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OXIDATION RESISTANCE AND THERMAL STABILITY OF Ti-Al-Si ALLOYS PRODUCED BY REACTIVE SINTERING

Pavel Novák Filip Průša Jan Šerák

Dalibor Vojtěch Alena Michalcová

Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technicka 5, 166 28 Prague 6, Czech Republic

e-mail: panovak@vscht.cz Abstract

Alloys formed by Ti-Al intermediary phases have low density and very good high-temperature oxidation resistance. Further improvement of high-temperature behaviour can be achieved by a suitable addition of silicon. In this work, Ti-Al-Si alloys containing 10 - 20 wt. % of both silicon and aluminium produced by a reactive sintering technology were tested. Oxidation tests were carried out at 800 - 1000°C in air. The oxidation rate was determined from weight gains caused by oxide formation. Microstructure and phase composition of the oxide layers were studied. Microstructure and hardness changes after various annealing durations were described.

Keywords

titanium aluminide, titanium silicide, reactive sintering, powder metallurgy, oxidation resistance, thermal stability

1. INTODUCTION

Nickel alloys and heat-resistant steels are currently the most widely used metallic high-temperature materials. Their main advantage is relatively simple production, while their main disadvantage is a high density. Where low weight of the part is required, as for example in the aerospace industry, constructors tend to replace these materials by lighter ones. Very promising materials with low density are intermediary phases of titanium with aluminium. In addition, these materials have also very good thermal stability and oxidation resistance at the temperatures of 600 – 800°C [1]. The main disadvantage complicating the utilization of these materials is their problematic melt-metallurgy production due to high melting points of intermetallics and a high reactivity of the melts with the most of the crucible materials [2]. Powder metallurgy using reactive sintering seems to be a very promising production route. In this process, powders of pure metals or other suitable precursors are blended and pressed. Formation of intermediary phases proceeds during sintering via thermally-activated chemical reactions. It has been found that the reactive sintering progress and product structure are positively influenced by the appropriate silicon addition [3,4,5]. Titanium and silicon form stable Ti5Si3 silicide, which is used also in the surface treatment of titanium alloys against high-temperature oxidation [6,7]. In this work, the oxidation resistance and thermal stability

METAL 2009 19. – 21. 5. 2009, Hradec nad Moravicí ___________________________________________________________________________

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of Ti-Al-Si alloys with various contents of aluminium and silicon produced by reactive sintering were studied.

2. EXPERIMENTAL

In this work, Ti-Al-Si alloys produced by reactive sintering of titanium, aluminium, silicon and AlSi30 powders were studied. Powders of titanium, aluminium and AlSi30 with the particle size 200 – 1000 µm were prepared by mechanical machining. Silicon powder with the particle size up to 50 µm was obtained by mechanical milling. Green bodies of Ti-Al-Si ternary alloys containing 10 - 20 wt. % of both aluminium and silicon were 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 was carried out at the temperature of 900°C for 30 min in the electric resistance furnace in evacuated silica ampoules. Microstructure of the prepared materials was observed by Olympus PME3 light microscope and Hitachi S-450 scanning electron microscope equipped with EDS analyser. Phase composition was determined by x-ray diffraction (XRD) using a Philips X’Pert Pro x-ray diffractometer. Hardness of the prepared materials was tested by the Vickers method with the load of 10 kg (HV 10).

Oxidation tests were carried out at 800 - 1000°C in air. Oxidation rate was determined from the weight gains caused by the oxide formation on the surface of thermally-exposed samples. Microstructure of the oxide layers was documented by the light microscope, chemical and phase composition were determined by XRD and EDS. Thermal stability was studied by the hardness measurement (HV 10) and by the microstructure observation after a long-term annealing at the temperatures given above. 3. RESULTS AND DISCUSSION

Fig.1 shows the microstructure of the studied materials prepared by reactive sintering. Materials were prepared according to our previous experiments [3] to achieve the lowest possible porosity and the absence of unreacted components. All alloys are formed by Ti5Si3 particles surrounded by titanium aluminide. TiAl20Si20 contains TiAl3 phase (Fig.1c), the other materials contain TiAl aluminide. The average size of silicide particles reaches 6 - 7 µm in TiAl20Si10 and TiAl20Si15 alloys and approx. 11 µm in the alloys with 20 wt. % of silicon.

