate.

## 3. Lattice parameters

Lattice parameters of the phases found in NiCrBSi alloys are rarely discussed. However, according to the
<figure><img src="image_9.png" /><figcaption>Fig. 8—Lattice parameters vs temperature of the powder during cooling of (a) γ (Ni), (b) γ' Ni₃Si β1, (c) CrB, (d) Cr₇C₃, (e) Ni₃B and (f) Ni₃AlSi₁₂. A diffractogram every 50 K is used to draw the general view, but a diffractogram every 9 K is used to picture the transformations between 950 °C and 1050 °C.</figcaption></figure>

different databases, these lattice parameters can vary (Table IV). In this study, the Rietveld refinement method accurately determines lattice parameters on diffraction patterns obtained via synchrotron XRD. The lattice parameters of the nickel-rich γ (Ni) matrix are on the upper side compared to various databases. Elements in solid solutions such as iron, silicon or chromium can explain this phenomenon. The lattice parameters of CrB and γ' Ni₃Si β1 are in line with those found in various databases. However, for Cr₇C₃, the **b** and **c** axes are more extended, while **a** is shorter. For Ni₃B, the **a** and **c** axis are shorter, while the **b** axis is longer. Finally, for Ni₃AlSi₁₂, the **c** axis is longer than the value given in various databases. Those differences between measured

lattice parameters and those provided by the databases might be explained by the matrix surrounding those crystals, differences in the chemical composition of those crystals or by inhomogeneous strain. Indeed, it has been shown that for M₇C₃, depending on the amount of chromium and iron, lattice parameters vary.[73] Furthermore, differences between the lattice parameters measured on the powder and the cast sample can be noticed. The **b** axis of CrB and the **a** axis of Cr₇C₃ are larger for a low cooling rate (cast), while the **c** axis of CrB, the **b** axis of Cr₇C₃ and the **a** axis of Ni₃B are larger for a high cooling rate (powder). Those changes can be attributed to lattice strain or stoichiometric composition. However,
rejecting one hypothesis favouring the other is impos-
sible without further local analyses.

4. Phase fraction

The phase fraction's determination of NiCrBSi alloys
(visible in Figure 14) has not been published yet. A low
cooling rate favours γ' Ni₃Si β1 and CrB, while a high
cooling rate favours Ni₃B and Ni₃₁Si₁₂ (Figure 14). This
has to be clarified by dedicated investigations. However,
it is not easy to explain this behaviour without thermal
simulation. Due to the lack of representativity (0.12
mm² analysed in EBSD vs 5 mm³ in XRD) and the
impossibility of indexing γ' Ni₃Si β1 in EBSD, compar-
ison of the phase fraction found in XRD and in EBSD is
not investigated.

<figure><img src="image_10.png" /><figcaption>Fig. 9—DTA curve of the powder during cooling between 900 °C and 1100 °C at different cooling rates.</figcaption></figure>

B. *Transformation Sequence*

Based on Li *et al.*, [46] the transformation sequence is
determined through *in situ* XRD analyses using a
synchrotron source. Furthermore, the transformation's
temperature is determined by DTA. The following
paragraphs comment on these results in relation to the
bibliography.

1. *Liquid-solid transformations*

According to the nature of the transformation deter-
mined by the *in situ* XRD synchrotron, those transfor-
mations can be associated with peaks on the DTA
signal. Figure 15 is a proposal of DTA signal decom-
position. The blue curve represents the part of the signal
depicting transformations of γ (Ni)-Ni₃₁Si₁₂ while the
green curve represents transformations of γ (Ni)-Ni₃B.
Indeed, during heating, γ (Ni)-Ni₃B and γ (Ni)-Ni₃₁Si₁₂

<figure><img src="image_11.png" /><figcaption>Fig. 11—Diffraction pattern of the cast sample.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10—Thermodynamic computing of the studied alloy: (a) equilibrium phase fraction, and (b) Scheil–Gulliver solidification model.</figcaption></figure>

