Oxidation Behavior Of TiAl Research Articles

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.

Oxidation Behavior of TiAl Based Alloys Prepared by Spark Plasma Sintering

The spark plasma sintering (SPS) microstructure and high temperature oxidation behavior of TiH2-45Al-0.2Si-5Nb(at.%) alloy were investigated.Emphasis was placed on the effect of SPS microstructures, obtained by blend powder and mechanical alloying powder. The mass gain due to oxidation was measured using an electro balance. The oxide layers as well as its micro-structure were examined by SEM and EDS, and XRD. The results show that sintered microstructure of blend powder is composed of fully lamellar TiAl/ Ti3Al phase, and that of the mechanical alloying powder is composed of finer granular TiAl/Ti3Al phase. The latter oxidation rate is lower, and forms continuous mixed oxide layer of Al2O3 and TiO2. Both SPS microstructure of blend powder and mechanical alloying powder are superior in oxidation behavior to ordinary vacuum sintering.

Effect of niobium on the oxidation behavior of TiAl

Two TiO2layers formed in TiAl oxidation for 50 h at 900 °C were studied using scanning transmission electron microscopy. The main efforts were placed on the investigation of the distribution of niobium. It was found that Nb enriched in TiO2grains of mixture layer but did not exist in the outer TiO2layer. High-resolution electron microscopy (HREM) Z-contrast image revealed that Nb substitute for Ti site leading to Nb enrichment in TiO2grains of the mixture layer. The formation mechanism of the two TiO2layers and the potential effect of Nb doping in the mixture layer were also discussed.

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 Al 2 O 3 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 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.

TiAlの高温酸化特性に及ぼすイオン注入の影響

TiAlの高温酸化特性に及ぼすイオン注入の影響

Oxidation behavior of TiAl(Cr, Ag) at 900 °C in water vapor

Ti-(48,52)Al-5Cr- xAg ( x=0, 2, 3 at.%) alloys were made by Ar-arc melting and casting, followed by a homogenization treatment at 1100 °C in vacuum. The coupon samples were oxidized at 900 °C in room air and air with 30.8 vol.% H 2O for 340 h. Ti-52Al-5Cr was unable to resist the oxidation in either “dry” or “wet” air. Ti-52Al-5Cr-2Ag was an alumina former in room air, but in wet air it formed non-protective oxides (directionally grown TiO 2 and the mixture of TiO 2 and Al 2O 3). Water vapor showed little effect on the oxidation resistance of Ti-(48,52)Al-5Cr-3Ag, which formed continuous α-Al 2O 3 scales in both atmospheres, with no sub-layers composed of the Laves phase observed beneath the Al 2O 3 scales. As compared with their as-cast alloys, Cr in γ-phase had partitioned to the Laves and/or L1 2 phase and Ag to the precipitates of Ag in γ-phase or the L1 2 phase after oxidation exposures, which was believed to have prevented Cr fluxing to the oxide/alloy interface to form the Laves phase sub-layers. The anisotropic diffusion of hydrogen in TiO 2 was considered to be the main reason for the formation of directionally grown TiO 2, which had deteriorated the oxidation resistance.

Influence of Micro-alloying on Oxidation Behavior of TiAl

ABSTRACTThe influence of a wide range of elements on oxidation behavior of TiAl was investigated by micro-alloying using ion implantation with ion doses of 1019 to 1022 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 Al2O3 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 Al2O3 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 TiO2 growth due to doping effect, (2) lattice defects induced by the implantation, and (3) decreased scale strength and enhanced scale spallation.

Effect of Ion Implantation of High Melting Point Elements on the Oxidation Behavior of TiAl

Cyclic oxidation behavior of TiAl implanted with W or Ta was investigated in a flow of purified oxygen at 1200K. The specimens implanted with W or Ta at an acceleration voltage of 100keV or 180keV with a dose of 10 21 ions/m 2 formed adherent alumina scales on the surface, resulting in an excellent oxidation behavior. However, implantation with W with a dose of 10 19 ions/m 2 was insufficient to improve the oxidation resistance of TiAl. The oxidation resistance of the implanted alloys with a dose of 10 21 ions/m 2 was comparable to that of the alloys containing W or Ta 1 at% or higher. The TiO 2 suppression by doping effect is believed to be a reason for the improvement. As the Z-phase was identified by X-ray diffraction with the specimens which showed excellent oxidation behavior, another reason for the improved oxidation resistance may be a stabilization of Al 2 O 3 layer formed on the specimen by this phase. The ion implantation with high melting point elements has turned out to be a useful surface treatment to improve the oxidation resistance of TiAl without increasing alloy density.

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Twenty-four ternary element additions were made to a binary TiAl alloy (Ti−34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al2O3 barrier being formed. This is probably due to the doping of those elements in TiO2. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al2O3 layer as the second layer from the outer surface and the relatively massive precipitation of Al2O3 in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.

Effect of Hf addition (0.24 w/o) on the oxidation behavior of TiAl at high temperatures

The isothermal oxidation behavior of TiAl coupons containing 0.24 wt.% Hf has been studied in the temperature range 1100 to 1400 K in a flow of purified oxygen at atmospheric pressure. The addition of Hf is very effective in decreasing the overall oxidation rate at temperatures up to 1300 K, although the oxidation rate was slightly increased during the initial period of about 10 ks, after which a very low oxidation rate was maintained. However, at 1350 K the effect becomes small, and at 1400 K it is inverted. The excellent oxidation resistance obtained is attributable to the formation of two alumina-rich layers in the scale; one is beneath the outer rutile layer and the other at the scale/substrate interface. In addition, no internal oxides were formed in the substrate up to 1300 K.

TiAl金属間化合物の耐酸化性に及ぼすMo添加の影響

TiAl金属間化合物の耐酸化性に及ぼすMo添加の影響

The influence of Cr on oxidation behavior of TiAl at 1173K

The influence of Cr on oxidation behavior of TiAl at 1173K

Oxidation behavior of TiAl coated with a fine-grain Co-30Cr-4Al film

The oxidation behavior of TiAl coupons coated with a fine-grain Co-30Cr-4Al (mass %) film of about 30-μm thickness has been studied at 1100–1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and accelerated stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The activation energy for parabolic oxidation agrees with reported values for the oxidation of alumina-former alloys, although the mass gains during the parabolic stages are relatively small at 1200 and 1300 K. Micropores developed mainly at the scale/coating and coating/substrate interfaces as oxidation proceeded. This is attributable to recrystallization of the coating during oxidation and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of rutile mounds on the scale.