In Situ Measurement of Stresses and Phase Compositions of the Zirconia Scale During Oxidation of Zirconium by Raman Spectroscopy

Raman spectroscopy was used to determine the stress and phase compositions of the zirconia scale in situ during the oxidation of zirconium at 600–900 °C in ambient air. The results show that the compressive stresses in the zirconia scale vary with the oxidation temperature and the oxidation time. The tetragonal (t) phase forms at the metal/oxide interface and the t to monoclinic phase transformation occurs far away from the interface during the oxide scale growth. The compressive growth stress at the oxidation temperature is favourable to the formation of t phase.     

Oxidation properties of Zr-Nb alloys at 500-600 °C under low oxygen potentials

Zr-Nb alloys with 1-10 wt.% Nb content were oxidized at 500-600 °C in the CO-CO₂ gas mixtures. The oxidation weight gain increased with Nb content and the kinetics except for Zr-1Nb alloy changed from cubic rate to linear one at 600 °C for a long period of time, 7 d. The cubic rate constant was almost insensitive to oxygen potential of oxidizing atmosphere. As the oxidation resistance deteriorated, the volume ratio of tetragonal to monoclinic zirconia phase and the relevant compressive stress in oxide film decreased with increase of Nb content. Before and after oxidation, Nb re-distribution could not be observed under the present experimental condition.     

Internal Oxidation of Ternary Alloys Forming a High Oxygen Conductive Oxide

Five ternary alloys consisting of a noble base metal (Ni, Co, Fe, Cu) and two reactive metals (Zr + Y, Ce + Gd) being able to form a high oxygen ion conductive oxide were internally oxidized under low oxygen partial pressures. All alloys developed either a continuous yttria-stabilized zirconia phase or a continuous gadolinia-doped ceria phase behind the front of internal oxidation. A Ni–Ce–Gd alloy showed extraordinarily high internal oxidation rates of up to 120 µm²/s at 900 °C. High internal oxidation rates in these ternary alloys were not limited to low concentrations of the reactive metals. The type of the internal oxide phase was found to be more important for the internal oxidation kinetics than the noble base metal.     

Tetragonal phase stability in ZrO2 film formed on zirconium alloys and its effects on corrosion resistance

A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO₂ film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO₂ grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.     

Influence of minor additions on the emissivity of the oxide scale of FeCrAlY alloys during resistance heating

Chromium aluminum yttrium (FeCrAlY) alloys owe their low oxidation rate to the formation of a slow growing α‐alumina scale. For material used for heating elements not only the life time and the behavior of the resistance during the life time is of relevance, but also the emission coefficient of the oxide scale. The power density J_S produced by resistance heating of strip with 50 µm thickness and about 5–6 mm width at 1050 °C is approximately equal to the radiant flux density, which is according to Stefan–Boltzmann's law proportional to the total emission coefficient ε_g. Resistance heating tests were performed on samples made from FeCrAlY alloys with different zirconium and carbon content. The “high zirconium” containing FeCrAlY alloys (zirconium > about 0.10%) have a higher power density/emissivity than the “low zirconium” alloys. In parallel with this, all samples with higher power density/emissivity have internal oxidation and therefore a “rough” metal–oxide interface. Thus, one cause for the increase of the emissivity of the scale could be this rough metal–oxide interface; other causes could be a higher amount of zirconium incorporated into the scale, more pores and/or different grain structure in the scale. Additionally the carbon content influences the appearance of a higher emissivity and the internal oxidation.     

Cyclic Oxidation of Heat Resisting Steels

Standard cast, heat resisting steels containing 25–29 w/o (weight percent) chromium and 30–36 w/o nickel together with cast alloys containing 45 or 60 w/o nickel plus low levels of aluminium were subjected to cyclic oxidation in air at 1000 and 1150°C. The standard materials suffered rapid weight loss which was somewhat mitigated by the presence of cerium. The 45 w/o nickel alloys were much more resistant and the 60 w/o nickel alloys showed superior resistance to cyclic oxidation. This improvement was due to alumina formation at or near the alloy surface. In the absence of aluminium, alloys underwent subsurface chromium carbide oxidation at a rate independent of alloy chromium content. This effect is shown to be a consequence of rapid oxygen diffusion along internal phase boundaries.     