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 12—EBSD-EDS analysis: EBSD phase map (step size = 0.5 µm, 96.6 pct indexation), and associated EDS spectrum of γ (Ni), CrB, Cr<sub>7</sub>C<sub>3</sub>, Ni<sub>3</sub>B and Ni<sub>31</sub>Si<sub>12</sub> phases.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 13—Electron micrographs on etched samples: (a) general view (SE detector), and (b) high-resolution imaging from the area denoted in general view (SE In-lens detector).</figcaption></figure>

start to melt at 972 °C. The melting of γ (Ni)–Ni₃₁Si₁₂ ends at 1006 °C and the melting of γ (Ni)–Ni₃B ends at 1019 °C. During cooling, γ (Ni)–Ni₃B starts to solidify at 1008 °C and γ (Ni)–Ni₃₁Si₁₂ starts to solidify at 988 °C. Finally, their solidification end at 959 °C. However, it should be mentioned that the peak related to the

precipitation or the dissolution of γ' Ni₃Si β1 can be masked by other transformations. Therefore, this experimental study determined that liquid-solid transformations up to 1200 °C concern only γ (Ni)–Ni₃₁Si₁₂ and γ (Ni)–Ni₃B. On the other hand, γ (Ni), CrB and Cr₇C₃ are still solid up to 1200 °C. It should be mentioned that
<table><caption>Table III. Crystal Structure and Space-Group of CrB, Cr<sub>7</sub>C<sub>3</sub>, Ni<sub>3</sub>B, Ni<sub>31</sub>Si<sub>12</sub> and Ni<sub>3</sub>Si β1 According to the Database TMP<sup>[56]</sup></caption><thead><tr><th>Phase</th><th>Crystal Structure</th><th>Space-Group</th></tr></thead><tbody><tr><td>CrB</td><td><u>orthorhombic</u></td><td>Cmcm</td></tr><tr><td rowspan="2">Cr<sub>7</sub>C<sub>3</sub></td><td><u>tetragonal</u></td><td><u>I</u>4<sub>1</sub>/<u>amd</u></td></tr><tr><td><u>orthorhombic</u></td><td>Pnma</td></tr><tr><td>Ni<sub>3</sub>B</td><td><u>hexagonal</u></td><td><u>P</u>6<sub>3</sub><u>mc</u></td></tr><tr><td>Ni<sub>31</sub>Si<sub>12</sub></td><td><u>orthorhombic</u></td><td>Pnma</td></tr><tr><td rowspan="2">Ni<sub>3</sub>Si β1</td><td><u>trigonal</u></td><td>P321</td></tr><tr><td><u>cubic</u></td><td>Pm-3m</td></tr><tr><td></td><td><u>trigonal</u></td><td><u>R</u>-3m</td></tr></tbody></table>

Crystal structure and space groups found in this study are underlined.

