Spark plasma sintered alloys FeAl₂₀Si₂₀ with ternary additions – microstructure and phase composition

J. Kopeček^1,*, F. Laufek², P. Haušild³, M. Karlík³, K. Nová⁴, J. Šesták⁴, B. Severa⁴, P. Novák4, F. Průša⁴

¹ Department of Functional Materials, Institute of Physics of the CAS, Prague, Czech Republic.

² Czech Geological Survey, Prague, Czech Republic.

³ Department of Materials, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Czech Republic.

⁴ Department of Metals and Corrosion Engineering, University of Chemistry and Technology Prague, Prague, Czech Republic.

^*Corresponding author, kopecek@fzu.cz

Ordered aluminides and silicides of transient metal have been known for decades as complicated materials, which interesting high-temperature properties are counterbalanced by their brittleness at room temperature and by tricky thermomechanical processing. Nevertheless, the growing demand for new materials with limited usage of “strategic elements” (as chromium) brings back old ideas and hints. For this purpose, significant support is given by spark plasma sintering – the powder metallurgy compaction method, which provides compact samples using low pressure (couple tens of MPa) and high electric current (tens of kA) passing through the green body. Thus, the sample is compacted by Joule heat within a few minutes suppressing the undesirable microstructural coarsening.

The FeAl20Si20 (in wt. %) was selected as a basic alloy for our study. We vary stoichiometry of alloy with respect of silicon and aluminum and we add ternary and quaternary additions, usually as substitutes of iron. The wide set of materials was created within the project [1,2]. The compacts were found homogeneous, isotropic and having small void density. The grain size observed by SEM was in the order of hundreds of nanometers, but XRD showed real nanocrystalline material with crystallite size varying between 10 to 30 nm depending on the phase. Such parameters are expected in SPS processing.

The attempts to characterize phase composition of the samples become a problem. The basic estimation from EDS and phase diagram knowledge was not sufficient, as tabulated structure types for EBSD did not give reasonable results. It was necessary to prepare model alloys by conventional arc-melting. Such samples were ground and diffraction patterns of model alloys were evaluated to prove phases presented in SPS samples. We use model samples to check the stability of EBSD evaluating routine and apply it on SPS samples with a various level of success.

The three phases were found in FeAl20Si20 alloy including ternary, triclinic Fe₃Al₂Si₃ phase (space group ) and two cubic phases: FeSi () and Fe₃Si (). All observed phases were strongly off-stoichiometric due to an amount of components not fitting to stoichiometric phases, competition between aluminum and silicon in phases’ creation and, of course, due to non-equilibrium conditions introduced by SPS. It was found, that silicon and aluminum are presented in all binary phases despite the tendency to create silicon reach areas. SPS compacts are extremely brittle as two phases have lower symmetry (FeSi is cubic, but do not have any four-fold axis). It became plastic at higher temperatures, experiments were performed at 800 °C. Mechanical properties were investigated by nanoindentation too [3]. A significant problem was caused by complicated thermal/magnetic properties – during EBSD mapping samples were strongly drifting and enormous amount of work was invested to obtained results.

Prepared samples have good corrosion properties as was expected. These behavior was improved by nickel addition. Samples containing nickel contain four phases, beyond three mentioned already there appears FeAl () with a different (lower) type of order compared to D0₃. We expect, the B2 () ordered areas contain a higher amount of nickel, which promote structure of aluminide against B20 () structure of silicide.

Despite, investigation of functional properties did not show any surprising results, careful investigation of microstructure and phase composition pointed complexity of the investigated system.

This research was carried out in the frame of the project 17-07559S and of the project No. CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF; in part by the MEYS SAFMAT CZ.02.1.01/0.0/0.0/16_013/0001406, LO1409 and LM2015088 projects (SEM Tescan FERA 3 maintenance).

1. K. Nová, P. Novák, F. Průša, J. Kopeček, J. Čech, Synthesis of Intermetallics in Fe-Al-Si System by Mechanical Alloying, Metals, 9:(1), 20-1 – 20-14, (2018); https://www.mdpi.com/2075-4701/9/1/20

2. P. Haušild, M. Karlík, J. Čech, F. Průša, K. Nová, P. Novák, P. Minárik, J. Kopeček, Preparation of Fe-Al-Si Intermetallic Compound by Mechanical Alloying and Spark Plasma Sintering, Acta Phys. Pol. A, 134:(3), 724-728, (2018); http://przyrbwn.icm.edu.pl/APP/ABSTR/134/a134-3-25.html

3. P. Haušild, J. Čech, M. Karlík, F. Průša, P. Novák, J. Kopeček, Nanoindentation Characterization of Mechanically Alloyed Fe-Al-Si Powders, Key Eng. Mater., 784, 15-20, (2018); https://www.scientific.net/KEM.784.15