Corrosion behavior of the resistance sintered TiAl based intermetallics induced by two different molten salt mixture
Introduction
Components of gas turbine engines (like turbine blades, disks, stator blades, guide vanes and rotors) used in aerospace, marine and industrial applications must be able to withstand many harsh conditions, including cyclic stresses in oxidizing and/or corrosive environments during operation at elevated temperatures [[1], [2], [3]]. Extensive studies have carried out on the reduction of NOx emission gases and low fuel consumption to provide higher efficiency of gas turbine engines. This can be achieved in two ways: the first one is increasing the combustion temperature and pressure in the gas turbine, and the second one is applying developments in cooling systems. However, higher service temperatures expose components to high-temperature corrosion and thus considerably decrease their service life [4]. The γ-TiAl based intermetallic compounds have drawn attention materials for high-temperature applications due to a combination of superior features such as high melting point, lower density, adequate high specific strength and elastic modulus as well as good oxidation and corrosion resistance [[5], [6], [7], [8], [9]]. The efforts to develop titanium aluminides have led the use of these aluminides as low-pressure turbine (LPT) blades in GEnx ™ 1B (Boeing 787) and the GEnx ™ 2B (Boeing 747-8) engines [10,11]. Furthermore, γ-TiAl based intermetallics are lighter than nickel-based superalloys and offer the opportunity of weight-saving up to 50 % for the hot sections of the gas turbine, resulting in engine weight reduction, performance enhancement and reduction of the centrifugal force applied to the turbine disk [[11], [12], [13]].
During the combustion of a gas turbine engine, the hot section components can expose to an aggressive form of corrosion related to the presence of condensing various salts (e.g., Na2SO4, K2SO4, NaCl, V2O5) on the external surface of components or oxide scales. This type of corrosion, causing the degradation of materials, is called hot corrosion [1,4,6,14]. The deposition of fused salts results from infiltrated air and residual fuels in gas turbine engines [15]. The high concentrations of sulfur (wt.% 4), sodium (wt.% 0.01) and vanadium (0.05 wt.%) are the results of low-quality fuel. These impurities and the chlorine from ingested air react with the oxygen surrounding the hot-section region (combustion chambers and turbine blades) and form mostly volatile oxides such as SO2, SO3, NaOH, NaO, Na2O, VO(OH)3, V2O5 and V2O4. The mentioned compounds condense at the temperatures range from 500 to 900 °C to form molten salt deposits [16].
There are various processing techniques to produce TiAl alloys, such as vacuum arc melting [17,18], hot isostatic pressing [19] and hot press sintering [20]. However, the manufacturing costs are high due to the several problems in casting processes, as an example, considerable effort has been devoted to minimizing the composition segregation and texture during solidification [21]. Besides, the TiAl alloys have a low castability, large solidification shrinkage rate and high chemical reactivity, resulting in misrun defects on the surface, porosity and crack [22]. The above considerations have restricted the applications of these alloys in the industrial area. Therefore, scientists and engineers are faced with the fact of finding alternative processes to conventional methods to produce TiAl based alloys. It was reported that the coarse lamellar structures and segregation faced by the casting alloys could be prevented by using the powder metallurgy route with rapid solidification [23]. The resistance sintering (RS) technique has been attracted attention as a new powder metallurgy approach for producing intermetallics, ceramics and composites materials. The RS process is regarded as an ever-growing and effective producing technology. The most significant property of the RS is that the powder or green compact are heated by the Joule effect and thus, the materials can be synthesized uniformly and rapidly. As a result, materials with high dense and fine microstructure can be achieved in very short processing time [6,24,25].
TiAl alloys inevitably suffer severe hot corrosion in the actual environment. That is to say, it is important to investigate the corrosion resistance of TiAl in the molten salt environments. However, very little amount of research has been focused on the hot corrosion behavior of TiAl and reported in the literature. In view of this, the present study was undertaken. Thus, the aim of this study is to investigate cyclic hot corrosion behavior of TiAl based intermetallics produced by RS technique in two different molten salts consisting of wt.% Na2SO4-25K2SO and Na2SO4-25NaCl.
Section snippets
Sample preparation
Titanium aluminides used in this study have the nominal composition of (at.%) Ti-48Al, Ti-48Al-2Cr, Ti-48Al-2 Mn, Ti-48Al-2Cr-2Mo and Ti48Al-2 Mn-2Mo were prepared by RS technique. The elemental materials used in the present study were titanium (purity 99.5 %, 40 μm), aluminum (purity 99.5 %, 7−15 μm), chromium (purity 99.2 %, 10 μm), molybdenum (purity 99.55 %, 3−7 μm) and manganese (purity 99.6 %, 10 μm).
All elemental powders were provided by Alfa Aesar company. Prior to the RS process, the
Characterization of the RSed intermetallics
Titanium and aluminium may react during the sintering in the following manner [26]:Ti + 3Al = TiAl34Ti + TiAl3 = 2TiAl + Ti3Al
In the first stage, Al and Ti react to form TiAl3 which is an intermediate phase, under the melting point of Al (∼600 °C). It was reported that the free energy of TiAl3 is the lowest in comparison to Ti3Al and TiAl phases in all temperatures. It is thus expected that the first product to be developed will be TiAl3 phase during sintering. Besides, since the diffusion rate
Discussion
In conventional powder sintering, the sample is usually heated by a furnace. In order to obtain a uniform temperature distribution and microstructural homogeneity, slow heating rates and long sintering cycles (several hours, even days) are mostly required [35]. In contrast, since an intense electric current flows directly through the compact during RS process, the heat is produced within the sample and as a result, very rapid heating rates achieve. Regarding the main drawbacks of the RS
Conclusions
In this study, resistance sintering (RS) technology was effectively applied for production of TiAl based intermetallics added Cr, Mn and Mo alloying elements and the influence of these additions on the hot corrosion behavior of alloys in the molten salt environment consisting of % wt. Na2SO4-25K2SO4 and Na2SO4-25NaCl was explored. The following main conclusions can be drawn from this study:
- 1
TiAl based intermetallics prepared by RS consisted of γ-TiAl and α2-Ti3Al phases.
- 2
The weight losses of
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
CRediT authorship contribution statement
Y. Garip: Conceptualization, Investigation. O. Ozdemir: .
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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