Effects of Nb and Si on high temperature oxidation of TiAl

The isothermal oxidation behavior of TiAl based alloys at 900℃ in air with a combination of Nb (5%-10%, mole fraction) and Si (1%-5%, mole fraction) was investigated. The microstructure and the composition of the oxidation scale were studied by using XRD, TEM and EPMA. The results show that the combination of Nb and Si can improve the oxidation resistance of the alloys significantly. The element Si can change the typical microstructure of oxidation scale on TiAl based alloys. In alloys with Si addition, the compact Al2O3 forms in the interior side of oxidation scale. When x (Si)3%, the Ti5Si3 phase forms and the coarse crystal TiO2 forms on Ti5Si3 phase after oxidation. The increase of Nb content in the TiAl based alloys impedes the growth of Ti5Si3 phase, and the formation of TiO2 on surface and on Ti5Si3 phase is also impeded.     

Effect of Nb on oxidation behavior of NiTiNb alloys

The oxidation behavior of NiTi and NiTiNb alloys containing different amounts of Nb （7%, 9%, mole fraction） were studied at 800℃ in air. It is found that the oxidation resistance of NiTi alloy can be effectively increased by the Nb addition. Under the same oxidation condition, the mass gain of NiTi is about 7 mg/cm^2, while the inass gains are only 3 mg/cm^2 for Ni47Ti44Nb9 alloy and 2.4 mg/cm^2 for Ni52Ti41NbT. Moreover the oxidation resistance of single phase NiTiNb alloy is better than that of the dual-phase alloy with large amount of Nb precipitates. On the basis of thermodynamics and kinetics of oxidation, the effect of Nb alloying element on the oxidation behavior of NiTi-based alloys was discussed.     

Oxidation behavior of a novel multi-element alloyed Ti_2AlNb-based alloy in temperature range of 650-850 ℃

The oxidation behavior of a novel multi-element alloyed Ti_2AlNb-based alloy (Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si) was studied in the temperature range of650-850℃. X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy-dispersive spectroscopy (EDS) were used to identify the phase constituents and microstructure of the scales formed on the specimens after oxidation at different temperatures.Isothermal oxidation tests show that the oxidation rate of the alloyed Ti_2AlNb-based alloy is obviously reduced at all temperatures, and the mass gains are very low for this alloy in comparison with those of Ti-22Al-25Nb alloy. The alloying elements Mo, V, Zr and Si have an obvious affect on the oxidation products of the alloys. The improved oxidation resistance for the alloy is ascribed to the introduction of Mo, V, Zr and Si elements, which are beneficial to the selective oxidation of Al to form protective oxides while are disadvantage of the formation of poor oxidation resistance oxides such as AlNb0_4.     

Microstructure and High-Temperature Oxidation Behavior of Dy-Doped Nb–Si-Based Alloys

Microstructures and oxidation behaviors of four Dy-doped Nb–Si-based alloys at 1250℃ were investigated. The nominal compositions of the four alloys are Nb–15Si–24Ti–4Cr–2Al–2Hf–xDy(at.%), where x = 0, 0.05, 0.10 and 0.15,respectively. Results showed that the four alloys all consisted of Nbss, αNb_5Si_3 and γNb_5Si_3, and the addition of Dy produced no obvious effect on the phase constitution and the microstructures of Nb–Si-based alloys. After oxidation at 1250℃ for 58 h, it was found that the addition of Dy accelerated the oxidation rate of Nb–Si-based alloys and caused a larger weight gain, accompanied by the formation of a more porous and less protective oxide scale. The oxides of Nb_2O_5,Ti_2Nb_(10)O_(29), TiNb_2O_7, Ti_(0.4)Cr_(0.3)Nb_(0.3)O_2 and glassy SiO_2 were formed on Dy-doped Nb–Si-based alloys. The hightemperature oxidation mechanism of Dy-doped Nb–Si-based alloys was discussed.     

FORMATION OF SILICONIZED LAYER ON TiAI-BASED ALLOY AND THE OXIDATION RESISTANCE

The Ti-48Al alloy was pack siliconized with 15%Si+85%Al2O3. The microstructure of the siliconized coating on the TiAl-based alloy was analyzed and its effect on oxidation resistance was investigated. The specimens before and after cycle oxidation were examined by XRD and SEM equipped with XEDS. The results showed that the coating is composed of a thin Al2O3 outer layer and a composite inner layer of Ti5Si3 with an appropriate amount of Al2O3 dispersed in. Cycle oxidation tests showed that the high temperature oxidation resistance of TiAl-based alloy was greatly improved by forming such composite coating. No spaliation and crack happened and the weight gain was very small after cycle oxidation at 900℃ for 314h.     

