Oxidation Behavior Of TiAl Alloys Research Articles

High temperature oxidation behavior of TiAl alloy with electrodeposited SiOC coating

SiOC coating was electrodeposited on TiAl alloy to improve the high temperature oxidation resistance. The mass gain of the compact SiOC coating is 0.06 ± 0.026 mg cm-2, which is one order of magnitude lower than that of TiAl alloy. The SiOC coating exhibits good thermal stability and partial amorphous SiOC transforms into crystal SiO2 or SiC. After oxidation, no obvious pore can be found in the coating. Moreover, an Al2O3 layer is generated due to the enrichment of Al and the low oxygen partial pressure at the coating/substrate interface, therefore improving the barrier property of the oxide scale.

Investigation on the initial oxidation behavior of TiAl alloy

In this paper, the initial oxidation behaviors of α2, γ and β phase in the newly developed TNM alloy (Ti-44Al-4.0Nb-1.5Mo-0.1B-0.01Y) were investigated. The oxidation tests were performed at 800 °C for 3 min, 5 min, 20 min, 1 h, 2 h and 4 h respectively. The alloy has a good oxidation resistance below 800 °C. The antioxidant capacity of the three phases decreases as follow order: γ > α2 > β. Within the lamellar colony, the values of atom ratio (Ti/Al) in α2 lath and γ lath remain within the range of 1.11-1.16, indicating that atom diffusion occurs at the initial stage of oxidation. The microstructure observations also confirmed the segregation of Y element at grain boundaries of primary coarse α grains and segregation of Nb element in massive γ phases. The peeling off of the oxide from β phase and oxygen diffusion at grain boundaries are detrimental to the oxidation resistance of the alloy.

Oxidation behavior of TiAl alloy for various times

PurposeThe purpose of this paper is to investigate the oxidation behavior of Ti‐48Al‐8Cr‐2Ag (at.%) at 900°C and 1000°C for various different times.Design/methodology/approachLaboratory tests were performed to determine growth process of the oxide scale at 900°C and 1000°C for various different times with SEM/EDX, XRD and TEM.FindingsMerely Al2O3 occurred on the Laves phase at the initial stage at 900°C, while a mixture of Al2O3+TiO2 formed at the initial stage at 1000°C. Oxidation rate of the alloy at 900°C after long‐term oxidation was higher than that at 1000°C because a dense Al2O3 scale formed on the surface at 1000°C.Originality/valueThe paper shows that the oxidation behavior of TiAl alloy at initial stage is the basis of the revealing mechanism of oxidation. It is necessary to further investigate the oxidation of Ti‐Al‐Cr‐Ag alloy in more detail to clearly understand its oxidation process and growth process of the oxide scale.

Oxidation behavior of TiAl alloy treated by plasma surface chromizing process

TiAl–Cr alloy was prepared on surface of TiAl alloy by plasma chromizing process. The oxidation kinetics of the TiAl–Cr surface alloy and the bulk TiAl were examined. The morphology and constituent of the oxide scales were observed. The results indicated that the oxidation resistance of TiAl was improved significantly after the alloying treatment. The scale of TiAl–Cr surface alloy was mainly consisted of Cr2O3 initially, then combination of Cr2O3, Al2O3 and TiO2, and eventually the mixture of Al2O3 and TiO2. The oxide scale was dense and integrated throughout the oxidation process. The improvement is owing to the enhancing of scale adhesion and the preferential oxidation of aluminum brought by the alloying effects of chromium.

Effect of Nb on oxidation behavior of high Nb containing TiAl alloys

The isothermal oxidation behavior of Ti-45Al-8Nb and Ti-52Al-8Nb alloys at 900 °C in air was investigated. The early oxidation behaviors were studied by using XPS and AES. And the microstructure and the composition of the oxidation scale were studied by using XRD and SEM. The results show that the oxidation behavior of TiAl alloy is significantly improved by Nb addition. Nb substitutes for Ti in TiO 2 as a cation with valence 5, and thus to suppress TiO 2 growth. The (Ti,Nb)O 2-rich layer is a dense and chemically uniform which is more protective than the TiO 2 layer. Nb addition also lowers the critical Al content to form an external alumina. Nb 2Al phase is formed in the metallic matrix at the oxide–metal interface on the high Nb containing TiAl alloys.

