Papers by Alexander G . Donchev
![Research paper thumbnail of [Ga18(SitBu3)8] and [Ga22(SitBu3)8]—Syntheses and Structural Characterization of Novel Gallium Cluster Compounds](https://attachments.academia-assets.com/116346932/thumbnails/1.jpg)
Chemistry: A European Journal, Aug 3, 2001
The novel neutral gallium cluster compounds [Ga 18 R* 8 ] (1) and [Ga 22 R* 8 ] (2) are obtained ... more The novel neutral gallium cluster compounds [Ga 18 R* 8 ] (1) and [Ga 22 R* 8 ] (2) are obtained by warming up a metastable solution of gallium(i) bromide in THF/C 6 H 5 CH 3 after addition of equimolar amounts of supersilyl sodium NaR* from À 78 8C to room temperature (R* SitBu 3 supersilyl). From X-ray structure analyses, the observed arrangements of the 18 and 22 Ga atoms in 1 and 2, respectively, are comparable with an 18 atom section of the b-Ga modification, or show at least some kind of relationship to a 22 atom section of the Ga-III modification. This allows a description of both the clusters as metalloid. The topology of the Ga atoms in 2 is also well explained by the Wade ± Mingos rules as an eightfold capped closo-Ga 14 cluster, whereby the Ga atoms of Ga 14 occupy the center and the corners of a cuboctahedron with one Ga 3 face replaced by a Ga 4 face. Some concepts are presented about the formation mechanism, the cluster growth, and the metalloid character of the two Ga cluster compounds.
MRS Proceedings, 2008
Alloys based on aluminium and titanium are possible materials for several high temperature applic... more Alloys based on aluminium and titanium are possible materials for several high temperature applications. The use of TiAl would increase the efficiency of e.g. aero turbines, automotive engines and others due to their properties, among others low specific weight and good high temperature strength. The oxidation resistance is low at temperatures above approximately 800°C so that no long term use of TiAl-components is possible without improvement of the oxidation behaviour. Small amounts of halogens in the surface zone of TiAl-samples lead to a dramatic improvement of the oxidation resistance at temperatures up to 1100°C for more than 8000 hours in air. In this paper results of the work on the halogen effect over the last years are presented. The results of thermogravimetric measurements, thermocyclic oxidation tests of small coupons and thermodynamic calculations for different atmospheres (e.g. air, H2O, SO2) are shown and the halogen effect mechanism is discussed. The postulated mechanism is in good agreement with the results of the oxidation tests. The limits of the halogen effect will also be mentioned. Predictions for the halogenation of TiAl-components can be given so that the processing can be planned in advance.
Steel Research International, Aug 1, 2012
Titanium is a widely used structural material for applications below approximately 5008C but righ... more Titanium is a widely used structural material for applications below approximately 5008C but right now it cannot be used at higher temperatures. Titanium forms a fast growing rutile layer under these conditions. Furthermore enhanced oxygen uptake into the metal subsurface zone leads to embrittlement which deteriorates the mechanical properties. To overcome this problem a combined Al-plus F-treatment was developed. The combination of Al-enrichment in the surface zone so that intermetallic Ti x Al y-layers are produced which form a protective alumina layer during high temperature exposure plus stabilization of the Al 2 O 3-scale by the fluorine effect led to significantly improved resistance against increased oxidation and embrittlement in high temperature exposure tests of several Ti-alloys. In this paper, the experimental procedures and achieved improvements are described. The results will be discussed for the use of Ti-alloys at elevated temperatures.
MRS Proceedings, 2012
ABSTRACTIntermetallic titanium aluminides are potential materials for application in high tempera... more ABSTRACTIntermetallic titanium aluminides are potential materials for application in high temperature components. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, their practical use at temperatures as high as 800°C requires improvements of the oxidation resistance. This paper reports on the fluorine effect on a multi-phase TiAl-alloy in the cast and hot-isostatically pressed condition at 800°C in air. The behavior of the so-called TNM material (Ti-43.5Al-4Nb-1Mo-0.1B, in at %) was compared with that of two other TiAl-alloys which are Nb-free and contain different amounts of Mo (3 and 7 at%, respectively). The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. After fluorine treatment all alloys exhibit slow alumina kinetics indicating a positive fluorine effect. Results of isothermal and thermocyclic oxidation tests at 800°C in air are presented and discussed in the view of composition and microstructure of the TiAl-alloys investigated, along with the impact of the fluorine effect on the oxidation resistance of these materials.
