Shigeji Taniguchi

Influence of additional elements on the oxidation behaviour of TiAl

The influence of alloying additions on the high-temperature oxidation behaviour of TiAl and materials based on it is critically reviewed in the light of currently proposed mechanisms, i.e. the valence-control rule, Wagners scaling model and the formation of a barrier layer in the scale. In addition, our newly obtained results are presented, where additions of 0·2 Zr and 0·2 Hf and Nb-ion implantation are dealt with. All the treatments resulted in the formation of highly protective alumina scales which were sound even under thermal cycling conditions. Modification of the initially formed alumina scale is proposed as a fourth mechanism on the basis of our results. Fundamental knowledge on the oxidation of TiAl is also provided for a better understanding.

Characteristics of scale/substrate interface area of Si-containing low-carbon steels at high temperatures

Low-carbon steels containing Si of up to 1.5 mass% were oxidised in air at 1416 and 1476 K, i.e. below and above the eutectic temperature of FeO and Fe2SiO4. An acoustic emission technique was used to assess the temperature at which a major scale spallation takes place during cooling in static air. This allowed evaluation of the thermal stress. The stress for the scale spallation during cooling from 1416 K increases as the Si content increases, while it is almost constant for cooling from 1476 K. This difference is attributable to the formation of liquid phase in the latter case. The conventional metallographic examinations revealed that scales formed at 1416 K consist mainly of two FeO layers except for 0.01%Si steel; the inner layer is porous FeO and the outer layer is mainly dense FeO. The porous layer consists of a mixture of FeO and Fe2SiO4 grains. A row of large micropores separates the dense FeO layer. These micropores become larger as the Si content increases. At 1476 K the eutectic liquid phase is formed between the scale and the substrate, and penetrates very deep along grain boundaries of the FeO and slightly to the substrate.

Influence of siliconizing on the oxidation behavior of a γ-TiAl based alloy

Abstract Ti–48Al–1.3Fe-1.1V–0.3B (at.%) alloy was siliconized to improve its oxidation resistance by burying it in Si powder and heating at 1073, 1123, 1173 and 1273 K for 18 ks in a vacuum. The isothermal oxidation behavior of all the treated specimens was tested at 1173 K for 349.2 ks (97 h) in air using a thermobalance. To evaluate the practical performance of this measure, the cyclic oxidation behavior of the specimen, siliconized at 1273 K, was also examined at 1123 K for 1260 ks (350 h) in a simulated exhaust gas. The element distribution, phase composition, and morphology of the Si-modified layer, and the oxide scale were characterized by AES, XRD, GAXRD and SEM. The results indicated that the TiAl alloy siliconized above 1173 K shows excellent isothermal and cyclic oxidation resistance in air or simulated exhaust gas. When siliconizing temperature is below 1123 K, however, this beneficial effect was limited to the early stage of oxidation. A Si-rich layer which was identified as Ti5Si3 and the following an Al- rich zone are the major constituents in the Si-modified layer on the TiAl substrate if the siliconizing temperature is above 1173 K. The thickness of the above constituent layers increases with rising siliconizing temperature. Existence of Si and Al with high concentrations is detected in the oxide scale even after long-term oxidation. It is concluded that the existing Si-rich layer, probably amorphous silica, and large amounts of Al2O3 in the oxide scale along with the remaining of the newly formed phase such as Ti5Si3 are responsible for the significant improvement in the oxidation resistance.