<table><caption>Table IV. Lattice Parameters of CrB, Cr<sub>7</sub>C<sub>3</sub>, Ni<sub>3</sub>B, Ni<sub>31</sub>Si<sub>12</sub> and γ' Ni<sub>3</sub>Si β1 According to Our Study, to ICDD,<sup>[55]</sup> TMP,<sup>[56]</sup> COD,<sup>[57]</sup> and Different References and Our Study</caption><thead><tr><th>Phase</th><th>a (Å)</th><th>b (Å)</th><th>c (Å)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="4">γ (Ni) (Fm-3m)</td><td>3.48</td><td></td><td></td><td>TMP</td></tr><tr><td>3.45</td><td></td><td></td><td>COD</td></tr><tr><td>3.535</td><td></td><td></td><td>ICDD</td></tr><tr><td>3.53820 (3)</td><td></td><td></td><td>this study (powder)</td></tr><tr><td>3.53571 (8)</td><td></td><td></td><td>this study (cast)</td></tr><tr><td rowspan="6">CrB (Cmcm)</td><td>2.812</td><td>7.562</td><td>2.812</td><td>TMP</td></tr><tr><td>2.960 to 2.969</td><td>7.810 to 7.858</td><td>2.932 to 2.940</td><td>COD</td></tr><tr><td>2.966 to 2.993</td><td>7.867 to 7.925</td><td>2.932 to 2.954</td><td>ICDD</td></tr><tr><td>2.884</td><td>7.732</td><td>2.874</td><td>Wang et al.<sup>[74]</sup></td></tr><tr><td>2.97 (3)</td><td>7.8 (1)</td><td>2.94 (3)</td><td>this study (powder)</td></tr><tr><td>2.97 (1)</td><td>7.86 (3)</td><td>2.9257 (9)</td><td>this study (cast)</td></tr><tr><td rowspan="5">Cr<sub>7</sub>C<sub>3</sub> (Pnma)</td><td>4.480</td><td>6.900</td><td>11.940</td><td>TMP</td></tr><tr><td>4.526</td><td>7.010</td><td>12.142</td><td>ICDD</td></tr><tr><td>4.488 to 4.560</td><td>6.850 to 7.012</td><td>11.732 to 12.144</td><td>Sobolev et al.<sup>[73]</sup></td></tr><tr><td>4.4424 (3)</td><td>7.05 (2)</td><td>12.20 (3)</td><td>this study (powder)</td></tr><tr><td>4.48 (1)</td><td>7.04 (7)</td><td>12.2 (1)</td><td>this study (cast)</td></tr><tr><td rowspan="6">Ni<sub>3</sub>B (Pnma)</td><td>4.34</td><td>5.15</td><td>6.55</td><td>TMP</td></tr><tr><td>5.222</td><td>6.617</td><td>4.392</td><td>COD</td></tr><tr><td>5.211 to 5.232</td><td>6.609 to 6.617</td><td>4.389 to 4.390</td><td>ICDD</td></tr><tr><td>5.222</td><td>6.617</td><td>4.392</td><td>Gumeniuk et al.<sup>[75]</sup></td></tr><tr><td>5.3086 (1)</td><td>6.6079 (1)</td><td>4.4081 (1)</td><td>this study (powder)</td></tr><tr><td>5.264 (1)</td><td>6.606 (1)</td><td>4.398 (1)</td><td>this study (cast)</td></tr><tr><td rowspan="6">Ni<sub>31</sub>Si<sub>12</sub> (P321)</td><td>6.600</td><td></td><td>12.150</td><td>TMP</td></tr><tr><td>6.671</td><td></td><td>12.288</td><td>COD</td></tr><tr><td>6.670</td><td></td><td>12.280</td><td>ICDD</td></tr><tr><td>6.671</td><td></td><td>12.288</td><td>Connétable et al.<sup>[76]</sup></td></tr><tr><td>6.6648 (3)</td><td></td><td>12.3102 (9)</td><td>this study (powder)</td></tr><tr><td>6.6655 (9)</td><td></td><td>12.30 (3)</td><td>this study (cast)</td></tr><tr><td rowspan="4">γ' Ni<sub>3</sub>Si β1 (Pm-3m)</td><td>3.460</td><td></td><td></td><td>TMP</td></tr><tr><td>3.551 to 3.542</td><td></td><td></td><td>COD</td></tr><tr><td>3.515</td><td></td><td></td><td>ICDD</td></tr><tr><td>3.506 to 3.520</td><td></td><td></td><td>Connétable et al.<sup>[76]</sup></td></tr><tr><td></td><td>3.5148 (1)</td><td></td><td></td><td>this study (cast)</td></tr></tbody></table>

during the heating there is a small peak at around 970 °C which is not attributed to any transformation in Figure 6. According to the *in situ* XRD analyses, it can be associated either with the dissolution of γ' Ni<sub>3</sub>Si β1 or the precipitation of Ni<sub>31</sub>Si<sub>12</sub>. However, evidence to support one hypothesis over the other is yet to be found. Those results are not in line with transformation sequences determined by DTA<sup>[8,25]</sup> found in the literature. Indeed, according to Lebaili *et al.*<sup>[8]</sup> the solidification of a similar alloy (14.0 Cr pct<sub>at.</sub> 14.0 B pct<sub>at.</sub> 6.3 Si

pct<sub>at.</sub> 1.5 C pct<sub>at.</sub>) begins by CrB at 1025 °C followed by γ (Ni), Cr<sub>7</sub>C<sub>3</sub> and Ni<sub>3</sub>B at 990 °C and ends by Ni<sub>5</sub>Si<sub>2</sub> at 955 °C. It should be mentioned that compared to the alloy used in this paper (12.7 Cr pct<sub>at.</sub> 15.3 B pct<sub>at.</sub> 6.6 Si pct<sub>at.</sub> 2.6 Fe pct<sub>at.</sub> 2.5 C pct<sub>at.</sub>), the alloy studied by Lebaili *et al.*<sup>[8]</sup> does not contain iron. Iron is used in those alloys to saturate the matrix. Therefore, it is very unlikely that the lack of iron explains the lower solidification temperature of CrB, γ (Ni) and Cr<sub>7</sub>C<sub>3</sub> found by Lebaili *et al.*<sup>[8]</sup> However, by taking into consideration that
<figure><img src="image_16.png" /><figcaption>Fig. 14—Phase fraction of the powder and the cast sample determined by Rietveld.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 15—Summary of the liquid-solid transformations sequence during heating and cooling at 15 K·min⁻¹ of the powder analysed by DTA and *in situ* XRD. The blue curve represents the part of the signal depicting transformations of γ (Ni)–Ni₃₁Si₁₂ while the green curve represents transformations of γ (Ni)–Ni₃B (Color figure online).</figcaption></figure>

