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Fe-Al-Si-X ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

Pavel NOVÁKa, Václav HOŠEK, Lucie MEJZLÍKOVÁ, Jan ŠERÁK, Dalibor VOJTĚCH

Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technicka 5, 166 28 Prague 6, Czech Republic, apanovak@vscht.cz

Abstract

Iron aluminides are promising high-temperature materials. Previous research showed that silicon improves the high-temperature oxidation resistance. In this work, the effect of alloying elements (Cr, Co, Ti, Cu) on microstructure, phase composition, hardness, wear resistance and high-temperature oxidation resistance of Fe-Al-Si alloys was studied. Alloys were prepared by the powder metallurgy technique using reactive sintering.

Keywords: iron aluminide, high-temperature oxidation, powder metallurgy

1. INTRODUCTION

Iron aluminides (Fe3Al, FeAl) are modern structural materials for high-temperature applications. Their favourable properties are good corrosion resistance together with lower density (5.7 g.cm-3 for FeAl [1]) than that of currently used iron- or nickel-based alloys and low cost of the constituents. These alloys show excellent resistance against high-temperature oxidation, sulfidation or the high-temperature corrosion in highly oxidizing salts [1,2]. Possible production routes of iron aluminides concern casting technology [2,3] or powder metallurgy processes. In the case of powder metallurgy technologies, alloyed Fe-Al powders show poor compressibility and sinterability. Therefore, reactive sintering is considered as a promising alternative production route. Reactive sintering is a technology, where pure elements or other suitable precursors are transferred into desired compounds by a thermally activated in-situ chemical reaction during sintering process [4]. By a pressureless reactive sintering of Fe-Al pressed powder mixtures, relative density of maximum 75% can be obtained [5]. A promising solution of the problem with porosity consists either in pressure-assisted reactive sintering or alloying with other element, modifying the reaction mechanism. In our previous work, silicon was found to reduce the porosity of Fe-Al alloys produced by reactive sintering [6,7].

Silicon added to Fe-Al alloys completely modifies the intermediary phases’ formation mechanism. Silicon-containing phases (Fe-Si, Fe-Al-Si) grow towards the molten Al-Si alloy while the iron aluminides follow the direction to the core of the iron particles as in the Fe-Al binary system. It helps to fill the spaces between iron particles in the pressed powder mixture. In addition, silicon forms a eutectic with aluminium which melts at lower temperature than aluminium (577°C). Therefore the time of the melt existence during heating prior the formation of intermediary phases is prolonged. Due to this fact, melt is able to fill the pores between iron particles. Optimum alloy composition was previously determined as FeAl20Si20 (given in wt. %) [6]. In this work, the effect of partial replacement of iron by other transition metals on the structure and properties of Fe-Al-Si alloys was studied.

2. EXPERIMENTAL

Fe-Al-Si-X alloys produced by reactive sintering of iron, AlSi30 alloy, silicon and X (X=Cu, Co, Cr, Ti) powders were studied. Powder of AlSi30 alloy with particle size of 200 – 600 µm was prepared by mechanical machining. Silicon powder with the particle size below 50 µm was obtained by mechanical milling. Iron and X powders were used in a form of commercially available powder of p.a. purity and a grain size below 10 µm. Green bodies of Fe-Al-Si-X alloys containing 20 wt. % of aluminium, silicon and X were

18. - 20. 5. 2011, Brno, Czech Republic, EU

produced by blending of the above mentioned powders and uniaxial pressing at the laboratory temperature by a pressure of 260 MPa using Heckert FPZ100/1 universal loading machine. Reactive sintering of Fe-Al-Si pressed powder mixtures was carried out at the temperature of 1100°C for 30 min in the electric resistance furnace according to previous results published in [6]. Microstructure of the prepared materials was observed by Olympus PME3 light microscope. AxioVision 4.7 software was applied for the digital image recording and processing. Phase composition was determined by x-ray diffraction (XRD) analysis using a PANalytical X’Pert Pro x-ray diffractometer and Tescan Vega 3 LMU scanning electron microscope equipped with Oxford Instruments INCA 350 EDS analyser.

Hardness of the prepared materials was tested by the Vickers method with the load of 10 kg (HV 10). The abrasive wear resistance was evaluated by using a modification of the “pin-on-disc” method, where “pin” was the tested material and “disc” was a P1200 grinding paper. The applied load was 5.8 N and the sliding distance was defined as 2.5 km. The wear rate was calculated from the measured weight losses by the equation (1) [7]:

lmw

⋅⋅∆

=ρ

1000 (1)

where w, Δm, ρ and l are wear rate [mm3m-1N-1], weight loss [g], density [g.cm-3] and sliding distance on the grinding paper [m], respectively. The density of samples was determined by the Archimedes method.

