The initial experiments were performed with samples cooled at 2 K/s from fully liquid conditions at 1523 K to 1073 K (1350 °C to 800 °C). Within this range, complete solidification was found to have taken place and there was no residual liquid at 1073 K (800 °C). An example of the typical microstructure formed under these conditions is given in Figure 4.
Using FE-SEM and EPMA measurements, the phases present in the sample were identified to be hematite (H, Fe2O3), dicalcium Silicate (C2S, 2CaO.SiO2), calcium ferrite (CF, CaO·Fe2O3), and calcium diferrite (CF2, CaO·2Fe2O3). Notably, the SFC phase was absent in all the samples examined. The preliminary analysis of the phases and microstructures demonstrated that those present were different from those anticipated for both equilibrium and Scheil–Gulliver cooling.
To provide a clear understanding of the sequence of reactions, a series of experiments were performed. These experiments were undertaken with samples cooled from the same starting temperature (1350 °C), and the same cooling rate (2 K); samples were quenched from selected temperatures between 1543 K and 1453 K (1270 °C and 1180 °C).
There are significant experimental difficulties in preparing and characterizing the samples that have been cooled under controlled conditions to these temperatures; very high cooling rates are required to retain the liquid as amorphous or microcrystalline phase on quenching. The high proportion of solids present provide sites for heterogeneous nucleation and growth of new and existing phases on quenching of the sample. Care must be exercised to ensure that structures formed on quenching are not interpreted as part of the controlled cooling sequence.
Between a fully homogeneous liquid and the final microstructure after complete solidification, four intermediate stages of solidification were observed in these experiments, each resulting in the formation of specific phases and phase assemblages. Within a single sample, multiple assemblages were able to be observed for a range of temperatures. The formation of new phases did not occur simultaneous in all regions of the sample at a single temperature, suggesting that nucleation occurred independently in these regions. The four, in the order of formation, are as follows, Liquid + H (Assemblage I), Liquid + H + C2S (Assemblage II), Liquid + C2S + calcium diferrite (Assemblage III), and finally C2S + calcium diferrite + calcium ferrite (Assemblage IV). Although the resultant total phase assemblage is additive, consisting of all solids that formed in the previous stages, for ease of description of the solidification phenomena, each of the phase assemblages are presented as occurring independently of the pre-existing solids. The temperatures at which the different phase assemblages were observed are illustrated in Figure 5.
L + H (Assemblage I)
Assemblage I, involving the formation of hematite crystals, was the first phase assemblage observed during cooling from 1533 K to 1528 K (1260 °C to 1225 °C). Figure 6 shows a micrograph illustrating the typical hematite primary phase and microstructure.
The hematite was observed to be in the form of large, faceted dendritic (skeletal) crystals (>100 μm long, ~ 30 μm wide), distributed throughout the liquid; a larger proportion of solids was observed to be present near the platinum substrate. The hematite crystals increased in size and number with decreasing temperature. At the highest temperatures, just below the liquidus, these crystals were only observed in association with the substrate.
The composition of the liquid perpendicular to a planar hematite crystal was measured by point analysis with EPMA. The concentration profile is illustrated in Figure 7 with data from Table II, which shows the liquid composition as a function of distance from hematite. The composition profile was measured with the composition measured as close to the melt-hematite interface as possible without the excitation volume encompassing both melt and hematite.
The liquid composition was found to vary with distance from the hematite/liquid interface. The composition of the liquid remote from the hematite found to have a Fe2O3 concentration close to that predicted by the liquidus surface between hematite and liquid (i.e., the hematite and liquid equilibrium) at the temperature from which it was quenched. The liquid close to the hematite surface was observed to be poor in Fe2O3 relative to the high temperature equilibrium. This indicated that the concentration profiles observed are a result of crystal growth during the quenching processes rather than present during crystal growth during the controlled cooling.
It was observed that the liquid composition was closest to equilibrium when measured at least 40 μm from hematite and within 40 μm of the surface of the sample. The liquid composition close to the sample surface and far from hematite was measured to have typically 5 wt pct more Fe2O3 than the other regions of the sample. This is illustrated in Figure 6, where Line 2 was measured to have an average Fe2O3 concentration of 70.7 wt pct and Line 1 with 65.2 wt pct. On quenching, the surface of the sample is the first to solidify while the center of the sample was still molten, allowing a short time for mass transfer to the liquid in the center of the sample and for further solidification of hematite to take place.
