Michael Schütze

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO2 (rutile), while the inward-growing part is composed of a mixture of TiO2 and α-Al2O3. A barrier layer of Al2O3 on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al2O3 barrier dissolved and re-precipitated in the outer TiO2 layer. This “shift” leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al2O3 layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al2O3 barrier layer formed; large Al2O3 particles were part of the outward-growing layer. A brittle α2-Ti3Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti2AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al2O23 formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.

Mechanical properties of oxide scales

At high temperatures most alloys rely on the protective effect of oxide scales formed by the reaction between oxygen from the environment and components of the alloy. The protective effect of these scales may, however, be impaired if stresses lead to cracking or spalling. Therefore, the mechanical properties of the scales play a vital part in protection, in particular under service conditions of the materials where the presence of stresses cannot be excluded. The paper provides a survey of the existing models that describe the mechanical properties of scales (the emphasis being on mechanical scale failure), the measuring techniques and the data. It will be demonstrated that, in the case of tensile conditions, the situation is relatively well understood and that a fair amount of data exists. With regard to compressive conditions scale failure consists of a combination of different failure steps, which can each be described by a quantitative model, but there is still clearly a need for experimental verification. While the measuring techniques for the properties under tensile stresses are fairly well developed, those for the behavior under compressive stresses, in particular those characterizing scale adhesion, are still under discussion and relatively few experimentally determined data are available in this case. Nevertheless, existing knowledge can be put to good use in assessing scale behavior under the effect of stresses, based on the models and data given in the paper.

Deformation and cracking behavior of protective oxide scales on heat-resistant steels under tensile strain

The deformation and cracking behavior of oxide scales formed in air on four heat-resistant steels and on nickel 99.6 have been studied in constant-extension-rate tests at 800°C. The strain rates in the experiments ranged between 10−6 and 10−9 s−1. Acoustic emission (AE) was used as an instrument for detecting the beginning of scale cracking. Additionally, metallographic, SEM, and micro-probe investigations were performed which supported the results from the AE measurements. The strain-to-cracking of the scales did not exceed 0.5% except when lateral growth effects in the oxide scales occurred, leading to critical strains of up to nearly 2.5%. Also the crack distribution in the scales was measured. The deformation and cracking behavior of the scales investigated could be explained by model like considerations.

TEM investigations of the early stages of TiAl oxidation

The early stages of TiAl oxidation at 900°C and 1000°C in air have been investigated by transmission electron microscopy (TEM). The investigations revealed that at the beginning of oxidation, i.e., after 4 min, TiO2 and Al2O3 grow in a preferential orientation on the γ-TiAl substrate. After 4 h of oxidation an oxide scale structure can already be found similar to that known from long-term oxidation. In addition, besides α-Al2O3, the formation of a second aluminum oxide phase and of titanium nitrides is observed. The processes at the metal-oxide interface of oxidation in the early stages, consisting of a repeated cycle of Al2O3 formation, Al2O3 dissolution, outward migration of Al through the scale, and reprecipitation of Al2O3 in the outer scale, are described by a model. The four stages observed in the kinetics of TiAl oxidation are explained on the basis of the results obtained and the structure of the oxide scale.

The Oxidation Behavior of Several Ti-Al Alloys at 900°C in Air

Ti-23Al, Ti-50Al and Ti-50Al-2Nb (at.%) wereoxidized in air at 900°C for times up to 1130 hr.The resulting oxide scale structures were analyzed ingreat detail by metallographic and microprobe investigations and theAl2O3 structures in the complexoxide scales were correlated with the course of thethermogravimetric curves. It appears that in order toachieve long-term protective behavior of the scales, it is necessary to stimulate theformation of a thin Al2O3 barrierat the scale-metal interface and not at a position inthe outer part of the scale. The Nb effect seems to bemostly due to this stimulation of anAl2O3 layer at theinterface.

