In situ observation of phase transformation in iron carbide nanocrystals

Accepted Manuscript Title: In Situ Observation of Phase Transformation in Iron Carbide Nanocrystals Authors: Le Thanh Cuong, Nguyen Duc Dung, Ta Quoc Tuan, Nguyen Thi Khoi, Pham Thanh Huy, Ngo Ngoc Ha PII: DOI: Reference:

S0968-4328(17)30203-2 https://doi.org/10.1016/j.micron.2017.10.009 JMIC 2492

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Please cite this article as: Cuong, Le Thanh, Dung, Nguyen Duc, Tuan, Ta Quoc, Khoi, Nguyen Thi, Huy, Pham Thanh, Ha, Ngo Ngoc, In Situ Observation of Phase Transformation in Iron Carbide Nanocrystals.Micron https://doi.org/10.1016/j.micron.2017.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

In Situ Observation of Phase Transformation in Iron Carbide Nanocrystals

Le Thanh Cuong1, Nguyen Duc Dung1,*, Ta Quoc Tuan2, Nguyen Thi Khoi2, Pham Thanh Huy2, and Ngo Ngoc Ha3 1

Electron Microscopy and MicroAnalysis Laboratory (BKEMMA), Advanced Institute

of Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No.1 Dai Co Viet, Vietnam. 2

Nano Optoelectronics Laboratory, Advanced Institute of Science and Technology

(AIST), HUST, No.1 Dai Co Viet, Vietnam. 3

Micro-nano Systems and Sensors Laboratory, International Training Institute for

Materials Science (ITIMS), HUST, No.1 Dai Co Viet, Vietnam. *

e-mail: [email protected]

Highlights - in situ observation of phase transformation at nanoscale; - The formation and switching of h-Fe7C3 and o-Fe7C3 nanocrystals; - Structural analysis of iron carbide nanocrystals by FFT from HR-TEM images; - Experimental evidence for the stability of the crystal structures.

Abstract: This paper reports on the in situ observation of phase transformation in an iron carbide nanocrystal encapsulated in a graphitic shell by means of high resolution transmission electron microscopy (HR-TEM). A Fe7C3 nanocrystal in orthorhombic (oFe7C3) structure with carbon graphitic cover is captured at the initial time of the experiment. Under the projection of a high-energy electron beam (200 kV), the graphitic carbon layer evaporates gradually and structural changes in orthorhombic (oFe7C3) crystal manifests simultaneously. Specifically, changes in crystal direction 1

happens first and then the crystal structure switching between orthorhombic and hexagonal (h-Fe7C3) follows. Details analysis and conclusive evidences of the phase structure and transformation are presented and discussed. The appearance of o-Fe7C3 structure is captured for about 92 min over 100 min of observation, indicating the preference of o-Fe7C3 form over h-Fe7C3 form. Keywords: Iron carbide; phase transformation; nanocrystals; in situ TEM; structural stability.

INTRODUCTION Iron carbide is the most important binary alloy that can be found in many aspects of human lives, e.g. diversified steels and cast irons. Especially, it is the primary constituent of the Earth’s core and involves in its magnetic and electric fields, gravity characteristics, and heat flow (Litasov and Shatskiy, 2016; Raza et al., 2015; Weerasinghe et al., 2015, 2011). Under extreme conditions of the inner core, i.e. high pressure and temperature, Fe7C3 iron carbide is reported to be the first phase crystallizing from a liquid iron-carbon alloy (Chen et al., 2014; Li et al., 2016; Nakajima et al., 2011). Studies of magnetic properties of Fe7C3 have shown a Curie temperature Tc ~ 250°C and high saturation magnetization (Carpenter and Carpenter, 2003; Moustafa et al., 2000; Serna et al., 2006). The crystal structure of Fe7C3 has been intensively investigated both experimentally and theoretically with two major morphologies identified: hexagonal lattice (h-Fe7C3) and orthorhombic lattice (o-Fe7C3) (BauerGrosse et al., 1981; Fang et al., 2009; Senczyk, 1993; Sluiter, 2007). Development of advanced technologies, especially high resolution transmission electron microscopy 2