Materials Science Forum, Aug 1, 2004
Intermetallic alloys based on titanium aluminides, especially γ-TiAl (Ti-50at.%Al), are of great ... more Intermetallic alloys based on titanium aluminides, especially γ-TiAl (Ti-50at.%Al), are of great attraction for structural high temperature applications e.g. in aero engines because of their low specific weight (about 4g/cm3) and their good mechanical properties at elevated temperatures. However, the use of TiAl alloys is still limited to a temperature of approximately 750°C due to their poor oxidation resistance at higher temperatures. The oxidation behaviour of TiAl alloys has to be improved before these alloys can be used at temperatures of 800°C and above. The improvement of the oxidation resistance of γ-TiAl can be achieved by small amounts of halogens F, Cl, Br and I (so called halogen effect). The halogens can be incorporated in several ways e.g. by ion implantation. In this paper results of a technical TiAl alloy called γ-MET [Ti-46.5Al-4(Cr, Nb, Ta, B)] with and without implantation are presented.
Plasma Processes and Polymers, Jul 18, 2009
ABSTRACT Titanium aluminides are promising light weight materials for several high-temperature ap... more ABSTRACT Titanium aluminides are promising light weight materials for several high-temperature applications, e.g., in aero engines but due to their insufficient oxidation resistance at temperatures above roughly 800 degrees C they cannot be used yet despite of their good mechanical high-temperature properties. The oxidation behavior of TiAl-alloys can be improved significantly by adding small amounts of fluorine into the subsurface zone of the components (microalloying). One possibility to apply fluorine to the surface of complex TiAl-components is the PI(3)-technique (plasma-immersion-ion-implantation). The use of an Ar/CH(2)F(2)-plasma for the F-PI(3) into small coupons led to a positive effect which was as good as the beamline implantation of elemental fluorine gas into samples of the same geometry. Turbine blades, as examples for real TiAl-components, were implanted with an optimum set of parameters. Post-exposure investigations like scanning electron microscopy revealed a thin protective alumina scale on the surface of the implanted samples in contrast to a thick mixed oxide scale (TiO(2)/Al(2)O(3)) growing on untreated samples during high-temperature oxidation in air. The high-temperature oxidation resistance of several TiAl-alloys was improved by plasma-immersion-ion-implantation of fluorine. Small coupons showed a much lesser oxidation during high-temperature exposure after fluorine treatment than untreated samples. The performance of turbine blades for the low pressure compressor of a new generation of jet engines was also improved. Fluorine treated samples are covered with a thin, protective alumina scale after high-temperature exposure in air instead of a fast growing, nonprotective mixed oxide (TiO(2)/Al(2)O(3)) scale which is found on untreated samples.
Surface & Coatings Technology, Apr 1, 2012
ABSTRACT Experiments have been undertaken to explore the possibility of creating an oxygen barrie... more ABSTRACT Experiments have been undertaken to explore the possibility of creating an oxygen barrier coating, which is effective in preventing oxidation and oxygen embrittlement of Ti and several low-Al content Ti-base alloys during exposure to oxidizing environments at elevated temperatures. The fabrication process has involved three steps, namely co-deposition of Ti and Al by magnetron sputtering onto a substrate material to be protected, followed by vacuum annealing and plasma immersion ion implantation of fluorine. The first two steps produce an overlay of γ-TiAl while the last step provides the necessary conditions for bringing about the halogen effect upon subsequent high-temperature oxidation. Analysis techniques such as cross-sectional transmission electron microscopy (XTEM) in conjunction with electron energy loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and elastic recoil detection (ERD) have been used to study the microstructure, phase formation and depth distribution of the elements in the coating material. Following oxidation in air at 600 °C for 100 h, specimens have been prepared for metallographic analysis, and their cross sections have been characterized by scanning electron microscopy (SEM) in combination with EDX, and electron probe microanalysis (EPMA). The results obtained show that during oxidation exposure the coating is capable of forming a protective alumina-containing scale which serves as an oxygen barrier, thereby preventing oxygen embrittlement. In addition, since the only constituents of the coating are Ti and Al, it exhibits excellent chemical substrate compatibility.
Journal of Alloys and Compounds, Mar 1, 2008
... Coat. Technol. 125 (2000), p. 89. Article | PDF (288 K) | View Record in Scopus | Cited By in... more ... Coat. Technol. 125 (2000), p. 89. Article | PDF (288 K) | View Record in Scopus | Cited By in Scopus (18). [8] A. Donchev, B. Gleeson and M. Schütze, Intermetallics 11 (2003), p. 387. ... [14] A. Donchev, E. Richter, M. Schütze and R. Yankov, Intermetallics 14 (2006), p. 1168. ...