Degradation of SOFC Anodes upon Redox Cycling: A Comparison Between Ni/YSZ and Ni/CGO

In this experimental study, electrolyte-supported solid oxide fuel cells (SOFCs) with two different anodes were investigated. Specifically, the stability of cells with a nickel/8 mol % yttria-stabilized zirconia (Ni/8YSZ) cermet anodes was compared to those based on nickel/40 mol % gadolinia-doped ceria (Ni/CG40). For this, the cells were characterized by impedance spectroscopy as well as by four-point electrical conductivity measurements. A high frequency process was observed in the Ni/8YSZ anode, which was not detected in the Ni/CG40 anode. After eight redox cycles at 950°C, the cell with the Ni/8YSZ anode showed an increase in the polarization resistance mainly in the high frequency domain. However, the cell with the Ni/CG40 anode showed an increase in both ohmic and polarization resistances, the latter mainly in the low frequency domain. Compared with Ni/CG40, the degradation in Ni/8YSZ upon redox cycling was higher at 850°C but lower at 950°C. For the Ni/8YSZ anode, a significant degradation was seen in the first 3 h after a redox cycle. The increase in the ohmic resistance of the Ni/CG40-based cell is believed to correlate with a decrease in the electrical conductivity of the anode. The latter showed a strong decrease upon a subsequent redox cycling at 950°C. For the Ni/CG40 anode, the degradation in both the conductivity and electrochemical performance significantly improved by decreasing the operation temperature from 950 to 850°C.

Microalloying effects in the oxidation of TiAl materials

Abstract The influence of microalloying on the oxidation behaviour of γ-TiAl based alloys was studied. The microalloying elements were added by ion implantation. Oxidation tests at 1173 K in air showed that the addition of chlorine into TiAl improves the oxidation resistance resulting in a decrease of the oxidation rate by about 2 orders of magnitude compared to unalloyed TiAl. Microstructural investigations revealed that the formation of a protective alumina layer on top of Cl-implanted TiAl is the cause for the decrease in the oxidation rate. AES measurements in the initial stage of oxidation showed that chlorine is located under the alumina layer in the metal phase. Thermodynamic calculations, investigation of the temperature dependence of the chlorine effect and of the oxidation kinetics of preoxidized Cl-implanted samples support the model of a selective Al-transport via AlCl. Furthermore, the influence of small additions (in the ppm range) of P, B, C and Br on the oxidation kinetics of γ-TiAl-based alloys has been investigated. P, B and C implanted TiAl showed a different oxidation behaviour and oxide scale microstructure compared to Cl and Br microalloyed TiAl. Especially the P implanted sample revealed an extensive nitride formation connected with a breakaway oxidation after 100 h.

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. I. Scales formed in oxygen and air

The acoustic-emission (AE) technique has been applied to study scale-damage processes during thermal cycling of a tube, preferentially between 600 and 300°C in air, oxygen, and air + 0.5% SO2. The AE measurements were accompanied by optical and electron-optical investigations on tube rings exposed to the same cycling conditions. During the first period of cycling, a scale rich in hematite is formed. It suffers compressive stresses during cooling. The result is a buckled multilayered scale with separated lamellae. The scaling rate is lower than under isothermal conditions. AE signals start after 175°C cooling. After longer exposure times, the scale contains an increasing amount of magnetite and becomes more compact. The scaling rate increases and is comparable to that under isothermal conditions. AE signals are already observed after 50°C cooling and are correlated with crack formation in the magnetite caused by tensile stresses there. The addition of SO2 to air enhances the crack-healing process due to higher Fe diffusion in FeS. The scale is more compact.

Improvement of the oxidation resistance of TiAl alloys by using the chlorine effect

Abstract Most TiAl alloys lack from their insufficient oxidation resistance in air at temperatures above 800°C. Recently it has been found that small amounts of Cl in the alloy may significantly improve the oxidation resistance. However, the mechanisms responsible for this effect were not yet understood. In the paper oxidation experiments with TiAl alloys are reported where Cl was alloyed to the material by the P/M route or applied to the material surface either by ion implantation or by PVD. The results of the investigations revealed that the Cl-effect is due to the formation of volatile metal chloride species. It is shown that chlorine can have a beneficial effect on oxidation resistance in a certain Cl-range which is quantified by thermodynamic calculations. It is assumed that the Cl-effect offers a significant potential for improvement of the oxidation resistance of technical TiAl alloys.

Mechanical aspects of the rare-earth effect

In order to gain a better understanding of the reactive-element effect (REE), the improvement of the oxidation behavior of chromia- or alumina-scale-forming alloys by the addition of small amounts of elements with higher affinity to oxygen than the scale-forming element, it is necessary to clearly distinguish between isothermal oxidation and the behavior of the metal/oxide composite system during cooling. An approach is presented based on fracture-mechanical considerations which correlates critical differential strain between scale and substrate, fracture toughness of the metal/scale interface, scale thickness, defect size and interfacial amplitude. This approach allows a quantitative assessment of the REE for scale adhesion, and although the necessary experimental data are yet lacking, it describes the reported REE in a qualitatively correct manner.