PSI - Issue 23

Petr Haušild et al. / Procedia Structural Integrity 23 (2019) 179–184 Haušild et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Search for the replacement of stainless steels and/or nickel based superalloys is stimulated by the content of potentially critical raw materials such as nickel, chromium and cobalt that are of high importance to the (not only) European economy and of high risk associated with their supply. From that reason, iron based intermetallic alloys (especially iron aluminides and iron silicides) have been under investigation for decades as these alloys show excellent corrosion resistance in oxidation and sulfidation environment even at high temperatures. However, the alloys based on iron aluminides and iron silicides suffer from the room temperature brittleness caused mainly by hydrogen embrittlement (Stoloff (1998)). Due to this drawback, effects of various alloying element on the mechanical properties were investigated in order to improve the plasticity of these alloys. Only little attention has been paid to the ternary Fe-Al-Si system with high content of aluminum and silicon. Due to the lack of plasticity, ternary iron-aluminium-silicon alloys can hardly be prepared by conventional processing routes (melting, casting, rolling or forging). The processing is difficult especially in the presence of τ 1 phase (Al 2+x Fe 3 Si 3-x ). To overcome this problem, ternary FeAl20Si20 alloy was successfully prepared by mechanical alloying in previous research (Haušild et al. (2018a) , Novák et al. (2018)). The mechanical alloying was carried out from elemental powders in a high energy ball-mill. Relatively short processing time (~4h) was achieved by applying extremely high ball-to-powder ratio (50-70:1) and high rotational velocity (at least 400 rpm). Such processing condition led to the development of Fe 3 Si phase supersaturated by Al. The formation of equilibrium τ 1 phase (Al 2+x Fe 3 Si 3-x ) was completely suppressed. The aim of this paper is to compare the alloys prepared from elemental powders and from the pre-alloyed powders (FeAl, FeSi and AlSi) in order to characterize the effect of pre-alloying on kinetics of mechanical alloying as well as on final microstructure and mechanical properties of so prepared alloys. The microstructure, phase composition and mechanical properties during each step of mechanical alloying were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and nanohardness measurements. The results obtained on batches prepared from pre-alloyed powders are compared with the results obtained on batches prepared from elemental powders. Milling was performed in Retsch PM 100 high-energy ball mill. Aimed final chemical composition of the alloy was FeAl20Si20 (60 wt.% Fe + 20 wt.% Al + 20 wt.% Si). The feedstock was pure Fe, Al, Si (purity>99.5%) powders and pre-alloyed FeAl, FeSi and AlSi powders (see Table 1). Powders were blended in corresponding amount (in the case of AlSi30 initial powder, the pure silicon powder was added to balance) and inserted into a mould from AISI 420 stainless steel. Milling balls were also from stainless steel, ball-to-powder mass ratio was 60:1, rotational speed of 400 rpm. In order to prevent oxidation during mechanical alloying, the milling vessel was flushed with inert gas (argon) for 5 min. The milling time varied from 1 to 8 h. After 1, 2, 4, 6 and 8 h, small samples of mechanically alloyed powders were extracted for SEM and XRD analysis as well for nanohardness measurement. XRD analysis was performed using PANalytical X'Pert Pro X-ray diffractometer (Bragg-Brentano geometry, Cu cathode). For microstructural observation and nanohardness measurements, the powder particles were embedded in a hard conductive resin (filled with carbon filler) and polished down to 0.04 µm colloidal silica suspension. SEM was carried out in Jeol JSM 5510LV scanning electron microscope equipped with iXRF 500 energy dispersive X-ray spectrometer. Nanoindentation measurements were performed on Anton Paar NHT 2 Nanoindentation Tester equipped with Berkovich indenter. The load – depth of penetration records were treated following the ISO 14577 standard (based on Oliver and Pharr (1992)). At least 15 indentations were carried out (in 15 different embedded powder particles) in order to obtain statistically representative results. 2. Experimental details

Table 1. Composition and designation of feedstock powder. Powder composition (wt.%) Designation Fe (60%), Al (20%), Si (20%) Fe_Al_Si