Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

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.     

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 third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

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.     

Water Vapor and Oxygen/Sulfur-Impurity Effects on Oxidation and Nitridation in Single- and Two-Phase Cr–Nb Alloys

The oxidation and nitridation behavior of Cr(Nb) solid solution and Cr2Nb Laves phase was studied individually and in combination at 1100°C and 950°C in dry air, humid air, and N₂–4H₂ for alloys containing high or low levels of oxygen and sulfur impurities. At 1100°C, humid-air exposure of alloys in which the Cr₂Nb phase was the matrix resulted in greatly increased subscale nitridation as compared with exposure in dry air. At 950°C, little difference between dry vs. humid-air exposures was observed; however, greatly increased subscale nitridation was observed in the Cr₂Nb-matrix eutectic alloy when the levels of oxygen and sulfur impurities in the alloys were reduced. In contrast, unalloyed Cr and Cr(Nb) solid-solution alloys were relatively insensitive to water vapor or impurity oxygen/sulfur effects under the conditions studied, although the addition of Nb as a solute in Cr did increase the extent of subscale nitridation. Possible synergistic effects between sulfur impurities and water vapor in the Cr₂Nb alloys and the possibility for similar effects in other intermetallic systems are discussed.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Preparation and oxidation resistance of a crack-free Al diffusion coating on Ti22Al26Nb

A crack-free Al diffusion coating has been developed to improve the oxidation resistance of Ti22Al26Nb. It was produced by a two-step method; an Al film was deposited on the substrate alloy by arc ion plating followed by a diffusion process conducted at 873 K in pure Ar to form the Al diffusion coating. The two-step method lowers the temperature required to form the diffusion coating, which dramatically decreases the thermal stress developed in the coating and results in it being crack-free. The oxidation resistance of the non-coated Ti22Al26Nb alloy in isothermal and cyclic tests in air at 1073 K was poor, but the coated specimens possessed excellent oxidation resistance because a protective α-Al₂O₃ scale formed. The life of the Al diffusion coating greatly depends upon the rapid initial formation of a protective Al₂O₃ scale and interdiffusion between coating and substrate. Once the stable Al₂O₃ scale has formed and the composition changes from (Ti, Nb)Al₃ into (Ti, Nb)Al₂, the coating has a long life.     

Sulphidation/oxidation behaviour of TiAlCr and Al2Au coated Ti45Al8Nb alloy at 750 °C

In this paper, we present the sulphidation/oxidation behaviour of a Ti45Al8Nb (at%) alloy coated with different protective surface films. Two intermetallic coatings are considered; TiAlCr and Al₂Au deposited by physical vapour deposition. The coated alloy was subjected to a H₂/H₂S/H₂O yielding pS₂ - 10⁻¹ Pa and pO₂ - 10⁻¹⁸ Pa potentials at 750 °C for up to 1000 h. The corrosion kinetics were determined by means of discontinuous gravimetry and the as-received and exposed samples were characterised using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The materials showed the development of a multilayered structure. In the case of the TiAlCr coated Ti45Al8Nb - base alloy, Al₂O₃, TiO₂ and Cr₂S₃ developed. For the Al₂Au coated Ti45Al8Nb samples an Al₂O₃ scale containing TiO₂ nodules was observed at the surface.     

Oxidation behavior of Ti–46Al–8Nb alloy with boron and carbon addition under thermal cycling conditions

Oxidation behavior of Ti–46Al–8Nb (in at.%) alloy with boron and carbon addition under thermal cycling conditions was investigated. Oxidation of Ti–46Al–8Nb, Ti–46Al–8Nb–1B and Ti–46Al–8Nb–1B-0.25C (in at.%) alloys was carried out at 1073 K in laboratory air for 42 cycles (1 cycle, 24 h), 1008 h in total. The mass loss rates of Ti–46Al–8Nb and Ti–46Al–8Nb–1B measured during the oxidation were similar. The best oxidation resistance was found for Ti–46Al–8Nb–1B-0.25C alloy with the smallest mass change. XRD and SEM-EDS investigations showed that in all cases, the oxide scales compositions were substantially similar. The scale consisted of an outer layer built of Al₂O₃ with the presence of some amounts of TiO₂, an intermediate layer of the scale consisting of TiO₂, an inner layer composed of oxides and nitrides. Additionally, niobium rich particles at the scale/substrate interface were present. The oxidation mechanism of Ti–46Al–8Nb was studied via two-stage isothermal oxidation (24 h in ¹⁶O₂ followed by 24 h in ¹⁸O₂) at 1073 K combined with secondary neutral mass spectroscopy (SNMS). These results indicate that the oxidation mechanism depends on a mixed diffusion process, consisting of outward transport of cations and simultaneous oxygen inward transport.     

