Characterization of Ni-25 wt.% Cr alloys containing reactive elements and the oxide scales grown on them at high temperatures using transmission electron microscopy

Nickel-base alloys of nominal composition Ni-25 Wt.% Cr-(0-0.6 Wt.%) RE (RE=Y, La, and Ce) were prepared by conventional arc-melting Ni, Cr, and Y metal. The microstructure of the alloys was characterized by using electron diffraction and X-ray energy dispersive spectroscopy (XEDS) to determine the structure, morphology, and distribution of second-phase particles. Selected alloys were oxidized at 900°C and 1000°C in 1-atm air, and the resultant oxide scales were characterized using the same analytical techniques. The experimentally determined electron-diffraction data were compared with a JCPDF-EDD database, and several compounds were matched. The observed phases were RE oxysulfide, cerium orthochromite-CeCrO₃, yttria-Y₂O₃, yttrium orthochromite-YCrO₃, and Ni₅Y.The significance of these observations is discussed with respect to the current level of knowledge on the role of sulfur in the reactive-element effect. The ability of the reactive elements to getter sulfur is examined with respect to the thermodynamic stability of the oxysulfide species.     

The reactive element effect on the growth rate of nickel oxide scales at high temperature

The restriction of oxide-scale growth by an incorporated reactive element has been investigated for the oxidation at 900°C of CeO₂-coated and Ce-alloyed Ni. Analytical electron microscopy of scales in transverse section revealed that significant inhibition of diffusion along a network of grain-boundary pathways in NiO was associated with segregation of Ce at a high concentration. The development of this form of Ce distribution depended critically on the provision of sufficiently small and closely spaced CeO₂ source particles within the scales.     

The Partial Ionic and Electronic Conductivity of Y-Containing and Y-Free Chromia Scales

This work reports on the ionic and electronic partial conductivity, determined in situ by the Wagner asymmetry-polarization technique, of chromia scales thermally grown in 1 atm flowing oxygen at 900°C. Results show an activation energy of 0.3 eV for ionic conductivity and of 0.6 eV for electronic conductivity in the temperature range 300–700°C. After treatment with yttria, both the ionic and electronic conductivity were reduced; however, the activation energy was the same as the Y-free scales. This is consistent with previous results obtained from chromia scales grown in H₂/H₂O vapor. Comparison of the data between scales grown at low and high P_O2 suggests that hydrogen doping in the scale can reduce both ionic and electronic conductivity by approximately one order of magnitude.     

The Role of Silicon in the Reactive-Elements Effect on the Oxidation of Conventional Austenitic Stainless Steel

In this work we evaluate the influence of silicon on the high-temperature oxidation of austenitic stainless steels and propose a mechanism that explains the Reactive-Element Effect (REE) in terms of a synergistic action between the reactive element and the silica layer that forms in the innermost areas of the scale. To do this we have studied the oxidation at 900°C of austenitic commercial alloys (AISI-304, AISI-316 and AISI-310S) and a laboratory-designed high-silicon stainless steel (AISI-304). Lanthanum was selected as the reactive element which was surface deposited by means of ion interchange. Results obtained in this work allowed us to state that the reactive element would enhance the formation of a silica layer that shows diffusion through the scale. The reactive element also changes the expansion coefficient at the scale-alloy interface, increasing the adherence of the oxide layer to the metal.     

Preparation and oxidation behaviour of electrodeposited Ni–CeO2 nanocomposite coatings

Ni–CeO₂ nanocomposite coatings with different CeO₂ contents were prepared by codeposition of Ni and CeO₂ nanoparticles with an average particle size of 7 nm onto pure Ni surfaces from a nickel sulfate. The CeO₂ nanoparticles were dispersed in the electrodeposited nanocrystalline Ni grains (with a size range of 10–30 nm). The isothermal oxidation behaviours of Ni–CeO₂ nanocomposite coatings with two different CeO₂ particles contents and the electrodeposited pure Ni coating were comparatively investigated in order to elucidate the effect of CeO₂ at different temperatures and also CeO₂ contents on the oxidation behaviour of Ni–CeO₂ nanocomposite coatings. The results show that the as-codeposited Ni–CeO₂ nanocomposite coatings have a superior oxidation resistance compared with the electrodeposited pure Ni coating at 800 °C due to the codeposited CeO₂ nanoparticles blocking the outward diffusion of nickel along the grain boundaries. However, the effects of CeO₂ particles on the oxidation resistance significantly decrease at 1050 °C and 1150 °C due to the outward-volume diffusion of nickel controlling the oxidation growth mechanism, and the content of CeO₂ has little influence on the oxidation.     

