Heterogeneous Nucleation Behavior in Al Deoxidized Liquid Iron
2018 Volume 59 Issue 12 Pages 1949-1951
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2018 Volume 59 Issue 12 Pages 1949-1951
As an important deoxidization agent, Al and its deoxidization products are rarely investigated in terms of nucleation catalyzer for liquid iron itself. Here, we tracked the atomic structure evolution of Al deoxidized liquid iron using in situ high energy X-ray diffraction method to figure out the effect of Al and its deoxidization product for the nucleation in liquid iron. The identified icosahedral short-range ordering (ISRO) and its enhancement with decreasing temperature in both the liquids investigated suggest that the type and number of ISRO is not the sole reason responsible for the variation of undercooling. A decreased nearest neighbor atomic distance (r1) of liquid was detected with the enhanced nucleation after adding 1 mass%Al for de-oxidization. It indicates that the difference between SRO structure of Fe–1 mass%Al liquid and the corresponding crystal structure becomes small, resulting in a lower undercooling.
Deoxidation process is an important practice in steelmaking, accompanying with the formation of various oxide inclusions. These oxide inclusions aggregate together or uniformly disperse in the melt affecting the nucleation behavior of liquid metal. Some common oxide inclusions, such as MnO, SiO2, Al2O3, Ti2O3 and Fe3O4,1–3) were extensively investigated, and conclusions were drawn that the undercooling was closely related to the calculated value of lattice misfit between oxides and the primary crystal of δ-Fe. It means that these oxides could be potential nucleation agents for liquid. But most of the discussions4–6) based on the Turnbull’s hypothesis7) on lattice misfit rather than in situ observation on nucleation itself. Based on up-to-date understanding, liquid structure plays a significant role in nucleation process, mainly reflected in the similarity of structure between clusters and crystal nuclei. During nucleation process, the substrate can tune the melt structure adjacent to the interface, leading to in-plane ordering and layer ordering.8) But whether this ordering structure promotes or suppresses nucleation behavior depends on the substrate template effect.9) In this paper, we investigate the heterogeneous nucleation behavior of Al deoxidized liquid iron. Liquid structure evolution is tracked using in situ synchrotron high energy X-ray (HE-XRD) method. The mechanism for the undercooling variation after deoxidization is discussed based on the atomic structural similarity between ordering of liquid and corresponding crystal nuclei.
To simulate deoxidation process of liquid iron, 1 mass%Al was added into pure iron in an arc melting furnace with oxygen atmosphere (400 Pa). To eliminate the impurity effect on nucleation, high purity metals (99.99 mass%) were used in this study. After arc melting, the ingot was cut into small samples of 1.5 × 1.5 × 1.5 mm3 for the HE-XRD test. The measurements were performed on BL13W1 beamline at Shanghai synchrotron radiation facility (SSRF) using a monochromatic beam. The beam had an energy level of 72.095 keV and a wavelength of 0.17199 Å. The schematic diagram of synchrotron experimental setup is shown in Fig. 1. The diffraction patterns were recorded by a Perkin Elmer Si 1621 detector, and the exposure time for data collection was 6 s. The samples were melted on a corundum substrate protected by Ar2–5 vol% H2 atmosphere in case any more oxidization interfering the investigation. The undercooling was recorded by a pyrometer with an accuracy of ±1 K. The raw 2D data was integrated from X-ray intensity profiles using FIT2D program,10) and transformed to 1D profile. Measurement was acquired over a range of Q values from 1 to 16 Å−1 using a rapid-acquisition pair distribution function (PDF) technique, where Q is the scattering vector magnitude. The nature of the oxide particles was examined using a JEOL JSM7600F thermal field emission scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) operated at an accelerating voltage of 15 kV. The oxygen content of sample after HE-XRD analysis was detected by oxygen & nitrogen analyser, and the entire oxygen content in the sample was 20 ppm.
Schematic diagram of experimental device.
Figures 2(a), (b) show the typical cooling curves for both pure iron and Fe–1 mass%Al, where the result of pure iron is presented as a comparison. From the cooling curves, the undercooling values can be detected as 293 K for pure liquid and 48 K for Fe–1 mass%Al, respectively. The inset in Fig. 2(b) shows the morphology of oxide dispersed in Fe–1 mass%Al sample, appears a square shape with a mean size of 1.67 ± 0.30 µm, and the number density is 2.6 × 1014 m−3. According to the EDS analysis, we can infer the particle is the deoxidization product, Al2O3. The residual Al content in the as-melted sample was measured by EDS from the analysis for random 10 regions, giving an average value (0.47 ± 0.05) mass%.
Cooling curves for pure iron (a) and Fe–1 mass%Al (b), and the inset in (b) displays the SEM and EDS of oxides, S(Q) of (c) pure iron melt, (d) Fe–1 mass%Al melt, and the insets show the enlarged first and second peaks.