(HR-TEM), enables research of new and improved nanostructures, utilizing unique atomic-scale properties (Börrnert, 2016; Gonnissen et al., 2016; Jinschek, 2016; Mu et al., 2016; Taheri et al., 2016). By using a first-principles method, Sluiter (Sluiter, 2007) reported the preference of o-Fe7C3 phase to h-Fe7C3 phase. In another report (Fang et al., 2009), total energy and electronic structure of the two basic lattices of Fe7C3 were determined. The calculations also showed that the Fe sublattices in the oFe7C3 structure exhibit higher stability than the Fe sublattice in the h-Fe7C3 form. The differences between o-Fe7C3 and h-Fe7C3 lattices are often too small for the ease of distinguishing of the two polymorphs, given that in the o-Fe7C3 form, the latice index b = 6.8790 Å is not much different from the latice indexes a = b = 6.8820 Å of the hFe7C3 form. In such situation, the observations of the two polymorphs from one certain direction are often the same, with the marginal error δdhkl below 0.13%, thus can not be distinguished by just HR-TEM images. However, the two polymorphs can be identified by the diffraction spot intensity of selected area electron diffraction (SAED) or Fast Furrier Transform (FFT) images. In this work, we report conclusive evidence of in situ observation of phase transformation of Fe7C3 nanostructure under highenergy electron irradiationover a time span of 100 min. The switching between two crystal structures, h-Fe7C3 and o-Fe7C3 are visualized and the preference of o structure over h structure is observed.

EXPERIMENTAL Pure graphite was ground in a tungsten mortar and under a continuous flow of argon gas. The pressure and temperature of the mortar was maintained at 300 kPa and 750° C, respectively. After grinding, the mortar temperature was reduced gradually to room 3

temperature in 15 h. A small amount of iron from the milling ball added to graphite powder as an impurity during the grinding process, facilitating the formation of iron carbide nanostructures under the projection of an electron beam. The morphology and crystal structure of the iron carbide were investigated by means of high-resolution transmission electron microscopy (HRTEM), model FEI Tecnai GF20. In situ observation of phase transformation of the material was enabled by the continuous high-energy electron beam (200kV) of the HRTEM in a total 100 min. The SAED mode of the HRTEM was utilized for structure analysis with few tens of nanometers in each direction. Gatan Digital Micrograph software was used to analyze crystal structure through FFT. Compositions of the sample were determined by Energy-dispersive X-ray spectroscopy (EDX) which shows only iron and carbon in the studied sample.

RESULTS AND DISCUSSION Figure 1(a) shows a HRTEM image of a nano-sized crystal encapsulated in a onionlike shell at the beginning of our observation. The enlarged image of the shell (Fig. 1b) presents a stack of graphitic layers with an average distance of 3.35 Å between layers. This value is close to the distance between carbon layers in graphite (Banhart et al., 1998; Ugarte, 1992). Shown in Fig. 1(c) and Fig. 1(d) are the enlarged images of nanocrystal and its FFT analysis, respectively. These results indicate an iron carbide nanocrystal with o-Fe7C3 structure and in [010] direction. The distances of various crystal plane families tabulated in Table 1 are in good agreement with the data in Ref. (Fruchart and Rouault A., 1969). Fig.1(c) and 1(d) indicate clearly crystal planes and their corresponding diffractions spots, in which the plane distance is d101 = 4.24 Å. 4

This is the unique structural characteristic existing only in o-Fe7C3 form that can not be found in the h-Fe7C3 form.

This iron carbide nanocrystal was selected for further observation. The onion-like layer was gradually evaporated (melted and faded away) under the projection of the electron beam, whereas the nanocrystal stays in o-Fe7C3 structure. At the observation time close to t = 87 min, changes in HRTEM image of the nanocrystal can be seen. The HRTEM image and its FFT pattern of the iron carbide nanocrystal at t = 87 min are shown in Fig. 2. The distances between crystal planes are presented in Table 2, inferring the o-Fe7C3 structure. The crystal direction perpendicular to the HRTEM image plane has rotated from [010] to [122]. We note that particle movement or tilt of the specimen did not occur.

