Cyclic oxidation behavior of TiAl composites incorporated with TiB2 dispersoids

TiAl alloys incorporated in (0,3,5,10) wt.% TiB₂ dispersoids were manufactured via mechanical alloyingspark plasma sintering (MA-SPS), and their cyclic oxidation characteristics were studied at 800, 900 and 1000°C in air. The cyclic oxidation resistance of the prepared TiAl-TiB₂ composites effectively increased with increases in TiB₂ content. The oxide scale formed consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (Al₂O₃+TiO₂) mixed layer. The scale adherence was relatively good, and much thinner oxide scales, when compared to TiB₂-free TiAl alloys, were formed on the prepared composites. The incorporated TiB₂ dispersoids oxidized to TiO₂ and B₂O₃ which evaporated during oxidation.     

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.     

The Effect of Si3N4 Additions on the Oxidation Resistance of TiAl Alloys

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si₃N₄ particles were studied at 1173 and1273 K in 1 atm of air. The Si₃N₄ dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti₅Si₃ and dissolvednitrogen. The oxide scale formed on TiAl–Si₃N₄alloys consisted of an outer TiO₂, an intermediate(Al₂O₃+TiO₂), and an inner(TiO₂+Al₂O₃) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO₂ particlesbelow the outer TiO₂ layer, and the improved scale adhesion bySi₃N₄ dispersions were attributable mainly to theincreased oxidation resistance compared to the Si₃N₄-freeTiAl alloys. Marker experiments showed that, for TiAl–Si₃N₄ alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.     

Effect of Fe on the high temperature oxidation of TiAl alloys

An alloy of 51.23Ti−48.73Al−0.4Fe (at.%) was oxidized at 800, 900 and 1000°C in air to determine the effect of Fe on oxidation. The scales formed consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner mixed (TiO₂ Al₂O₃) layer, typical of conventional TiAl alloys. A small amount of dissolved Fe ions was weakly segregated in the outer TiO₂ layer and also in the inner (TiO₂−Al₂O₃) mixed layer. Ti₂AIN and TiN were detected in the scale in some instances. A thin, oxygen-affected Ti₃Al sublayer formed at the oxide-substrate interface. The overall oxidation kinetics and the scale morphology were not affected by Fe-addition.     

Oxidation of Powder Metallurgy (PM) Ti–48%Al–2%Cr–2%Nb–(0–1%)W Alloys Between 800 and 1000°C in Air

Three powder metallurgy (PM) TiAl alloys with a fully lamellar structure were oxidized isothermally and cyclically between 800 and 1000°C in air in order to find the effect of W on the oxidation behavior of Ti–48Al–2Cr–2Nb alloys. The alloys oxidized parabolically during isothermal oxidation. Tungsten improved the isothermal and cyclic oxidation resistance. The oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃-rich layer, and an inner TiO₂-rich layer. The alloying elements of Cr, Nb, and W tended to segregate in the lower part of the scale owing to their thermodynamic nobility. In the vicinity of the scale/matrix interface, TiN and Ti₂AlN coexisted.     

Effects of Cr and Nb on the high temperature oxidation of TiAl

From isothermal and cyclic oxidation tests on thermomechanically treated Ti-51%Al, Ti-47%Al-4%Cr, and Ti-48%Al-2%Cr-2%Nb alloys at 800, 900, 1000°C in air, it was found that Ti-48%Al-2%Cr-2%Nb and Ti-47%Al-4%Cr had the best and the worst oxidation resistance, respectively. The oxide scales consisted primarily of TiO₂ and Al₂O₃, with and without a small amount of dissolved Cr and Nb ions, depending on the alloy composition. These ions were slightly enriched inside the inner oxide layer, and strongly enriched around the scale-matrix interface. The outer TiO₂-rich layer was formed by the outward diffusion of Ti ions, while the inner (TiO₂+Al₂O₃) mixed layer was formed by the inward transport of oxygen. The outward movement of Al ions formed the intermediated Al₂O₃-rich layer, above the prepared alloys.     

High temperature oxidation of Ti−48%Al−2%W intermetallics

Powder metallurgically produced Ti-48% Al-2%W alloys were oxidized between 800 and 1050°C in air. The W-addition was quite effective in providing isothermal and cyclic oxidation resistance. The alloys oxidized parabolically up to 1050°C during isothermal oxidation, with small weight gains. The scales were adherent up to 900°C during cyclic oxidation. Oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. Tungsten was present below the intermediate Al₂O₃ layer. and also at the scale-matrix interface as W-enriched compounds. Below the oxide scale, a Ti₃Al zone containing some W and O existed.     

