Effect of impurities on the oxidation mechanism of nickel at 800°C

Abstract

The effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800°C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiO/outer NiO interface is planar without any segregation, while the NiO/Ni interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiO/Ni interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiO/Ni interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity.     

A Mössbauer study of the oxidation of Fe-Ni alloys at 535 and 635°C

Fe⁵⁷ transmission Mössbauer spectroscopy, supported by metallography, SEM and X-ray diffraction analysis, has been employed to study the oxidation of Fe-Ni alloys at 535 and 635° C in 1 atm. of air. With increasing Ni content of the alloy, the composition of the scale changed and the oxidation rate decreased. For an alloy containing 0.9% Ni, the oxide scale produced at 535° C was Fe₃O₄ covered by a thin outer layer of-Fe₂O₃, while at 635° C FeO was additionally present as a major phase. The scale formed on a 10% Ni alloy at both 535 and 635° C was similar to that observed for the 0.9% Ni alloy oxidized at 535° C (i.e. of Fe₃O₄ and-Fe₂O₃), although the-Fe₂O₃ layer tended to be relatively thicker. For a 49% Ni alloy, the scale at both 535 and 635° C comprised an inner layer of Ni _x Fe_3–x O₄ (withx0.5, on average) and an outer layer of-Fe₂O₃, of similar thickness. Finally, on an 83% Ni alloy oxidized at 635° C, the scale consisted of roughly equally thick layers of NiO (next to the metal) and NiFe₂O₄, and a thin outer covering of-Fe₂O₃. The decrease in oxidation rate with increasing Ni content of the alloy is discussed briefly in relation to the changing composition of the scale and diffusion in the alloy.     

Microstructure and oxidation behaviour at 1100°C of HfC containing Cr rich iron based cast alloys

Three alloys containing 0·25–0·50 wt-%C, 26–28 wt-%Cr and 4–6 wt-%Hf were elaborated by foundry. They contained a dendritic matrix and HfC carbides, which are expected to strengthen the alloy at high temperatures. They were exposed in air at 1100°C during 46 h. The aged microstructures displayed coarsened chromium carbides but no significant changes to the fraction or morphology of the HfC carbides. The surface characterisation of the oxidised samples showed that the alloys behaved well despite some localised instances of fast oxidation. The hardness was modified by the microstructure stabilisation achieved during the high temperature exposure. A preliminary test showed that the reinforcement by HfC may indeed lead to interesting creep resistance at 1100°C as had been hoped.     

High-temperature degradation mechanism of aluminide coatings on titanium alloys under cyclic oxidation at 750°C

Aluminide coatings prepared on Ti-6Al-4V substrate were able to improve oxidation resistance of the alloy under cyclic oxidation at 750°C both in dry and moist air conditions due to aluminide’s ability to form a stable alumina oxide scale. However, degradation of the coating due to spallation, cracking, internal oxidation and formation of voids with increased cyclic oxidation reduced the lifespan of the coating and the underneath substrate. The main cause of coating degradation for hot-dip specimens is cracks that initiated and propagated perpendicular to the surface. For the plasma spray specimens, the cracks are parallel to the surface. Initiation of cracks in hot-dip coatings are more accredited to residual stresses due to cooling and presence of brittle intermetallic phases TiAl₂ and TiAl. For plasma spray coatings, initiation and propagation of cracks are attributed to presence of entrapped oxides, pores and grain boundaries of the deposited splats whose flattened edges are parallel to the surface of the coating. Presence of water vapor, too, acts as an oxygen carrier and thus promotes oxidation internally, inhibits growth of continuous protective alumina oxide scales and weakens the scale/alloy interfacial toughness. Water vapor therefore accelerates degradation by increasing spallation and cracking rate of the coating.     

Characterization of alloy 3033 after exposure to superheated steam at 800°C

Alloy 3033 was evaluated in superheated steam (SHS) at 800°C for a duration of 3000 hours. The SHS was able to simulate the supercritical water (SCW) condition at higher temperatures which no commercial SCW rig is currently capable of reaching. After exposure to the SHS, the weight change and surface oxide formation of Alloy 3033 were analyzed. Alloy 3033 had an initial weight gain after 1000 hours; however, the net weight gain reduced after 2000 and 3000 hours of exposure, suggesting oxide spallation. Formation of both Cr₂O₃ and MnCr₂O₄ was observed on the surface after 2000 and 3000 hours of SHS exposure. However, as exposure progressed, the XRD peak intensity ratio of MnCr₂O₄ to Cr₂O₃ decreased, in addition to the observation of more exposed Cr₂O₃. Based on this preliminary investigation, Alloy 3033 may not be suitable for extended use in SHS due to weight loss associated with oxide spallation.     