At the observation time close to t = 92 min, changes of the HRTEM image of the nanocrystal can be seen again. Details of the crystal observation at t = 92 min and analyses are shown in Fig.3. We see that the o-Fe7C3 crystal structure now transforms to h-Fe7C3 structure. The new nanocrystal form has [001] crystal direction perpendicular to the HRTEM image plane. Some parameters of the new form of the iron carbide nanocrystal in h-Fe7C3 structure are shown in Table 3.

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At this point, we see that the h-Fe7C3 [001] is quite like o-Fe7C3 [100]. As mentioned in the introduction, both morphologies give almost similar diffraction spot positions with a marginal error δdhkl below 0.13 %, thus they can not be distinguished. However, by considering the diffraction spot intensity, the problem can be solved. In the o-Fe7C3 [100] form, the diffraction spots corresponding to diffraction plane (020) would fade away, while such diffraction spots in a similar description but different phase, h-Fe7C3 [001], from (21̅0), (110) and (1̅20) appear equal. And that is the case here, we see the diffraction spots corresponding to the diffraction planes (21̅0), (110) and (1̅20) in Fig.3(c). This is the conclusive evidence for the phase transformation from o-Fe7C3 form to h-Fe7C3 form.

At the observation time close to t = 97 min, we observe again the changes of the HRTEM image as shown in Fig. 4. The distances between crystal planes are presented in Table 4 and are in good agreement with the previous report for h-Fe7C3 structure (Herbstein and Snyman, 1964). These again refer that the iron carbide nanocrystal stays in h-Fe7C3 structure, only the crystal direction perpendicular to the HRTEM image plane has rotated from [001] to [041]. We see that the FFT images present a straight lines refering to an enomous phase transformation occuring in the material.

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At an observation time close to t = 100 min, we observe another structure transformation of the nanocrystal h-Fe7C3 structure with [041] direction likely transforms back to o-Fe7C3 crystal structure but with the [12̅2] direction. This is determined from the enlarged HRTEM image and diffraction pattern of the new crystal structure shown in Fig. 5. The distances between crystal planes are also compared to data reported in Ref. (Fruchart and Rouault A., 1969) and presented in Table 5.

In summary, over 100 minutes of observation, the iron carbide nanocrystal experiences several phase transformations from o-Fe7C3 to h-Fe7C3 crystal structure and back. However, the existing time of h-Fe7C3 crystal structure of around 5 min in 100 min is minor in comparison with the existing time of o-Fe7C3 crystal structure of 95 min. We can conclude that the o- Fe7C3 crystal structure is favored over the h-Fe7C3 crystal structure. This is also supported by another report (Fang et al., 2009) on the total formation enthalpy of h-Fe7C3 (38.9 meV/atom) which is higher than that for o-Fe7C3 (22 meV/atom).

CONCLUSION For the first time, in situ observation of phase transformation of Fe7C3 nanocrystal encapsulated in graphitic shell under high-energy electron irradiation has been presented. The switching back and forth between two basic crystal structures, h-Fe7C3 and o-Fe7C3, has been visualized. Conclusive evidence for the existence of the two polyphors have been presented and analyzed. Moreover, the appearance of o-Fe7C3 is captured for about 95 minutes over 100 minutes of observation, which indicates that o7

Fe7C3 is more stable than h-Fe7C3 structure. Our results help to build a more comprehensive knowledge of the phase diagram of iron carbide with a high concentration of carbon.

ACKNOWLEDGMENTS This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.01-2011.54.

