Identification of diffusing species and the dynamic nature of diffusion paths during oxidation of a dilute Ni-Cr alloy

The results of an investigation of oxidation of a Ni-1 at.% Cr alloy are presented. Photolithographic marker experiments revealed that the markers were found to reside at the interface between a predominantly columnar outer NiO layer and a very fine grain inner layer of NiO, indicating that substantial oxygen ingress had occurred through the columnar scale. New oxide growth at the metal-oxide interface requires the oxidant to be transported across the oxide layer. Since the measured diffusion rate of oxygen ions along grain boundaries and through the lattice is much too slow to account for the observed microstructural growth (1: 1 ratio of inner and outer layers), it is necessary to postulate that the oxidant traverses the scale along some type of short-circuit path other than grain boundaries. Extensive formation of elongated pores and pipelike channels was observed along columnar oxide grain boundaries. Thus, it appears that the transport of oxygen occurs via voids (pores) formed by vacancy coalescence at the columnar grain boundaries. These pores appear to open and close continuously. Formation of new fine-grained oxide in these pores was observed to have sometimes completely resealed the void, suggesting a dynamic nature of the voids.     

High-Temperature Oxidation of Ni–20 wt.% Cu from 700 to 1100°C

Ni–20 wt.% Cu was oxidized in different oxygen pressures from 1×10^–5 to 1 atm at 700–1100°C. The oxidation consisted of an initial transient period in which a composite scale of NiO and Cu oxides formed, and a subsequent quasi steady-state regime during which parabolic growth of NiO determined the overall oxidation rate. Based on the oxide composition and the oxygen- pressure dependence of the parabolic rate constant, it is concluded that outward transport of Ni via vacancies determines the growth rate of the oxide during the steady-state period, either in the grain boundaries or in the lattice. The influence of Cu dissolved in NiO on the oxidation rate and the oxygen-pressure dependence of the parabolic rate constants is discussed.     

Grain boundary diffusion in Cr-doped NiO and the oxidation of Ni-Cr alloy

We have measured the diffusion of ⁶³Ni radiotracer into polycrystalline NiO, nominally doped with 0.1% Cr (Cr/Ni ratio) in the temperature range 600 to 900°C. The experiments show that Cr doping increases diffusion of Ni in the oxide lattice, but decreases diffusion of Ni along grain boundaries provided that the grain boundary Cr/Ni ratio is sufficiently large (greater than about 1%). This is believed to be due to the formation of immobile Cr-vacancy pairs which block fast boundary diffusion. The oxidation rate of Ni 0.1% Cr alloy is, however, slightly faster than that of pure Ni at 700°C, at which temperature grain boundary diffusion should be dominant. This apparent discrepancy between oxidation rates and diffusion studies, it is argued, is due to the complicating effects of low Cr mobility, a duplex film structure and inward oxygen transport during oxidation of the alloy.     

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.     

A Microstructural Study of Preferential Oxidation at the Grain Boundaries of Ni–Cr Alloys

The oxidation behavior of alloy grain boundaries in model Ni–Cr alloys was investigated. Two binary alloys with nominal wt.% compositions of Ni–10Cr and Ni–20Cr were used. Oxidation was performed in air for 50 hr at 1000°C. The grain boundaries intersecting the alloy surface in Ni–10Cr did not exhibit oxidation, whereas the alloy formed a thick (60 m) oxide layer which formed inwardly. The grain boundaries in this alloy showed a passivating influence at the adjacent regions and retarded oxide formation. An examination of the Ni–20Cr cross section revealed preferential oxidation to a depth of 65 m at the alloy grain boundaries intersecting its surface, while the oxide at the surface was a few micrometers thick. It is noted that the extent to which the grain-boundary oxidation differs from the alloy surface oxidation depends on the Cr content of the alloy. It is also considered that the grain-boundary oxidation behavior in different Ni–Cr alloys changes as a function of Cr content.     

