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Journal of Nanomaterials
Volumeย 2012ย (2012), Article IDย 953828, 4 pages
http://dx.doi.org/10.1155/2012/953828
Research Article

Solubility of Carbon in Nanocrystalline ๐›ผ-Iron

Institute of Materials Science, Technische Universitรคt Dresden, 01062 Dresden, Germany

Received 24 February 2012; Accepted 3 May 2012

Academic Editor: Grรฉgoryย Guisbiers

Copyright ยฉ 2012 Alexander Kirchner and Bernd Kieback. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A thermodynamic model for nanocrystalline interstitial alloys is presented. The equilibrium solid solubility of carbon in ๐›ผ-iron is calculated for given grain size. Inside the strained nanograins local variation of the carbon content is predicted. Due to the nonlinear relation between strain and solubility, the averaged solubility in the grain interior increases with decreasing grain size. The majority of the global solubility enhancement is due to grain boundary enrichment however. Therefore the size effect on carbon solubility in nanocrystalline ๐›ผ-iron scales with the inverse grain size.

1. Introduction

An enhancement of solid solubility has been found in many bulk nanocrystalline materials, for instance [1, 2]. Often the bigger part of the effect can be attributed to nonequilibrium processing such as mechanical alloying [3]. However quantifying the impact of grain refinement on equilibrium solid solubility is important for understanding the behavior of nanocrystalline alloys upon thermal activation. In this context equilibrium shall be constrained by the assumption of a stable grain size.

The scope of this work is to develop a thermodynamic model of nanocrystalline alloys. It has to comprise both the nanoscale grains and the grain boundary regions, since the fraction of atoms located in those cannot be neglected. The description is based upon the previously published thermodynamic treatment of alloy grain boundaries [4]. There the grain boundaries are approximated by a uniformly dilated lattice retaining the symmetry of the bulk material [5, 6] and are characterized by their volumetric strain ฮ”๐‘‰/๐‘‰0.

The interstitial solution of carbon in ๐›ผ-iron is chosen as an alloy of practical relevance and experimental accessibility. In pure iron the nanocrystalline state is retained even after prolonged annealing at 650โ€‰K [7]. At this temperature and a time of 3600โ€‰s, the diffusion length of carbon in bulk iron exceeds 50โ€‰ฮผm [8]. Since the carbon diffusivity will be larger in nanocrystalline iron, this enables chemical equilibration without strong grain growth.

2. Thermodynamic Model of Nanocrystalline Fe-C

The thermodynamic description of a binary Fe-C alloy under volumetric strain is based upon Kaufman and Schlosser's expression [9] for the Helmholtz free energy ๐น. The free energy of the strained solid consists of ๐นโˆ˜ at zero pressure and a second term that accounts for the elastic energy depending on the parameter ๐œ’=(๐‘‰/๐‘‰0)1/3 using the Vinet et al. universal equation of state [10]. The isothermal bulk modulus ๐ต0, the molar volume ๐‘‰0, and the anharmonicity parameter ๐œ‚0 are given at zero pressure: ๐น=๐นโˆ˜+9๐ต0๐‘‰0๐œ‚20๐œ‚๎€บ๎€ฝ0๎€พ๐‘’(1โˆ’๐œ’)โˆ’1๐œ‚0(1โˆ’๐œ’)๎€ป.+1(1) A two-sublattice model Fe(Va,C)3 is used to describe the interstitial solution of carbon in ๐›ผ-iron. In the second sublattice carbon substitutes normally vacant octahedral sites. Here ๐‘ฆC marks the molar fraction of carbon in the sublattice. Its relation to the carbon content given as the molar fraction ๐‘ฅC is ๐‘ฆC=๐‘ฅC/(3(1โˆ’๐‘ฅC)). Conveniently choosing pure ๐›ผ-iron and graphite as the reference state, the molar free energy ๐นโˆ˜ of the unstrained alloy is stated by [11] in units of Joule where ๐‘‡ is the temperature in Kelvin: ๐นโˆ˜=(322050+75.667๐‘‡)๐‘ฆC๎€ท๐‘ฆ+3๐‘…๐‘‡Cln๐‘ฆC+๎€ท1โˆ’๐‘ฆC๎€ธ๎€ทln1โˆ’๐‘ฆC๎€ท๎€ธ๎€ธโˆ’190๐‘‡1โˆ’๐‘ฆC๎€ธ๐‘ฆC.(2) Assuming a dilute alloy, ๐ต0 and ๐œ‚0 of pure ๐›ผ-iron are used in (1). Values of ๐ต0=178.6GPa, ๐‘‰0=7.09โ‹…10โˆ’6m3/mol [12], and ๐œ‚0=5.16 [13] at ๐‘‡=298K are employed. Their temperature dependence is calculated according to Vinet and coworkers [10] using the coefficient of volumetric thermal expansion ๐›ผ0=36.9โ‹…10โˆ’6Kโˆ’1 [12]. At low solute concentration the molar volume ๐‘‰0 of the alloy varies linearly with ๐‘ฅC, and ๐œ’=3๎ƒŽ1+ฮ”๐‘‰/๐‘‰01+๐‘ฅCฮฉCFe,bcc(3) is obtained. The volume size factor of carbon in ๐›ผ-iron ฮฉCFe,bcc=0.825 is averaged from two publications [14, 15]. The equilibrium between carbon in strained ๐›ผ-iron and the reference state graphite is given by ๐น+(1โˆ’๐‘ฅC)โ‹…(๐œ•๐น/๐œ•๐‘ฅC)=0.