Effect of silicon on oxidation of Ni-15Al alloy

1　INTRODUCTIONChromium, aluminum, and silicon can formsatisfactory protective scales on Ni based alloys.Chromium is expensive and not suitable for use attemperatures above 1 000℃ due to the evaporationof CrO3. It has also been well established that theincorporation of Si in many alloy systems has abeneficial effect on their oxidation resistance[1, 2].In addition, silicon is abundant and cheap. More over, Si has one of the largest solubility in Ni3Alwhere it …     

Corrosion behavior of Zr–Nb–Cr cladding alloys

Zr-Nb-Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corro- sion resistance of ZrxNb0.1Cr （x = 0.2, 0.5, 0.8, 1.1; wt%） alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zrl.lNb alloy, which is ascribed to Zr（Cr,Fe,Nb）2 precipitates with a much larger size than β-Nb.     

Effects of Mo substituted Nb on magnetic properties of Finemet alloy

Different Mo contents have been added into traditional Finemet alloy to form Fe73.5Cu1Nb3-x MoxSil3B9.5( x = 0 - 3) alloys. The change in DC and AC magnetic properties with Mo for Nb substimtlon was investigated. The results show that, with adding Mo, although the DC relative permeability decreases and the coercive force increases slightly,the saturation flux density Bs can be increased, and the core loss of the alloy can be decreased. The AC permeability of samples contained Mo is higher than that of alloy without Mo content. Fe73.sCu1Nb1Mo2Si13B9.5 alloy has the highest saturation flux density Bs. Fe73.sCu1Nb2Mo1Si13B9.5 alloy has the best frequency dependence on the AC permeability and core loss.     

Direct electroless Ni-P deposition on AM50 magnesium alloy from sulfate bath

A bright electroless Ni-P deposition on AM50 magnesium alloy in a sulfate plating bath was proposed by using direct plating process with non-chromate pretreatment. The electroless Ni-P plating on AM50 magnesium alloy has an admirable appearance and good adhesion. The results indicate that the electroless Ni-P deposition with non-chromate pretreatment has better adhesion than that of zinc immersion coating. Anodic polarization curves indicate that the electroless Ni-P deposition obtained from the sulfate bath has similar corrosion-resistance to that obtained from basic nickel carbonate bath. The deposition process generates less pollutant by a non-chromate plating bath and is suitable for the magnesium alloys manufacture because of its low cost. The hardness of the electroless Ni-P plated AM50 is about HV 720.6 and HV 969.7 after heat treatments at 180℃ for 2 h. The wear resistance of Ni-P plated magnesium alloy specimens is about 5 to 9 times as high as that of bare magnesium alloys.     

Static oxidation behaviour of silicide coating on niobium alloy at high temperature

A silicide coating was prepared on the surface of the Nb521 alloy by the complex pack cemented method. The oxidation resistance properties of the present coating were exeamed by the static oxidation tests at 1 700 ℃in air. The compositions and the microstructures of the coating before and after test were characterised and analysed through scanning electron microscopy （SEM）, X-ray diffraction analysis （XRD）, energy dispersive X-ray spectrometry （EDS） and electron probe microanalysis （EPMA）, respectively. The present silicide coating can provide an effective protection for the Nb alloy for 25 h at 1 700 ℃ in air. The results show that the oxidation kinetics of the present silicide coating is parabolic. The diffusion of Si leads to the phase transformation and evolution during the oxidation.     

Oxidation of molybdenum-tungsten-chromium-silicon alloys

The oxidation kinetics of (50–60) wt.% Mo-(35–47) wt.% Cr-(2–5) wt.% Si and (30–40) wt.% Mo-(30–40) wt.% W-(27–37) wt.% Cr-(0–3) wt.% Si alloys were studied between 1000 and 1200°C in a pure oxygen atmosphere. The oxidation of Mo-W-Cr-Si alloys resembled that of Mo-Cr-Si alloys but was much more oxidation resistant. In general, oxidation resistance increased with increasing Cr and Si content. Alloys with good oxidation behavior had a thin outer Cr₂O₃ layer and an internal oxidation zone (in both Mo and Mo-W alloys). Alloys displaying poor oxidation behavior had a porous Cr₂O₃ layer (in Mo alloys) or layers of oxides of W and Cr (in Mo-W alloys). Although the alloy systems were not truly oxidation resistant, definite improvement in oxidation resistance was achieved.     

