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Characterisation of non-stoichiometric cementite (Fe3C)
CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 68 (2020) 101689
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Characterisation of non-stoichiometric cementite (Fe3C) Andre Schneider a, *, Martin Palm b a b
Vallourec Research Center Germany, Vallourec Deutschland GmbH, Düsseldorf, Germany Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Cementite Iron-carbon Experimental
In order to measure the composition range of cementite, a sample has been prepared by carburisation at 700 � C at a high carbon activity aC ¼ 100. Such strongly carburising conditions normally result in a quick onset of the hightemperature corrosion process metal dusting, which leads to cementite decomposition at the surface. The effect of sulphur to retard this corrosion process was used by adding a small amount of 1 ppm H2S to the gas mixture. The measurements of the carbon contents of the cementite particles formed at the sample surface were per formed by using electron-probe microanalysis (EPMA). It was found that cementite (Fe3C) which was formed at a very high carbon activity aC » aC (Fe3C/α-Fe) reveals a measurable carbon gradient. The homogeneity range of cementite at 700 � C in the investigated sample was determined to be of at least 0.6 at.% carbon. Note: this study has been performed at Max-Planck-Institut für Eisenforschung already two decades ago, but it was not yet published internationally. However, the respective experimental results were already described in [1].
1. Introduction Cementite is generally described as a stoichiometric compound. The structure of cementite can be described as an arrangement of trigonal prisms with iron atoms at their corners and every second prism contains a carbon atom. Thermodynamically, a certain disorder structure can be expected also in stoichiometric compounds [2]. For cementite this is discussed by Kayser et al. and also Hume-Rothery et al. [3–5]. Hume-Rothery et al. determined the carbon content of cementite which was extracted from steel samples to be 6.52 � 0.10 wt.% (»24.5 � 0.5 at. %) at room temperature [5]. The studies of Kayser et al. [3,4] also focus on measurements of cementite composition at room temperature. Den sity measurements of Fe-C alloys containing ferrite and cementite in metastable equilibrium were performed for various carbon contents of the alloys up to 4.17 wt.% (» 16.83 at.%) and the results were extrapo lated to 6.6895 wt.% carbon (¼ 25.0 at.% C in Fe3C), i.e. the carbon content of stoichiometric cementite. The density of cementite, which was determined by this extrapolation, is slightly lower than the value calculated based on lattice parameters determined by means of X-ray diffraction analyses (density that is calculated using the mass of the atoms in the unit cell and the lattice constants) in the same work [4]. Therefore, Kayser et al. concluded that cementite in equilibrium with ferrite is under-stoichiometric due to carbon vacancies with a chemical
composition of 6.65 � 0.02 wt.% carbon (» 24.88 � 0.10 at.%) at room temperature [3,4]. Recently, Leineweber et al. presented new results on nonstoichiometric cementite which was not obtained by metal dusting, but by electrolytic etching of ferrite or martensite of Fe-C alloys [6]. Before etching, the Fe-C alloys with 0.63, 1.6, and 4.2 wt.% C were equilibrated at various temperatures from 550 to 1050 � C to obtain the temperature dependent metastable equilibria of cementite with α-Fe or γ-Fe, respectively. Then the samples were quenched to room tempera ture. Lattice-parameter determination of the extracted cementite pow der was performed using X-ray powder diffraction (XRD). The decrease of the lattice parameters a and c and the simultaneous increase of b with increasing annealing temperature was attributed to an increased con centration of carbon vacancies in Fe3C1-x [6]. Therefore, the resulting C content of cementite in equilibrium with α-Fe or γ-Fe decreases with temperature. At 700 � C the C concentration of cementite can be inter polated to approximately 24.8 at.% C. Walker et al. investigated the homogeneity range of Fe3C at high pressures [7]. At 1 and 7 GPa they found a maximum width at 1150 � C of about 21.5–26.3 at.% C, strongly decreasing with decreasing tempera ture. As the homogeneity range is the same at both pressures, it seems as if the solid solubility does not much depend on pressure. Though it is of course open, whether a comparable large solid solution range exists also
* Corresponding author. E-mail address: [email protected] (A. Schneider). https://doi.org/10.1016/j.calphad.2019.101689 Received 16 September 2019; Received in revised form 23 October 2019; Accepted 24 October 2019 Available online 19 November 2019 0364-5916/© 2019 Elsevier Ltd. All rights reserved.
