Microstructural analysis of interfacial reaction between molten aluminium and solid iron
Introduction
The process of aluminizing [1], permanent mould casting [2], master-alloy production [3] and solidification of iron containing aluminium alloys [4], [5], bimetal formation, and welding and brazing [6], [7], and more recently the fabrication of iron aluminide-based intermetallic matrix composites [8], [9], [10], are principally dependent upon the interfacial reaction between the molten aluminium and solid iron. It would therefore be quite beneficial to have a thorough knowledge and understanding of the reaction products and kinetics to devise, design and control experiments to achieve products with the required characteristics in such diverse fields as mentioned above.
The last published Fe–Al equilibrium phase diagram [11] is shown in Fig. 1. The system is characterized with an iron-based solid solution and six non-stoichiometric intermetallic compounds of Fe3Al, FeAl, FeAl2, Fe2Al3, Fe2Al5 and FeAl3. Table 1, Table 2, Table 3 give the physical, thermodynamical and phase reactions for this phase diagram with special emphasis on the intermetallic phases, respectively [11], [12], [13].
There have been several studies on the microstructural analysis of the interface layer when molten aluminum comes in contact with solid iron [12], [14], [15], [16], [17], [18], [19], [20], [21]. The initial study of the interface layer was reported by Maruin [14] who had claimed the formation of more than one intermetallic phases while in a later report Agiew and Viwr [15] could only confirm the formation of θ-FeAl3 at the interface layer. Such early findings were later clarified by Gebhardt and Obrowski [16] who identified the major intermetallic phase as Fe2Al5. Although subsequent investigation principally supported Gebhardt and Obrowski finding, but also reported of formation of other minor phases [1], [17], [18], [19], [20]. The latest report [21] has identified the interfacial reaction products as Fe2Al5 and FeAl3, with FeAl3 as the minor phase. Although Bouche et al. [21] have conclusively reported the formation of both Fe2Al5 and FeAl3 and went on to characterize the kinetics of reaction, they did not elaborate much on FeAl3 and what would happen to this phase as reaction time is extended.
In other words, there still exist some ambiguities on the microstructure of interface layer for molten aluminum–solid iron system. Therefore, the current investigation was devised to clarify such discrepancies as part of a wider research program on the in situ fabrication of iron aluminides and manufacture of near net or net-shape aluminide parts [22]. This is the first part of a report on the microstructure while part II of this report is concentrated on the kinetics of reaction [23].
Section snippets
Experimental
In order to investigate the interfacial reaction and resultant microstructure of a solid iron–molten aluminum couple, rectangular iron coupons were submerged in a molten aluminium bath using the experimental set-up shown in Fig. 2.
Rectangular iron coupons, 60 mm long, 20 mm wide and 1.7±0.2 mm thick, were ground and polished down to 0.5 μm diamond paste, degreased, pickled and washed in alcohol before immersion in molten aluminium bath. A 200 g commercially pure aluminium was melted in a graphite
Results
The optical micrographs in Fig. 3 illustrate the overall microstructure of the reaction product at 800 °C after 185 and 3000 s, respectively. The aluminium and iron regions are easily recognizable with the distinct ferritic–pearlitic structure of iron coupon and white aluminium cells. The interface layer has a cellular-dendritic characteristics, where at longer times, i.e. 3000 s, the coarsened dendrites have overlapped and formed a massive block of intermetallic phase at the interface. It may
Discussion
As the BSE micrographs, Fig. 4, and EDS and EPMA analysis, Fig. 5, Fig. 6, show there are two stable phases of Fe2Al5 and FeAl3 within the experimental time scale in this study, and Fe2Al5 is the major phase in this system. The EPMA graph confirms the darker phase as FeAl3, higher aluminium content and Fe2Al5 as the lighter one in Fig. 4. This is in full support of the results reported by Bouche et al. [21].
The fact that Fe2Al5 is the major phase and also detected with FeAl3 right from 90 s
Conclusions
The microstructure of solid iron and molten aluminum couple was examined at the interface to identify the phases as Fe2Al5 and FeAl3. The former grows faster and is the major phase while the latter is easily detectable at the later stages of reaction and dissolves partially within molten aluminium. This is in contradiction with thermodynamic principles, where FeAl3 is expected to form before Fe2Al5, which may suggest the importance of kinetics in this system. It was further shown that it is
Acknowledgements
The authors are grateful to the ministry of mines and metals of Iran for providing financial support to one of us, H.R.S., to pursue his Ph.D. studies. Special thanks go to Mr. R. Ashraf-Semanai, acting deputy minister for his continuous support during the course of this study. Dr. Aboutalebi, head of Metallurgy Department of the Iran University of Science and Technology is acknowledged for providing research facilities and granting R. Ghomashchi a visiting professor position at DMSE-IUST. Dr.
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