Two data sets are used to compare the phase formation trend of the Fe-Al-Si system. HSC software was applied to calculate the Fe-Si compound. Unfortunately, the software does not provide information on Fe-Al compounds. Therefore, reference [14] was used to extract the Gibbs free energy of formation for Fe-Al compounds.
The results of these calculations are shown in Fig. 1. It is clear from this curve that the minimum amount of free energy is associated with Fe5Si3. FeAl3 also has the lowest energy among aluminide compounds. However, according to the stoichiometry of Fe and Al at the synthesis temperature used in the present study, the FeAl phase should be the dominant phase [15–18].
Iron silicides have been reported to have good oxidation resistance [19]. Iron silicides are either stable such as Fe3Si, FeSi, and β-FeSi2 [20–23], or unstable such as α-FeSi2, Fe5Si, and Fe2Si [24]. It was found that the addition of silicon to iron aluminides achieves higher oxidation resistance due to the formation of a silicide phase under the aluminum layer formed on the oxidized surface [25].
Yeh et al. [26] found that silicon increases the porosity of iron aluminides. To investigate this issue, electron micrographs of samples with varying amounts of silicon were compared with samples without silicon (Fig. 2). As can be seen from this figure, different amounts of silicon can create different morphologies. Sample S2 shows reduced porosity and a more consistent structure. Differences between the results of the current study and those of other studies can be explained by the type of method used. For instance, other studies performed the SHS [26] method or mechanical alloying [27].
Both due to the creation of numerous defects in the mechanical alloying method and as the result of the possibility of increasing the dissolution of silicon over the equilibrium amount, different results may be obtained.
Silicon undergoes a eutectic transformation with aluminum [28], with a maximum melting temperature drop at 12.6 wt% of silicon. Silicon can also vary the degree of porosity [26]and the surface tension [29]. This also leads to a change in the morphology of the structure. It has been explained that in the ratio of Fe-20% to Fe-30% Si, the only phase that can be produced is Fe3Si. Between 45 and 55% silicon, the FeSi phase is the dominant phase. The main phase of α-FeSi2 is formed when the silicon content is between 66.7 and 75% [26]. From the obtained electron microscope images (Fig. 2), it can be recognized that with a small amount of silicon increase (S1), the morphology has changed and the amount of holes has increased compared to sample 0. But in the S2 sample, the amount of added silicon is so much that it has made the structure cohesive and minimized the holes (S2).
However, further increasing the amount of silicon (S3), the number of holes increases again. This probably depends on various factors such as a change in surface tension, defect count change, and phase composition type for each sample.
XRD analysis was applied to determine the produced phases in each sample. Based on the obtained results, it was found that when no silicon was added to the sample, the only aluminide phase obtained was the FeAl phase, since the stoichiometric ratio is 1:1 Nevertheless, the FeAl3 phase offers better production conditions in the presence of silicon. Comparing these results with the thermodynamic predictions in Fig. 1, it is clear that Fig. 1 indicates that the free energy of the formation of FeAl3 is more negative than that of FeAl.
However, phase formation depends on achieving the required stoichiometric ratio of iron and aluminum in the raw materials. Therefore, FeAl phase is formed when the molar ratio of iron and aluminum is the same[15–18]. When silicon is added to the system, some of the iron content reacts with silicon to form various types of iron silicide phases. Therefore, more aluminum remains for unreacted iron, leading to the formation of FeAl3.
Also, in Fig. 1, at 950°C, the formation probability of Fe3Si was higher than that of FeSi, and ultimately both were predicted to be higher than that of FeSi2. However, these predictions can also be achieved even where stoichiometric amounts of iron and silicon elements are available. As can be seen from the XRD results obtained in Fig. 3, Fe2Si and Fe3Si phases form when insufficient silicon is available. However, as the silicon content in the system increases, the FeSi2 phase becomes the dominant phase.
This phenomenon can also be confirmed by the elemental distribution map obtained from the sample product. At low silicon levels, iron silicide forms as islands between iron aluminides, as shown in Fig. 4. With increasing silicon content, the aluminide phase type changed to FeAl3. A red color (FeAl3) appeared clearly in the iron aluminide part. Finally, various silicide phases cover a wide range of microstructures. A notable clue observed in the microstructures of Fig. 3, S3 and Fig. 4, S3 is the breakdown of the silicide within the sample.
This is probably related to the difference in the molar volumes of the compounds obtained. Since the molar volume of FeAl3 is larger than that of FeSi2 (calculated based on the molar mass and density), the increase in molar volume due to the formation of FeAl3 limits the formation of FeSi2 and may break this phase.
Sritharan et al. [30] also used elemental powders of iron, aluminum, and silicon to produce intermetallic compounds. The effects of various parameters on the types of phases formed and their properties were investigated using the SHS method. The proportions in this study are based on the purpose of forming a specific ternary compound formulation. Ternary Fe-Al-Si compounds are used in the electronics industry [31].
Sritharan et al. [30] also showed that the formation of this type of intermetallic compound occurs after the formation of the molten aluminum phase.
They also found that melting and endothermic reactions occur as the temperature rises to about 872°C, but the subsequent reactions are highly exothermic. Aluminum melts have been found to provide a favorable environment for iron and silicon to dissolve. This dissolution continues until the stoichiometry required for the reaction is reached.
Finally, the Fe-Al-Si ternary diagram was investigated at a temperature of 1000°C, which is very close to the current research temperature. The stoichiometry of the samples used in this study is shown in Fig. 5.
Table 2 shows the composition obtained in this study and the composition predicted from the Fe-Al-Si ternary diagram. As can be seen, the results for the free silicon sample (sample 0) are consistent with the ternary diagram. This means that the FeAl phase with the B2 structure is obtained [34].
However, in sample S1, different types of iron aluminide and iron silicide phases are predicted. The reason for this may be that materials and elements are more accessible in the molten state than in the solid powder state. On the other hand, the activation energy in solid-state reactions is higher than in reactions that can be carried out in the presence of liquids. Compounds with the symbol τ in the Fe-Si-Al system are ternary compounds [35, 36] and are predicted to exist in ternary diagrams, but have not been discovered in the current study. Sritharan [30] also recognized that the pure Fe-Si-Al ternary compound is difficult to obtain in powder form purely.
Table 2
A comparison of the phases predicted based on the ternary phase diagram and the phases obtained in the current study.
Sample
|
Predicted phases based on the ternary phase diagram
|
Obtained phase in the current research
|
0
|
B2
|
FeAl
|
S1
|
FeSi, Fe2Al5, B2, τ 1,L
|
FeAl, FeAl3, Fe3Si, Fe2Si
|
S2
|
FeSi, τ1,L
|
FeAl3, Fe3Si, FeSi, FeSi2,
|
S3
|
FeSi2,L
|
FeAl3, FeSi2,
|