Intermetallics, Oct 1, 2006
Titanium aluminides are of great interest for several structural high temperature applications be... more Titanium aluminides are of great interest for several structural high temperature applications because of their low specific weight (about 4 g/ cm 3) and their excellent high temperature strength. They could replace the much heavier high temperature steels or Ni-based superalloys (up to 9 g/cm 3) which are usually in service. The implementation of this new group of intermetallic alloys in e.g. the aerospace or automotive industry is therefore due to economic and ecologic reasons. The use of TiAl-based alloys is still limited to a temperature of about 750 C because of their poor oxidation resistance despite of their good mechanical properties which would allow the use at higher temperatures. The oxidation resistance can be improved significantly by small amounts of halogens such as fluorine, chlorine, bromine and iodine (so called halogen effect). A defined dose of these halogens has to be provided at the metal/oxide interface of the component. The halogens promote the selective formation of gaseous Al-halides at temperatures above 700 C which are oxidised to Al 2 O 3 during their outward diffusion through the naturally grown oxide scale. So finally a protective alumina scale is formed which is stable for long times even under thermocyclic exposure and wet atmospheres. In this paper the results of isothermal and thermocyclic high temperature oxidation tests of technical TiAl-alloys with and without halogen treatment are shown. Additionally the results of high temperature creep tests of halogen treated TiAl-alloys are presented and compared with the untreated alloys.
John Wiley & Sons, Inc. eBooks, May 18, 2012
Materials and Corrosion-werkstoffe Und Korrosion, Jan 12, 2011
Titanium is a widely used structural material because of its low specific weight, good mechanical... more Titanium is a widely used structural material because of its low specific weight, good mechanical properties and excellent corrosion resistance at ambient temperature. As a result of increased oxidation at elevated temperatures and environmental embrittlement the maximum operation temperature of standard Ti-alloys is only about 600 8C. The oxidation behaviour can be improved by different methods, e.g. coatings. This leads to an improvement which is, however, often limited. The combination of Al-enrichment in the sub surface zone, so that a TiAl-layer is formed, plus F-treatment gives impressively good results because a protective alumina scale is formed due to the fluorine effect. This alumina scale prevents oxygen inward diffusion which causes embrittlement and protects the material against environmental attack. The procedure is applied to alloys with a very low Al-content or even no Al at all. In the paper results of oxidation tests of a-Ti without any treatment, with Al-treatment and with a combination of Al-þ F-treatment are presented. Aluminium was diffused into the samples by a powder pack process. Fluorine was applied by a liquid phase process. The formation of an alumina scale on treated samples was revealed by post experimental investigations. The results are discussed referring to the fluorine effect model for TiAl-alloys.
Surface & Coatings Technology, 2021
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Solar Energy Materials and Solar Cells, Aug 1, 2021
Molten salt corrosion in the receiver's piping system and storage tanks is still one of the major... more Molten salt corrosion in the receiver's piping system and storage tanks is still one of the major drawbacks of concentrated solar power (CSP) plants and is currently covered by using expensive high-alloyed steels or Ni-base alloys. The employment of cheaper structural materials combined with protective coatings is an attractive alternative to increase cost-efficiency. The present study investigates the corrosion resistance of three different coatings (a pure Ni, a Cr and a combined Ni+Cr coating) deposited on ferritic-martensitic X20CrMoV12-1 steel during isothermal immersion in molten solar salt in comparison to uncoated Ni-base alloy Haynes 230. Exposure tests were conducted at 600°C for up to 1000 h. To identify the individual role of the main elements, exposure tests of pure Fe, Ni and Cr were undertaken as well. In addition to the crosssectional investigations via light-optical microscopy and EPMA, X-ray diffraction measurements were conducted to identify the corrosion products. These were complemented by weight change measurements and chemical analysis of the salts after the exposure tests. The study reveals differences between the corrosion behaviour of the different coating approaches, which is discussed as a function of coating composition and degradation mechanisms. Whereas pure Cr or pure Ni coatings offer hardly any improved protection of the uncoated X20CrMoV12-1 substrate, the combined Ni+Cr coating performs very well and improves the scaling behaviour of X20CrMoV12-1 significantly. The outcome highlights the correlation between the solubility of the pure metals Fe, Ni and Cr in molten solar salt and the corrosion resistance of the investigated coatings.
Materials Science Forum, 2010
The oxidation resistance of TiAl-alloys can be improved by several orders of magnitude by treatin... more The oxidation resistance of TiAl-alloys can be improved by several orders of magnitude by treating the surface of the materials with small amounts of halogens especially Cl and F. The oxidation mechanism changes due to the so called halogen effect. The formation of a fast growing mixed oxide scale on untreated alloys is suppressed, instead a thin protective alumina scale is formed on samples after optimum treatment. The different methods only influence the surface region of the components so that the bulk properties are not affected. Recent results achieved with complex TiAl-samples showed the potential that the fluorine effect could be used for TiAl-components in several high temperature applications e.g. jet engines. TiAl-specimens were treated with fluorine and chlorine in several ways and their performance during high temperature oxidation tests in air was investigated. Results of isothermal and thermocyclic oxidation tests are presented. The long term stability of the fluorine effect lasted for at least one year under thermocyclic exposure at 900°C in laboratory air. The results are discussed in terms of later use of the fluorine effect for technical applications.
