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 effect of alloy grain size on the transient oxidation behavior of an alumina-forming alloy

In the early stages of alloy oxidation, diffusion of solute through the metal to the surface is important in determining the composition of the oxide scale that forms during the transient stage. Rapid solute diffusion to the interface will promote the formation of a protective scale, thereby suppressing the formation of base-metal oxide. The effect of alloy grain size on the formation of the transient oxide scale has been studied using a very fine grained NiCrAlY alloy produced by plasma spraying. The long-term oxidation behavior of this alloy was found to be independent of the grain size of the underlying alloy. However, the short-term, transient oxidation rate was found to decrease with decreasing alloy grain size. This is attributed to the rapid grain boundary transport of Al and Cr to the oxide/metal interface which promoted the formation of Cr₂O₃ and Al₂O₃.     

Grain boundary diffusion and precipitates in B2 Ti−50.2 at.% Ni alloy

Grain boundary diffusion of ⁴⁴Ti and ⁶³Ni in the B2 Ti−50.2 at.% Ni polycrystalline alloy was measured in Harrison's B regime (573−923 K) using the radiotracer technique. The triple product P = sδD_gb (s is the segregation factor, δ the grain boundary width, and D_gb the corresponding grain diffusion coefficient) for Ti and Ni was determined. Although the absolute values of the triple products P are typical for the B2-ordered alloys, both Ti and Ni GB diffusion in NiTi reveals a unique behavior with significant deviations from a linear Arrhenius-type temperature dependence. Transmission electron microscopy analysis of GB structures at 673 K and 923 K substantiated the occurrence of different interface types which may provide the slower and faster grain boundary diffusion paths in agreement with the experimental data. The influence of different types of precipitates on grain boundary diffusion and possible diffusion mechanisms in the different regimes are discussed.     

Oxidation of nickel-manganese alloys in the temperature range 873–1273 K

Ni-Mn alloys containing up to 38% Mn have been oxidized in pure oxygen between 873 and 1273 K and the parabolic rate constants measured. The scale morphologies and oxide compositions are interpreted in terms of modifications to the scale on pure Mn caused by the presence of Ni. The scales are composed predominantly of two layers at all temperatures, giving the sequences of phases alloy/cubic monoxide (Ni, Mn)O/ternary spinel, with the cubic (Ni, Mn)O layer always having the greater thickness. There is limited evidence for a third, very thin, outer layer in the scales on all alloys at 873 K and for Ni-38%Mn at 1073 K, which is tentatively considered to be Mn₂O₃, giving layers in the order alloy/cubic monoxide/ternary spinel/Mn₂O₃, by analogy with the scale formed on pure Mn. The distribution of the alloy components in the scale is discussed in relation to the Ni-Mn-O phase diagram and in terms of recent theoretical treatments of solid solution scale formation on binary alloys, as far as the available diffusion data allow. The occurrence of internal and intergranular oxidation and the formation of a Mn-depleted zone coincident with the band of uniform internal oxide are considered briefly.Deceased.     

Growth mechanism of Ni3Sn4 in a Sn/Ni liquid/solid interfacial reaction

The chemical interfacial reaction of Ni plates with eutectic Sn–3.5Ag lead-free solder was studied by microstructural observations and mathematical calculations. Compared with the Sn–3.5Ag–0.75Ni/Ni interfacial reaction, based on a simple model of the growth of the liquid/solid chemical compound layer, the growth mechanism of Ni₃Sn₄ in the Sn–3.5Ag/Ni interfacial reaction is discussed and presented. The growth process of Ni₃Sn₄ in the Sn/Ni liquid/solid reaction interface involves the net effect of several interrelated phenomena, such as volume diffusion, grain boundary diffusion, grain boundary grooving, grain coarsening, and dissolution into the molten solder. The growth time exponent n and morphology of Ni₃Sn₄ were found to be dependent on these factors.     

