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

THE HIGH TEMPERATURE MECHANICAL PROPERTIES OF Zr-ALLOYED IRON ALUMINIDES

Pavel KEJZLAR a, Petr KRATOCHVIL b, Zuzana ANDRSOVA a

a Department of Material Science, Faculty of Mechanical Engineering, Technical University of Liberec, Studentska 2, 461 17 Liberec 1, Czech Republic, pavel.kejzlar@tul.cz

b Department of Physics of Materials, Charles University, Ke Karlovu 5, 121 16 Praha 2, Czech Republic, pekrat@met.mff.cuni.cz

Abstract

Iron aluminides are promising materials as candidates for high temperature structural applications because of their excellent oxidization resistance, good high temperature strength and low price of raw materials. The mechanical properties of Fe-Al alloys could be enhanced by addition of Zr which leads to formation of λ1 or/and τ1 intermetallic phases corresponding to Fe-Al-Zr ternary diagram. In this paper is examined the structure of the samples in the as cast state with different concentration of Zr in Fe-30at.%Al-xZr, the volume fraction of phases is calculated and the effect of Zr addition on the high temperature mechanical properties is described.

Keywords: Iron Aluminides, High temperature mechanical properties, Intermetallics, Structure

1. INTRODUCTION

The impulse of iron aluminides research is development of technical materials for high temperature (H.T.) applications which are made from inexpensive and available raw materials. While the mechanical properties of most technical alloys are influenced mainly by size of particles (µm dimensions), the behaviour of intermetallics depends on their much finer structure - the arrangement of atoms in the elementary cell (dimensions of nm). The covalent bonds created between the metal atoms are responsible for some special properties of the alloy, for example high mechanical strength, melting temperature and elastic modulus. Intermetallics of Fe-Al type are promising candidates as materials for structural applications due to their outstanding H.T. oxidation resistance, good mechanical strength, relatively low weight, good wear resistance and low raw material costs [1-3].

Considerable problems of intermetallics lay in their brittleness, limited fabricability, low ductility at room temperature (R.T.) and their low creep strength at H.T. [1-7]. Current research activities are focused on improving H.T. tensile and creep properties to enable applications up to 600 – 900 °C. The improving o f mechanical and metallurgical properties can be achieved by using of alloying additions, microstructural control and optimization of material´s processing. High temperature strength and creep resistance could be enhanced for example by hardening of solid solution, strengthening with coherent or incoherent precipitates including carbide and boride precipitates or diverse phases which are originating because of ternary addition of Zr, Ta, Nb or Ti [1-7].

Zirconium was found as a noticeable alloying element for the Fe3Al matrix. If the matrix contains carbon or boron, zirconium carbides or borides can be formed [6, 7]. It is known, that Zr has nearly zero solubility in all Fe-Al phases, independently on the temperature. It has been shown that even a small addition of Zr to the Fe-Al alloy leads to a formation of λ1-Laves phase Zr(Fe,Al)2 or τ1-phase Zr(Fe,Al)12 depending on the Al content [8-13]. The part of the ternary diagram Fe-Al-Zr which shows the phase equilibrium at 1000°C i s on Fig. 1 [8]. The structure and stability of Laves phases are closely described in [14]. The presence of λ1 and τ1 phases may enhance the mechanical properties [5, 12, 13]. The R.T. ductility of Fe3Al was found to be decreased by alloying with 5 at.% Zr [3, 13]. In [12] there was studied and described the deformation behaviour of Fe-10Al-2.5Zr and Fe-20Al-2.5Zr. Effect of different Zr content on the structure, mechanical

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

properties and oxidation behaviour of some Fe-10Al, Fe-20Al and Fe-40Al was studied in [13]. It has been shown, that 0.2%yield stress increases with increasing amount of the λ1 respectively τ1 phase at all temperatures and decreases with increasing temperature for each composition.

2. EXPERIMENTAL

As the samples were prepared four prismatic casts with dimensions of 20 x 40 x 150 mm by vacuum induction melting; Zr was added in metallic form. Their composition is marked in Tab 1. and in Fig. 2. The mean concentrations of the technological impurities coming from the metals used for the preparation of the alloys were: 0.1 at.% Cr, 0.01 at.% B, 0.1 at.% Mn, 0.06 at.% C.

Tab. 1: Chemical composition of the investigated alloys

Alloy Al [at.%] Zr [at.%] Fe

30_0 29.3

0.4 Bal.

30_1 29.2

0.9 Bal.

30_2 28.9

1.9 Bal.

30_5 30.1

5.2 Bal.

The phase structure and grain boundaries were visualised using mechanical-chemical polishing with the OP-S emulsion. For examination of the alloy`s structure in the cast state were used the light optical microscope (LOM) with differential interference contrast (DIC), scanning electron microscope (SEM) equipped with the energy dispersive X-ray analysis system (EDX), X-ray diffraction (XRD) as well as measurement of Vickers micro-hardness. The approximate volume fractions of phases were calculated using the image analysis from SEM taken in chemical contrast mode (BSE).

The prismatic samples for H.T. compression tests were cut from the bulk by electrical discharge machining and their faces were subsequently carefully polished. Dimensions of the samples were 6x6x10 mm.

