Michiko Yoshihara

Improvement in oxidation resistance of the intermetallic compound titanium aluminide by heat treatment under a low partial pressure oxygen atmosphere

AbstractThe improvement in oxidation resistance of an intermetallic compound TiAl was investigated by means of a new type of surface treatment: heat treatment under a low partial pressure oxygen atmosphere. The specimens treated by this method showed superior oxidation resistance compared with a nickel-base superalloy Inconel 713C during cyclic heating to a temperature of 1173 K in static air. The best conditions for the heat treatment under a low partial pressure oxygen atmosphere were found to be: pressure, 6.7 X 10-3 Pa; temperature, 1273 K; time, 7.2 ks (2 h) or more. It was presumed that the excellent oxidation resistance resulting from this method is due to preferential formation of a thin, strong Al203 surface layer.

Influence of Zr Addition on Oxidation Behavior of TiAl-Based Alloys

The alloys based on γ-TiAl have attractive properties as light weight heat-resisting material. The poor oxidation resistance is one of the drawbacks to limit their application. This results from the high growth rate of TiO2 in the scale and the spallation of the oxide scale during cooling. In the present study, the influence of Zr addition on the oxidation behavior of TiAl has been investigated with emphasis on scale adherence. The oxidation behavior of Ti-45Al (in mol %), Ti-45Al-6Nb alloys containing 0 - 0.2% Zr, Ti-Nb and Ti-Zr binary alloys were investigated. The oxide scale was analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The addition of Nb to TiAl results in a remarkable decrease in mass gain, while the Zr addition to TiAl results in a slight decrease. The Ti-45Al-Nb alloy with Zr shows scale spallation after long term exposure. Concerning the binary alloys, the addition of Zr to Ti more than 20 mol% accelerates the oxide growth significantly. The XRD for Ti-45Al-6Nb-0.2Zr after oxidation shows the formation of Zr oxides and Ti-Zr complex oxides in the scale. The scale spallation observed in this alloy is due to the enrichment of Zr at scale / alloy interface and formation of Zr oxide in the scale.

INFLUENCE OF ION IMPLANTATION ON OXIDATION BEHAVIOR OF TiAl

The influence of alloying elements on oxidation behavior of TiAl has been investigated using an ion-implantation technique and the mechanisms were discussed. The influence can be classified into several groups according to their effects. The implantation of β-forming elements, halogens, Cu and Zn results in a significant improvement of the oxidation behavior through formation of Al2O3 layer in the initial stage of oxidation. The improvement by Zn is attributed to the formation of complex oxide of Zn and selective oxidation of Al beneath the layer. The implantation of Al, Si or P is also effective. On the other hand, implantation of Ag, Se and other several elements enhance the oxidation. The deterioration by Ag or Se is explained in terms of Al depletion in the implanted layer.

Influence of Micro-alloying on Oxidation Behavior of TiAl

The influence of a wide range of elements on oxidation behavior of TiAl was investigated by micro-alloying using ion implantation with ion doses of 10 19 to 10 22 m −2 and at acceleration voltages of 40 to 340kV. The oxidation resistance was assessed by a cyclic oxidation test at 1200K in a flow of purified oxygen under atmospheric pressure. The implanted elements can be classified into several groups according to their effect and mechanism. The mechanisms by which the oxidation resistance is improved are as follows: (1) Formation of a protective Al 2 O 3 layer through β-phase formation, which was confirmed by TEM observations, in the modified surface layer by the implantation. (2) Reduction of TiO2 growth rate due to doping effect of the implanted element. (3) Protective Al 2 O 3 layer formation through migration of volatile halide. (4) Enrichment of oxide of the implanted element in the scale. On the other hand, the oxidation resistance is decreased by (1) enhanced TiO 2 growth due to doping effect, (2) lattice defects induced by the implantation, and (3) decreased scale strength and enhanced scale spallation.

Development of Surface Treatment Techniques to Improve Oxidation Resistance of Titanium Aluminide

The oxidation resistance of intermetallic compound TiAl was greatly improved in cyclic heating at 900°C by means of a new type of surface treatment, which is the heat treatment under a low partial pressure oxygen atmosphere. The treatment becomes more effective with increasing aluminum content in the specimen. It was presumed that the excellent oxidation resistance resulting from this method is due to preferential formation of a thin and tighit Al 2 O 3 surface layer, which is formed during the surface treatment at a temperature vicinity of 1000°C and air pressure of about 10 −3 Pa. The oxidation resistance of Al 2 O 3 film formed, however, was found to be insufficient at 950°C. A combined treatment consisting of the heat treatment under a low partial pressure oxygen atmosphere followed by a diffusion coating with aluminum showed an excellent resistance to cyclic oxidation even at 950°C.

Influence of ion implantation of several elements on oxidation behavior of TiAl

TiAl-based alloys have attractive properties as light weight heat-resisting material. In the present work, the influence of Cu, Zn, Ag and Se on the oxidation behavior of TiAl was investigated by ion implantation at acceleration voltage of 50 kV and ion doses of 1019 to 2x1021 ions/m2. The oxidation behavior was assessed by a cyclic oxidation test at 1200 K in a flow of purified oxygen under atmospheric pressure. The oxidation products were analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The implantation of Zn and Cu improves the oxidation resistance significantly by forming virtually Al2O3 scales, while Ag and Se enhance the oxidation. The improvement by Zn is attributable to the formation of complex oxide of Zn in the initial stage of oxidation. The oxygen partial pressure under the layer seems to be very low, resulting in the formation of alumina scale due to a selective oxidation of Al. The influence of Cu is not certain. The influence of Ag and Se is explained in terms of Al depletion in the implanted layer.

TEM Observation of the Initial Stage Oxidation of TiAl Based Alloys in a Simulated Combustion Atmosphere

The oxidation behavior of Ti-48Al-2Cr-2Nb, Ti-48Al-2Cr-2W and Ti-48Al-2Cr-2Fe was studied in a simulated combustion gas, 10O2-7CO2-6H2O-bal.N2 (vol%), at 1173 K and TEM observation was performed for understanding the initial stage of oxidation behavior. Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2W show excellent oxidation resistance in the test gas by forming thin and protective Al2O3-rich scales, while Ti-48Al-2Cr-2Fe shows poor oxidation resistance. The superior oxidation resistance of W-containing alloy is explained in terms of the formation of a bcc phase with low Al content in the alloy which was confirmed by TEM observation and also possible enhanced Al diffusion from the substrate to the scale in this phase. The oxidation resistance of the former two alloys in the test gas is better than in laboratory air, due to the lower O2 content in the test gas. The presence of H2O and CO2 in the test gas enhances the oxidation of Ti-50Al, while it has almost no influence on the oxidation behavior of these two alloys, indicating that these gases are influential to a TiO2-rich scale but not to Al2O3-rich scales.

Oxidation Behavior of TiAl Based Alloys

The cyclic oxidation behavior was investigated for Ti-50at%Al-(0–20)at%Nb and Ti-50at%Al-(0–2)at%W at 900° and 950° in air. The addition of Nb up to 10at% and W greatly improved oxidation resistance of TiAl. This is attributed to both the suppression of TiO2 growth and the promotion of protective Al2O3 layer formation.