Effect of addition of N on L10 transformation and atomic diffusion in FePt films

The effect of nitrogen incorporation on the kinetics of L1₀ transformation in FePt alloy films was studied. Films with a maximum possible amount of nitrogen were prepared by sputtering a FePt compound target with pure nitrogen. As-prepared films consisting of nano-grains of Pt and Fe_0.7N_0.3 transformed to face-centered cubic FePt phase with a certain amount of nitrogen incorporated in the grains after annealing at 573 K. It was found that the presence of N results in faster kinetics of L1₀ transformation compared with pure FePt alloy films, as well as a substantial reduction in grain size. Detailed structural and diffusion measurements were carried out to elucidate the mechanism of enhanced transformation kinetics as well as grain size reduction. The activation energy for volume diffusion of Fe in FePtN films was found to be 0.27 ± 0.14 eV, while that in FePt was 0.50 ± 0.17 eV. Faster atomic diffusion in nitrogen-containing films was the cause of an enhanced rate of L1₀ transformation. Further studies reveal that, in partially transformed FePtN alloy films, N is incorporated mainly in the grain boundary region, which hinders grain growth and results in a smaller grain size.     

Densification and grain growth during isothermal sintering of Mo and mechanically alloyed Mo–TZM

Isothermal sintering behavior of pure molybdenum (Mo) and mechanically alloyed Mo–TZM (Mo–0.6Ti–0.2Zr–0.02C) has been investigated in the temperature range 1000–1800 °C. A linear relationship has been found to exist between logarithms of increment in density and time. Although the volume diffusion has been found to be the dominant sintering mechanism, a significant contribution from grain boundary diffusion is also identified. Both the diffusion coefficients (D_v) obtained from shrinkage data and the grain boundary mobility (M_b) during grain growth are found to be lower for Mo–TZM due to the presence of carbides in the microstructure. The grain boundary migration is restricted due to the presence of carbides and porosities in the microstructure.     

Phase field modeling the growth of Ni3Al layer in the β/γ diffusion couple of Ni–Al binary system

The growth behavior of Ni₃Al phase layer in the β/γ diffusion couple of Ni–Al binary system, including the shape evolution and growth kinetics, have been simulated by using the KKS multiphase field model. Simulation results indicate that, when Ni₃Al layer growth is controlled completely by volume diffusion, it could be regarded as parabolic growth. However, if the fast grain boundary diffusion is taken into account, the growth rate of Ni₃Al phase is accelerated, and the growth kinetics deviates from parabolic growth, which is consistent with experiments and other simulation results. Simulation results also demonstrate some details of shape evolution of grains, such as the uneven Ni₃Al/γ and Ni₃Al/β interfaces and the grain boundary migration of Ni₃Al grains caused by fast grain boundary diffusion.     

Influence of chemical composition and alloying elements on microdefects and electron density in Ni—Al alloys

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Oxidation Behavior of Ni-3,6, 10 wt.% Al Alloys at 800°C

The oxidation behavior of Ni and Ni-3, 6, and 10Al alloys at 800°C in an N₂–O₂ gas mixture was investigated. The mass gain of each alloy depended on both the oxidation periods and Al content. NiO scale was formed on all alloy substrates accompanied by internal oxides of Al₂O₃. Many cavities were formed at the NiO/substrate interface at shorter oxidation times, and these cavities were found to be filled by metallic Ni(Al) from the matrix in the internal-oxidation zone by the development of internal oxides. The filling of cavities by Ni(Al) was more significant on higher Al alloys, which had a higher density of internal Al₂O₃. Once metallic Ni(Al) formed along the entire NiO/substrate interface, the oxidation kinetics became the same as pure Ni. It was concluded that pure Ni filling the cavities at the interface provided a diffusion path of Ni from the substrate to the NiO scale, and that controlled the oxidation kinetics.     