Further aspects of the oxidation of binary two-phase alloys

The corrosion behavior of binary, two-phase alloys is considered in which the matrix contains mostly the less-noble metal that forms a fast-growing oxide, while the second phase is rich in a component that forms a more stable but slowly-growing oxide. It is assumed that the second phase exists as a dispersion of isolated, rod-like particles. It is further assumed that both phases form external films with no internal oxidation. It is shown that the oxidation behavior of this type of alloy depends on both the oxidation time and the size of the second-phase particles. In particular, for short oxidation times and large second-phase particles the matrix will oxidize faster than the dispersed phase, so that the dispersed particles will be only partly corroded or even incorporated into the matrix-oxide scale as unoxidized islands, forming an irregular alloy-scale interface. On the contrary, for long times and small particle sizes the two phases will tend to oxidize at approximately the same rate, leading to the formation of regular alloy-scale interfaces. The time for the transition between the two corrosion regimes depends not only on the ratio between the rate constants for the growth of the two oxides but also on the size of the dispersed-phase particles, smaller sizes producing shorter transition times. Eventually, under favorable conditions the formation of the fast-growing oxide may even stop, leading to the formation of a protective layer of the most-stable oxide.     

The oxidation of Ni-70 wt.%Cr in oxygen between 1073 and 1473°K

Oxidation of the relatively simple, two-phase alloy Ni-70 wt.%Cr in oxygen between 1073 and 1473°K results in the formation of a Cr₂O₃ scale containing less than O.5 wt.% Ni in solid solution. The oxidation kinetics are irreproducible for an initial period, which is brief at 1073 and 1273°K but much more pronounced at 1473°K, both in duration and degree. This behavior is associated with the failure of the protective Cr₂O₃ scale. However, after longer periods a compact layer of Cr₂O₃ becomes established under isothermal conditions and results in a change to more reproducible kinetics, especially at 1073 and 1273°K. Oxidation causes chromium depletion and the formation of a single-phase zone which separates the scale and the two-phase bulk alloy. The depth of Cr₂O₃ internal oxide coincides with this zone. The oxidation behavior is compared with that of more Ni-rich, single-phase Ni-Cr alloys, with particular reference to the effects of the constitution of the underlying alloy and the integrity of the protective oxide.     

Preferential oxidation of carbon during the initial stage of oxidation of an Fe-0.8%C alloy at 400°C

The preferential oxidation of carbon in an Fe-0.8%C alloy during the first 60 min of oxidation in air at 400°C was studied by a thermogravimetric method and by measuring the quantity of evolved carbon oxides. The morphology of the external oxide surface depended on the type of exposed phase—a rosettelike oxide grows over ferrite, whereas the oxide surface over cementite is relatively smooth. A possible mechanism for the preferential oxidation of carbon and its subsequent cessation is proposed.Formerly of Institute of Materials Engineering, Academy of Mining and Metallurgy, Cracow, Poland.     

The transition from the formation of mixed scales to the selective oxidation of the most-reactive component in the corrosion of single and two-phase binary alloys

The conditions for the transition from the formation of mixed scales to the exclusive oxidation of the component B, forming the most stable oxide, are examined for both single-phase and two-phase binary A-B alloys by taking into account the displacement of the alloy-scale interface due to the growth of the protective oxide. This procedure eliminates the inconsistencies arising from Wagner's classical treatment for single-phase alloys when the interdiffusion coefficient in the alloy is small with respect to the parabolic rate constant for outer-scale growth; but the same procedure leads to a significantly-improved treatment also for two-phase alloys. For the latter systems, the transition is shown to depend also on the solubility of B in the A-rich phase.Moreover, the exclusive growth of the most-stable oxide is more difficult than for single-phase alloys because it requires higher average concentrations of B in the alloy and may even become impossible if the parabolic rate constant of oxidation is large with respect to the interdiffusion coefficient in the alloy.     

The internal oxidation of two-phase binary alloys beneath an external scale of the less-stable oxide

The internal oxidation of two phase binary A-B alloys by a single oxidant at high temperatures, under partial pressures sufficient to also form external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of single-phase alloys. It is shown that, depending on various factors, particularly on the solubility and diffusivity of the most-reactive component B in the most-noble component A, this process may or may not involve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an increase of the rate of internal oxidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to single-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.     