A. Schneider and M. Palm
Calphad 68 (2020) 101689
at normal pressure, the results show that Fe3C exists with off-stoichiometric compositions, both, leaner or richer in C. Due to missing experimental data in the past, the main theoretical studies on the Fe-C system consider cementite as stoichiometric com pound [8–13]. Narahi et al. also treat cementite as stoichiometric compound, but mention that cementite may exist over a narrow composition range deviating from the stoichiometric Fe3C composition [13]. In their theoretical work on point defect structure of cementite Jiang et al. took note of the experimental evidence that cementite can indeed exist with non-stoichiometric composition [14]. By combining the statistical mechanical Wagner-Schottky model and first-principles calculations they considered carbon Frenkel pairs in C-depleted (xC < 25 at.%) and stoichiometric cementite. For C-rich (xC > 25 at.%) cementite Frenkel pairs, Schottky and interbranch defects are theoreti €hring cally predicted to be dominant depending on temperature [14]. Go et al. recently developed a new thermodynamic model for non-stoichiometric cementite [15], based on the CALPHAD-formalism, by also taking into account the new results of Leineweber et al. [6]. With this new information the carbon content of cementite in metastable equilibrium with α-Fe or γ-Fe deviates from the stoichiometric compo sition of 25 at.%. Employing their new thermodynamic model for €hring et al. determined a carbon con non-stoichiometric cementite, Go tent of 24 at.% in cementite being in metastable equilibrium with austenite at 1400 K, for instance [15]. Auger electron spectroscopy (AES) was also applied by Schneider et al. to measure the carbon gradient in Fe3C close to the sample surface [1]. Though the AES spectra showed the presence and extent of carbon gradients within Fe3C, it is difficult to quantify the carbon content and therefore a more accurate method was employed [1]. New techniques such as atom probe tomography (APT) could be applied in future studies to measure carbon profiles in Fe carbides with a higher lateral resolu tion. The complex procedures of quantitative carbon determination by APT in Fe3C were demonstrated by Takahashi et al. [16] and Kitaguchi et al. [17]. An advantage of the APT could be related to the possibility to receive APT samples from the desired sample locations being of interest by focused ion beam milling (FIB). In this study a pure iron sample was carburised at 700 � C at a high carbon activity aC ¼ 100 to form cementite at the surface. Such strongly carburising conditions normally result in a quick onset of the hightemperature corrosion process metal dusting, which leads to cementite decomposition at the surface [18]. The effect of sulphur to retard this corrosion process [1,19,20] was used by adding a small amount of 1 ppm H2S to the gas mixture. The measurements of the carbon contents of the cementite particles formed at the sample surface were performed by using electron-probe microanalysis (EPMA).
EPMA is not a routine application, the applied procedure is explained in detail in the following paragraph. The basic challenges of carbon content determination by EPMA were precisely investigated by Weisweiler [23–28]. One major problem is the fact that the carbon Kα-radiation shows a relatively low intensity. Additionally, this Ka-radiation is strongly absorbed in the material. Another difficulty stems from contaminations on the sample surface, which can lead to wrong results with too high carbon contents. The cause of this contamination is the presence of hydrocarbons, which can be present also with a very good vacuum in the EPMA chamber. Pump-oil, plasticisers, or vacuum grease can be sources of the hydro carbons [29]. The electron beam hits the hydrocarbons on the sample surface and leads to cracking of the molecules and subsequent deposi tion of carbon-bearing crack products on the surface. These deposits are usually visible as black dots or circles in a secondary-electron (SE) image at the spot where the analysis has been performed. Therefore, various effective procedures to minimise the contamination impact were applied in this study. A cooling finger cooled with liquid nitrogen is used to reduce the amount of hydrocarbons in the EPMA chamber. Inflating oxygen towards the sample surface using a gas-jet leads to almost complete removal of the contamination layer. Combined use of cold finger and oxygen gas-jet leads to a very low contamination after a certain time period, with minor subsequent increase of this signal due to continuous contamination with crack products. Based on experiences with carbon content determination of carbides, a measuring instruction for the carbon measurement in cementite (Fe3C) was defined as follows [30]: At first the electron beam is switched-on at the measuring point with activated gas-jet for 4 minutes (pre-burning), since after this time period the minimum of the C-Kα signal is reached. Subsequently, the C- and Fesignals were measured for 100 s. Even in case of incomplete removal of the contamination the total amount of carbon will be in fact incorrect, but the various measurements will be well comparable to each other due to the application of the same condition for every measurement. A socalled PC1-pseudo crystal was applied for the measurement of the CKα-peak – this crystal shows an especially low background signal in the range of the C-Ka-peak. For the standardisation procedure iron with Fe3C precipitates is chosen as reference sample, in that carbon has the same bonding state as in the samples which were investigated. Thus a shift of the maximum of the Kα-line between standard and sample is avoided, as the energy, i.e. the position, of the Kα-line depends on the bonding of the C atoms [26]. The so-called ZAF correction program PAP [31] for the analysis of light elements (Z < 11) was applied for this study which was already successfully tested in a previous study [30]. Based on experi ences with the applied EPMA equipment and on a error estimation for C € [32] it can assumed that the ac measurements by EPMA by Runnsjo curacy of the C measurements is better than � 0.4 at.% carbon. For the C measurements a very careful surface preparation has to be performed without using carbon bearing preparation substances. After carburisation treatment the samples were polished with aluminium oxide and then cleaned by flushing with distilled water. It has to be taken care that during contacting sample on the EPMA sample holder the carbon bearing solvent of the conduct silver does not contaminate the sample surface. In order to make sure that the sample is free of contamination several EPMA measurements on the iron parts of the sample in the vicinity of the analysed cementite particles were per formed with the result that carbon is below detection limit. This proofs that the sample preparation does not lead to any surface contamination.