Ni-22Cr-20Co-18W合金在1100℃时氧化膜的演变

采用扫描电子显微镜和X射线衍射分析仪研究了1100℃下Ni-22Cr-20Co-18W合金氧化膜的演变规律。结果显示,在氧化初始阶段,表面形成了Cr2O3,NiO和(Co,Ni,Mn,Cr)3O4混合氧化膜,后者为M3O4型氧化物。长时间氧化后,氧化膜由单层转变为双层,在内层形成连续的Cr2O3膜,在外层形成可以抑制内层Cr2O3挥发的致密NiO氧化膜;同时氧化空位在氧化膜与合金基体界面处形成,并且Al元素的内氧化也在该处发生。     

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr₂O₃, except for a narrow band adjacent to the surface where NiCr₂O₄ particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr₂O₃ which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr₂O₃-rich surface bands.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Oxidation of Two Ternary Cu–Ni–20 wt.% Cr Alloys at 700–800°C in 1 atm O2

Two ternary Cu–Ni–Cr alloys containing approximately 20 wt.% chromium, but with a different Cu and Ni content, have been oxidized in 1 atm of pure oxygen at 700–800°C. The alloy containing about 60 wt.% nickel (Cu–60Ni–20Cr) was composed of a single solid-solution phase and formed external scales of chromium ocide with an outermost layer containing a mixture of copper and nickel oxides. The alloy comprised of about 40 wt.% nickel (Cu–40Ni–20Cr) contained a mixture of two metal phases and formed complex external scales, containing copper oxide and a nickel–chromium spinel plus a region where islands of the metallic phase richer in chromium surrounded by a thin chromia layer were mixed with oxidized islands rich in copper and nickel, producing a situation out of equilibrium. With time, a very irregular and thin but essentially continuous layer of chromia formed at the base of the mixed internal region for this alloy, producing a gradual decrease of the corrosion rate down to very low values. The oxidation behavior of the two alloys is interpreted in terms of their different microstructure. In particular, the fast initial oxidation of Cu–40Ni–20Cr, associated with the formation of large amounts of copper oxides, is attributed to restrictions in chromium diffusion in the alloy due to the simultaneous presence of two metal phases.     

Air Oxidation of Two Nanophase Ni–Ag Alloys at 600–700°C

Two nanophase Ni-base alloys containing 50 and 25 at.% Ag prepared by mechanical alloying, denoted Ni–50Ag and Ni–25Ag were oxidized in air at 600 and 700°C for 24 hr. Ni–50Ag underwent internal oxidation of nickel, associated with the formation of a continuous outermost layer of silver metal with scaling rates larger than those for pure nickel. On the contrary, Ni–25Ag formed a continuous NiO layer surmounted by a discontinuous silver layer and internal oxidation was suppressed. The oxidation rate of Ni–25Ag decreased with time much more rapidly than predicted by the parabolic rate law during the initial stage and eventually became parabolic, with rate constants much lower than those for the oxidation of pure nickel. These results are attributed to the two-phase nature and, particularly, to the very small grain size of the two alloys.     

Segregation beneath oxide scales

Studies are reported and discussed on Auger analyses of the region beneath Cr₂O₃, Al₂O₃, or NiO layers on their metal substrate. Small concentrations of S, C, and P were detected in areas which had been connected to the oxide layer, most probably due to segregation in defects, such as misfit dislocations, microvoids, grain boundaries, etc. For high oxygen pressures at the interface (Ni–NiO) P also can be enriched in the inner layer as phosphate. Sulfur starts to segregate to the free-metal surface as soon as the scale and metal separate, stabilizing voids and accelerating their growth to cavities or favoring the detachment of scale in the case of growth stresses. In this surface segregation S displaces C and P from the metal surface.     

