Alexander Donchev

The halogen effect for improving the oxidation resistance of TiAl-alloys

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.

The effect of water vapor in increasing growth stresses in the oxide scale on martensitic steam plant alloys

Abstract Investigation of the oxidation behavior of 9% Cr steels at 650°C in dry air and air + 10% H2O have shown that oxide scale growth stresses may play a significant role in breakaway. For this reason, preliminary studies on the development and characteristics of growth stresses in oxides on these materials have begun. These studies include in situ acoustic emission (AE) for monitoring scale cracking, deflection testing in monofacial oxidation (DTMO) and in situ X–ray measurements. The measurements were complemented by detailed post-experimental metallographic investigations. From the DTMO measurements the stresses in the oxide on the specimens tested in humid environment are increased by about a factor of 5 compared to dry air at the beginning of oxidation for P91. In a humid environment the stresses decrease with oxidation time while in dry air they remain almost constant. Significant acoustic emission (AE) occurs in humid air for oxidation coupons while virtually no AE is observed for thin foils in a humid environment and for coupons in dry air. These first results seem to be in good agreement with weight gain data characterizing the breakaway behavior in these environments, indicating that, indeed, oxide growth stresses play a key role for oxidation resistance and, thus, component lifetimes of such steels. The first X–ray results indicate that for E911 the scale structure and composition changes completely between the two environments. Furthermore, in humid air, a breakaway effect is observed with a change from protective spinel type oxide to locally non-protective Fe–rich oxide.

Comparison of Different Fluorine Treatments for the Protection of TiAl-Alloys Against High Temperature Oxidation

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.

Cyclic Oxidation Behaviour of Halogen Implanted Ti-46.5Al-4(Cr, Nb, Ta, B)

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.

Optimization of the Fluorine Effect for Improving the Oxidation Resistance of Tial-Alloys

The oxidation resistance of TiAl-alloys can be improved drastically by treating the surface of the components with small amounts of fluorine. The oxidation mechanism is changed. Hence, 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 fluorine treatment. The different methods only influence the surface region of the components so that the bulk properties are not affected. Recent results achieved with F-containing inorganic compounds showed that the fluorine effect can be improved even further. TiAl-specimens were treated only with fluorine and with F-containing compounds 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 results are discussed in terms of a later use of the fluorine effect for technical applications.

Oxidation protection of several intermetallic TixAly alloys by fluorine

Abstract The oxidation resistance of γ-TiAl alloys can be dramatically improved by the fluorine effect. The Al content is a critical factor. It has to be above at least 40 at.% to get the fluorine effect to operate. Alloys with an Al content near this value or below have to be enriched with Al in advance. For this purpose, aluminisation is one possibility. The Al content within the diffusion layers can be determined by variation of the pack parameters, i.e. Al activity, time and temperature. These Al-rich diffusion layers form a protective alumina layer during high-temperature exposure in oxidising environments, but they have to be treated subsequently with fluorine to stabilise their protective effect. Results of high-temperature exposure tests of several untreated and treated alloys will be presented in this paper. Post experimental metallographic investigations reveal the formed oxide scales so that the effects can be rated. Finally, the results will be discussed in view of the known fluorine effect for technical TiAl alloys.

Economic Surface Treatment of Ti-Alloys to Improve their Resistance against Environmental High Temperature Attack

High temperature Ti-alloys are usually sophisticated and hence expensive. To allow the use of cheaper alloys at elevated temperatures an economic and easy to apply procedure was developed to improve their high temperature capability. The treatment consists of a combination of Al-enrichment in a shallow surface region plus additional fluorination. The Al-enrichment at elevated temperatures leads to the formation of intermetallic TiAl-phases. These phases improve the oxidation resistance of Ti-alloys but not to a sufficient extent. An additional fluorine treatment of the Al-enriched surface leads to the formation of a protective alumina scale due to the fluorine effect. In this paper results from high temperature exposure tests performed on different Ti-alloys without any treatment and with a combination of Al-treatment plus fluorination are presented. The results are discussed in the view of the use of the optimized Ti-components for several high temperature applications.

Effective Fluorine Treatment for Improved High Temperature Oxidation Behavior of Novel Cu, Mo and Si Containing TiAl-Alloys

Intermetallic light weight TiAl-alloys are expected to replace the heavy Ni-based super alloys in several high temperature applications. However until now they cannot be used at temperatures above 700°C for longer times due to their insufficient oxidation resistance. The high temperature oxidation behavior can be improved drastically for the use at temperatures up to at least 1050°C by small amounts of fluorine in the surface region of TiAl-components. A thin protective alumina layer is formed after an optimized fluorine treatment during exposure in oxidizing high temperature environments. Results of isothermal and thermocyclic high temperature oxidation tests of untreated and halogen treated TiAl-samples of new types of TiAl-alloys containing Mo, Cu and Si will be presented in this paper. These results will be compared and discussed considering the beneficial effect of fluorine for a later use as e.g. turbine blades in jet engines. Key words: Titanium aluminides, high temperature oxidation, halogen effect,