according to Connétable *et al*.,[76] Ni₅Si₂ (space-group P6₃/mmc) should be replaced by Ni₃₁Si₁₂ (space-group P321), this study is in line with the lower part of the solidification sequence concerning Ni–Si–B eutectics proposed by Lebaili *et al*.[8] According to Hemmati

*et al*.,[25] the solidification begins with CrB, followed by Cr₅B₃, followed by Cr₇C₃ and by the formation of Ni–Si–B eutectics at under 1000 °C. The authors do not specify the solidification temperature for CrB, Cr₅B₃ and Cr₇C₃. First, in this study, Cr₅C₃ is not identified by synchrotron XRD or EBSD. It should be mentioned that the alloy studied by Hemmati *et al*.[25] (12.5 Cr pct_at. 13.4 B pct_at. 6.9 Si pct_at. 3.4 Fe pct_at. 2.8 C pct_at. 1.0 Mo pct_at. 1.3 Cu pct_at.) contains molybdenum and copper unlike the alloy used in this study. According to Hou *et al*.,[77] adding molybdenum does not modify the nature of chromium boride. Therefore, it is difficult to explain the lack of Cr₅B₃ in this study. This experiment does not permit to confirm that the solidification begins with CrB and Cr₇C₃ (still solid up to 1200 °C). However, this experiment demonstrates that the solidification ends with Ni-Si-B eutectics. This is partially in line with thermodynamic computations presented in Figure 10. Indeed, Ni₃B does solidify at around 1000 °C and CrB is still stable up to 1200 °C. However, Ni₃₁Si₁₂ is not depicted confirming the metastable nature of this phase and the solidus of γ (Ni) and γ' Ni₃Si β1 is lower than that observed experimentally. Finally, CrC and B₄C are depicted by thermodynamical simulations instead of Cr₇C₃ which is found experimentally. This might be explained by the diffusivity of carbon at solid state. According to thermal simulations by Pereira *et al*.[45] on a very similar alloy (12 Cr pct_at. 11 B pct_at. 6.6 Si pct_at. 3.6 Fe pct_at. 1.9 C pct_at.), the solidification begins with the formation of chromium carbide Cr₇C₃ at 1180 °C, the nickel-rich matrix at 1050 °C and Cr₃C₂ at 1000 °C. The solidification ends with the formation of Ni₃Si, Ni₃B, Ni₅Si₂ and Cr₂B at 870 °C, 860 °C, 840 °C and 830 °C, respectively. Despite Cr₂B, the phases observed in this simulation are similar to those found in our experimental study. Furthermore, while the solidification of Cr₃C₂ is not observed via synchrotron XRD, a small amount of Cr₃C₂ is observed via EBSD on the cast sample. However, according to our experimental study, Ni₃₁Si₁₂ or Ni₅Si₂ is formed before Ni₃Si and after Ni₃B. Furthermore, the temperatures of transformation are higher in our study. Therefore, our work does not match with thermodynamical simulations by Pereira *et al*.[45]

## 2. Solid-solid transformations

This study determined several solid-solid transformations between Ni₃₁Si₁₂ and γ' Ni₃Si β1 occurring in a NiCrBSi alloy. Indeed, during heating, between 600 °C and 700 °C, γ' Ni₃Si β1 precipitates while the metastable Ni₃₁Si₁₂ is dissolved. In the literature, the precipitation of γ' Ni₃Si β1 from the metastable Ni₃₁Si₁₂ phase is depicted.[65,78–82] However, given that γ' Ni₃Si β1 are found in the γ (Ni) matrix, it seems that Ni₃₁Si₁₂ dissolves, engendering the supersaturation of the γ (Ni) matrix. Finally, γ' Ni₃Si β1 precipitate from the supersaturated γ (Ni) matrix. Furthermore, at 970 °C, γ' Ni₃Si β1 are dissolved and Ni₃₁Si₁₂ precipitates. The decomposition of γ' Ni₃Si β1 to Ni₃₁Si₁₂ + γ (Ni) is depicted in the literature.[83] However, given that γ' Ni₃Si β1 are found in the γ (Ni) matrix, it seems that Ni₃Si β1 dissolves engendering the supersaturation of the γ (Ni) matrix. This supersaturation engenders the eutectic
transformation, leading to $\gamma$ (Ni) + Ni₃Si₁₂. During cooling, $\gamma'$ Ni₃Si β₁ precipitate from the matrix at 950 °C. Those transformations have rarely been studied in the literature concerning NiCrBSi alloys. Indeed, only Lebaili et al.[8] refer to the precipitation of $\gamma'$ Ni₃Si β₁ during the cooling.