The Effect of Niobium-Ion Implantation on the Oxidation Behavior of γ -TiAl Alloys in Static and Flowing Air

-TiAl (Ti–50Al at.%) alloys were implanted with Nb ions at an acceleration energy of 50 keV, at a dose of 1.2×10¹⁷ ions/cm². The cyclic-oxidation behavior of the unimplanted and Nb⁺-implanted TiAl specimens was investigated at 850°C in static air and in air with a flow velocity of 12.0 m/s (1000 ml/min). In static air, the unimplanted TiAl specimen showed rapid oxidation during a transition period of about 80 hr, after which partial scale spallation occurred and a net mass loss was observed. In flowing air, the whole scale spalled off after each cycle. On the other hand, Nb-ion implantation led to the formation of an adherent protective Al₂O₃ scale during oxidation in both static and flowing air, thereby significantly improving the cyclic-oxidation resistance of -TiAl alloys. A remarkable deference in the initial-oxidation behavior between unimplanted and Nb⁺-implanted specimens was also observed. A mixed TiO₂/A₂O₃ scale on the unimplanted specimen developed at a high growth rate during the very initial stage of oxidation. In contrast, the initial scale growth rate was significantly decreased by Nb-ion implantation and an Al₂O₃-rich layer was found present as the inner part of the initial scale on Nb⁺-implanted TiAl. Flowing air appeared to cause severe scale spallation during oxidation of unimplanted TiAl, but not to have any influence on the adhesion of the scale on Nb⁺-implanted TiAl.     

High temperature oxidation of thermomechanically treated Ti-47 Al−2Cr−2Nb

Thermomechanically treated Ti−47Al−2Cr−2Nb alloys were oxidized at 800°C, 900°C and 1000°C in air. Various heat treatments done to obtain duplex microstructure and stabilize microstructures did not affect the overall oxidation kinetics and the oxide morphology of the alloys. Alloys oxidized at similar rates formed oxide scales consisting primarily of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. A small amount of dissolved Cr ion was preferentially segregated in the inner oxide layer. Niobium, however, was segregated in not only the inner, but also the outer oxide layer.     

Improving the oxidation resistance of γ‐titanium aluminides by halogen treatment

Intermetallic alloys based on TiAl are candidates for several structural high temperature applications but their oxidation resistance is limited to temperatures below 800 °C. In this paper the results of high temperature oxidation and creep tests will be presented and discussed. The treatment with halogens improves the oxidation resistance of these alloys up to 1050 °C. A thin protective Al₂O₃‐layer is formed after treatment with halogens instead of the mixed TiO₂/Al₂O₃/TiN scale typically grown on these alloys. This alumina layer protects the component under isothermal and thermocyclic conditions. The protective effect is stable up to at least 8760 h. Creep tests of halogen treated materials at high temperatures showed no effect on the creep behaviour. Automotive turbocharger rotors were exposed at 1050 °C in air with and without fluorine‐treatment for demonstration of real parts.     

Isothermal oxidation behavior of powder metallurgy beta gamma TiAl–2Nb–2Mo alloy

A new TiAl–2Nb–2Mo beta gamma alloy was synthesized by powder metallurgy process. HIP’ed and vacuum heat treated specimens were isothermally oxidized at 800 °C and 900 °C in air up to 500 h. The TiAl–2Nb–2Mo alloy oxidized parabolically up to 500 h at both 800 °C and 900 °C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer, and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of lamellar colonies formed a diffusion zone just below the oxide/substrate interface consisting of γ-TiAl matrix and dispersed beta phases which contained high concentration of Nb and Mo. The oxidation rate of the TiAl–2Nb–2Mo alloy is more sensitive to temperature than those of the Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.     

The corrosion behavior of Ni-Al alloys and Ni-Nb-Al alloys in a H2/H2O/H2S gas mixture

The corrosion behavior of two Ni-Al alloys and four Ni-Nb-Al alloys was studied over the temperature range of 600° C to 1000° C in a mixed-gas of H₂/H₂O/H₂S. The parabolic law was generally followed, although linear kinetics were also observed. Multiple-stage kinetics were observed for the Ni-Al alloys. Generally, the scales formed on Ni-13.5Al and Ni-Nb-Al alloys were multilayered, with an outer layer of nickel sulfide with or without pure Ni particles and a complex inner scale. The outer scale became porous and discontinuous with increasing temperature. Very thin scales formed on Ni-31Al. The reduction in corrosion rate with increasing Al content is ascribed to the formation of Al₂O₃ and Al₂S₃ in the scale. Platinum markers were found at the interface between the outer and inner scales.     

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

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 -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 2-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

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.     

Sulfidation processing and Cr addition to improve oxidation resistance of TiAl intermetallics in air at 1173 K