Internal Oxidation of a Ag-1.3 at.% Se Alloy. Part II: Kinetics of Internal Oxidation

The kinetics of internal oxidation of a two-phase Ag-1.3 at.% Se alloy were studied at 750, 800 and 830°C in pure oxygen. The parabolic rate law was followed to a reasonable extent for the first 2 hr of oxidation. The rate of internal oxidation was much faster than expected from the classical Wagner model. Three major factors affected this: (i) the non-uniform distribution of Ag₂Se particles in the as-cast alloy caused non-uniform internal oxidation of the alloy; (ii) the formation of a liquid phase during in-situ oxidation of Ag₂Se particles accelerated the diffusional processes and lowered the activation energy of the oxidation reactions (a sort of catastrophic internal oxidation); (iii) gradual oxidation of the alloying element in the form of second-phase particles caused the extension of the single internal-oxidation front (IOF) to the internal-oxidation volume (IOV) between the inner and outer IOFs. The IOV requires a much lower oxygen quantity for its advancement than the (single) IOF thereby enabling much faster internal oxidation of two-phase alloys that oxidize in the in situ (diffusionless) mode. The kinetics are, also discussed according to the treatment which was proposed for internal oxidation of two-phase alloys by Gesmundo.⁷     

Zirconia Layer Formed by High Temperature Oxidation of Pure Zirconium: Stress Generated at the Zirconium/Zirconia Interface

Oxidation of pure zirconium metal at high temperature (500 and 600 °C) under air at normal atmospheric pressure was investigated using the Raman Spectroscopy technique. Analysis of the absolute intensities as well as the positions of the Raman bands for the tetragonal and the monoclinic zirconia phases was performed. Evolution of the thermal stress has been presented and discussed in comparison to the Raman mode shift recorded in situ during cooling. Ex-situ analyses of cross-sections confirm the presence of tetragonal phase preferentially located close to the metal/oxide interface and show the existence of a relaxed and highly disordered tetragonal phase preferentially located in the outer part of the scale. Using a micro tension––compression machine, it is shown that compression loads lead to a significant intensity change of the Raman peaks for the tetragonal zirconia. The effect of tension load appears less clear which demonstrates that the relation between Raman peak shift and stress is not as simple as generally considered.     

锆微弧氧化表面处理技术研究进展

锆及锆合金是重要的核结构材料和有潜力的生物医用材料,但在实际应用中,腐蚀、磨损易造成其失效,而适当的表面改性是提高它们服役性能的有效手段。重点介绍了锆及锆合金微弧氧化（MAO）表面处理技术的研究现状,讨论微弧氧化过程中电压电流特征及微弧放电机理,总结电解液体系及电参数对锆微弧氧化膜生长及膜层性能的影响规律,最后指出目前存在的问题和后续的研究方向。锆微弧氧化膜硬度高,致密性好,能大幅度提升基材的抗磨损和抗腐蚀性能。因此,锆微弧氧化技术在核电及生物医学领域有着很好的应用前景。此外,电解液中铝、硅元素进入微弧氧化膜后可以稳定膜层中高温氧化锆相（t-ZrO2）,避免膜层中应力集中和微裂纹的产生。用P和Ca元素修饰后的锆微弧氧化膜具有较好的生物活性、抗体液腐蚀和抗菌性能。     

Penetration of oxygen along grain boundaries during oxidation of alloys and intermetallics

Fast penetration of oxygen into grain boundaries and intergranular oxidation of -NiAl has been observed. Since the solubility of oxygen in NiAl is virtually nil, special ways of oxygen ingress at grain boundaries have to be presumed. Selective intergranular oxidation of binary alloys and fast penetration of oxygen along grain boundaries were analyzed by computer simulation. Interdiffusion caused by consumption of the less-noble component by oxidation at the metal-oxide interface leads to deviation of the alloy composition from the original value. When the diffusivity of the less-noble component is higher than the diffusivity of the other component, a grain-boundary Kirkendall effect may lead to void-chain formation. Experimental evidence for this phenomenon is presented. The deviation in composition and void formation were considered as processes influencing the effective oxygen diffusivity. Both processes were found to allow penetration of oxygen as fast as grain-boundary interdiffusion occurs. In addition, oxygen penetration during intergranular internal oxidation when oxides form at voids beneath the metal-oxide interface was analyzed and treated as a self-propagating process. In this case, fast oxygen penetration is accompanied by fast internal oxide formation and fast displacement of the metal-oxide interface.     