2. Experimental Samples of pure iron were ground with silicon paper of grits 400, 600, and 1000. Final polishing was performed using diamond paste (3 and 1 μm). Before inserting the sample into the carburisation furnace it was cleaned in acetone in an ultrasonic bath. The sample was carburised at 700 � C in a CH4-H2-H2S gas mixture at aC ¼ 100 with 1 ppm H2S for 114 h and then quenched in an air stream. The carbon activity of the CH4-H2-H2S gas mixture was calculated ac cording to [21]. H2S was formed by passing H2 through a mixture of iron and its sulphide. The equilibrium content of H2S was established by controlling temperature [22]. The gas velocities were controlled by capillary flow meters. The phases on the carburised samples were identified by X-ray diffraction (XRD) with Cr-Kα radiation and observed in metallographic cross sections. Measurements of carbon content of the cementite particles at the sample surface were performed by means of electron probe microanalysis (EPMA) using a CAMECA SX50 microprobe with wavelengthdispersive X-ray analysis (WDX). Since carbon determination with
3. Results The sample for the carbon measurements was cut out of a melt of pure iron by spark erosion. The chemical composition of the respective melt (denoted as VA 1009) is shown in Table 1. The X-ray diffraction (XRD) spectrum in Fig. 1 shows that the surface of the carburised iron sample consists of α-Fe, cementite Fe3C, and 2
A. Schneider and M. Palm
Calphad 68 (2020) 101689
Table 1 Chemical composition of the pure iron melt VA 1009 (in wt.%). VA 1009
C
Si
Mn
P
S
N
O
Al
0.0053
0.0071
0.0024
<0.0001
<0.0010
-
0.0029
0.0015
graphite. For the carbon measurements the sample was ground perpendicu larly to its carburised surface to prepare a cross-section (nonembedded). EPMA carbon measurements have been performed in cementite particles at the sample surface according to the procedure described above. It is assumed that cementite Fe3C at the phase interface Fe3C/α-Fe is stoichiometric and shows a carbon content of cC ¼ 25 at.%. This assumption was done and used for the representation of the results with respect to the fact that the absolute values of the C content deter mined by EPMA are less accurate than differences of the single values to each other. However, this assumption targeting a defining a sort of reference composition is to be questioned (for this it is referred to the discussion). Starting with the reference composition of cC ¼ 25 at.%, the various carbon profiles that have been measured perpendicular to the carburised surface are shifted so that the theoretical and measured C content at the Fe3C/α-Fe interface are close to each other. By this approximation the differences between the various carbon contents of the profiles are not changed. The following metallographic cross-section images (Figs. 2, 4 and 6) show the focal spots of the measurement points. The Figs. 3, 5 and 7 show the measured carbon contents at the respective cementite particles. The coordinates were taken from the EPMA sample stage: x axis is parallel and y axis is perpendicular to the carburised sample (see Fig. 2). Independent of the length (thickness) of the cementite particles in ydirection (perpendicular to the surface) all carbon profiles show gradi ents of decreasing carbon contents. At the interface coke/Fe3C the profiles start with carbon contents between 25.5 and 25.8 at.% and decrease down to 25.0 at.% at the interface Fe3C/α-Fe (see also above described procedure of data representation). Despite the relatively high uncertainty of measurement of the absolute carbon content value of �0.4 at.%, it can clearly be deduced from the measurements with respect to the relative differences of the carbon contents that cementite shows a composition profile with a difference between the two in terfaces coke/Fe3C and Fe3C/α-Fe of approximately 0.6 at.%. The Figs. 4 and 6 show the further investigated cementite particles. The respective carbon profile measurements in Figs. 5 and 7 are quali tatively and also quantitatively in good agreement with the first ones (Fig. 3).
Fig. 2. Light-optical microscopy image of the firstly investigated cementite particle (non-etched cross-section). Line profiles analysed by EPMA are visible by black circles stemming from residual contamination.