The internal oxidation of two-phase binary alloys under low oxidant pressures

The main features of the internal oxidation in two-phase binary alloys are examined for insignificant and important diffusion of the most-reactive component and are compared with the behavior of corresponding single-phase systems. It is shown that two-phase alloys may have two different types of internal oxidation, one of which is similar to that of the single-phase alloys (classical type), producing a uniform distribution of small oxide particles in the zone of internal oxidation, while another is typical of two-phase systems and involves the in situ conversion of the most-reactive component into its oxide. It is also shown that, under the same values of all the relevant parameters, the classical internal oxidation of two-phase alloys involves faster kinetics and smaller degrees of enrichment of the most-reactive component in the zone of internal oxidation than for single-phase alloys. As a consequence of this, the transition to the external oxidation of the most-reactive component in these systems involves higher overall concentrations of the most-reactive component than in corresponding single-phase alloys.     

Factors affecting the high-temperature oxidation behavior of some dilute nickel- and cobalt-base alloys

Oxidation of nickel- and cobalt-base alloys, containing small additions of a higher valent second metal, in oxygen or air at high temperatures results in the formation of relatively complicated scale morphologies which change subtly with increasing additions of the second element and its characteristics. The various factors that can influence the oxidation behavior of such alloys are assessed and correlated with the oxidation kinetics and scale morphology types. For very dilute alloys the increase in oxidation rate compared with that of the corresponding pure metal (nickel or cobalt) is largely due to doping of the external oxides. However, once the solubility limit of the second metal in this oxide is exceeded, additional increases in second metal content of the alloy can either increase further or decrease the oxidation rate. The exact behavior depends on the relative interplay of factors such as internal oxide formation and coalescence, blocking effects of incorporated internal oxide or pores in the scale, short-circuit paths through the scale, doping, and the relative diffusion rates of the two metals in the scale. Probable rate-determining steps for oxidation of different alloy composition ranges are proposed.     

The formation of solid solution oxides during internal oxidation

The diffusion processes occurring when binary alloys react with oxygen to form an oxide that contains both alloy components in solid solution, either exclusively as internal oxide or in combination with a surface scale, have been analyzed and compared with experimental results for Fe-Mn and Ni-Co alloys. The experimental results available for the Fe-Mn system were obtained under conditions of exclusive internal oxidation, and good agreement was obtained between calculated and experimental results. In the Ni-Co system, a surface scale and a zone of internal oxidation develop. Agreement between calculated and experimental depths of internal penetration is acceptable if the diffusivity of oxygen in the alloy is 3.8×10^–6 cm²/sec at 1100°C. Agreement between calculated and experimental concentration profiles is not very good.List of Principal Symbols B alloy component with higher affinity for oxygen - BO more stable scale component - a _O activity of oxygen - D _O diffusivity of oxygen in the alloy - D _O ^eff effective diffusion coefficient of oxygen in alloy - f volume or mass fraction of internal oxide - f _max maximum volume or mass fraction of internal oxide - G _AO , G _BO free energies of formation of oxides AO and BO, respectively - N _B mole fraction of component B in the alloy - N _B ^O bulk mole fraction of component B in the alloy - N _BO mole fraction of oxide BO in oxide phase - N _O atomic fraction of oxygen dissolved in alloy - N _O ^I ,N _O ^II atomic fraction of oxygen dissolved in alloy at the internal oxide-surface scale and alloy-internal oxide interfaces, respectively - R gas constant - r ratio of number of moles of precipitated oxide to total number of moles of metallic constituents in the alloy - T temperature - t time - X ₁,X ₂ positions of internal oxide-surface scale and internal oxide-alloy interfaces, respectively - x position coordinate - defined as [–1/RTG _BO ] - ₁, ₂ dimensionless rate constants describing rate of displacement of the internal oxide-surface scale and internal oxide/alloy interfaces, respectively - _O Henry's law activity coefficient for oxygen dissolved in alloy - defined as [–1/RT(G _BO –G _AO )]     

Stability of protective oxide films on Ti-base alloys

Thermodynamic calculations are performed to estimate isothermal sections of Ti-Al-O, Ti-Si-O, and Ni-Al-O phase diagrams. Very small aluminum levels (<10^–10 at. %) are needed to stabilize alumina on Ni-Al alloys. However, much higher aluminum (50%) and silicon (40%) levels are needed to stabilize alumina and silica on Ti-Al and Ti-Si alloys, respectively. These calculations suggest that the mechanism of formation of the protective oxide films on titanium-based alloys is radically different from that on nickel-based alloys. The aluminum levels needed to form a continuous film of alumina on nickel-based alloys are dominated by kinetic factors. On the other hand, thermodynamic factors appear to dominate the alloy compositions needed to form protective films of alumina and silica on titanium-based alloys. Further work is needed to evaluate any possible role of kinetic factors.