### 3. Lattice parameters' evolution

As expected, lattice parameters increase when the temperature increase and decrease with the temperature decrease. However, during the heating, the evolution of the **a** axis of Ni₃B is perturbated between 700 °C and 800 °C (Figure 5(e)). This is in line with perturbations on the **b** axis of CrB (Figure 5(c)). As evidenced by EDS Figure 12, Ni₃B and CrB could be alloyed with chromium and nickel, respectively. Therefore, those perturbations can be correlated with solute exchanges of chromium and iron between Ni₃B and CrB. However, it is not easy to confirm it. Finally, it is also possible that both phases contain carbon which start to diffuse significantly at those temperatures leading to variations of lattice parameters, but this is even more difficult to confirm.

## V. CONCLUSION

This paper contributes to a better understanding of NiCrBSi alloys from a metallurgical point of view. Powerful characterisation tools are used to answer open questions of the literature regarding the microstructure and the transformation sequence.

* The non-equilibrium microstructure of the gas-atomized powder is composed of the nickel-rich $\gamma$ (Ni) matrix (Fm$\bar{3}$m) with CrB (Cmcm), Cr$_7$C$_3$ (Pnma), Ni$_3$B (Pnma) and Ni$_{31}$Si$_{12}$ (P321) crystals.

* The near-equilibrium microstructure of cast samples is composed of the nickel-rich $\gamma$ (Ni) matrix (Fm$\bar{3}$m) with CrB (Cmcm), Cr$_7$C$_3$ (Pnma), Ni$_3$B (Pnma) and Ni$_{31}$Si$_{12}$ (P321) and further $\gamma'$ Ni$_3$Si β₁ (Pm$\bar{3}$m) precipitates in the matrix.

* The transformation sequence is determined during heating as: Ni$_{31}$Si$_{12}$ dissolution and Ni$_3$Si β₁ precipitation at approx. 620 °C, Ni$_3$Si β₁ dissolution and Ni$_{31}$Si$_{12}$ precipitation at approx. 970 °C, $\gamma$ (Ni)-Ni$_{31}$Si$_{12}$ melting between 972 °C and 1006 °C and $\gamma$ (Ni)-Ni$_3$B melting between 972 °C and 1019 °C.

* The transformation sequence is determined during cooling as: $\gamma$ (Ni)-Ni$_3$B solidification between 1008 °C and 959 °C, $\gamma$ (Ni)-Ni$_{31}$Si$_{12}$ solidification between 988 °C and 959 °C and $\gamma'$ Ni$_3$Si β₁ precipitation in the matrix at approx. 980 °C.

## ACKNOWLEDGMENTS

This paper has been written within the context of the SPARTAN Project CP/2020-DEC/09.04 financed by

the Region Occitanie. The authors acknowledge the European Synchrotron Radiation Facility (ESRF) for providing synchrotron radiation facilities under Proposal Number MA-5643, and we would like to thank all the staff at the ID22 Beamline. We would also like to thank Jonathan Wright for his assistance concerning phase determination and Romain Garlet who built the furnace for in situ XRD experiments. We would like to thank Arnaud Proietti's technical assistance regarding EBSD and A. Nickolas from the Thermocalc® Company for providing thermodynamical computations. We are indebted to Jean-Marc Cloué, Jérôme Rocchi and Jordan Rigaud for their technical expertise. Finally, the authors sincerely thank Jade Pécune and Nathalie Aubazac for their technical support with sample preparation and SEM observations, respectively.

## CONFLICT OF INTEREST

The authors declare that they have no conflicts of interest. For the purpose of Open Access, a CC-BY public copyright licence has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission

## FUNDING

Open access funding provided by Institut National Polytechnique de Toulouse.

## OPEN ACCESS

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