Fig. 3. Carbon profiles in cementite particle no. 1 (see Fig. 2) measured by EPMA and represented according to the above described procedure. Fig. 1. X-ray diffraction (XRD) spectrum of the investigated sample. 3
A. Schneider and M. Palm
Calphad 68 (2020) 101689
4. Discussion The determination of the carbon content was done with an uncer tainty of measurement of �0.4 at.% for each measurement point. Due to the fact that every measurement was performed according to the same procedure and that every composition profile shows the same tendency, it can be assumed that the differences of carbon content between the various measurement points show a better accuracy. As a result of this study it can be concluded that cementite, which was assumed to be a stoichiometric compound Fe3C, shows a homoge neity range of about 0.6 at.% at 700 � C. This result refers to iron after carburisation at a carbon activity aC ¼ 100 at 700 � C. During carburisa tion and before the onset of graphite deposition and subsequent metal dusting reaction, the high carbon activity of the gas is in direct contact with the Fe3C surface. A certain decrease of this value will be given due to the transfer reaction of carbon from the gas phase to the metal so that a carbon activity of 1.33 << aC (gas/Fe3C) < 100 (with aC ¼ 1.33 at the interface Fe3C/α-Fe, see also [33]) will be active at the gas-cementite interface, i.e. the cementite surface. It can be assumed that for higher carbon activities in the gas phase even higher carbon contents can be reached in cementite close to its surface. The determination of the carbon content is a result of a comparison of the measured intensities with those of the cementite standard sample with an assumed carbon content of 25.0 at.%. However, due to this assumption the question on the absolute carbon content remains open. In the studies of Hume-Rothery [5] and Kayser et al. [3,4] the carbon content of cementite in (metastable) equilibrium at room temperature was determined to be 24.5 or 24.9 at.%.. Kayser et al. discuss the pos sibility of a decreased carbon content below the stoichiometric compo sition of Fe3C (cC < 25.0 at.%) due to vacancies in the carbon sub-lattice. Following this argumentation the carbon content of cementite will then decrease with increasing temperature due to an increased vacancy concentration in the carbon sub-lattice. When combining the above mentioned results and those of Leine weber et al. [6] with the ones presented in this work the homogeneity range of cementite at 700 � C could be (at least) 24.8 up to 25.4 at.%. In addition to vacancies (VC) also carbon atoms on interstitial sites (Ci) must be considered as another possible defect structure. Thus, the following defect equilibria must be considered for cementite formation in a CH4-H2 gas mixture (CC ¼ carbon concentration in the carbon sublattice):
Fig. 4. Light-optical microscopy image of the secondly investigated cementite particle (non-etched cross-section).
Fig. 5. Carbon profiles in cementite particle no. 2 (see Fig. 4) measured by EPMA and represented according to the above described procedure.
CH4 þ VC ¼ CC þ 2H2
(1)
and CH4 ¼ Ci þ 2H2 (2) Fig. 6. Light-optical microscopy image of the thirdly investigated cementite particle (non-etched cross-section).
Studies on the defect structure of cementite will significantly depend on the absolute measurement values of composition. In principle, results concerning defect structure of cementite could be obtained by a gravi metric determination of cementite composition under equilibrium con ditions between gas and sample as it was done by Grabke in his study on the defect structure of the γ0 - iron nitride [34], but there is the main difficulty that cementite tends to decompose before thermodynamic equilibrium is reached. 5. Conclusion Measurements of carbon contents of cementite were performed using electron-probe microanalysis (EPMA). Cementite (Fe3C), which grows during carburisation at high carbon activity aC » aC (Fe3C/α-Fe), reveals a measurable carbon gradient. The homogeneity range of analysed cementite particles which formed at 700 � C was determined to be of at least 0.6 at.% carbon. When combining the literature results with the ones presented in this work, the homogeneity range of cementite at 700 � C could be (at least) 24.8 up to 25.4 at.%.
Fig. 7. Carbon profiles in cementite particle no. 3 (see Fig. 6) measured by EPMA and represented according to the above described procedure.
4
A. Schneider and M. Palm
Calphad 68 (2020) 101689
References
[19] A. Schneider, H. Viefhaus, G. Inden, H.J. Grabke, E.M. Müller-Lorenz, Mater. Corros. 49 (1998) 336. [20] A. Schneider, H. Viefhaus, G. Inden, Mater. Corros. 51 (2000) 338. [21] H.J. Grabke, D. Grassl, H. Schachinger, K.H. Weissohn, J. Wünning, U. Wyss, H€ art.Tech. Mittl. 49 (1994) 306. [22] W.H. Herrnstein, F.H. Beck, M.G. Fontana, Trans. Metall. Soc. AIME 242 (1968) 1049. [23] W. Weisweiler, Quantitative Elektronenstrahl-Mikroanalyse Leichter Elemente (Habilitationsschrift), Karlsruhe, 1972. [24] W. Weisweiler, Mikrochim. Acta (1972) 145. [25] W. Weisweiler, Mikrochim. Acta (1975) 365. [26] W. Weisweiler, Mikrochim. Acta (1975) 611. [27] W. Weisweiler, Mikrochim. Acta (1975) 161. [28] W. Weisweiler, Mikrochim. Acta (1975) 179. [29] S. Baumgartl, A.R. Büchner, K. Dreyer, P. Schwaab, H. Stender, H. Vetters, Arch. für das Eisenhuttenwes. 50 (1979) 85. [30] M. Palm, Konstitutionsuntersuchungen Zum System Fe-Al-C, Doctoral Thesis, Dortmund, 1990. [31] J.L. Pouchou, F. Pichoir, La Recherche A� erospatiale, 1984, p. 13. [32] G. Runnsj€ o, Scand. J. Metall. 9 (1980) 199. [33] A. Schneider, Corros. Sci. 44 (2002) 2353. [34] H.J. Grabke, Berichte der Bunsengesellschaft für physikalische Chemie, vol. 73, 1969, p. 596.