Liquid + H + C2S (Assemblage II)
Assemblage II was observed to form between 1508 K and 1473 K (1235 °C and 1200 °C) and was the second phase assemblage observed on cooling following the formation of the initial hematite dendrites. Figure 8 shows typical micrographs illustrating this phase assemblage.
In this phase assemblage, the C2S phase was observed to be present as both individual crystals and in a coupled microstructure with hematite. The different microstructures were either formed concurrently or separately as cooling occurred. The C2S/liquid interface in all cases was non-faceted.
The C2S was observed as a thin layer on the primary hematite (IIa), individual C2S dendrites (IIb), and a coupled H + C2S microstructure (IIc). The layer of C2S on hematite (IIa) was observed on most of the hematite crystals, and was only 2 to 3 micrometers in thickness.
The coupled microstructure (H-C2S) (IIc) was found to surround all the individually formed hematite crystals, forming on either the hematite or the thin layer of C2S. In the coupled microstructure, hematite is the continuous phase and C2S is present as either rods or as an irregular unfaceted microstructure. The phase ratio is approximately 2 H:1 C2S by volume. The volume fraction of this coupled microstructure increased with decreasing temperature.
The individual C2S (IIb) appeared as unfaceted crystals either isolated in the liquid, or dendritic structures surrounded by either the coupled (H-C2S) microstructure or the liquid. Analysis of the 2D microstructure indicates that the individual and isolated C2S crystals are in fact part of the dendritic structures. The individual crystals were not observed in all regions of assemblage II, increasing in proportion and occurrence with decreasing temperature. These individual crystals appear to occasionally split to form two main branches. The crystals themselves are smaller than the hematite, and are observed to be up to 10 μm in width and 50 μm in length. The isolated crystals are not directly associated with the hematite, with only a Liquid/C2S interface observed. The crystals themselves are larger than the C2S crystals in the coupled microstructure.
L + C2S + CF2 (Assemblage III)
Phase assemblage III was observed to form between 1215 K and 1473 K (1215 and 1200 °C). Figure 9 shows a micrograph illustrating this phase assemblage. The high proportion of solids present in this temperature interval lead to difficulties in retaining liquid on quenching the sample. An increase in the proportion of solids increased the area of surfaces on which further solidification and nucleation is able to occur on quenching.
The microstructure observed consists of individual CF2 crystals and a coupled C2S-CF2 microstructure. The individual CF2 crystals were observed to form on the hematite and the coupled C2S-CF2 microstructure on the individual CF2 crystals. No change in crystal size or shape, or reduction in volume fraction of hematite primary phase crystals. This indicates no measurable dissolution of previously solidified hematite has taken place during this stage.
The individual CF2 crystals (IIIa) were observed to form thin faceted needles or plates (up to ~ 80 μm long and 5 μm wide), with an interface with both hematite, liquid and occasionally C2S. Some of the individual CF2 crystals were curved, providing an indication of the direction of growth or solidification. The individual crystals were observed to be thin and formed splitting tips. The splitting tips potentially indicated that the CF2 formed as a series of parallel plates, resulting in the appearance as an individual needle or plate. The length of the crystals appeared to be physically limited by the presence of solids formed following solidification at higher temperature. The smallest crystals were observed to form an interface with one solid phase, Hematite. The CF2 crystals were observed to have similar orientations and associations with other solids, but differed in size. The CF2 crystals were observed to form around the individual C2S crystals.
The coupled C2S-CF2 microstructure (IIIb) was only observed in some regions of assemblage III. The coupled microstructure is illustrated with Figure 10, as this microstructure is more clearly illustrated in Assemblage IV. The CF2 formed the continuous phase and C2S as rods (< 1 μm diameter), indicating that CF2 formed a larger volume proportion of the microstructure. The coupled microstructure was always located in association with the individual CF2 crystals, forming both in close proximity to and far from the solidified hematite. At the interface with the liquid, the C2S formed ahead of the CF2 with an unfaceted interface while the CF2 formed a faceted interface.
L + C2S + CF2 + CF (Assemblage IV)
The final microstructures to form consisted of individual CF crystals (IVa), coupled C2S+CF microstructures (IVb) and a coupled C2S+CF2+CF microstructure (IVc). These phases are the last to form during or before the complete solidification of the liquid at between 1200 °C and 1195 °C.