Intermetallics, Oct 1, 2014
Intermetallic titanium aluminides are potential materials for a number of high-temperature compon... more Intermetallic titanium aluminides are potential materials for a number of high-temperature components used in aero and automotive engines. In particular, alloys solidifying via the b-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic b o-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, the practical use of such alloys at a temperature as high as 800 C requires improvement of their oxidation resistance. Various attempts have been made including alloying with additional elements such as Nb, Cr, Mo etc. or applying the so-called fluorine effect. However alloying could not provide a sufficient oxidation resistance above 850 C whereas the fluorine effect protects the base material against environmental degradation up to over 1000 C. This paper aims to investigate the influence of the phase composition on the oxide scale morphology without and with fluorine effect. The results refer to the oxidation behavior of three b-solidifying g-TiAl-based alloys in the cast and hot-isostatically pressed condition at 800 C in air. The behavior of the TNM alloy (Tie43.5Ale4Nbe1Moe0.1B, in at.%) was compared with that of two Nb-free TiAl alloys which contain different amounts of Mo (3 and 7 at.%, respectively) and hence a different microstructure (a 2 /b o /g vs. b o /g). During testing in dry synthetic air at 800 C a mixed oxide scale develops on all three alloys. This behavior was changed via the fluorine effect, as demonstrated for previously investigated TiAl alloys with an Al-content higher than 40 at.% based on a 2 /g and a 2 /b o /g phases. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. The reason for this is the change in the oxidation mechanism triggered by the small additions of fluorine in the subsurface zone of the investigated alloys. The results of isothermal oxidation tests at 800 C in air are presented and discussed in view of chemical composition and microstructure, along with the impact of the phase composition on the efficiency of the fluorine effect. From a microstructural perspective the fluorine effect leads to the formation of an even thinner oxide scale on the b-phase compared to the g-phase.
Materials at High Temperatures, 2005
Abstract Alloys based on TiAl intermetallics are potential candidates for high temperature applic... more Abstract Alloys based on TiAl intermetallics are potential candidates for high temperature applications in e.g. aero engines or automotive engines because of their low specific weight and good high temperature strength. To improve their oxidation resistance at temperatures up to 1000°C the halogen effect offers an innovative and cost-effective way. The addition of small amounts of halogens into the surface leads to the preferential formation of gaseous aluminium halides which are oxidised to aluminium oxide during their outward migration forming a dense, protective and slowly growing alumina scale on the surface. In this paper two methods were used to apply halogens to the surface, ion implantation (F and Cl) and a liquid phase process (F). Ion beam analysis with detection limits in the ppm-range was applied to quantify the needed amount of halogens to achieve the halogen effect. Thermocyclic oxidation experiments at 900°C were performed in laboratory air and wet air. Depth concentration profiles of fluorine were measured by PIGE within the first 1.4 μm without destruction of the sample before and after oxidation. Furthermore, the loss of fluorine during heating up and oxidation was measured characterising the stability of the effect. Simultaneous RBS-measurements of the O-, Al- and Ti-depth profiles prove the formation and growth of an almost pure alumina scale. Correlation with the fluorine profiles validates the proposed model for the halogen effect. Furthermore, metallographic methods, REM, EPMA, AES and the proton micro beam (PIXE) were applied to study cross-sections. A virtually pure alumina scale was found after F-treatment and oxidation up to 1500 hours at 900–1000°C in air. The fluorine depth profiles after ion implantation and liquid phase treatment, respectively, show similar levels for both methods before and after oxidation. The development of the fluorine interfacial concentration underneath the oxide scale as a function of oxidation time and temperature was recorded. The results are discussed in the light of the existing model considerations on the halogen effect and with regards to differences in the behaviour between F- and Cl-doping.
Materials Science Forum, 2012
Advanced Engineering Materials, Sep 9, 2013
JOM, 2010
Titanium aluminides suffer from non-protective mixed-oxide scale formation during high-temperatur... more Titanium aluminides suffer from non-protective mixed-oxide scale formation during high-temperature exposure in oxidizing environments, so that they cannot be used at temperatures above approximately 800° C for longer times without additional treatment. A fluorine treatment on γ-TiAl alloys leads to the formation of a pure protective alumina scale and allows their use at service temperatures above 800°C. This thermally grown
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Papers by Alexander G . Donchev