S. Becker

Bio: S. Becker is an academic researcher from DECHEMA. The author has contributed to research in topics: Oxide & Spallation. The author has an hindex of 3, co-authored 3 publications receiving 444 citations.

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

Abstract: 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.

Cyclic-oxidation behavior of TiAl and of TiAl alloys

Abstract: The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO2-Al2O3 and an outward-growing coarse-grain TiO2 layer or TiO2+Al2O3 mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al2O3 layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al2O3 layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.

Fundamentals of TiAl oxidation: a critical review

Abstract: The paper critically reviews the state of the art on the oxidation of TiAl. At the beginning differences compared to the oxidation of conventional Fe, Ni and Co alloys are discussed. The Knowledge of the TiAlO phase diagram as a basis for an understanding of the processes occurring during oxidation is very incomplete. Three different scale structures can be distinguished as functions of time or scale thickness. In the sub-surface zone two phases with high oxygen solubilities can be identified after a certain time. These phases do not follow from the existing phase diagram. Nitrogen can have two different effects depending on the time it is added to the oxidation environment. The micro-structure also affects the oxidation behaviour significantly, so this effect can lead to a misinterpretation of the influence of other parameters such as alloying elements. The influence of alloying elements is not yet understood. Mechanical loading leads to scale cracking, however, the system has a significant scale repair capacity, though about twothirds of the scale grows by inwards transport. Grundlagen der TiAl-Oxidation – Eine kritische Ubersicht Die Arbeit gibt eine kritische Betrachtung der Kenntnisse uber die Oxidation von TiAl. Einleitend wird auf Unterschiede im Vergleich zur Oxidation konventioneller Fe. Ni und Co-Legierungen hingewiesen. Die derzeitigen Kenntnisse uber das Phasendiagramm TiAlO als Basis fur das Verstandnis der bei der Oxidation ablaufenden Prozesse sind sehr unvollstandig. Im Aufbau der Oxidschicht konnen drei Grenzfalle unterschieden werden, die als Funktion der Zit nacheinander durchlaufen werden. Stickstoff kann offensichtlich zwei Effekte auslosen, abhangig vom Zeitpunkt der Zugabe zum Reaktionsgas. Die Mikrostruktur kann einen starken Einflus haben und andere Einflusse wie die von Legierungselementen verdecken. Der Einflus von Legierungselementen wird derzeit nicht verstanden. Damit sind auch keine sinnvollen Vorhersagen moglich. Mechanische Belastungen verursachen ein Aufreisen der Oxidschicht, die jedoch eine beachtliche Ausheilfahigkeit fur Oxidschichtrisse hat.

High-Temperature Oxidation Behavior of Ti2AlC in Air

Abstract: The isothermal oxidation behavior of bulk Ti2AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS The results demonstrated that Ti2AlC had excellent oxidation resistance The oxidation of Ti2AlC obeyed a cubic law with cubic rate constants, kc, increasing from 238×10-12 to 213×10-10 kg3/m6/sec as the temperature increased from 1000 to 1300°C As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner α-Al2O3 layer and a discontinuous outer TiO2 (rutile) layer formed on the Ti2AlC substrate A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys In addition, the scales had good adhesion to the Ti2AlC substrate during thermal cycling