Combined Al- plus F-treatment of Ti-alloys for improved behaviour at elevated temperatures (original) (raw)
Related papers
Suppressing the oxygen ingress into Ti-alloys by a one-step Al- plus F-treatment
MATEC Web of Conferences, 2020
Oxygen ingress into technical Ti-alloys during high temperature exposure in oxygen containing environments leads to an enriched surface zone due to the high oxygen solubility in titanium. This affected zone is known as alpha case and is embrittled compared to the substrate material. Therefore, the operating temperature of these alloys is limited. An enrichment of Al in the surface zone of Ti-alloys leads to an improvement by the formation of intermetallic TixAly-phases with decreased oxygen solubility. This one-step Al-enrichment plus a F-treatment is proposed for the formation of a continuous stable alumina scale which prevents oxygen inward diffusion. In this paper the results of high temperature exposure tests in air of technical Ti-alloys are presented without any treatment and treated with Al-enrichment including fluorination. Post experimental investigations such as SEM reveal the formation of thin alumina layers on treated samples. Hardness measurements confirm that no embrit...
Intermetallics, 2021
Current limitations to a wider use of intermetallic TiAl alloys in aircraft and automotive engines arise from an insufficient oxidation resistance at temperatures above approximately 800°C. In this paper, the high temperature oxidation behavior of three engineering -TiAl-based alloys at 900°C in air is reported. The performance of the TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B), the 4822 alloy (Ti-48Al-2Cr-2Nb), and the Nb-free IRIS alloy (Ti-48Al-2W-0.08B) is compared (all chemical compositions are given in at.%). During testing in air non-protective mixed oxide scales developed on all untreated samples, but with different compositions and thicknesses. These different oxide layers are characterized and their formation mechanisms are discussed. The presence of W in the IRIS alloy leads to a better oxidation behavior compared to untreated TNM and 4822. This behavior was changed in the direction of a protective alumina layer formation via the so-called "fluorine effect". The above-mentioned alloys were treated with fluorine via a liquid phase process by evenly spraying a fluorine containing polymer on all faces of the specimens. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated specimens. Due to the fluorination all treated test coupons exhibited slow oxidation kinetics. The results of isothermal as well as thermocyclic exposure tests are presented and discussed in the view of the chemical composition and processing conditioned microstructure of the three investigated γ-TiAl-based alloys. Keywords A. intermetallics (aluminides, silicides) B. oxidation C. coatings F. microscopy, various G. aero-engine components; automotive uses, including engines (and other transportation uses) Highlights Comparison of the engineering γ-TiAl-based alloys TNM, 4822 and IRIS with regard to their oxidation resistance Positive effect of W in the IRIS alloy is leading to reduced spallation Improvement of the oxidation resistance for all tested technical γ-TiAl-based alloys via the fluorine effect No effect of the different microstructures and chemical compositions on the fluorine effect efficiency Possible application at high temperatures above 800°C after a fluorine treatment
The Importance of the Fluorine Effect on the Oxidation of Intermetallic Titanium Aluminides
Intermetallic Compounds - Formation and Applications, 2018
Due to the low Al activity within technical titanium aluminides and the similar thermodynamic stabilities of Al-and Ti-oxide these alloys always form a mixed oxide scale at elevated temperatures consisting of TiO 2 ,A l 2 O 3 and also nitrides if the exposure takes place in air. This mixed scale does not provide any oxidation protection especially under thermocyclic load or in water vapor containing environments. Thus accelerated oxidation occurs. Alloying of additional elements such as Nb improves the oxidation behavior if the additions stay within a certain concentration range but such additions cannot suppress non-protective mixed scale formation. Coatings are another way to protect these materials but several obstacles and new degradation mechanisms exist such as delamination e.g. due to CTE mismatch or development of brittle intermetallic phases due to interdiffusion. Therefore, other suitable protective measures have to be undertaken to make sure a protective oxide scale will develop. The so called halogen effect is a very promising way to change the oxidation mechanism from mixed scale formation to alumina formation. After optimized halogen treatment the alumina layer is very protective up to several thousand hours even under thermocyclic load and in atmospheres containing water vapor or SO 2 .