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for a single optimal design. Ultramicroscopy 170, 128–138. doi:10.1016/j.ultramic.2016.07.014 Herbstein, F.H., Snyman, J. a., 1964. Identification of Eckstrom-Adcock Iron Carbide as Fe7C3. Inorg. Chem. 3, 894. doi:10.1021/ic50016a026 Jinschek, J.R., 2016. Achieve atomic resolution in in situ S/TEM experiments to examine complex interface structures in nanomaterials. Curr. Opin. Solid State Mater. Sci. doi:http://dx.doi.org/10.1016/j.cossms.2016.05.010 Li, Y., Vočadlo, L., Brodholt, J., Wood, I.G., 2016. Thermoelasticity of Fe7C3 under inner core conditions. J. Geophys. Res. Solid Earth 121, 5828–5837. doi:10.1002/2016JB013155 Litasov, K.D., Shatskiy, A.F., 2016. Composition of the Earth’s core: A review. Russ. Geol. Geophys. 57, 22–46. doi:10.1016/j.rgg.2016.01.003 Moustafa, I.M., Moustafa, M.A., Nofal, A.A., 2000. Carbide formation mechanism during solidification and annealing of 17% Cr-ferritic steel. Mater. Lett. 42, 371– 379. doi:10.1016/S0167-577X(99)00213-X Mu, X., Kobler, A., Wang, D., Chakravadhanula, V.S.K., Schlabach, S., Szabo, D. V., Norby, P., Kubel, C., 2016. Comprehensive analysis of TEM methods for LiFePO4/FePO4 phase mapping: spectroscopic techniques (EFTEM, STEMEELS) and STEM diffraction techniques (ACOM-TEM). Ultramicroscopy 170, 10–18. doi:10.1016/j.ultramic.2016.07.009 Nakajima, Y., Takahashi, E., Sata, N., Nishihara, Y., Hirose, K., Funakoshi, K.I., Ohishi, Y., 2011. Thermoelastic property and high-pressure stability of Fe7C3: Implication for iron-carbide in the Earth’s core. Am. Mineral. 96, 1158–1165. doi:10.2138/am.2011.3703 10

Raza, Z., Shulumba, N., Caffrey, N.M., Dubrovinsky, L., Abrikosov, I.A., 2015. Firstprinciples calculations of properties of orthorhombic iron carbide Fe7C3 at the Earth’s core conditions. Phys. Rev. B - Condens. Matter Mater. Phys. 91, 1–7. doi:10.1103/PhysRevB.91.214112 Senczyk, D., 1993. Some polytypes of Fe7C3 carbides. Phase Transitions 43, 153–156. doi:10.1080/01411599308207809 Serna, M.M., Jesus, E.R.B., Galego, E., Martinez, L.G., Corrêa, H.P.S., Rossi, J.L., 2006. An overview of the microstructures present in high-speed steel - carbides crystallography. Mater. Sci. Forum 530–531, 48–52. doi:10.4028/www.scientific.net/MSF.530-531.48 Sluiter, M., 2007. Phase stability of carbides and nitrides in steel. Mater. Res. Soc. Symp. Proc. 979E, 43–48. Taheri, M.L., Stach, E.A., Arslan, I., Crozier, P.A., Kabius, B.C., LaGrange, T., Minor, A.M., Takeda, S., Tanase, M., Wagner, J.B., Sharma, R., 2016. Current status and future directions for in situ transmission electron microscopy. Ultramicroscopy 170, 86–95. doi:10.1016/j.ultramic.2016.08.007 Ugarte, D., 1992. Curling and closure of graphitic networks under electron-beam irradiation. Nature 359, 707–709. doi:10.1038/359707a0 Weerasinghe, G.L., Needs, R.J., Pickard, C.J., 2011. Computational searches for iron carbide in the Earth’s inner core. Phys. Rev. B - Condens. Matter Mater. Phys. 84, 1–7. doi:10.1103/PhysRevB.84.174110 Weerasinghe, G.L., Pickard, C.J., Needs, R.J., 2015. Computational searches for iron oxides at high pressures. J. Phys. Condens. Matter 27, 455501. doi:10.1088/09538984/27/45/455501 11

FIG. 1: (a) HRTEM image of an iron carbide nanocrystal with a graphitic “carbon onion” shell at the initial observation time t = 0 s; (b) Enlarged image of graphitic layers; (c) Enlarged image of the iron carbide indicating the o-Fe7C3 structure with [010] direction; (d) FFT image of the iron carbide with diffraction spots corresponding to the indicated crystal surfaces.