Deformation tests were realized by digitally controlled testing machine INSTRON 1186 at temperatures of 600, 700, 800 and 880°C. Compression tests were per formed at a strain rate of 1.5 x 10-4 s-1, the temperature was kept with the accuracy of 3 K.

Fig. 1. A part of the ternary diagram Fe-Al-Zr at 1000°C [8]

Fig. 2. A part of Fe-Al-Zr diagram. Compositions of investigated alloys

are marked by ” ”.

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3. RESULTS

3.1 Structure of cast alloys

Fig. 3a-d shows SEM images taken in chemical contrast. Their structure is composed of primary Fe-Al matrix and eutecticum which is composed of thin lamellae Fe-Al (B2/D03) and λ1 Zr(Fe,Al)2 Laves phase. The amount of the eutecticum increases linearly with the amount of Zr (Fig. 4). Presence of Zr(Fe,Al)2 was confirmed by XRD (Fig. 5) and EDX. Except Fe-Al and λ1 there is one more phase in the structure of cast samples. This phase was identified by EDX as ZrC (Fig. 6).

Fig. 3: Structure of alloys in BSE images a) 30_0; b) 30_1; c) 30_2; d) 30_5 in cast state. The dark phase is Fe-Al, grey phase is λ1 Zr(Fe,Al)2 and small bright particles are ZrC.

50 µm 50 µm

50 µm 50 µm

a b

c d

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Fig. 6: Line EDX analysis of ZrC particle. Signal received during line profiling from a) carbon, b) zirconium

3.2 H.T. mechanical properties

Fig. 7 a shows the temperature dependence of 0.2%yield stress, where the strong value drop of σ0.2 with increasing temperature for all Zr concentrations is pronounced. Fig. 7b shows the dependence of the 0.2% yield stress on the atomic concentration of Zr. Obviously addition of smaller amount of Zr has only insignificant effect on σ0.2 while its values for addition of 5 at.% Zr are approximately two times higher for the whole temperature range. In Fig. 8 there is the structure of the alloy 30_5 after the deformation at 800°C. From flattering of dark areas of Fe-Al is obvious, that deformation took place mainly in the soft matrix while the effect of the deformation on the hard eutecticum was only weak.

Fig. 5: The XRD spectrum of the sample 30_5 in cast state.

Fig. 4: The volume fraction of the eutecticum depending on the atomic concentration of Zr.

a b

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

Fig. 7: The dependence of a) 0.2% yield stress on the temperature for different content of Zr, b) 0.2% yield stress on the Zr content at different temperatures.

4. DISCUSION AND CONCLUSIONS

The addition of Zr to Fe-30Al material leads to formation of ductile (B2/D03) Fe-Al phase with the residual eutecticum which is composed of the fine lamellae of λ1-Laves phase and Fe-Al phase (Fig. 3). The eutectic usually have a lamellar structure. The volume of the eutecticum increases linearly with the amount of Zr (Fig. 4). Presence of a small amount of carbon in the raw Fe-material leads to formation of ZrC particles because of high affinity of C to Zr [6]. The Laves phase was identified by XRD (Fig. 5) and EDX analysis, ZrC particles were identified only by the EDX (Fig. 6), because of their volume (≤0.03 vol.%) is under the detection limit of XRD. The hard λ1-Laves phase act as a skeleton in the soft Fe-Al matrix and enhances significantly the H.T. mechanical properties (Fig. 7). Addition of small amount of Zr into the Fe-30Al alloy has only insignificant effect on the H.T. mechanical properties, but it is worth to notice, that the addition of 5 at.% Zr is very effective to enhance the strength of the material. It is obvious that values of σ0.2 are approximately two times higher after the addition of 5 at.% of Zr compared to other alloys for the temperature range between 600 – 800°C. Deformation during compression tests took place preferably in the soft Fe-Al phase while the

Fig. 8: The structure of the 30_5 alloy after the pressure deformation at

800°C

Fig. 9: Graph showing the effect of Al amount on the 0.2%yield strength in Fe-xAl-5Zr alloys

at different temperatures [13]

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

deformation of the eutecticum was very slight (Fig. 8). In Fig. 9 the present data obtained for the alloys 30_5 are given together with those obtained in [13]. Present data are marked by “�” symbols. It is obvious, that values of the 0.2% yield strength of the alloy with 30 at.% Al and 5 at.% Zr at temperatures 600 – 800°C are higher. This may be because of the matrix which contains B2 order in comparison with the A2 matrix in compared materials. Structural changes of alloy 30_5 after the heat treatment were described at [14]. Subject of the next research will be the influence of phase transformation which was initiated by the long-lasting high operating temperature exposure to the H.T. mechanical properties. Despite of increase of the room temperature brittleness caused by addition of Zr, alloying by 5 at.% Zr seems to be very effective way to enhance the H.T. mechanical properties.

ACKNOWLEDGEMENT

The author wish to thank to Prof. Ing. Ivo Schindler, DrSc., who supported the alloys, to Doc. RNDr. S. Danis, Ph.D., who kindly did the XRD phase analysis and to Dr. rer. nat. Robert Kral, Dr.,

who realized H.T. compression tests. This work is a part of research plans MSM 4674788501 that is financed by the Ministry of Education, Youth and Sport of the Czech Republic.

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