Full length articleCoarsening kinetics of lamellar microstructures: Experiments and simulations on a fully-lamellar Fe-Al in situ composite
Graphical abstract
Introduction
Fe-Al alloys with up to 50 at.% Al show excellent corrosion resistance and have a low density compared to steels, and hence, they became promising materials for structural applications [1], [2], [3], [4]. When the Al content is above 50 at.%, the beneficial corrosion behavior persists [5] and the alloy density will further decrease. According to the Fe-Al phase diagram in Ref. [6], Fe-Al alloys in the range from 53.0 to 64.4 at.% Al contain the body-centered cubic phase Fe5Al8 (Cu5Zn8-type crystal structure [7]) at high temperature, which decomposes into cubic B2 FeAl and triclinic FeAl2 through a eutectoid reaction at 1095 °C [6]. The orientation relationship between the two phases can be described by and [8]. The eutectoid transformation is a rapid reaction leading to the formation of a fine-scaled, fully lamellar microstructure at the eutectoid point (where the Al content is 60.9 at.% [6]), see, e.g., Fig. 1 a showing the microstructure after 10 min holding at 1000 °C. Similar lamellar microstructures are known from α2 + γ Ti-Al based alloys, where they were found to exhibit balanced properties with regard to creep, ductility and strength at elevated temperature [9]. In case of the Fe-Al system, there is no information in the literature about the mechanical behavior of such microstructures. When thinking about the capability for high-temperature structural applications, especially the stability of the lamellar structure is of high relevance. For this reason, the kinetics of lamellar coarsening in a fully lamellar FeAl + FeAl2 alloy is studied in detail here. Another motivation to perform these investigations is that the extended, nearly perfect lamellar microstructures in the present samples offer an excellent possibility to obtain a broad and reliable database for evaluating existing theoretical models for the kinetics of lamellar coarsening. To this end, heat treatments were performed at 600, 700, 800, and 1000 °C for various times ranging from 10 min to 7000 h. The effect of the colony size on lamellar coarsening was investigated as well. Finally, the experimental observations are complemented by simulation results on the lamellar coarsening behavior applying phase field modeling.
Section snippets
Overview of coarsening theories
Eutectoid or eutectic lamellar microstructures can coarsen either continuously or discontinuously (see e.g., Ref. [10], [11], [12]). Models for the latter case are not included in the following overview as the samples investigated in the present study solely show continuous coarsening.
The following brief overview will show that until today there exists no generally accepted model to describe the coarsening of lamellar microstructures. This is also related to the fact that there are only
Experimental
An Fe-Al alloy containing 60.9 at.% Al with a fully lamellar microstructure was produced by vacuum induction melting from high purity Fe (99.98 wt%) and Al (99.99 wt%) under argon and was cast in a cold copper mould with a diameter of 18 mm [6]. The impurity content of this alloy was determined by chemical analysis with P being measured by inductively coupled plasma-optical emission spectrometry (ICP-OES), C, O and S by infrared absorption spectroscopy and N by thermal conductivity measurement.
Kinetics of lamellar coarsening
Fig. 3 presents SEM-BSE images of the lamellar microstructures in both as-cast and various heat-treated states at 1000 °C showing that the lamellae coarsen significantly with increasing heat treatment time. Concurrently, the amount of terminations and branches decreases clearly indicating that fault migration and annihilation contribute to the lamellar coarsening. Lamellae start shrinking and some of them even lose their initial shape and begin to spheroidize after only 36 h. During the later
Phase field modeling of lamellar coarsening
Phase field methods are frequently used for the modeling of microstructure evolution, see e.g. Refs. [38], [39], [40], [41], [42] for recent reviews. Concerning the formation of lamellar structures, in particular eutectic solidification has been studied frequently [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], but contrary to coarsening with spherical inclusions [53], [54], the focus is typically on the growth instead of the coarsening regime.
The experimental results of the present
Conclusions
An Fe-Al alloy with 60.9 at.% Al was produced, which according to the phase diagram [6] undergoes a eutectoid transformation Fe5Al8 → FeAl + FeAl2 at 1095 °C during casting and cooling to room temperature. Due to this reaction, the as-cast alloy shows a fully lamellar microstructure with volume contents of about 30% FeAl and 70% FeAl2. The kinetics of coarsening of the lamellar microstructure was studied by both experiments and simulations yielding the following findings and conclusions:
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Acknowledgements
The authors would like to thank Mr. G. Bialkowski for cutting samples by EDM, Mrs. I. Wossack for EPMA analysis, and Mr. D. Kurz for chemical analysis. The authors are also grateful for the computing time granted on the supercomputer JURECA at Jülich Supercomputing Centre (JSC). The financial support by the Deutsche Forschungsgemeinschaft (DFG) within the project STE 1077/2 is gratefully acknowledged. In addition, R.S. acknowledges financial support by the DFG via the priority program 1713.
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