Oxidation kinetics of Y‐doped FeCrAl‐alloys in low and high pO2 gases

A model Fe‐20Cr‐5Al‐0.05Y alloy was oxidized in Ar‐20%O₂ and Ar‐4%H₂‐7%H₂O at 1200–1300 °C. Two‐stage oxidation experiments using oxygen isotope tracers showed that inward oxygen diffusion was predominant in both gases, but more isotope exchange was observed in the H₂/H₂O gas reaction. The alumina scales formed in both gases were composed of columnar grains, the lateral size of which increased linearly with depth beneath the scale surface. Thermogravimetric measurement of oxygen uptake revealed kinetics which were intermediate to parabolic and cubic kinetic rate laws. A model based on grain boundary diffusion control coupled with competitive oxide grain growth accounts satisfactorily for the results when the requirement for a divergence‐free flux within the scale is imposed. This treatment shows that the oxide grain boundary diffusion coefficient is lower when H₂O is the oxidant. It is concluded that hydrogen slows the grain boundary diffusion process by altering the nature of the diffusing species.     

Effect of grain size on high-temperature oxidation behavior of Cu-80Ni alloy

1　INTRODUCTIONTheoxidationofunalloyedcopperandnickelhasalreadybeenstudiedindetail.Whilethehigh tem peratureoxidationofCu Nialloys ,alsostudiedanumberoftimestodate[13] ,isanexampleofarela tivelysimpleclassofscalingofbinaryalloysbyasin gleoxidant,becausethemetalsformacontinuousse riesofsolidsolutions ,whiletheiroxides ,CuO ,Cu2 OandNiO ,exhibitsmallmutualsolubilitiesandshowsignificantdifferencesinthethermodynamicstabilityandparabolicgrowthrates .Thus ,copper richalloysformexternalscales…     

Analysis of oxygen-18 tracer profiles in two-stage oxidation experiments (I): predominant oxygen diffusion in the growing scale

Typical two-stage oxidation experiments in high-temperature oxidation studies on metals are analyzed. Two cases of predominant oxygen diffusion in the scale are studied: pure volume diffusion and simultaneous transport via grain boundaries and via the bulk. An analytical expression for the growth of the oxide layer is given for the assumption that the chemical potential of the oxygen varies linearly over the oxide layer. The numerical treatment of the differential equation is improved so that the calculation is possibly faster and/ or more accurate compared to a method given in the literature. The experimental profiles are described by four parameters, the grain boundary width, the grain radius, and the volume and grain boundary diffusivities. Two equations correlating these parameters can be extracted from the profiles. Two benchmark tests are described for testing the program. An analytical solution is presented which approximately describes the distribution of O-18 in the oxide layer for pure volume diffusion. Experimental SIMS profiles on Fe-Cr-Al alloys are explored on the basis of our calculation.     

Solid-state reactive diffusion between Ni and W

Various alloys are interconnected with W using a Ni intermediate layer by the diffusion bonding (DB) technique. During solid-state heating in the DB technique, however, reactive diffusion occurs at the interconnection between Ni and W. In order to examine the kinetics of the reactive diffusion, sandwich Ni/W/Ni diffusion couples were isothermally annealed at temperatures of T = 1023-1173 K for various times up to t = 366 h. Owing to annealing, Ni₄W is produced as a layer at the Ni/W interface in the diffusion couple and grows predominantly towards Ni. The thickness of the Ni₄W layer increases in proportion to a power function of the annealing time. The exponent of the power function is close to 0.5 at T = 1173 K and gradually decreases with decreasing annealing temperature. Furthermore, grain growth takes place in Ni₄W due to annealing. Therefore, volume diffusion is the rate-controlling process for the growth of Ni₄W at T = 1173 K. At T < 1173 K, however, boundary diffusion contributes to the rate-controlling process, and the contribution of boundary diffusion increases with decreasing annealing temperature. On the other hand, a region alloyed with W is formed in Ni from the Ni₄W/Ni interface by diffusion-induced recrystallization (DIR). The growth rate of the DIR region is much greater than the penetration rate of W into Ni by volume diffusion, and the concentration of W in the DIR region at the Ni₄W/Ni interface is equal to the solubility of W in Ni. Such growth behavior of the DIR region was numerically analyzed using a mathematical model. The analysis indicates that the growth of the DIR region is controlled by the interface reaction at the moving boundary of the DIR region as well as the boundary diffusion along the grain boundaries across the DIR region under the present annealing conditions.     