Cyclic oxidation behavior of Ni3Al-0.1B base alloys containing a Ti, Zr, or Hf addition

Ni3Al and Ni3Al-0.1B, with and without additions of about 2% Ti, Zr, or Hf were subjected to a thermal cycling oxidation test in pure flowing oxygen at atmospheric pressure at temperatures cycled between 400 and 1300 K. The scales formed on Ni3Al and Ni3Al-0.1B spalled repeatedly, resulting in a considerable mass loss of the specimen. The Ti addition to Ni3Al led to a repeated scale spollation, whereas Ti added to Ni3Al-0.1B resulted in a very adherent scale, although the oxidation kinetics were linear and the formation of deeply penetrating Al2O3 along the alloy grain boundaries took place. The scales were very adherent on alloys containing Zr and Hf. This was attributed to the so-called keying mechanism, because uneven penetration of Al2O3 into the alloy took place, leading to irregularly shaped scale/alloy interfaces. ZrO2 and HfO2 particles were incorporated into the Al2O3 layer and protrusions, and some of them were formed ahead of the Al2O3. The shape of these particles was not stringerlike as found with other alloys. The Ti, Zr, and Hf additions tended to decrease the density of voids formed at the scale/alloy interface, but the extent of the change seems to be insufficient to support the vacancy-sink mechanism. The Hf addition was found to be most effective in forming a protective scale.

Influence of silicon ion implantation and post-implantation annealing on the oxidation behaviour of TiAl under thermal cycle conditions

Abstract TiAl coupon specimens measuring 15×10×1 in mm were implanted with Si ions at varying acceleration voltages ranging from 80 to 260 keV with a constant dose of 2.5×10 21 ions·m −2 . In addition, three-step implantation was performed to obtain a wider and flat distribution of Si along a depth direction. Their oxidation resistance was assessed by a cyclic oxidation test with temperature varying between room temperature and 1200 K in a flow of purified oxygen under atmospheric pressure. The holding time at temperature was 72 ks (20 h). Conventional metallographic examinations were performed for implanted specimens and oxidised specimens using X-ray diffractometry, glancing angle X-ray diffractometry, AES, SEM and EDS. The implantation under all the conditions examined does not improve the oxidation resistance. The scales on all the specimens partially spall after a few oxidation cycles. The post-implantation annealing at 800 K for 600 s in an Ar atmosphere enhances the oxidation with the formation of scales spalling earlier. However, the vacuum annealing at 1100 K for 3.6 ks decreases the oxidation rates and improves the scale adherence. The best oxidation resistance is obtained by vacuum annealing at 1100 K for 36 ks. The scales are very adherent and oxidation becomes very slow. This excellent oxidation resistance is attributable to the formation of a layer consisting mainly of SiO 2 and Al 2 O 3 in the scale during the initial periods of oxidation. This initially formed SiO 2 layer seems to have resulted in the enrichment of Al 2 O 3 beneath it. The influence of acceleration voltage is small and the three-step implantation is not so effective.

Influence of implantation of Al, Si, Cr or Mo ions on the oxidation behaviour of TiAl under thermal cycle conditions

Abstract TiAl specimens measuring 15×10×2 in mm were implanted with Al, Si, Cr or Mo ions of a dose of 1021 or 1.2×1021 (for Cr only) ions m−2 at an acceleration voltage of 50 keV. Their oxidation resistance was assessed by cyclic oxidation tests with temperature varying between room temperature and 1200 K in a flow of purified oxygen under atmospheric pressure. The holding time at temperature was 72 ks (20 h) or 3.6 ks (1 h). Conventional metallographic examinations were performed for implanted specimens and oxidised specimens using glancing angle X-ray diffractometry (XRD), Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The implantation of Al or Si is effective to decrease the oxidation rate during the first near parabolic period of 130 ks, after which the implantation of Al results in breakaway oxidation, while the implantation of Si gives low oxidation rate for at least up to 800 ks. The implantation of Cr accelerates the oxidation during the initial period and then the partial scale spallation takes place leading to occasional mass losses. The implantation of Mo results in very slow oxidation rate for up to 1500 ks when the experiment was terminated. The same effect was obtained for the implantation of Mo under conditions of 40 keV and 2×1021 ions m−2. The excellent oxidation resistance obtained by the implantation of Si or Mo is attributable to the formation of virtually Al2O3 scales during the initial oxidation periods.