[1] A. Schneider, Einfluß von H2S auf die Bildung und den Zerfall von Eisenkarbiden beim Metal Dusting, Doctoral thesis, VDI-Verlag, Düsseldorf, 1999. [2] H. Schmalzried, A. Navrotsky, Festk€ orperthermodynamik, Chemie des festen Zustands, Verlag Chemie, Weinheim, 1975. [3] F.X. Kayser, A. Litwinchuk, G.L. Stowe, Metall. Trans. A 6A (1975) 55. [4] F.X. Kayser, Y. Sumitomo, J. Phase Equilibria 18 (1997) 458. [5] W. Hume-Rothery, G.V. Raynor, A.T. Little, J. Iron Steel Inst. 145 (1942) 143. [6] A. Leineweber, S.L. Shang, Z.K. Liu, Acta Mater. 86 (2015) 374. [7] D. Walker, R. Dasgupta, J. Li, A. Buono, Contrib. Mineral. Petrol. 166 (2013) 935–957. [8] J. Chipman, Metall. Trans. 3 (1972) 55. [9] H. Okamoto, in: T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 1990, p. 842. [10] J. Agren, Metall. Trans. A 10 (1979) 1847. [11] P. Gustafson, Scand. J. Metall. 14 (1985) 259. [12] B. Hallstedt, D. Djurovic, J. von Appen, R. Dronskowski, A. Dick, F. K€ ormann, T. Hickel, J. Neugebauer, Calphad 34 (2010) 129. [13] R. Naraghi, M. Selleby, J. Ågren, Calphad 46 (2014) 148. [14] C. Jiang, B.P. Uberuaga, S.G. Srinivasan, Acta Mater. 56 (2008) 3236. [15] H. G€ ohring, A. Leineweber, E.J. Mittemeijer, Calphad 52 (2016) 38. [16] J. Takahashi, K. Kawakami, Y. Kobayashi, Ultramicroscopy 111 (2011) 1233. [17] H.S. Kitaguchi, S. Lozano-Perez, M.P. Moody, Ultramicroscopy 147 (2017) 51. [18] H.J. Grabke, Mater. Corros. 54 (2003) 736.
5
Contents lists available at ScienceDirect
Calphad journal homepage: http://www.elsevier.com/locate/calphad
Characterisation of non-stoichiometric cementite (Fe3C) Andre Schneider a, *, Martin Palm b a b
Vallourec Research Center Germany, Vallourec Deutschland GmbH, Düsseldorf, Germany Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Cementite Iron-carbon Experimental
In order to measure the composition range of cementite, a sample has been prepared by carburisation at 700 � C at a high carbon activity aC ¼ 100. Such strongly carburising conditions normally result in a quick onset of the hightemperature corrosion process metal dusting, which leads to cementite decomposition at the surface. The effect of sulphur to retard this corrosion process was used by adding a small amount of 1 ppm H2S to the gas mixture. The measurements of the carbon contents of the cementite particles formed at the sample surface were per formed by using electron-probe microanalysis (EPMA). It was found that cementite (Fe3C) which was formed at a very high carbon activity aC » aC (Fe3C/α-Fe) reveals a measurable carbon gradient. The homogeneity range of cementite at 700 � C in the investigated sample was determined to be of at least 0.6 at.% carbon. Note: this study has been performed at Max-Planck-Institut für Eisenforschung already two decades ago, but it was not yet published internationally. However, the respective experimental results were already described in [1].
1. Introduction Cementite is generally described as a stoichiometric compound. The structure of cementite can be described as an arrangement of trigonal prisms with iron atoms at their corners and every second prism contains a carbon atom. Thermodynamically, a certain disorder structure can be expected also in stoichiometric compounds [2]. For cementite this is discussed by Kayser et al. and also Hume-Rothery et al. [3–5]. Hume-Rothery et al. determined the carbon content of cementite which was extracted from steel samples to be 6.52 � 0.10 wt.% (»24.5 � 0.5 at. %) at room temperature [5]. The studies of Kayser et al. [3,4] also focus on measurements of cementite composition at room temperature. Den sity measurements of Fe-C alloys containing ferrite and cementite in metastable equilibrium were performed for various carbon contents of the alloys up to 4.17 wt.% (» 16.83 at.%) and the results were extrapo lated to 6.6895 wt.% carbon (¼ 25.0 at.% C in Fe3C), i.e. the carbon content of stoichiometric cementite. The density of cementite, which was determined by this extrapolation, is slightly lower than the value calculated based on lattice parameters determined by means of X-ray diffraction analyses (density that is calculated using the mass of the atoms in the unit cell and the lattice constants) in the same work [4]. Therefore, Kayser et al. concluded that cementite in equilibrium with ferrite is under-stoichiometric due to carbon vacancies with a chemical
composition of 6.65 � 0.02 wt.% carbon (» 24.88 � 0.10 at.%) at room temperature [3,4]. Recently, Leineweber et al. presented new results on nonstoichiometric cementite which was not obtained by metal dusting, but by electrolytic etching of ferrite or martensite of Fe-C alloys [6]. Before etching, the Fe-C alloys with 0.63, 1.6, and 4.2 wt.% C were equilibrated at various temperatures from 550 to 1050 � C to obtain the temperature dependent metastable equilibria of cementite with α-Fe or γ-Fe, respectively. Then the samples were quenched to room tempera ture. Lattice-parameter determination of the extracted cementite pow der was performed using X-ray powder diffraction (XRD). The decrease of the lattice parameters a and c and the simultaneous increase of b with increasing annealing temperature was attributed to an increased con centration of carbon vacancies in Fe3C1-x [6]. Therefore, the resulting C content of cementite in equilibrium with α-Fe or γ-Fe decreases with temperature. At 700 � C the C concentration of cementite can be inter polated to approximately 24.8 at.% C. Walker et al. investigated the homogeneity range of Fe3C at high pressures [7]. At 1 and 7 GPa they found a maximum width at 1150 � C of about 21.5–26.3 at.% C, strongly decreasing with decreasing tempera ture. As the homogeneity range is the same at both pressures, it seems as if the solid solubility does not much depend on pressure. Though it is of course open, whether a comparable large solid solution range exists also
* Corresponding author. E-mail address: [email protected] (A. Schneider). https://doi.org/10.1016/j.calphad.2019.101689 Received 16 September 2019; Received in revised form 23 October 2019; Accepted 24 October 2019 Available online 19 November 2019 0364-5916/© 2019 Elsevier Ltd. All rights reserved.