填充Co的CoCrCuFeNi高熵合金扩散焊接头的组织和扩散机制

在850,950,1050和1100℃下,填充Co中间层对CoCrCuFeNi高熵合金（HEA）进行了扩散焊接,并对接头微观组织和扩散机制进行了分析。结果表明,在各温度下接头均形成了牢固的结合,接头无金属间化合物生成,高熵合金侧界面周围残留部分柯肯达尔孔。对Cr、Fe、Cu和Ni在Co填充层中的扩散系数进行了计算,排序如下：Cu>Cr>Fe>Ni。所有元素的扩散速度均在相同水平,CoCrCuFeNi高熵合金和Co填充层之间的扩散是在空位机制和晶界扩散机制的共同作用下发生的。     

纯铜表面纳米化对镍扩散的影响

利用表面机械研磨处理(SMAT)在纯铜表面制备出纳米结构表层,并对其表面进行了电镀镍处理,采用扫描电子显微镜分析了镍原子在纳米晶铜中的扩散行为.结果表明:表面纳米晶层内存在有大量非平衡态缺陷和晶界,尤其是三叉晶界数量增加,降低了镍原子扩散的激活能,提高了其扩散系数,从而加快了镍原子的扩散.     

Grain shrinkage driven by surface and grain boundary energy in Ba5Nb4O15 powder

The shrinkage and disappearance of small Ba₅Nb₄O₁₅ grains in a large grain matrix at 1133 K observed by transmission electron microscopy (TEM), included grain volume and boundary shrinkage. Rate equations for these processes were formulated based on the concept that total excess free energy directly stimulates material transport in volume, surface, or grain boundary diffusion. Based on these equations, grain vanishing was simulated and it was found that volume diffusion combined with boundary diffusion occurred, and high grain boundary energy and low grain boundary diffusivity made grains vanish while maintaining a truncated spherical shape.     

The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

An 8–9 μm thick Pt layer was coated on a superalloy and transformed to a Ni–Pt alloy layer by the interdiffusion of Ni and Pt at 1050 °C for 3 h. The surface of the Ni–Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 °C were studied. The treated Ni–Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni–Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 °C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 °C were composed of only a two-phase (NiAl + PtAl₂) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 °C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 °C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 °C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 °C.     

On the Role of Yttrium in Alumina Formers: Comparative Oxidation Behavior of (Ni–Cr–Al)- and (Ni–Cr–Al–Y)-Based Alloys

It is shown that the addition of Y to an alloy based upon the Ni–Cr–Al system slightly reduces the growth rate of Al₂O₃ scale during isothermal oxidation in air at temperatures in the range of 950–1150 °C. However, Y segregation at grain boundaries of the oxide is found to refine its grain structure down to the nanoscale with improved mechanical strength as compared to the Y-free alloy. It is concluded that Y can have the effect of decelerating the kinetics of diffusion processes leading to grain growth of the oxide.     

The influence of sulfur pressure on Sulfidation behaviour of NiCoCrAl(Y) alloys at high temperature

Sulfidation of alloy having nominal composition Ni-23Co-19Cr-12Al (wt%) with and without the addition of 0.6% yttrium was studied at temperatures 1073–1273 K in sulfur vapor at atmospheric pressure and in H₂/H₂S gas mixtures at sulfur pressure of 10^?3 and 10^?1.5 Pa. Sulfidation runs were followed thermogravimetrically. Phase and chemical composition of sulfide scales and scale morphologies were determined by means of XRD, EDX, EPM and SEM analyses. After certain initial period sulfidation of both materials followed approximately a parabolic rate law. The estimated sulfidation rates for each alloy increased with sulfur pressure and temperature. The sulfide scales on both materials showed complex microstructures and compositions, depending on sulfidation conditions, with several sulfide and sulfospinel phases present, such as (Ni,Co)S, (Ni,Co)₃S₄, (Ni,Co)Cr₂S₄, (Cr,Ni,Co)Al₂S₄ or (Cr,Ni,Co)S and (Cr,Ni,Co)₃S₄. There was no evidence of yttrium segregation either to the grain boundary regions in the scale or to the alloy/scale interface. Yttrium dissolved in the sulfide phases and accelerated the sulfidation process. This behaviour was ascribed to the doping effect.