M. Schütze

Oxidation-Resistant Aluminide Coatings on γ-TiAl

The long-term application of TiAl alloys based on the γ-phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl3, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of γ-TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl3 coating and the γ-TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl2 phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.

Role of Growth Stresses on the Structure of Oxide Scales on Nickel at 800 and 900°C

The development and relief of intrinsic growth stresses in oxide scales on nickel of different purity have been investigated by combining the deflection test in monofacial oxidation (DTMO) with acoustic-emission analysis (AE). Parallel metallographic analysis gave information about the development of the physical- defect structure and all other structural features. The investigations were performed for 100 hr at 800 as well as 900°C in air. The assumption of elastic behavior led to the best correlation between models, literature data, and results of the present investigations. Microcracks are responsible for the relief of growth stress and inward oxygen penetration leads to the typical NiO duplex scale. The growth of the microcracks is initiated at large pore populations at the oxide–metal interface that most probably form due to outward cation-diffusion and vacancy condensation. The pore formation is increased by the presence of impurities. An equilibrium of microcracking and crack healing is finally reached, leading to a continuous growth of the inner and outer NiO layers of the duplex scale. The scale growth stresses are mainly compressive and can reach maximum values of −560 MPa at 900°C. An estimation of the fracture toughness of the oxide–metal interface assuming a wavy interface led to a KIc value of about 2 MNm-3/2 at 900°C

The halogen effect in the oxidation of intermetallic titanium aluminides

Abstract Stoichiometric TiAl and two quasi-technical TiAl alloys were investigated in the temperature range of 800–1000°C with regard to the halogen microalloying effect on oxidation resistance. The halogens Br, Cl, F and I were introduced into the material surface by ion implantation with different doses and energies before the thermogravimetric oxidation experiments were started. The results show that a very low “homeopathic” amount of these halogens at the oxide scale/metal interface activates a mechanism of selective Al-oxidation which changes oxidation behavior from fast mixed titania/alumina kinetics to slow pure alumina kinetics. A model is described which explains this change in kinetics supported by quantitative data from thermodynamic calculations. The halogen microalloying effect offers a high potential for the improvement of oxidation resistance of TiAl alloys for technical applications.

Mechanical properties of oxide scales on mild steel at 800 to 1000°C

In the hot-rolling process of steels the oxide scales play a key role with regard to surface quality of the sheet. Therefore, a quantitative knowledge of the mechanical properties of oxide scales at rolling temperature can provide a significant improvement of sheet quality. In the present paper the mechanical properties of the oxide scales formed on mild steel were investigated in 4-point bend tests at 800, 900, and 1000°C in dry air, humid air (7–19.5 vol.% H2O) and laboratory air at different deformation rates. It turns out that the environment has a significant influence on scale thickness and structure as well as on adhesion of the oxide scales. The mechanical measurements show plastic-creep deformation of the oxide scales. Water vapor did not have any significant effect on the creep properties of the oxide scales. In the measurements the secondary-creep-stress values of the oxide were determined as a function of the strain rate and plotted into an Ashby map for FeO. This plot can serve for an extrapolation of the data for even higher strain rates as relevant for the hot-rolling process.

Investigation of the Fluorine Microalloying Effect in the Oxidation of TiAl at 900°C in Air

High-temperature oxidation resistance of gamma titanium aluminides can be achieved by the formation of a continuous scale of slowly growing Al2O3. The formation of such a scale was favored by the addition of small amounts of fluorine. It is shown that fluorine can have a beneficial effect on oxidation resistance in a certain F-range which is quantified by thermodynamic calculations and by experimental investigations. It is assumed that the F-effect offers a significant potential for improvement of the oxidation resistance of technological TiAl alloys.