A. Schneider and M. Palm
Calphad 68 (2020) 101689
at normal pressure, the results show that Fe3C exists with off-stoichiometric compositions, both, leaner or richer in C. Due to missing experimental data in the past, the main theoretical studies on the Fe-C system consider cementite as stoichiometric com pound [8–13]. Narahi et al. also treat cementite as stoichiometric compound, but mention that cementite may exist over a narrow composition range deviating from the stoichiometric Fe3C composition [13]. In their theoretical work on point defect structure of cementite Jiang et al. took note of the experimental evidence that cementite can indeed exist with non-stoichiometric composition [14]. By combining the statistical mechanical Wagner-Schottky model and first-principles calculations they considered carbon Frenkel pairs in C-depleted (xC < 25 at.%) and stoichiometric cementite. For C-rich (xC > 25 at.%) cementite Frenkel pairs, Schottky and interbranch defects are theoreti €hring cally predicted to be dominant depending on temperature [14]. Go et al. recently developed a new thermodynamic model for non-stoichiometric cementite [15], based on the CALPHAD-formalism, by also taking into account the new results of Leineweber et al. [6]. With this new information the carbon content of cementite in metastable equilibrium with α-Fe or γ-Fe deviates from the stoichiometric compo sition of 25 at.%. Employing their new thermodynamic model for €hring et al. determined a carbon con non-stoichiometric cementite, Go tent of 24 at.% in cementite being in metastable equilibrium with austenite at 1400 K, for instance [15]. Auger electron spectroscopy (AES) was also applied by Schneider et al. to measure the carbon gradient in Fe3C close to the sample surface [1]. Though the AES spectra showed the presence and extent of carbon gradients within Fe3C, it is difficult to quantify the carbon content and therefore a more accurate method was employed [1]. New techniques such as atom probe tomography (APT) could be applied in future studies to measure carbon profiles in Fe carbides with a higher lateral resolu tion. The complex procedures of quantitative carbon determination by APT in Fe3C were demonstrated by Takahashi et al. [16] and Kitaguchi et al. [17]. An advantage of the APT could be related to the possibility to receive APT samples from the desired sample locations being of interest by focused ion beam milling (FIB). In this study a pure iron sample was carburised at 700 � C at a high carbon activity aC ¼ 100 to form cementite at the surface. Such strongly carburising conditions normally result in a quick onset of the hightemperature corrosion process metal dusting, which leads to cementite decomposition at the surface [18]. The effect of sulphur to retard this corrosion process [1,19,20] was used by adding a small amount of 1 ppm H2S to the gas mixture. The measurements of the carbon contents of the cementite particles formed at the sample surface were performed by using electron-probe microanalysis (EPMA).