Effect of carbide volume fraction on the oxidation of austenitic Fe‐Cr‐C alloys

A series of Fe‐15Cr‐(2‐3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M₂₃C₆ precipitates. The alloys were oxidized in the austenitic state at 850°C in pure oxygen, with and without a pre‐oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia‐rich scaling took place if the chromium concentration at the metal‐scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre‐oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide‐free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.     

Die Oxydation von intermetallischen Zirkoniumlegierungen in Sauerstoff und Wasserdampf bei 400 °C

The oxidation of intermetallic zirconium alloys in oxygen and steam at 400°C The author has studied the oxidation behaviour of some intermetallic zirconium alloys in oxygen and steam. Depending on their behaviour the alloys can be divided into three categories:

• 1 Selective oxidation of alloying elements in the case of the alloys ZrAl₃ et ZrSi
• 2 Selective oxidation of Zr produced by rearrangement and enrichment of alloying elements in the case of Zr₄Sn, ZrCU₃ ZrNi, ZrV₂ and ZrMo_2/
• 3 Oxidation control by mixed oxides in the case of ZrNb and ZrTa

The mechanism of the corrosion of ZrAl₃ and ZrSi₂ in steam is interpreted in terms of the selective attack of Al and Si respectively.     

The mechanism of oxidation of dilute NiAl alloys at 800–1200°C

The oxidation behaviour of dilute NiAl alloys at 800–1200°C in flowing oxygen at 1 atm pressure has been studied using kinetic measurements, optical and scanning electron microscopy and electron probe micro-analysis. The oxidation rates of Ni0.5 to 4%Al alloys are greater than the corresponding values for nickel at 1000 and 1200°C, but less at 800°C. The increased rates at the higher temperatures are largely due to increases in the total cation vacancy concentration in the scale, although internal oxide formation can make a significant contribution to the oxidation rate. The decreased rates at 800°C are almost certainly due to a build-up of Al₂O₃ particles at the oxide/alloy interface. The roles played in the oxidation processes by doping, internal oxidation, blocking effects in the oxide, dissociation of NiO and gaseous transport of oxygen within the scale are considered in detail and related to the oxidation rates of the various alloys.     

The corrosion of two NiNb alloys under 1 atm O2 at 600–800 °C

The oxidation of two NiNb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in pure oxygen under 1 atm O₂ at 600–800 °C. The scales formed on both alloys under all conditions show an external scale, generally duplex, containing an outermost layer of nearly pure NiO and an innermost region of NiO mixed with the double NiNb oxide NiNb₂O₆. Moreover, the samples corroded at all temperatures also show a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both alloy phases Ni₈Nb and Ni₃Nb. No important depletion of niobium was observed in the alloy close to the interface with the zone of internal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen pressures at the same temperatures. The corrosion mechanism of these alloys is examined with special reference to the effects of the low solubility of niobium in nickel.     

The oxidation of Ni-Nb alloys at low-oxygen pressures at 600–800°C

The oxidation of two Ni–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600° C and 10^–20 atm O₂ at 700 and 800° C, these pressures being less than the dissociation pressure of nickel oxide. The scales formed on both alloys at 600 and 700° C show only a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both the metal phases present, i.e., Ni₈Nb and Ni₃Nb. Only small, or even no, Nb depletion was observed in the alloys close to the interface with the zone of internal oxidation at these temperatures. On the contrary, samples of both alloys corroded at 800° C produced a continuous external scale of niobium oxides without internal oxidation. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in nickel.     