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FIG. 2: (a) HRTEM image of the o-Fe7C3 nanocrystal at the observation time t = 87 min; (b) Enlarged image of the o-Fe7C3 nanocrystal corresponding to the [122] direction; (c) FFT image of the o-Fe7C3nanocrystal with the indicated diffraction spots corresponding to different diffraction planes.

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FIG. 3: HRTEM image of the iron carbide nanocrystal at the observation time t = 92 min; (b) enlarged image of the nanocrystal indicating h-Fe7C3 structure with [001] direction; (c) FFT image of the nanocrystal with diffraction spots corresponding to different diffraction planes of the h-Fe7C3 structure.

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FIG. 4: HRTEM image of the iron carbide nanocrystal at the observation time t = 97 min; (b) enlarged image of the nanocrystal indicating the h-Fe7C3 structure with [041] direction; (c) FFT image of the nanocrystal with indicated diffraction spots corresponding different diffraction planes of the h-Fe7C3 structure.

FIG. 5: HRTEM image of the iron carbide nanocrystal at the observation time t = 100 min; (b) enlarged image of the nanocrystal indicating the o-Fe7C3 structure with [12̅2] direction; (c) FFT image of the nanocrystal with indicated diffraction spots corresponding different diffraction planes of the o-Fe7C3 structure.

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TABLE 1: Distances between crystal planes of the o-Fe7C3 structure at the observation time of t = 0. Distance between crystal planes of o-Fe7C3

Plane (hkl)

dhkl (Å)

dhkl (Å)

Following Ref.

Current work

(Fruchart and Rouault A., 1969) (002)

5.971

5.97

(101)

4.244

4.24

(101̅)

4.244

4.24

(103)

2.993

2.98

(004)

2.986

2.98

(200)

2.270

2.27

(202)

2.122

2.12

(006)

1.990

1.99

TABLE 2: Crystal structure analyses of the o-Fe7C3 structure at the observation time of t = 87 min. Distance between crystal planes of o-Fe7C3

Plane (hkl)

dhkl(Å)

dhkl(Å)

Following Ref.

Current work

(Fruchart and Rouault A., 1969) (011̅)

5.961

5.96

16

(2̅01)

2.230

2.23

(212̅)

2.028

2.03

Angles between two planes Planes (hkl)

Following Ref.

Current

(Fruchart and Rouault A., 1969) (011̅) and (212̅)

64.86o

64.9o

TABLE 3: Some parameters of the h-Fe7C3 nanocrystal structure at the observation time of t = 92 min. Distance between crystal planes of h-Fe7C3

Plane (hkl)

dhkl (Å)

dhkl (Å)

Following Ref.

Current work

(Herbstein and Snyman, 1964) (100)

5.959

5.96

(010)

5.959

5.96

(1̅00)

5.959

5.96

(110)

3.441

3.44

(1̅20)

3.441

3.44

(2̅10)

3.441

3.44

(020)

2.980

2.98

(120)

2.253

2.25

(030)

1.987

1.98

17

Angles between two planes Planes

Following Ref.

Current

(Herbstein and Snyman, 1964) (100) and (010)

60o

60o

(1̅10) and (010)

60o

60o

(100) and (11̅0)

60o

60o

TABLE 4: Distances between crystal planes of the h-Fe7C3 nanocrystal at the observation time of t = 97 min. Distance between crystal planes of h-Fe7C3

plane (hkl)

dhkl(Å)

dhkl (Å)

Following ref.

Current work

(Herbstein and Snyman, 1964) (100)

5.959

5.95

(200)

2.979

2.98

(300)

1.987

1.99

TABLE 5: Crystal structure analyses of the o-Fe7C3 nanocrystal at the observation time of t = 100 min. Distance between crystal planes of o-Fe7C3 dhkl(Å)

dhkl(Å)

18

Plane (hkl)

Following Ref.

Current work

(Fruchart and Rouault A., 1969) (011)

5.961

5.96

(210)

2.156

2.16

(221)

1.871

1.87

Angles between two planes Planes

Following Ref.

Current work

(Fruchart and Rouault A., 1969) (011) and (221)

56.66o

56.5o

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