EPMA is not a routine application, the applied procedure is explained in detail in the following paragraph. The basic challenges of carbon content determination by EPMA were precisely investigated by Weisweiler [23–28]. One major problem is the fact that the carbon Kα-radiation shows a relatively low intensity. Additionally, this Ka-radiation is strongly absorbed in the material. Another difficulty stems from contaminations on the sample surface, which can lead to wrong results with too high carbon contents. The cause of this contamination is the presence of hydrocarbons, which can be present also with a very good vacuum in the EPMA chamber. Pump-oil, plasticisers, or vacuum grease can be sources of the hydro carbons [29]. The electron beam hits the hydrocarbons on the sample surface and leads to cracking of the molecules and subsequent deposi tion of carbon-bearing crack products on the surface. These deposits are usually visible as black dots or circles in a secondary-electron (SE) image at the spot where the analysis has been performed. Therefore, various effective procedures to minimise the contamination impact were applied in this study. A cooling finger cooled with liquid nitrogen is used to reduce the amount of hydrocarbons in the EPMA chamber. Inflating oxygen towards the sample surface using a gas-jet leads to almost complete removal of the contamination layer. Combined use of cold finger and oxygen gas-jet leads to a very low contamination after a certain time period, with minor subsequent increase of this signal due to continuous contamination with crack products. Based on experiences with carbon content determination of carbides, a measuring instruction for the carbon measurement in cementite (Fe3C) was defined as follows [30]: At first the electron beam is switched-on at the measuring point with activated gas-jet for 4 minutes (pre-burning), since after this time period the minimum of the C-Kα signal is reached. Subsequently, the C- and Fesignals were measured for 100 s. Even in case of incomplete removal of the contamination the total amount of carbon will be in fact incorrect, but the various measurements will be well comparable to each other due to the application of the same condition for every measurement. A socalled PC1-pseudo crystal was applied for the measurement of the CKα-peak – this crystal shows an especially low background signal in the range of the C-Ka-peak. For the standardisation procedure iron with Fe3C precipitates is chosen as reference sample, in that carbon has the same bonding state as in the samples which were investigated. Thus a shift of the maximum of the Kα-line between standard and sample is avoided, as the energy, i.e. the position, of the Kα-line depends on the bonding of the C atoms [26]. The so-called ZAF correction program PAP [31] for the analysis of light elements (Z < 11) was applied for this study which was already successfully tested in a previous study [30]. Based on experi ences with the applied EPMA equipment and on a error estimation for C € [32] it can assumed that the ac measurements by EPMA by Runnsjo curacy of the C measurements is better than � 0.4 at.% carbon. For the C measurements a very careful surface preparation has to be performed without using carbon bearing preparation substances. After carburisation treatment the samples were polished with aluminium oxide and then cleaned by flushing with distilled water. It has to be taken care that during contacting sample on the EPMA sample holder the carbon bearing solvent of the conduct silver does not contaminate the sample surface. In order to make sure that the sample is free of contamination several EPMA measurements on the iron parts of the sample in the vicinity of the analysed cementite particles were per formed with the result that carbon is below detection limit. This proofs that the sample preparation does not lead to any surface contamination.
2. Experimental Samples of pure iron were ground with silicon paper of grits 400, 600, and 1000. Final polishing was performed using diamond paste (3 and 1 μm). Before inserting the sample into the carburisation furnace it was cleaned in acetone in an ultrasonic bath. The sample was carburised at 700 � C in a CH4-H2-H2S gas mixture at aC ¼ 100 with 1 ppm H2S for 114 h and then quenched in an air stream. The carbon activity of the CH4-H2-H2S gas mixture was calculated ac cording to [21]. H2S was formed by passing H2 through a mixture of iron and its sulphide. The equilibrium content of H2S was established by controlling temperature [22]. The gas velocities were controlled by capillary flow meters. The phases on the carburised samples were identified by X-ray diffraction (XRD) with Cr-Kα radiation and observed in metallographic cross sections. Measurements of carbon content of the cementite particles at the sample surface were performed by means of electron probe microanalysis (EPMA) using a CAMECA SX50 microprobe with wavelengthdispersive X-ray analysis (WDX). Since carbon determination with
3. Results The sample for the carbon measurements was cut out of a melt of pure iron by spark erosion. The chemical composition of the respective melt (denoted as VA 1009) is shown in Table 1. The X-ray diffraction (XRD) spectrum in Fig. 1 shows that the surface of the carburised iron sample consists of α-Fe, cementite Fe3C, and 2
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Calphad 68 (2020) 101689
Table 1 Chemical composition of the pure iron melt VA 1009 (in wt.%). VA 1009
C
Si
Mn
P
S
N
O
Al
0.0053
0.0071
0.0024
<0.0001
<0.0010
-
0.0029
0.0015
graphite. For the carbon measurements the sample was ground perpendicu larly to its carburised surface to prepare a cross-section (nonembedded). EPMA carbon measurements have been performed in cementite particles at the sample surface according to the procedure described above. It is assumed that cementite Fe3C at the phase interface Fe3C/α-Fe is stoichiometric and shows a carbon content of cC ¼ 25 at.%. This assumption was done and used for the representation of the results with respect to the fact that the absolute values of the C content deter mined by EPMA are less accurate than differences of the single values to each other. However, this assumption targeting a defining a sort of reference composition is to be questioned (for this it is referred to the discussion). Starting with the reference composition of cC ¼ 25 at.%, the various carbon profiles that have been measured perpendicular to the carburised surface are shifted so that the theoretical and measured C content at the Fe3C/α-Fe interface are close to each other. By this approximation the differences between the various carbon contents of the profiles are not changed. The following metallographic cross-section images (Figs. 2, 4 and 6) show the focal spots of the measurement points. The Figs. 3, 5 and 7 show the measured carbon contents at the respective cementite particles. The coordinates were taken from the EPMA sample stage: x axis is parallel and y axis is perpendicular to the carburised sample (see Fig. 2). Independent of the length (thickness) of the cementite particles in ydirection (perpendicular to the surface) all carbon profiles show gradi ents of decreasing carbon contents. At the interface coke/Fe3C the profiles start with carbon contents between 25.5 and 25.8 at.% and decrease down to 25.0 at.% at the interface Fe3C/α-Fe (see also above described procedure of data representation). Despite the relatively high uncertainty of measurement of the absolute carbon content value of �0.4 at.%, it can clearly be deduced from the measurements with respect to the relative differences of the carbon contents that cementite shows a composition profile with a difference between the two in terfaces coke/Fe3C and Fe3C/α-Fe of approximately 0.6 at.%. The Figs. 4 and 6 show the further investigated cementite particles. The respective carbon profile measurements in Figs. 5 and 7 are quali tatively and also quantitatively in good agreement with the first ones (Fig. 3).