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 Resistance of CrAlYN-Coated Ti45Al8Nb

The oxidation behaviour of Cr_0.45Al_0.53Y_0.02N (at.%) thin film deposited on ??-TiAl based alloy Ti?C45Al?C8Nb (at.%) was carried out. The tests were performed in the temperature range 750?C950???C for 500?h. The materials showed good corrosion resistance only at temperatures of 750 and 850???C where Al₂O₃ and Cr₂O₃ phases developed as a top oxide scale. A lack of corrosion resistance was observed at 950???C where a thick scale (100???m) that developed consisted of alternate layers of TiO₂ and Al₂O₃ phases. To investigate the oxidation behaviour, the corrosion kinetics were determined by means of discontinuous gravimetry of the exposed samples. Surface and cross-sectional analyses of exposed samples were conducted using scanning electron microscopy (SEM) and energy X-ray dispersive spectroscopy (EDS).     

NiCoCrAlY coatings with and without an Al2O3/Al interlayer on an orthorhombic Ti2AlNb-based alloy: Oxidation and interdiffusion behaviors

The mechanism of oxidation protection of NiCoCrAlY overlay coatings on the orthorhombic Ti₂AlNb-based alloy (O alloy) Ti–22Al–26Nb (at.%) is described. While the bare alloy exhibited poor oxidation resistance at 800 °C, adding NiCoCrAlY coatings significantly improved the oxidation resistance. However, serious interdiffusion between the coatings and the substrate resulted in rapid degradation of the coating system. Several reaction layers were formed at the coating/substrate interface by interdiffusion, and non-protective scales mainly of Cr₂O₃ and TiO₂ were formed due to the degradation of the coating. In order to solve this problem, an Al₂O₃/Al interlayer was sandwiched into the coating system as a diffusion barrier. The isothermal and cyclic oxidation protection of the multilayer coating system on the Ti–22Al–26Nb substrate was evaluated at 800 and 900 °C. The results indicated that the interdiffusion was much suppressed, and the duplex coating system demonstrated improved oxidation resistance on the Ti–22Al–26Nb substrate, with a thin and adherent protective α-Al₂O₃ scale forming on the surface.     

Oxidation Behavior of Cr2N,CrNbN, and CrTaN Phase Mixtures Formed on Nitrided Cr and Laves-Reinforced Cr Alloys

A series of single-phase Cr(X), single phase Cr ₂X Laves phase, and two-phase Cr(X) + Cr₂ X alloys (X = Nb or Ta) were thermally nitrided for 24 hr at 1100°C in N₂--4H₂ and then oxidized for 2 hr at 1100°C in air. The Cr(X) phase nitrided to form Cr₂N while the Cr₂X phases nitrided to form a complex local mixture of Cr₂N/Cr and CrNbN/CrTaN, Cr₃Nb₃N/Cr₃Ta₃N phases depending on the depth in the nitrided zone. The Ta only slightly increased the isothermal oxidation rate of nitrided Cr(Ta) and Cr₂Ta-reinforced Cr alloys, compared with nitrided, unalloyed Cr. Further, the nitrided two-phase alloys Cr--9.5Ta and Cr--20Ta exhibited improved Cr₂O₃ scale adherence relative to nitrided unalloyed Cr and Cr--1Ta. In contrast, Nb was detrimental to the oxidation resistance of the nitrided Cr(Nb) and Cr₂Nb-reinforced Cr alloys, resulting in the formation of nonprotective Cr--Nb oxides rather than continuous Cr₂O₃. A phenomenological explanation for these effects based on phase chemistry and microstructural distribution is presented. Implications of these results for understanding the oxidation behavior of developmental high-temperature, Laves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.     