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Fig.1. Microstructure of materials prepared by reactive sintering at 900°C for 30 min: a) TiAl20Si10, b) TiAl20Si15, c) TiAl20Si20, d) TiAl10Si20.

During annealing of the investigated alloys at 800 – 1000°C, gradual hardness

decrease can be observed. This hardness reduction grows with the annealing temperature, as shown in Fig.2. The hardness decrease is the most significant during first 200 h of annealing. After that hardness remains almost constant at 800 - 900 °C, at 1000°C it slightly decreases.

Fig.2. Hardness of TiAl15Si15 alloy vs. annealing time (annealed at 800 – 1000°C).

The softening during annealing strongly depends on the alloy composition (Fig.3).

The smallest hardness changes with the annealing time were detected in alloys containing 15 - 20 wt.% of silicon and 20 wt. % of aluminium.

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Fig.3. Hardness of investigated alloys vs. annealing time at 1000°C.

As it is evident when comparing the alloys microstructure before and after

annealing, the hardness decrease is probably caused by the coarsening of Ti5Si3

particles. In the case of TiAl20Si15 alloy shown in Fig. 4, annealing at 1000°C for 300 h caused an increase of the average silicide particle size from approx. 6 µm to nearly 12 µm.

Fig.4. Microstructure of TiAl20Si15 alloy: a) before annealing, b) after 300 h annealing at 1000°C.

The oxidation tests at 800 and 1000°C showed, that the oxidation rate depends

on both silicon and aluminium contents. The lowest weight gain by oxidation was detected in the alloys containing 20 wt. % of aluminium and 15 – 20 wt. % of silicon (Fig.5). TiAl20Si20 and TiAl10Si20 alloys did not exhibit any delamination of the oxide layers, while all alloys containing up to 15 wt. % of silicon showed the spallation of 250 – 800 g/m2 of oxides.

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Fig.5. Oxidation kinetics of Ti-Al-Si alloys at 1000°C.

XRD revealed the presence of corundum (Al2O3) and rutile (TiO2) in the oxidized

layer. EDS analysis also detected silicon-rich regions in the oxide layer, probably silicon oxide. Since this phase was not detected by XRD, its structure is probably amorphous.

Microstructure of layers formed by oxidation at 1000°C in air consists of one or two sub-layers. On the surface, layer (I.) formed by the mixture of TiO2, SiO2 and Al2O3 can be recognized, as confirmed also by chemical and phase analysis. Alloys with 10 – 15 wt. % of silicon contain also second sub-layer (II.) under the above mentioned one, containg all oxides as the layer I. and also a TiN nitride. In these alloys, oxide layers are extremely porous and their adhesion to the basic material is very poor (Fig.6a). On the other hand, alloys containing 20 wt. % of silicon form only one oxide layer consisting of TiO2, SiO2 and Al2O3 with good adhesion to the substrate. That is the reason for exceptional oxidation resistance of TiAl20Si20 alloy.

Fig.6. Microstructure of oxide layers on: a) TiAl20Si10, b) TiAl20Si20 alloys after oxidation at 1000°C for 125 h in air.

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4. CONCLUSION

In this work, the oxidation resistance and thermal stability of Ti-Al-Si alloys produced by reactive sintering were studied. Structure of the investigated materials is formed by Ti5Si3 and TiAl or TiAl3. During annealing, hardness of these alloys decreases due to a coarsening of silicides. The lowest hardness decrease was determined in the alloys containing 15 - 20 wt. % of silicon and 20 wt. % of aluminium. These materials also exhibit exceptional high-temperature oxidation resistance. The lowest oxidation rate without delamination of the oxide layers was achieved in TiAl20Si20 alloy. To ensure the advantageous high-temperature behaviour, sufficient contents of both silicon and aluminium are necessary.

ACKNOWLEDGEMENT

This work was financially supported by KJB201250801 research project of Grant Agency of AS CR and by the MSM 6046137302 research project of the Ministry of Education, Youth and Sport of the Czech Republic.

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