Oxidation of TiAl Alloys Coated by Fluorine Resin in SO<sub>2</sub> Containing Atmosphere

There are several ways to improve the oxidation resistance of TiAl alloys. One of them is alloying with elements such as Nb, Ta, W, Si, Ag, Zr or Hf. However, bulk alloying influences the mechanical properties. Surface treatment of TiAl alloys by the small amounts of halogens leads to the formation of the protective alumina scale (“halogen effect”). The halogens can be applied by ion techniques (ion implantation, plasma immersion implantation) but also by spraying or dipping with halogen-containing inorganic and organic compounds. Deposition of the fluorine-containing resin on the surface of TiAl alloys is the easiest way to achieve the best results. SO2 impurity in air significantly influences oxidation behavior of TiAl alloys. In this work the results of the oxidation of a Ti-48Al-2Cr alloy coated with a fluorine-containing resin in the synthetic air and air containing 1% SO2 were presented. The oxidation runs were carried out in the temperature range 800-1000°C.

Effect of Coatings on the Creep and Oxidation Behavior of TiAl Alloys at 1173K in Air

The effects of coatings on the creep and oxidation behavior of Ti-50Al alloy were investigated at 1173K in air at a constant loading of 30MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixture layer, and a Cr diffusion zone. Creep tests were also carried out with sole Cr or Al coated TiAl and also of uncoated TiAl. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed several exfoliations. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.

The improvement of the oxidation resistance of TiAl alloys by fluorine plasma-based ion implantation

Plasma-based ion implantation (PBII) with fluorine ions has been applied for improvement of the oxidation behavior of TiAl alloys. The plasma was produced from Ar-5 vol.%F 2 mixed gas with a RF power supplier. 15–30 keV fluorine ions were implanted into Ti50Al (at.%) alloys for 1 h. The oxidation behavior of TiAl alloys was then investigated in air at 850 °C. XPS measurements revealed that prior to oxidation, fluorine ions were located in a very shallow surface region with a peak concentration, and reacted mainly with aluminum to form aluminum fluoride. TiAl treated by fluorine PBII showed a very low oxidation rate, and a protective continuous Al 2O 3 scale formed on the surface. The beneficial effect of fluorine on oxidation resistance of TiAl was due to that the preferentially formed volatile aluminum fluoride attributed to formation of the continuous Al 2O 3 scale. Fluorine PBII treatment proves to be a promising surface modification method for improving the oxidation resistance of TiAl alloys.

Effect of SiC, Si3N4 and TiB2 additions on the oxidation resistance of TiAl alloys

The oxidation behavior of TiAl alloys containing dispersed particles of (5, 10, 15 wt.%) SiC, (3,5 wt.%) Si3N4 or (3, 5, 10 wt.%) TiB2 was studied between 800 and 1200°C in atmospheric air. The TiAl−(SiC, Si3N4) alloys oxidized to TiO2, Al2O3, and SiO2. The TiAl−TiB2 alloys oxidized to TiO2, Al2O3, and B2O3 which evaporated during oxidation. Improvement in oxidation resistance accompanied by thin, dense scale formation due to the addition of dispersoids originated primarily from the enhanced alumina-forming tendency, improved scale adhesion by oxide grain refinement owing to the beneficial effect of dispersoids, and the incorporation of SiO2 within the oxide scale in the case of TiAl−(SiC, Si3N4) alloys.

Effect of Ti–Al–Cr coatings on the high temperature oxidation behavior of TiAl alloys

The high temperature oxidation behavior of TiAl alloys coated with Ti–Al–Cr alloys using magnetron sputtering method was studied. The results showed that Ti–Al–Cr coatings improved the oxidation resistance of TiAl alloys. The weight gains at 950°C for Ti–Al–Cr coatings are more than those at 1000°C. Oxide formed on the surface of Ti–50Al–15Cr coating after oxidation consisted of Al 2O 3, whereas oxide formed on the surface of Ti–50Al–10Cr coating after oxidation is composed of a mixture of a large portion of Al 2O 3 and a small portion of TiO 2. The effect of Ti–Al–Cr coatings on the improvement of oxidation resistance of TiAl alloys results from thermodynamic and kinetics factors contributing to Cr effect.

Ｔｉ‐Ａｌ系化合物焼結体の耐酸化性

The objective of this work is mainly to elucidate the potential of TiAl type P/M alloys for higher temperature structural applications. TiAl alloys containing B-0.05% or Si-3% were previously reported to have a good combination of mechanical properties at high temperature. Therefore the high temperature oxidation behavior of TiAl alloys containing Si and B was investigated in this study.The results were summarized as follows:(1) Oxidation of TiAl type alloys substantially begins at 900°C or less, fairly sensitive to its alloying elements. Its oxidation resistance was increased by Si-addition up to 4.6% and decreased by B-addition of a few hundred ppm.(2) Si-addition to TiAl alloy leads to less weight gain in the hot air of the range 1000-1100°C, mainly due to the protective alumina-like oxide formed on the surface layer.(3) Siliconizing using the pack-cementation process improved the oxidation resistance of TiAl alloy by more than five times, probably relevant to the protective effect of stable silicides.