Y and Al modified silicide coatings on an Nb–Ti–Si based ultrahigh temperature alloy prepared by pack cementation process

Y and Al modified silicide coatings were prepared on an Nb–Ti–Si based ultrahigh temperature alloy by co-depositing Si, Al and Y at 1150 °C for up to 10 h, respectively. The deposition of Al and Si occurred in a sequential manner during the pack cementation process. At the initial stage, the element deposited was primarily Al with very little Si and an Al₃(Nb,X) (X represents Ti, Cr and Hf elements) layer formed preferentially. After a short period of holding time, Si started depositing and Si–Al co-deposition took place. However, this Si–Al co-deposition period was not long. When the holding time was longer than 1 h at 1150 °C, Si deposition dominated the coating growth process. The coating growth kinetics at 1150 °C followed a parabolic law. The coating prepared at 1150 °C for 10 h had a multi-layer structure, with a thick (Nb,X)Si₂ outer layer, a thin (Ti,Nb)₅Si₄ middle layer and an Al, Cr-rich inner layer. The coating could protect the Nb–Ti–Si based alloy from oxidation at 1250 °C in air for at least 100 h. The excellent oxidation resistance of the coating was attributed to the formation of a dense scale mainly consisted of TiO₂, SiO₂ and Al₂O₃.     

A contribution to the Al–Ni–Cr phase diagram

The Al–Ni–Cr phase diagram was specified at 1000 °C and partially at 900 °C. The results concerning the region below 60 at.% Al agreed qualitatively with the literature data. The binary Al–Cr phases μ and γ dissolve up to 1 and 3 at.% Ni, respectively, and Al₃Ni₂ up to 2.5 at.% Cr. Two ternary phases were revealed: hexagonal ζ (a ≈ 1.77, c ≈ 1.24 nm) in a wide range between Al₈₁Ni₃Cr₁₆, Al_76.5Ni₃Cr_20.5, Al_76.5Ni₉Cr_14.5 and Al_71.5Ni₉Cr_19.5, and high-temperature orthorhombic (a ≈ 1.26, b ≈ 3.48, c ≈ 2.02 nm) around Al_76.5Ni_2.0Cr_21.5.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Oxidation of Ni-base alloys in atmospheres with widely varying oxygen partial pressures

The oxidation behavior of the Ni-base alloys IN 617, IN 713 LC, Ni20Cr, and Ni20Cr+Si has been investigated in the temperature range from 850°C to 1000°C in air and at low-oxygen partial pressure p(O₂) (10^–19 to 10^–16 bar). With the exception of alloy IN 713 LC, the materials show no influence of p(O₂) on the oxidation mechanisms and the kinetics. This result can be explained by the formation of a dense Cr₂O₃ layer, the growth rate of which is controlled by the Cr ion interstitial concentration in Cr₂O₃ at the phase boundary oxide/alloy and the mobility of Cr ions in Cr₂O₃. For the alloy IN 713 LC which develops a dense Al₂O₃ layer in air, a modified transition mechanism at low p(O₂) leads to the formation of Cr₂O₃ at the surface and a strong internal oxidation of Al.     

High-Temperature Scaling of Cu–Al and Cu–Cr–Al Alloys: An Example of a Non-Classical Third-Element Effect

The oxidation of three Cu–xCr–2Al and three Cu–xCr–4Al alloys (x ≅ 0,4,8 at.%) has been investigated at 800°C in 1 atm O₂. Oxidation of a binary Cu–Al alloy containing 2.2 at.% Al produced external scales composed mainly of copper oxides with small amounts of Al-rich oxide in the inner region, while the internal oxidation of Al was almost absent. The addition of 3.9 at.% Cr to this alloy was able to decrease the oxidation rate but was insufficient to prevent the oxidation of copper. Conversely, addition of 8.1 at.% Cr to the same binary alloy promoted the rather fast formation of a protective Al₂O₃ layer in contact with the metal substrate, with a simultaneous large decrease in the oxidation rate, producing a form of third-element effect. On the contrary, all the Cu–xCr–4Al alloys formed an internal Al₂O₃ layer after an initial stage during which all the alloy components were oxidized, so that the only effect of the presence of chromium was to decrease the duration of the fast initial stage. The third-element effect due to chromium additions to Cu–2Al is related to a transition from the formation of external scales composed of mixtures of Cu and Al oxides to the external growth of Al₂O₃–rich scales as a consequence of a thermodynamic destabilization of copper oxides associated with the formation of solid solutions between Al₂O₃ and Cr₂O₃.     

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.