The strain of the crystallite interior is calculated using Weissmรผller's model [16]. Accordingly local strain in bulk nanocrystalline materials is caused by the fact that the cavities defined by the adjacent grains need to accommodate a finite number of lattice planes. The maximum linear strain ๐œ€max is given by the interatomic distance ๐‘ŸNN (for iron ๐‘ŸNN=0.252nm [12]) and the grain size ๐ท: ||๐œ€max||=14๎‚™32๐‘ŸNN๐ท.(4) The predicted root mean square strain is in good agreement with strain measured in nc-Fe by X-ray diffraction [17, 18]. Here the mean strain โŸจ๐œ€โŸฉ๐‘‰=โˆ’2โŸจ๐‘“โŸฉ๐ด/(3๐ต0๐ท) is neglected, because with a grain-boundary stress โŸจ๐‘“โŸฉ๐ด of 1.1โ€‰N/m [19] it is more than a magnitude of order smaller than |๐œ€max|. The volumetric strain is given by the superposition of the linear strain in three dimensions ฮ”๐‘‰/๐‘‰0โ‰…๐œ€๐‘ฅ+๐œ€๐‘ฆ+๐œ€๐‘ง.

Furthermore a thickness ๐‘ค of 0.7โ€‰nm representing 3-4 monolayers and a value of ฮ”๐‘‰/๐‘‰0=0.12 is used to describe average grain boundaries. This choice of parameters yields a good agreement with an experimental value of the interface free energy of 468mJ/m2 at 1723โ€‰K [20].

Finally the resulting global solubility ๐‘ฅCglobal is calculated from the grain boundary and interior compositions with their respective molar fractions as weighting factors: ๐‘ฅCglobal=๐‘“๐‘ฅCGB+(1โˆ’๐‘“)๐‘ฅCinterior,(5)๐‘“=(๐ท+๐‘ค)3โˆ’๐ท3(๐ท+๐‘ค)3+๐ท3๎€ทฮ”๐‘‰/๐‘‰0๎€ธ.(6) The molar fraction of the atoms located in grain boundaries ๐‘“ is corrected for the different atomic densities and converges for ฮ”๐‘‰/๐‘‰0=0 and large grain sizes toward 3๐‘ค/๐ท.

3. Results and Discussion

Recently ab initio methods based on density-functional theory (DFT) allow for highly accurate simulation of carbon solution in ๐›ผ-iron. The excess enthalpy of carbon in iron as defined by [21] depending on volumetric strain was calculated at ๐‘‡=0๐พ and infinite carbon dilution. The result is presented in Figure 1. Good agreement with the strain dependence of excess enthalpies computed by DFT and a modified embedded-atom method (MEAM) is observed. The calculated derivative of the excess enthalpy with respect to the atomic volume at ฮ”V/๐‘‰0=0 is โˆ’0.95eV/ร…3 as compared to โˆ’1.08eV/ร…3 (DFT) and โˆ’0.84eV/ร…3(MEAM).

953828.fig.001
Figure 1: Difference of the excess enthalpies of a carbon atom in strained and strain-free iron. Results from [21] are marked as filled circles (DFT) and open squares (MEAM).