水蒸气对NiCrAlY涂层抗高温氧化性能的影响

采用热重分析及现代表面分析方法研究了低压等离子喷涂Ni CrAlY涂层在纯氧以及含5%水蒸气的O2中的高温氧化行为,结果表明：在纯氧的氧化环境中 ,NiCrAlY涂层氧化动力学遵循抛物线规律,在含有5％的水蒸气的O2中,NiCrAlY涂层在氧 化至110 h后氧化动力学几乎呈直线规律.XRD及SEM分析显示,NiCrAlY涂层在O2中氧化180 h后,表面形成一层致密的Al2O3;而在含5%水蒸气的O2中氧化180 h后表面氧化层中 除了有Al2O3外,还有NiO和Cr2O3.其原因在于水蒸气中的氢在氧化物中的溶解,致 使Ni2+扩散 速度增加,使氧化层变得疏松,降低其抗氧化性.     

Oxidation and Degradation of EB–PVD Thermal–Barrier Coatings

Thermal–barrier coatings (TBCs) consist of a magnetron-sputtered Ni–30Cr–12Al–0.3Y (wt.%) bond coat to protect the substrate superalloy from oxidation/hot corrosion and an electron-beam physical-vapor deposited (EB–PVD) 7 wt.% yttria partially stabilized zirconia (YPSZ) top coat. The thermal cyclic life of the TBC system was assessed by furnace cycling at 1050°C. The oxidation kinetics were evaluated by thermogravimetric analysis (TGA) at 900, 1000, and 1100°C for up to 100 hr. The results showed that the weight gain of the specimens at 1100°C was the smallest in the initial 20 hr, and the oxide scale formed on the sputtered Ni–Cr–Al–Y bond coat is only Al₂O₃ at the early stage of oxidation. With aluminum depletion in the bond coat, NiO, Ni(Cr,Al)₂O₄, and other spinel formed near the bond coat. During thermal cycling, microcracks were initiated preferentially in the YPSZ top coat along columnar grain boundaries and then extended through and along the top coat. The growth stress of TGO added to the thermal stress imposed by cycling, lead to the separation at the bond coat–TGO interface. The ceramic top coat spalled with the oxide scale still adhering to the YPSZ after specimens had been cycled at 1050°C for 300 cycles. The failure mode of the EB–PVD ZrO₂–7 wt.% Y₂O₃ sputtered Ni–Cr–Al–Y thermal-barrier coating was spallation at the bond coat–TGO interface.     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×10¹⁶ Y ions/cm² was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using¹⁸O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr₂O₃ grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr₂O₃ scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×10¹⁰ Y ions/cm² was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr³⁺ and O^2– transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.     

A comparison of the effects of small additions of various second elements on the oxidation of nickel at 1200°C

The oxidation behaviour of Ni-4.2% Mo, Ni-4.0% W, Ni-2.5% Al, Ni-4.2% V and Ni-5.0% Cr (all wt. %) at 1200 °C in flowing oxygen at 1 atm. pressure has been studied using various techniques. In particular, the solubility of the second element in NiO, its distribution across the NiO scale and the effects of these on the oxidation rates and scale morphologies have been examined. The oxidation rates of all the alloys are greater than that of nickel, although for Ni-4.2% Mo, where incorporation of internal oxide into the scale does not occur and molybdenum does not dope the oxide, the small increase in weight gain during oxidation Compared with that for nickel is due to internal oxide formation only. As the internal oxide particles pileup at the alloy/oxide interface, they exert a blocking effect to outward diffusion of Ni²⁺ ions, especially in the later stages of oxidation. Ni-4.0% W behaves similarly, although a few internal oxide particles are incorporated into the scale and a small amount of doping of the oxide ensures that it thickens at a slightly faster rate than the scale on nickel and for Ni-4.2% Mo. The oxidation rates of the other alloys are significantly faster than that of nickel and increase in the order Ni-2.5% Al, Ni-4.2% V, Ni-5.0% Cr. These increased rates are largely caused by increases in the total cation vacancy concentrations in the scales, although internal oxide formation can make a significant contribution to the oxidation kinetics. The influence on the oxidation behaviour of a number of factors, namely doping of the scale, internal oxidation, dissociation of NiO and transport of gaseous oxygen within the scale, blocking effects in the oxide and at the alloy/oxide interface, and grain growth of the oxide, are considered in detail.     