The oxidation behaviour of austenitic FeCrNi alloys containing dispersed phases

The high temperature oxidation behaviour of FeCrNi austenitic alloys containing 1% Ti which, in some cases, had been converted into an oxide dispersion has been examined. The oxide dispersions were produced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. Generally, the effect of the dispersed oxide was much less pronounced than in corresponding nickel-free, ferritic alloys. Nevertheless, the time-to-breakaway of the protective Cr₃O₃ scale which developed on Fe18CrNi alloys was substantially increased, although the differences between the untreated and the internally oxidized alloys reduced with increasing nickel content. An Fe14Cr20Ni alloy did not show any improvement after internal oxidation. Unlike the ferritic alloys, no coarsening of the dispersoid phase was observed during exposure.     

Enhanced Oxygen Diffusion Along Internal Oxide–Metal Matrix Interfaces in Ni–Al Alloys During Internal Oxidation

A study of the internal oxidation of dilute Ni–Al alloys in an NiO/Ni Rhines pack was performed at 800, 1000, and 1100°C. Considerable deviations from the classical internal oxidation model have been observed. The rate of internal oxidation depends not only on the concentration of the alloying element but also on its nature, which contributes to determining the size, shape, orientation and distribution of the internal oxide precipitates. For instance, the precipitates in the Ni–Al alloys are continuous rods, arranged in a cone-shaped configuration that extends from the surface to the internal oxide front. The observed depths of internal oxidation for the various concentrations of aluminum are discussed and related to the morphologies of the internal oxide precipitates. The apparent N^(s) _oD_o values determined from internal oxide penetrations increase with increasing solute content in the alloy. It is postulated that diffusivity of oxygen is enhanced along the internal oxide–metal matrix interface compared with that in the metal matrix.     

锆合金氧化膜在真空退火过程中的氧扩散行为

针对已预腐蚀生成一定厚度氧化膜的Zr-Sn-Nb合金,研究了其在不同温度下进行真空热处理过程中的氧扩散动力学及亚稳相演变行为。结果表明,真空退火后氧化膜变薄,氧在氧化锆基体中的扩散增强,并计算了特定合金中氧的扩散系数。退火后微观化学分析表明亚稳相层厚度增加。固溶氧锆基体(Zr(O))层也明显增厚。针对该现象,讨论了对应氧扩散及亚稳相形成过程：该扩散极为可能是由ZrO₂和(Zr(O))之间存在的氧含量梯度以及锆基体的高氧溶解度造成,受抑制的氧化速率将促进亚稳相的生长。在实际水腐蚀情况下,氧化及氧化膜向基体溶解过程应该是共存的。当氧化速率受限时,氧化膜向基体溶解作用将增强,有利于形成较厚的亚稳层。     

Phase stability of the two isomorphs monoclinic zirconia and hafnia under MeV ion irradiation

Zirconia and hafnia are known to exhibit similar phase transitions under temperature, pressure or swift heavy (i.e., GeV) ion irradiation. In the present study, both monoclinic zirconia and hafnia were irradiated with 5-MeV I ions in order to comprehend the underlying mechanism governing phase transition by low-energy ion irradiation. Surprisingly, it was found that, although monoclinic zirconia can easily transform to the tetragonal phase, monoclinic hafnia does not show any transition to the tetragonal structure even after irradiation with ～19 displacements per atom. The absence of this phase transition in hafnia and its presence in zirconia clearly break the parallel between these two oxides in their response to intense excitation. Furthermore, this comparative study allowed severe constraints to be imposed on the possible mechanisms aimed at explaining phase transformation by low-energy ion irradiation. It is found that the monoclinic-to-tetragonal phase transition in these oxides is very likely driven by oxygen vacancies and occurs once their concentration reaches a certain value. The absence of phase transition in hafnia can thus be explained by the lower concentration level attained by these defects, owing to their higher diffusivity in this material. A similar mechanism can also be invoked to explain the remarkable phase stability of tetragonal zirconia under prolonged low-energy ion irradiation.     

Factors Affecting Chromium Carbide Precipitate Dissolution During Alloy Oxidation

Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr₂O₃ scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr₂O₃ scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr₂O₃scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr₂O₃ growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr₂O₃ scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.