High-temperature oxidation resistance was studied by the cyclic oxidation tests at 800°C for 432 h. During these tests, samples were heated for 48-hours cycles. Each cycle consisted of heating to the test temperature, air-cooling to the laboratory temperature, weighing and heating again to the test temperature. Oxidation rate of the alloys is presented as a dependence of the specific weight gain, i.e. the weight change over the exposed surface area, on the oxidation duration.

3. RESULTS AND DISCUSSION

FeAl20Si20 ternary alloy is formed by iron silicide (FeSi) particles surrounded by a mixture of Al2Fe3Si3 and FeAl phases. see Fig.1a. The porosity of this alloy reaches the average value of approx. 8 vol. % (Fig.2). Addition of chromium reduces the porosity down to 2 vol. %. On the other hand, copper and titanium makes the porosity to grow up to the values common for binary Fe-Al alloys produced by reactive sintering. Cobalt addition has no significant effect on the porosity. Due to the enormous porosity, titanium-alloyed material was excluded from the further tests.

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Fig.1: Microstructure of Fe-Al-Si-X alloys produced by reactive sintering at 1100°C: a) FeAl20Si20, b)

FeAl20Si20Ti20, c) FeAl20Si20Co20, d) FeAl20Si20Cr20, e) FeAl20Si20Cu20.

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Fig.2: Porosity of Fe-Al-Si-X alloys produced by reactive sintering at 1100°C.

The phase composition of all Fe-Al-Si-X alloys is very similar to the above described ternary one. Silicides, aluminides and ternary phases arise in these alloys. Titanium, cobalt and chromium were found predominantly in silicide particles (see Fig.1). In the case of titanium- and chromium-alloyed materials, Ti5Si3 or Cr5Si3 silicides was determined (Fig.1b,d), while cobalt was present in (Fe,Co)Si mixed silicide (Fig.1c). Copper was found to stabilize the aluminide phase (Fig.1e). Unfortunately, the chromium-alloyed alloy with the lowest porosity contains unreacted silicon particles which may have the negative impact on mechanical properties. Several residual particles of silicon were also found in cobalt-containing alloy. It can be highly expected that the occurrence of these particles can be eliminated by a slight modification of the chemical composition or by an optimization of the reactive sintering conditions.

All alloying elements increase both the hardness and wear resistance of the product, see Table 2. The most positive effect on hardness and wear resistance was found in the case of cobalt-containing alloy.

18. - 20. 5. 2011, Brno, Czech Republic, EU

Table 1: Mechanical properties of Fe-Al-Si-X alloys

FeAl20Si20 FeAl20Si20Co20 FeAl20Si20Cr20 FeAl20Si20Cu20

hardness [HV 10] 630 930 650 820

wear rate [x10-4 mm3m-1] 25 5 7 8

Oxidation tests at 800°C revealed that chromium and cobalt significantly improve the oxidation resistance of Fe-Al-Si alloys, while the effect of copper is detrimental, see Fig.3. The high oxidation rate of copper-alloyed material can be probably attributed to the above described increase of porosity. As proved by XRD, all of the oxide layers are composed of Al2O3 and small amount of Fe2O3. The effect of the alloying elements on the oxidation mechanism will be the subject of the future work.

Fig.3: Weight gain vs. duration of oxidation at 800°C.

4. CONCLUSION

In this work, the effect of alloying elements (Cr, Co, Cu) on microstructure, phase composition and oxidation resistance of Fe-Al-Si alloys produced by reactive sintering was studied. It has been found that chromium strongly reduces the porosity of Fe-Al-Si alloys, reaching the value of approx.2 vol. %. Chromium and cobalt

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improve the oxidation resistance of Fe-Al-Si alloys. In the case of chromium-containing alloy, the oxidation resistance reduces nearly 10 times. The effect of copper on the oxidation resistance was strongly detrimental. All of the tested alloying elements were found to increase the hardness and wear resistance of Fe-Al-Si alloy.

ACKNOWLEDGEMENT

The research of Fe–Al–Si intermetallics was financially supported by the MSM 6046137302 project.

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4. Encyclopaedia Britannica Online, (2011).

5. U. S. Patent No. 5,269,830.

6. NOVÁK, P., KNOTEK, V., ŠERÁK, J., MICHALCOVÁ, A., VOJTĚCH, D. Powder Metallurgy (2009), doi: 10.1179/174329009X449314.

7. NOVÁK P., VOJTĚCH, D., ŠERÁK, J. Surface and Coatings Technology 200 (2006) 5229 – 5236.