The use of photolithographic marker technique for the probe of the diffusing species in high-temperature oxidation

A microlithographic-marker experiment was conducted instead of a conventional kinetics study using thermogravimetry, to study the oxidation of pure Ni and a Ni-1 at.% Cr alloy. A composite marker, consisting of a 10-nm-thick Mo layer under a 150-nm-thick Pt layer, was designed, and the method for photolithographic deposition of inert markers is described. The micron scale of the marker eliminates any ambiguity in marker identification, which is frequently encountered with conventional markers, whose size may exceed that of the microstructural features controlling oxidation. Transverse-sectioning techniques were employed for TEM and SEM analysis to detect the markers. The regularity of the markers provides a more reliable demarcation of original interface positions. The position of the markers suggests that small additions of Cr significantly promote inward transport of oxygen. The presence of the markers appears to affect only marginally the oxidation kinetics and local oxide morphology.     

Ce对Ni—Cr—Cu合金抗氧化性的影响

作者研究了添加0.1％和0.8％Ce的Ni—Cr—Cu合金在空气中1200℃100小时等温氧化和500小时循环氧化。Ni—Cr—Cu合金中添加微量Ce后,显著降低了氧化速率,增加了氧化膜的剥落抗力。氧化速率降低是添加Ce后各种效应综合作用的结果。它们是:(1)由于Cr的扩散加快,富Cr保护膜更迅速形成;(2)聚集在膜/合金界面附近的含Ce氧化物与空位复合,减少了膜/合金界面的空洞;(3)固溶于氧化膜中的含Ce氧化物阻碍了Cr~(3+)沿氧化物晶界的短程扩散。 提高耐剥落抗力主要原因是:(1)添加Ce使氧化膜晶粒变细,从而改善了塑性变形和适应热应力的能力;(2)0.8Ce合金中稀土氧化物“钉扎”(Keying)作用改善了膜与合金粘附性,并改变了热应力的分布状态。     

Duplex scale formation during high-temperature oxidation of Ni-0.1 wt.% Al alloy

The development of a duplex NiO scale microstructure on a Ni-0.1 wt.% Al alloy at 900°C has been examined, principally using secondary-ion mass spectrometry and analytical transmission electron microscopy. The¹⁸O-tracer distribution following sequential oxidation in¹⁸O₂/¹⁸O₂ showed that the inner NiO layer formed as a result of gaseous-oxygen penetration of the scale. The provision of pathways for oxygen transport as well as the role of Al, Si, and Ce segregation at oxide grain boundaries in influencing the growth rate and spallation behavior of the scale are discussed.     

The morphological and structural development of internal oxides in nickel-aluminum alloys at high temperatures

The formation and development of internal oxides in Ni-Al alloys containing 1–4 wt.% Al in Ni-NiO packs and in 1 atm oxygen at 800 to 1100°C have been studied. The internal oxide particles were relatively fine, closely spaced, and mainly acicular, although more granular near the surface. They were identified as Al₂O₃ at the advancing front, but NiAl₂O₄ at the surface and at a significant distance from that surface. Growth of internal oxide particles resulted in the development of significant compressive stresses in the internal oxide zone when formed in Ni-NiO packs. These stresses led to grainboundary sliding at the higher temperatures and extrusion of weak, internal oxide-denuded zones adjacent to alloy grain boundaries. At the lower temperatures, these stresses also resulted in significant preferential penetration of oxides down grain boundaries and sub-grain boundaries. Stress development and resulting phenomena were much less significant during oxidation in 1 atm oxygen because vacancies injected from the external NiO scale accommodated the volume increase during growth of internal oxide particles.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.