The individual CF crystals (IVa) were observed to form interfaces with the individual CF2 crystals (IIIa), the coupled C2S-CF2 microstructure (IIIb), and the coupled C2S-CF (IVb) and coupled C2S-CF2-CF (IVc) microstructures. The interfaces between CF and CF2 were irregular and appeared as diffuse in SEM imaging. These individual crystals were smaller than the CF2 and C2S individual crystals present in the sample and constrained in all dimensions by the previously formed solids.
The fine interdispersion of the phases make it difficult to characterize individual phases in the coupled C2S-CF (IVb) and coupled C2S-CF2-CF (IVc) microstructures. The phases present in these two microstructures were determined by line analysis with EPMA. The diameter of the excitation volume created by the electron beam in these materials is of the order of 1 μm. As the position of the probe is moved across these fine microstructures, the mean composition therefore changes with the proportion of each material within the interaction volume. The measured compositions should however be on the binary join between the end members or within the alkemade triangles, assuming that the accuracy of the measurements is not affected by the heterogeneous nature of the material in the interaction volume. The line measurements were compared to the joins on the ternary phase diagram, as illustrated in Figure 11. Based on the line analysis, the four coupled microstructures were confirmed, C2S-Hematite (Microstructure IIc), C2S-CF2 (Microstructure IIIb), C2S-CF (Microstructure IVb), and C2S-CF2-CF (Microstructure IVc). The measured compositions for the C2S-CF2-CF coupled microstructure were found to be within the C2S-CF2-CF Alkemade triangle rather than along the C2S-CF or C2S-CF2 joins. This was attributed to either the presence of a ternary coupled microstructure or the interaction volume of the EPMA sampling two microstructures (e.g., both the C2S-CF2 and C2S-CF are sampled by the EPMA interaction volume) at the same time.
The C2S-CF2-CF coupled microstructure and the C2S-CF coupled microstructures were both observed to share interfaces with the CF and CF2 individual crystals and the C2S-CF2 coupled microstructure. Both coupled microstructures were observed to consist of either rods of C2S (< 1 μm diameter) in a CF or CF2/CF matrix or as an irregular coupled microstructure. The C2S in this microstructure extended from surface microstructures to the coupled microstructure.
In the current study, the compositions of the liquid at selected temperatures were measured with EPMA. The measured liquid compositions are shown in Figure 12 and given in Table I.
During the initial solidification of hematite (I), with decreasing temperature the liquid composition moved from the hematite primary phase field into the C2S primary phase field. As this occurred, some regions of the samples were observed to be of assemblage II (L + H + C2S). Once C2S was present, the coupled hematite + C2S microstructure (IIc) and individual C2S crystals (IId) solidified from the liquid. The liquid composition in Assemblage II (L + H + C2S) moved away from the C2S primary phase field as the temperature decreased. This continued as the liquid approached the CF2 primary phase field. In some regions of the sample, CF2 nucleated and these regions transitioned into Assemblage III (L + C2S + CF2). When compared to the composition of the liquid in the surrounding area (Assemblage II), only CF2 had solidified from the liquid in these regions (Assemblage III) (Table II).
The proportion of the phases in Assemblage II (L + H + C2S) at each temperature can be determined by undertaking a mass balance using the measured liquid composition, as summarized in Table 1. The calculated proportions of these phases present for this assemblage are shown in Figure 13. From this figure, it is seen that the proportion of hematite in the assemblage did not significantly change in the temperature range over which Assemblage II (L + H + C2S) was observed to form, only increased by 3 wt pct between 1508 K and 1498 K (1235 °C and 1225 °C). The C2S was observed to undergo a significant change, increasing from 0 to 10 wt pct between 1513 K and 1488 K (1240 °C and 1215 °C). The C2S increased from 0 to 8 wt pct between 1240 °C and 1225 °C. In this range, the increase in C2S with time was approximately constant, at approximately 1 wt pct/s. This could be taken to indicate that the C2S solidified independently from hematite from 1513 K and 1498 K (1240 °C to 1225 °C). At temperatures between 1498 K and 1488 K (1225 °C and 1215 °C), both the C2S and hematite solidified simultaneously.