Isothermal oxidation behavior of Ni3Al-0.1B base alloys containing Ti, Zr, or Hf additions

The isothermal oxidation behavior of Ni3Al-0.1B and of this alloy containing additions of approximately 2% Ti, Zr, or Hf was studied in purified oxygen at atmospheric pressure over the temperature range 1300 to 1500 K and for periods up to 400 ks. Ni3Al was also studied similarly for comparison. The oxidation of Ni3Al and Ni3Al-0.1B resulted in an extensive formation of geometric voids on the substrate surface leading to poorly adherent scales. The addition of B to Ni3Al did not change the oxidation behavior much, however, both the activation energy of oxidation and the relative thickness of the Al2O3 layer in the scale were increased. The addition of Ti led to the formation of adherent scales at 1300 K, but the oxidation was accelerated after an initial period at 1300 and 1400 K. At higher temperatures the scale was protective, and the parabolic rate constant, kp, decreased, however, the scale adherence was impaired by the formation of interfacial voids. The addition of Zr resulted in very adherent scales at all temperatures, but at the same time it increased kp considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased kp except at 1300 K. The improved scale adherence can be accounted for by the keying effect, since the additives resulted in roughened scale/alloy interfaces. A complex of oxide particle and an associated void was found on the exposed surface of the Hf-containing alloy. This supports the vacancy-sink mechanism.

Oxidation behavior of TiAl protected by Si+Nb combined ion implantation

Abstract The combined ion implantation of Si+Nb at room temperature and at 1173 K with C contamination was employed to improve the oxidation resistance of a γ-TiAl based alloy [Ti–48Al–1.3Fe–1.1V–0.3B (at%)]. The implantation was conducted with a dose of 3.0×1021 ions/m2 and at an accelerate voltage of 50 kV for each element. The isothermal oxidation behavior of above treated alloys was tested at 1173 K for 349.2 ks in air. The oxide scales formed by short-term and long-term oxidation were characterized by AES, SEM and XRD. It was found that the alloy implanted with Si+Nb at 1173 K with C contamination shows excellent long-term oxidation resistance in comparison to that of the room temperature Si+Nb implanted alloy, although the latter also significantly lowers the oxidation rate of non-implanted alloy. A continuous, compact and thus a protective Al2O3 layer was formed in the scale of the alloy implanted with Si+Nb at 1173 K with C contamination after long-term oxidation, and this layer contributed to the best oxidation resistance. It is also indicated that the introduction of Si into Nb modified layer and, in particularly C in conjunction with Nb and Si in the modified layer can further effectively favor the formation of the continuous Al2O3 layer in the scale on alloy during high temperature oxidation.

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

Abstract 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.%F2 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 Al2O3 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 Al2O3 scale. Fluorine PBII treatment proves to be a promising surface modification method for improving the oxidation resistance of TiAl alloys.

Improvement in the Oxidation Resistance of an Al-Deposited Fe–Cr–Al Foil by Preoxidation

The oxidation kinetics of conventional Fe–20Cr–5Al (in mass %) foil, Al-deposited foil and Al-deposited and preoxidized foil was studied at 1373 K in air. All the foils were 50-μm thick and contained minor additions of rare-earth elements. The oxide scales were observed with SEM and TEM combined with EDS and were characterized with X-ray diffractometry and electron diffraction. The deposition of Al onto the foil from the vapor phase improves oxidation resistance. The details regarding this matter were reported elsewhere. The combination of the Al deposition and the subsequent preoxidation at 1173 K for 90 ks in air further increases the oxidation resistance, i.e., the smallest parabolic rate constant among the three kinds of foils, and excellent scale adherence. Preoxidation enhances the growth of θ-Al2O3, which transforms to α-Al2O3 during subsequent oxidation. However, such α-Al2O3 grains are much larger than those formed on the conventional foil of similar chemical composition. Small closed voids and small spinel-type, oxide particles appear in α-Al2O3 grains with the progress of oxidation. The former is explained in terms of the volume decrease accompanying the phase transformation and the latter by the low solubility of Fe in α-Al2O3.