High temperature oxidation properties of TiAl- (1,2,4 and 10) Cr and 40Ti-56Al–4Cr alloys, which were sulfidized at 1173 K for 86.4 ks at 1.3 Pa sulfur partial pressure in a H₂–H₂S gas mixture, were investigated at 1173 K in air for up to 2.7 Ms. The sulfidation processing formed a (Cr,Ti)Al₂ layer between a TiAl₃ (TiAl₂ included) layer and a Ti-rich sulfide scale by selective sulfidation of Ti. Oxidation of the sulfidation-processed alloys was examined for up to 2.7 Ms in air under isothermal and room temperature to 1173 K heat cycle conditions. In both oxidation experiments the sulfidation processed TiAl–10Cr alloy showed very good oxidation resistance up to 2.7 Ms, due to the formation of a continuous Ti(CrAl)₂ Laves layer, which was changed from (Cr,Ti)Al₂ and has a composition of 28.7Cr–36.2Al–35.1Ti, between the layers of protective Al₂O₃ (TiO₂ included) and TiAl₂, which was changed from TiAl₃. The sulfidation processed TiAl, TiAl–4Cr, and 40Ti–56Al–4Cr alloys showed better oxidation resistance than conventional TiAl based alloys, but displayed localized oxidation. The Ti(Cr,Al)₂ Laves on the sulfidation processed TiAl–4Cr alloy was discontinuous, leading to a localized oxidation after long oxidation. The sulfidation processed 40Ti–56Al–4Cr alloy oxidized faster than the sulfidation processed TiAl–10Cr alloy due to the formation of an Al₂O₃ and TiO₂ mixture, although the TiAl₂ layer remains. It was concluded that the Ti(Cr,Al)₂ Laves layer between the oxide scale and alloy substrate caused the good oxidation resistance.     

Transient Oxidation of a γ-Ni–28Cr–11Al Alloy

γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al₂O₃ scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr₂O₃ layer, and an α-Al₂O₃ layer at the oxide/alloy interface. The evolution of such an NiO/Cr₂O₃/Al₂O₃ layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr₂O₃ outer layer and a discontinuous inner layer of Al₂O₃ initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al₂O₃, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr₂O₃ and Al₂O₃ to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al₂O₃ layer.     

Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe-25Cr-35Ni-2.5Al-XNb alloys at 1000°C and 1100°C

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-_XNb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the high-temperature oxidation resistance. A bilayer oxide scale with a Cr₂O₃-rich outer layer and Al₂O₃-rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al₂O₃ scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb₂O₅) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al₂O₃ scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr₂O₄, Cr₂O₃, and Fe₂O₃ outer layers and Al₂O₃ inner layer.     

The isothermal oxidation of Fe<Subscript>3</Subscript>Al−4%Cr−(0, 0.5, 1, 2%)Mo alloys at 1000°C

The air oxidation characteristics of Fe₃Al-4%Cr-(0, 0.5, 1, 2%)Mo alloys at 1000°C were studied using TGA, XRD, EPMA, and TEM/EDS. Molybdenum increased the oxidation resistance of Fe₃Al-4%Cr alloys. The whole Al₂O₃ grains that formed on the alloy surface contained a small amount of dissolved Fe ions. The Al₂O₃ grains next to the oxide-matrix interface additionally contained a small amount of dissolved Cr and Mo ions. Beneath the thin but non adherent Al₂O₃ layer, an Al-depleted, Fe-enriched matrix zone formed due to the consumption of Al in the scale.     

The oxidation behavior of Ti–46.5Al–5Nb at 900 °C

The early stages of Ti–46.5Al–5Nb (at%) oxidation at 900 °C have been investigated combined with TEM and STEM. The results reveal that a layer composed of polycrystalline TiO₂ and amorphous Al₂O₃ phase formed firstly after 5-min oxidation. The base alloy connected with the oxide scale has some deformation compared with the inner full lamellar TiAl structure. After 30-min oxidation, the phases of γ-Al₂O₃, κ-Al₂O₃, titanium nitrides and Ti₅Al₃O₂ were formed in the area from the nitride layer to base alloy. After 50 h of oxidation, Ti₅Al₃O₂ vanishes at the interface of oxide scale/base alloy in Ti–46.5Al–5Nb, contrary to the continuous formation of Ti₅Al₃O₂ in γ-TiAl at the interface of oxide scale/base alloy, Al₃Nb phase formed in this zone, which hinders the continuous formation of Ti₅Al₃O₂.     

Effect of Ion Plating TiN on the Oxidation of Sputtered NiCrAlY-Coated Ti3Al-Nb in Air at 850-950°C

A single sputtered NiCrAlY coating and a complexcoating of inner ion-plated TiN and outer sputteredNiCrAlY were prepared on the intermetallic compoundTi₃Al-Nb. Their oxidation behavior wasexamined at 850, 900, and 950°C in air by thermalgravimetry combined with XRD, SEM, and EDAX. The resultsshowed that Ti₃Al-Nb followed approximatelyparabolic oxi dation, forming an outer thinAl₂O₃-rich scale and an inner TiO₂-rich layer doped withNb at the three temperatures. The TiO₂-richlayer doped with Nb dominated the oxidation reaction.The single NiCrAlY coating did not follow parabolicoxidation exactly at 850 and 950°C, but oxidizedapproximately in a parabolic manner, because theinstantaneous parabolic constants changed slightly withtime. Besides the Al₂O₃ scale,TiO₂ formed from the coating surface at the coating-substrate interface. Thedeterioration of the coating accelerated with increasingtemperature. The NiCrAlY-TiN coating showed two-stageparabolic oxidation at 850 and 900°C, and anapproximate parabolic oxidation at 950°C. The TiN layerwas effective as a barrier to inhibit coating-alloyinterdiffusion.