Chemical-mechanical failure of oxide scales on 9% Cr steels in air with H2O

Abstract The oxidation behaviour of 9% Cr steels P91 and Nf616 has been investigated at 650°C in dry air and in air with water vapour, where particular attention was given to breakaway failure. Additional emphasis was given to the quantitative characterisation of the kinetics of chromium depletion in the metal subsurface zone resulting from scale growth, CrO2H4 evaporation, and scale cracking and healing, with scale cracking being monitored by acoustic emission measurements. While in dry air the steels show protective oxidation behaviour up to 10000 h, breakaway oxidation may occur already after 100 h in humid environments, which was correlated with the stronger Cr-depletion and the development of intrinsic oxide scale growth stresses exceeding a critical value, in the case of water vapour containing air. In the paper the different parameters that are responsible for breakaway oxidation were identified and discussed with regard to the role of water vapour in the environment. As a conclusion it turns out that breakaway is not a consequence of Intrinsic Chemical Failure (InCF) but of a Mechanically Induced Chemical Failure (MICF).

Protection of γ-based TiAl against high temperature oxidation using ion implantation of chlorine

Abstract The effect of ion beam implantation and plasma immersion ion implantation of chlorine on the high temperature oxidation of titanium aluminides above 800°C in air was investigated. Thermogravimetric oxidation tests (TGA) were performed to examine the long term protection. Depth profiling with Auger electron spectroscopy (AES) was used to investigate Cl diffusion and oxide formation during the first stage of oxidation. A microscopic model of the ‘Cl-effect’ will be discussed. A systematic variation of the implantation energy and fluence shows that there is a narrow regime of Cl concentration for optimum protective effect. The time to form a protective Al 2 O 3 layer depends on the local Cl concentration. The oxidation rate after this incubation time is reduced by about two orders of magnitude compared to untreated Ti50Al and is nearly independent of the fluence. The implantation energy is not a sensitive parameter because the implanted chlorine profile changes very quickly during high temperature oxidation.

Nanoparticle based inorganic coatings for corrosion protection of magnesium alloys

Abstract Inorganic nanoparticle based coatings for magnesium alloys were developed and tested for their performance in corrosion protection. Nanoparticles are characterised by a high sintering activity. This allows to obtain inorganic coatings by a sintering process at rather low temperatures which is suitable for magnesium alloys. Coating sols are based on silicon dioxide and sintering additives such as boron or sodium oxide. One technique is based on aqueous, commercial nanosols which can be applied by dip or brush coating to form layers on AZ31 and AZ91. Another technique is based on the electrophoretic deposition of silicon dioxide nanoparticles which also contain boron and phosphorus oxide. Crack free layers with a thickness of up to several micrometres could be obtained by a two step process including a bimodal particle distribution and polydiethoxysiloxane as adhesion promoter. The composition and surface structure of these novel coatings are characterised by modern analytical methods including SEM and atomic force microscopy and their applicability as protective coatings is investigated by using electrochemical impedance spectroscopy.

Growth Stresses in the Oxide Scales on TiAl Alloys at 800 and 900°C

In the oxidation of TiAl alloys, the role of scale-growth stresses formed during oxidation has, thus far, been unknown. In the present paper the oxide-growth stresses were investigated by the deflection-test method in monofacial oxidation (DTMO) accompanied by acoustic-emission measurements. On unmodified surfaces the growth stresses are compressive and reach levels of around −100 MPa. At the same time, significant acoustic emission occurs indicating that even under isothermal conditions, stresses are relieved by a scale-cracking mechanism. For oxide scales on TiAl surfaces, which had been ion implanted with chlorine before oxidation, a very thin protective alumina layer is formed which, however, develops growth stresses in the range of several GPa, accompanied by intensive acoustic emission. In all stress–time curves, a dynamic situation is observed. This consists of phases of stress relief by scale microcracking and phases of stresses increase due to crack healing and further oxide growth. As a result, the level of stress as a function of oxidation time, is characterized by an oscillating course.

Test methods and data on the mechanical properties of protective oxide scales

Abstract The present paper describes the key mechanical properties of oxide scales with regard to model equations and investigation methods. Modified test methods are introduced for the determination of values describing scale failure, growth stresses in oxides and creep of the scales. Furthermore, a summary of scale fracture data from the literature is given. As a conclusion it is discussed that failure strains of oxide scales can be estimated from the model equations and using basic mechanical property data like fracture toughness and geometrical scale parameters as physical defect size, interface roughness, scale thickness etc. without complicated mechanical tests. In the end a universal mechanical properties diagram is introduced, which describes fracture and creep behavior of the oxide scales.