High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na₂SO₄–60% V₂O₅) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr₂O₃, NiO and spinel NiCr₂O₄ has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Specific heats of commercial titanium alloys at 50–1100°C

Specific-heat measurements are reported for nine titanium alloys with and + structures; the temperature of the transition has been revised by DTA. A generalized approximating formula for the specific heat has been derived for the temperature range in which the structural state is stable, which contains a dimensionless parameter that includes the temperature of the transformation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 4, pp. 593–598, April, 1980.     

Isothermal embrittlement of Fe–8Mn alloys at 450°C

Abstract

Two Fe–8Mn alloys, one of which is alloy 193, stabilised with 0·17%Ti and 0·18%Al, were austenitised at 900°C, ice brine quenched and their DBTTs determined. In this condition, brittle fractures were predominantly cleavage, and thermodynamic calculations on alloy 193 showed that there were 0·0025 wt-%C and <0·03 ppm N in solid solution. Alloys were tempered for 6 min, 1 h and 10 h at 450°C and their DBTTs again determined; in this case, brittle fractures were mainly intergranular. In alloy 193, DBTT rose from 27 to 125°C in 6 min. Hardness values at 450°C were also monitored and the variation of hardness with time is discussed. It is thought that brittle fracture in alloy 193 is due to segregation of Mn per se to prior austenite grain boundaries, unlike an earlier investigation of a pure Fe–8Mn alloy (K1525), where embrittlement was due to a Mn–N and to a lesser extent a Mn–P interaction at prior austenite grain boundaries. The driving force for Mn segregation to prior austenite grain boundaries is thought to be the initial formation of reverted austenite at such sites.     

The isothermal and cyclic oxidation behaviour of Ti-48Al-2Cr at 700°C

The effect of the microstructure on the isothermal and cyclic oxidation behaviour of the intermetallic Ti–48A1–2Cr (at.%) alloy was investigated at 700°C in air up to 3000 h. Different microstructures, i.e., duplex, near gamma, nearly lamellar, and fully lamellar, obtained by various heat treatments, were used. Results of thermogravimetry showed a good oxidation resistance at 700°C against both isothermal and cyclic oxidation. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. After equal exposure times, cyclic oxidation induced a higher oxidation rate compared to isothermal oxidation. Oxidation of Ti–48A1–2Cr in air, initially resulted in the formation of α-A1₂O₃, TiO₂ (rutile), Ti₂A1N and TiN, with the latter two near the scale/substrate interface. After longer exposure times, the mixed corrosion scale was overgrown by fast growing TiO₂. The oxide scales, formed under isothermal as well as under cyclic conditions, were uniform.     

Pest disintegration of thin MoSi2 films by oxidation at 500° C

Thin molybdenum disilicide (MoSi₂) films have been produced by magnetron sputter deposition, and subjected to oxidation tests for the study of MoSi₂ pest-a phenomenon showing disintegration of a solid piece of MoSi₂ into powdery products. The as-prepared films were of an amorphous structure. Oxidation of the films in air at 500° C led first to cracking of the films, and then the cracked pieces eventually evolved into disintegrated powders with a yellowish appearance. Secondary electron microscopy and Auger electron spectroscopy revealed that the reaction products consisted of MoO₃ whiskers (platelets), Si-Mo-O fibres, SiO₂ clusters, and some residual MoSi₂. The disintegration of MoSi₂ films appeared to be independent of their crystal structure; a similar phenomenon was also observed in crystallized films, with a metastable hexagonal structure, oxidized under the same conditions. The disintegration of the MoSi₂ films is compared to and correlated with the pest reaction of bulk MoSi₂.     

Factors influencing long-term creep of type 316 steel at 600°C

Abstract

Creep tests have been carried out at 600°C on type 316 pipe and bar material, some of which was pre-aged to accelerate metallurgical changes. Solute concentrations were assessed by determining the lattice parameters and analyzing extract solutions. A transient strain-induced strengthening of ~3 × 10⁴ h duration was observed for material containing >0·025 wt-% dissolved carbon before testing; this was ascribed to a Mo–C interaction which progressively pinned dislocations. When the transient had decayed, a steady creep rate was achieved. The transient strain induced strength could be eliminated by pre-aging to reduce the dissolved carbon, and this occurred more rapidly in finer-grained material. In the steady creep regime viscous drag was rate controlling, but the effect of dissolved molybdenum was observed to become saturated in the range 0·8–2·0%. There is some evidence that this was because molybdenum acts in conjunction with nitrogen in the steady creep regime.