The structure factor S(Q) are displayed in Figs. 2(c), (d) and the corresponding first and second peaks are enlarged in the inserts. The temperatures were chosen from superheating to undercooling states of liquid for analysis. For all liquids investigated, S(Q) plots show the increased intensity and higher Q value for primary peaks with the decreased temperature. It means that the degree of order in the liquid significantly increased when liquid temperature was dropping. A shoulder on the right side of the second peak is observed for all measured S(Q) curves. Such a feature was identified as an icosahedral short range order (ISRO) from previous work,11) and indicating that both pure liquid iron and Al deoxidized liquid iron have a similar liquid structure.
The nearest neighbor distances (r1) of the atoms in liquid can be obtained from the location of the first maximum of the pair distribution function g(r), which was calculated from Fourier transformation of the S(Q). As shown in Fig. 3, for pure iron liquid, the r1 value shifted from 2.555 Å at 2083 K to 2.547 Å at 1653 K. As for Fe–1 mass%Al liquid, the r1 value also decreased at the same tendency with the decreasing temperature but indicating smaller values from 2.549 Å at 2035 K to 2.542 Å at 1773 K. At the melting point of liquid iron (Tm), the r1 values are 2.539 Å for pure liquid iron and 2.543 Å for Al deoxidized liquid iron. Both r1 values of the investigated liquids decrease linearly with the decreasing temperature, and the regression coefficient (R2) for the best fitting are 0.958 and 0.982 respectively. The coordination numbers (Z), calculated from the integral of the first peaks of g(r), were approximate 12 for both investigated liquids.
The nearest neighbor distances of pure iron melt (square symbol), Fe–1 mass%Al iron melt (circular symbol), iron crystal (star symbol) change with temperature.
From the melt structure evolution, we can see that the r1 of Fe–1 mass%Al liquid is obvious decreased comparing with pure liquid iron, and closer to that of iron crystal 2.539 Å at Tm, considering the thermal contraction of iron crystal.12) The reduction of r1 for Fe–1 mass%Al liquid might be affected by two factors, solute element, which are oxygen and aluminum in this research, and corresponding oxidization product, Al2O3 particles. Some researchers pointed out that the substrate particles were able to tune nucleation behavior through the substrate template effect.7,8,13) Here, we assume a four layers atomic ordering in the liquid adjacent to the particle surface. The contribution of such in-layer atomic ordering to the reduction of r1 can be estimated together with the effect of dispersed oxide particles. For Fe–1 mass%Al sample, the size of Al2O3 particle is measured as 1.67 µm and the number density is 2.6 × 1014 m−3. The total volume fraction of particles and assumed ‘ordered’ liquid would be approximately 5‰ volumetric fraction of the entire liquid, resulting in no difference in r1 for liquid. Thus, the in-layer ordering induced by the oxide itself make no contributions for liquid structure variation.
Solute element is another potential reason for r1 reduction. As we all knew, solute element can affect the nucleation of the crystals.14–17) The effect of the solute elements on liquid structures of iron were also reported by Roik.18) The result showed that the r1 of Fe–Al and Fe–Si melts had a certain dependence on the concentration, which were affected by chemical short-range order. The reduction of r1 was detected as 0.02 Å after adding 70 atom%Al.18) In our experiment, there is an amount of 0.47 mass% residual Al dissolved in liquid iron. According to the Al dexoidation equilibrium diagram,19) only a trace of oxygen content 1.68 ppm will exist in the sample (liquid state) at 1873 K. It means that the oxygen is mainly presented as oxides in liquid state at investigation temperature. These tiny amount of free oxygen atoms in the liquid may affect the liquid structure accompany with dissolved Al, but the contribution in reduction atomic distance is hard to be isolated from the effect of oxides in this study.
No matter the atomic distance in liquid iron is affected by solute element or oxide particles, this phenomenon results in a decreased undercooling for sure, which means a reduced nucleation barrier after deoxidization. The nucleation energy barrier depends on the atomic distance difference between SRO structure of the liquid and the corresponding crystal structure,20) a reduced r1 and an identical Z value in this study. The lower difference in atomic distance between SRO and crystal structure (Fig. 3), the lower energy required for nucleation. Hence a smaller undercooling (48 K) was obtained for 1 mass% Al deoxidized liquid iron. Although we still not clear which is the main reason for the reduced atomic distance in liquid, it can be confirmed that deoxidized liquid iron with Al has a reduced nucleation barrier comparing with pure liquid iron, which can be related to the modified nearest neighbor distance after de-oxidization.
In summary, the heterogeneous nucleation behavior of Al deoxidized iron melt was investigated. The results show that the de-oxidization does enhance the nucleation in liquid iron, showing a reduced nucleation undercooling comparing with that of pure liquid iron. The detected atomic structure through synchrotron HE-XRD indicates that the nearest neighbor distance (r1) of the atoms in the liquid was reduced after deoxidization with 1 mass%Al. Based on the same coordination numbers of the investigated liquids, the lower nucleation energy barrier of Al deoxidized liquid iron can be contributed to the closer r1 value of the deoxidized liquid and corresponding iron crystal.
This work is supported by The National Key R&D Program of China (2017YFA0403802), National Natural Science Foundation of China (No. 51474148, 51727802, U1660203). The support of synchrotron high energy X-ray diffraction by the BL13W1 beam line of Shanghai synchrotron radiation facility (SSRF), China, is gratefully acknowledged.