The maximum linear strain of the grain interior given by (4) can be used to calculate the local volumetric strain assuming three independent axes. For a grain size of 20โ€‰nm the linear strain varies locally between โˆ’3.8โ‹…10โˆ’3 and +3.8โ‹…10โˆ’3 and the volumetric strain between โˆ’0.011 and +0.011 with the frequency distribution shown in Figure 2. Close to ฮ”๐‘‰/๐‘‰0=0 the calculated carbon concentration in equilibrium with graphite at 673โ€‰K coincides with the linear theory of thermochemical equilibrium of solids under stress [22]. For large strains nonlinear behavior is observed. At lower values of ฮ”๐‘‰/๐‘‰0 indicating strong compression, the carbon concentration converges towards zero, which is the physically reasonable behavior.

953828.fig.002
Figure 2: Solubility of carbon in the strained grain interior at 673โ€‰K. The range and frequency distribution of the local strain is calculated for a grain size of 20โ€‰nm. The broken line represents the result of the theory by Larchรฉ and Cahn [22].

The expected variation in composition between individual nanograins can be deduced from the range of values calculated for ๐‘ฅCinterior. For a grain size of 20โ€‰nm the ratio between the maximum (9.8โ‹…10โˆ’4at.%) and the minimum carbon concentration (1.7โ‹…10โˆ’5at.%) is 59. This ratio will be even larger for smaller grain sizes. Another notable consequence of the nonlinear trend of ๐‘ฅCinterior is that weighted averaging yields a deviation from bulk solubility despite the strain distribution symmetry. Figure 3 shows the average carbon solubility in the grain interior as a function of inverse grain size. It increases with decreasing grain size slightly. ๐‘ฅCinterior is doubled with respect to the bulk solubility for 11โ€‰nm grains.

953828.fig.003
Figure 3: Average solubility of carbon in the grain interior ๐‘ฅCinterior and resulting solubility ๐‘ฅCglobal in nanocrystalline ๐›ผ-iron at 673โ€‰K as a function of grain size. Note that the axis of ๐‘ฅCinterior is scaled by a factor 1000.

The carbon concentration in grain boundaries was calculated in equilibrium with graphite assuming it to be independent of curvature and grain size. At a temperature of 673โ€‰K the computed value of ๐‘ฅCGB is 3.9at.%. This means that carbon is enriched by a factor of 3โ‹…104 in the grain boundaries with respect to the bulk. Atom probe microscopy measurements confirm the presence of a minimum of 2 at.% carbon in ๐›ผ-iron grain boundaries [23]. Other experiments at 873โ€‰K [24, 25] yielded higher values, expressed as excess densities of carbon at the interface of ฮ“CGBโ‰ˆ20โ€‰ฮผmol/m2. The corresponding theoretical prediction is lower at ฮ“CGB=5โ€‰ฮผmol/m2. The difference may be explained by the strong variation between individual grain boundaries and the information depth of the analytical methods employed. In the case of autoradiography it exceeds the grain boundary thickness of approximately 1โ€‰nm by far. The detected amount is likely to include carbon enriched at stress fields around grain boundaries as well. Then the concentration in the grain boundary core is lower than ฮ“CGB suggests.

Calculating the global solubility according to (5) yields an almost linear relation with the inverse grain size as Figure 3 illustrates. The reason is that the contribution of enriched grain boundaries dominates the size effect. Figure 4 shows the calculated solubility of carbon in nanocrystalline ๐›ผ-iron of various grain sizes. Thereafter a pronounced increase in equilibrium carbon solubility is to be expected in nanocrystalline iron in comparison to bulk iron.

953828.fig.004
Figure 4: Calculated solvus lines of carbon in nanocrystalline ๐›ผ-iron of 10, 20, and 50โ€‰nm grain size compared to bulk iron.

4. Conclusions

A thermodynamic model for nanocrystalline ๐›ผ-phase Fe-C alloys has been presented. Considering the strained grain interior in equilibrium with graphite local variation of the carbon concentration but only a weak size effect on the average carbon solubility is found. While the extent of enrichment at grain boundaries is not precisely established, grain boundary segregation dominates the solubility increase in the nanocrystalline state. It is concluded that the excess carbon solubility in nanocrystalline iron over bulk iron is proportional to the inverse grain size. At a given temperature the overall solubility follows ๐‘ฅCglobalโ‰ˆ๐‘ฅCbulk+const๐ท.(7)

Acknowledgments

The research of A. Kirchner was supported by Deutsche Forschungsgemeinschaft via the Emmy Noether Programme.

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