Oxidation protection behaviour of titanium aluminide coatings developed by TIG technique
Advances in Materials and Processing Technologies, 2015
Titanium alloys are attractive in aerospace applications for low density and good mechanical properties but they have poor in oxidation and wear resistance. A coating layer of titanium aluminide can mitigate these problems to some extent and make the alloys suitable for hot structure applications. This paper discusses the formation of titanium aluminide coatings on commercial purity titanium (CPTi) surfaces by melting a pre-placed aluminium and titanium powder mixture, using a tungsten inert gas (TIG) welding torch. Depending on powder composition and energy input, the resolidified melt layer produced a single phase 2-Ti 3 Al or a dual phase 2 and -TiAl microstructures of lamellar or columnar dendritic types. The microhardness varied from 400 to 600 Hv based on the distribution within the microstructure. Testing the resistance to oxidation, by heating and cooling through nine cycles at 750 0 C for a total of 100 h in air, gave a weight gain of 1.00 mg cm-2 for the 2-Ti 3 Al coating compared to 2.60 mgcm-2 for the CPTi specimen. The dual phase coating showed much improved oxidation resistance with a weight gain of 0.35 mg cm-2 after exposure at similar conditions.
ChemInform Abstract: Influence of Aluminum on the Oxidation of Titanium Between 550 and 750 °C
ChemInform, 1990
ChemInform Abstract The oxidation kinetics of binary Ti-Al alloys containing 1.65, 3, 5, and 10 wt.% Al are studied in the temp. range 500-750 rc C. The results obtained for oxidation in air confirm the beneficial role of Al on the oxidation resistance. Al modifies the internal structure of the oxide layer, reduces the amount of oxygen dissolved in the Ti substrate, and increases the adhesion of the oxide layer at higher (≥ 5 wt.%) Al contents.
Improvement of the oxidation behaviour of TiAl-alloys by treatment with halogens
Intermetallics, 2006
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.
Influence of aluminium on the oxidation of titanium between 550 and 750 °C
Journal of The Less Common Metals, 1990
In order to study the influence of aluminium on the oxidation resistance of titanium at high temperature, a range of binary alloys containing 1.65, 3, 5 and 10 wt.% of aluminium was prepared. Their oxidation kinetics were studied at temperatures between 500 and 750 "C using either continuous the~o~avimet~ or daily weighing for periods of up to several thousand hours. The results obtained for oxidation in air confirm the beneficial role of aluminium which has been observed previously for oxidation in oxygen. With regard to morphology and structure, aluminium modifies the internal structure of the oxide layers and their growth laws. A general dispersion of aluminium in rutile is observed, although a concentration of this phase is noted near the external interface; there is also a reduction in the amount of oxygen dissolved in the metal substrate which is related to the aluminium content in the alloy. Moreover, the presence of aluminium also modifies the adhesion of the oxide layers to the substrate.
Coatings, 2018
One of the major barriers limiting the suitability of TiAl intermetallic alloys for use in the demanding aircraft and automotive industries is their susceptibility to degradation as a result of oxidation at temperatures exceeding 760 • C. Paper presents the characteristics of resistance to cyclic oxidation at 950 • C for Ti-45Al-8Nb-0.5(B, C) alloy with and without protective coating obtained as a result of aluminizing using out of pack method. The characteristics of surface condition were determined by scanning electron microscope with EDS system, transmission electron microscope, and X-ray diffractometer. The favorable behavior of the Ti-45Al-8Nb-0.5(B, C) alloy with a protective coating under cyclic oxidation conditions is a result of a higher content of Al 2 O 3 in the microstructure of the scale and the presence of Al and Nb-rich phases at the substrate interface, which probably constitue a barrier for oxidation process. The high temperature oxidation test revealed that aluminide coating was responsible for a remarkable improvement in the oxidation resistance.
Enhancing the High Temperature Capability of Ti-Alloys
Steel Research International, 2012
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.