Fig. 2. Light-optical microscopy image of the firstly investigated cementite particle (non-etched cross-section). Line profiles analysed by EPMA are visible by black circles stemming from residual contamination.
Fig. 3. Carbon profiles in cementite particle no. 1 (see Fig. 2) measured by EPMA and represented according to the above described procedure. Fig. 1. X-ray diffraction (XRD) spectrum of the investigated sample. 3
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4. Discussion The determination of the carbon content was done with an uncer tainty of measurement of �0.4 at.% for each measurement point. Due to the fact that every measurement was performed according to the same procedure and that every composition profile shows the same tendency, it can be assumed that the differences of carbon content between the various measurement points show a better accuracy. As a result of this study it can be concluded that cementite, which was assumed to be a stoichiometric compound Fe3C, shows a homoge neity range of about 0.6 at.% at 700 � C. This result refers to iron after carburisation at a carbon activity aC ¼ 100 at 700 � C. During carburisa tion and before the onset of graphite deposition and subsequent metal dusting reaction, the high carbon activity of the gas is in direct contact with the Fe3C surface. A certain decrease of this value will be given due to the transfer reaction of carbon from the gas phase to the metal so that a carbon activity of 1.33 << aC (gas/Fe3C) < 100 (with aC ¼ 1.33 at the interface Fe3C/α-Fe, see also [33]) will be active at the gas-cementite interface, i.e. the cementite surface. It can be assumed that for higher carbon activities in the gas phase even higher carbon contents can be reached in cementite close to its surface. The determination of the carbon content is a result of a comparison of the measured intensities with those of the cementite standard sample with an assumed carbon content of 25.0 at.%. However, due to this assumption the question on the absolute carbon content remains open. In the studies of Hume-Rothery [5] and Kayser et al. [3,4] the carbon content of cementite in (metastable) equilibrium at room temperature was determined to be 24.5 or 24.9 at.%.. Kayser et al. discuss the pos sibility of a decreased carbon content below the stoichiometric compo sition of Fe3C (cC < 25.0 at.%) due to vacancies in the carbon sub-lattice. Following this argumentation the carbon content of cementite will then decrease with increasing temperature due to an increased vacancy concentration in the carbon sub-lattice. When combining the above mentioned results and those of Leine weber et al. [6] with the ones presented in this work the homogeneity range of cementite at 700 � C could be (at least) 24.8 up to 25.4 at.%. In addition to vacancies (VC) also carbon atoms on interstitial sites (Ci) must be considered as another possible defect structure. Thus, the following defect equilibria must be considered for cementite formation in a CH4-H2 gas mixture (CC ¼ carbon concentration in the carbon sublattice):
Fig. 4. Light-optical microscopy image of the secondly investigated cementite particle (non-etched cross-section).
Fig. 5. Carbon profiles in cementite particle no. 2 (see Fig. 4) measured by EPMA and represented according to the above described procedure.
CH4 þ VC ¼ CC þ 2H2
(1)
and CH4 ¼ Ci þ 2H2 (2) Fig. 6. Light-optical microscopy image of the thirdly investigated cementite particle (non-etched cross-section).
Studies on the defect structure of cementite will significantly depend on the absolute measurement values of composition. In principle, results concerning defect structure of cementite could be obtained by a gravi metric determination of cementite composition under equilibrium con ditions between gas and sample as it was done by Grabke in his study on the defect structure of the γ0 - iron nitride [34], but there is the main difficulty that cementite tends to decompose before thermodynamic equilibrium is reached. 5. Conclusion Measurements of carbon contents of cementite were performed using electron-probe microanalysis (EPMA). Cementite (Fe3C), which grows during carburisation at high carbon activity aC » aC (Fe3C/α-Fe), reveals a measurable carbon gradient. The homogeneity range of analysed cementite particles which formed at 700 � C was determined to be of at least 0.6 at.% carbon. When combining the literature results with the ones presented in this work, the homogeneity range of cementite at 700 � C could be (at least) 24.8 up to 25.4 at.%.
Fig. 7. Carbon profiles in cementite particle no. 3 (see Fig. 6) measured by EPMA and represented according to the above described procedure.
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References
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