MST/489     

Fatigue behavior of coated and uncoated cast Inconel 713LC at 800 °C

Cylindrical specimens of Inconel 713LC in as-received condition and with surface treatment by Al diffusion coating were cyclically strained under strain control at 800 °C in air. Surface treated layer was characterized and the hardness depth profile was measured. Cyclic stress–strain response and fatigue life of both materials were assessed. The stress response of the coated superalloy specimens is lower in comparison with the untreated specimens. Beneficial effect of surface treatment on the Manson–Coffin curve is documented. Specimen sectioning and fracture surface studies revealed fatigue damage mechanisms both in coated and uncoated specimens. Propagation path of the principal crack is predominantly interdendritic.     

The isothermal section of the Al-Si-B ternary system at 700 °C and its applications

The isothermal section of the Al−Si−B ternary system at 700 °C has been calculated using FactSage software and experimentally determined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray powder diffraction(XRD). The calculated results show that there are six three-phase regions in the isothermal section: Liquid-Al+AlB₂+(Si), AlB₂+AlB₁₂+(Si), (Si)+AlB₁₂+SiB₃, AlB₁₂+SiB₃+SiB₆, AlB₁₂+SiB₆+SiB₁₄, AlB₁₂+SiB₁₄+(B). Four three-phase regions of Liquid-Al+AlB₂+(Si), AlB₂+AlB₁₂+(Si), (Si)+AlB₁₂+SiB₃ and AlB₁₂+SiB_n+(B) were confirmed experimentally, which are consistent with calculated results. The small AlB₂ particles in the Al-3B alloy have good refining effect on the primary α-Al phase in the Al-10Si alloy, which greatly refines the microstructure and improves the mechanical properties of the Al-10Si alloy.     

Effect of ion implantation on the strength of sapphire at 300–600°C

Ion implantation with ¹¹B⁺ or ²⁸Si⁺ at 1000°C doubled the ring-on-ring flexure strength of c-plane sapphire disks tested at 300°C but had little effect on strength at 500 or 600°C. Disks were implanted on the tensile surface with 2 × 10¹⁷ B/cm² (half at 40 keV and half at 160 keV) or 1 × 10¹⁷ Si/cm² (80 keV). Sapphire implanted with 1 × 10¹⁸ B/cm² had only half as much flexure strength at 300° or 500°C as sapphire implanted with 2 × 10¹⁷ B/cm². Implantation with B, Si, N, Fe or Cr had no effect on the c-axis compressive strength of sapphire at 600°C. Boron ion implantation (2 × 10¹⁷ B/cm², half at 40 keV and half at 160 keV) induced a compressive surface force per unit length of 1.9 × 10² N/m at 20° and 1.4 × 10² N/m at 600°C. The infrared emittance at 550–800° of B-implanted sapphire at a wavelength of 5 m increased by 10–15% over that of unimplanted sapphire. Infrared transmittance of sapphire implanted with B, Si or N at either 1000°C or 25°C is within 1–3% of that of unimplanted material at 3.3 m. Implantation with Fe or Cr at 25°C decreases the transmittance by 4–8% at 3.3 m, but implantation at 1000°C decreased transmittance by only 2–4% compared to unimplanted material.     

Limits of the oxidation resistance of several heat-resistant steels under isothermal and cyclic oxidation as well as under creep in air at 650°C

In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidation leads to Cr subsurface zone depletion the question arises as to how the oxidation behaviour is affected by the decrease in Cr concentration with oxidation time. Four ferritic heat-resistant commercial steels containing 9–12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650°C in air to determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for times up to 3000 hours. In addition to isothermal tests, thermal cycling tests and creep tests were also performed. Surprisingly large differences in oxidation behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9Cr steels while the other one was best. Thermal cycling improved the oxidation behaviour of the steels which was worse under isothermal conditions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal subsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical value which had been determined by model calculations. The tendency observed under isothermal conditions is enhanced by superimposed creep deformation. It is concluded from the results that growth stresses in the oxide scales combined with the actual Cr-concentration in the metal subsurface zone play a major role in oxidation resistance.     

Elastic,piezoelectric, and dielectric properties of lanthanum-gallium tantalate crystals at temperatures of 20–600°C

Measurements of the temperature variations in the relative dielectric constants e₃₃^T/e₀ \varepsilon_{33}^T/{\varepsilon_0} , e₁₁^T/e₀ \varepsilon_{11}^T/{\varepsilon_0} ,and e₂₂^T/e₀ \varepsilon_{22}^T/\varepsilon {}_0 , conductivities g₁₁, g₂₂, and g₃₃, elastic constants C₁₁^D C_{11}^D , C₆₆^D C_{66}^D , and S₂₂^E S_{22}^E , and electromechanical coupling constants k_t, k₂₆, and k₁₂ of lanthanum-gallium tantalate over temperatures of 20–600°C are reported. Data are presented on the effect of lifetime (breakdown) testing on these characteristics of the piezoelectric crystals after retention at a temperature of 625°C for 250 h.