Influence of Preoxidation and Annealing Treatments on the Isothermal Oxidation in Air at 1173 K of Cerium-Implanted EN-1.4301 Stainless Steel

Improvement of the high temperature oxidation behavior of conventional EN-1.4301 (AISI type 304) stainless steel was achieved by implanting cerium ions. Implantation was performed in samples with two different surface treatments: commercial 2B mill finishing and preoxidation in synthetic air. One set of samples was annealed after implantation in order to recover the damage induced in the crystalline structure. Results show that the implantation of Ce greatly improves the oxidation resistance at 1173 K of the EN-1.4301 grade stainless steels and that the annealing treatment after the implantation is beneficial during the subsequent oxidation. Experiments confirm the presence of spinel-type compounds and Cr₂O₃ and Fe–Cr mixed oxides with high Cr content in the scale, and martensite being the dominating metallic phase. Chromium depletion in the near-surface layers is also described. Results are compared to previously published data on surface modification using La and Ce by pyrolysis of aerosols.     

The isothermal-oxidation behavior of several nickel-base single-crystal superalloys with and without coatings

Several commercial single-crystal superalloys (CMSX-2, CMSX-3, CMSX-4, CMSX-6, SRR 99) and some laboratory versions of one of them (CMSX-4) with various Y-additions were investigated concerning their oxidation resistance in air at temperatures between 800 and 1200°C. The investigations also included two materials (CMSX-6, SRR 99) with an RT-22 coating. Weight change was recorded for times of up to 1000 hr (in some cases up to 1600 hr). Oxidized coatings and substrates were characterized by metallography, SEM, and microprobe analysis. Most of the alloys showed good oxidation resistance up to 1000°C, while there was complete spalling during cooling after oxidation at 1150°C and 1200°C for the uncoated and Y-free alloys. Coated alloys were superior, however the best behavior was shown by a laboratory version of CMSX-4 containing between 10 and 60 ppm Y. Interdiffusion at 1000°C is tolerable for the coated alloys, but there was extremely rapid degradation of the coating by interdiffusion at 1200°C.     

Effect of Quaternary Additions on the Oxidation Behavior of Hf-Doped NiAl

Cast model alloys, based on -NiAl+0.05at.%Hf, were used to study the effects on oxidation behavior of elements that are commonly present in low-activity aluminide bond coatings on single-crystal, Ni-base superalloys. Single additions of Re, Ti, Ta, and Cr were examined in cyclic and isothermal exposures at 1100 to 1200°C in order to determine their effect on the oxide growth rate and resistance to scale spallation. With 1 at.% additions, all of these elements were found to be detrimental to the oxidation performance of the base NiAl+Hf alloy. Additions of Re and Cr were found to form second-phase precipitates in the alloy, which appeared to lead to scale spallation, while additions of Ti and Ta were internally oxidized and incorporated into the scale as grain-boundary segregants. These results suggest that it is necessary to minimize the levels of these types of elements that enter Hf-modified aluminide coatings by using process modifications or a diffusion barrier.     

The growth and structure of thin oxide films on ceria-sol-coated nickel

The influence of 14-nm thick ceria ceramic coatings deposited by the sol-gel technique on the early-stage oxidation of polycrystalline nickel at 973 K was studied by analytical electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and X-ray diffraction. The size of the ceria particles in the coating was modified prior to oxidation by vacuum annealing. It was found that ceria particle size is a crucial factor affecting the oxidation kinetics, oxide microstructure, and distribution of cerium within the oxide film. Coarse ceria particles applied to the nickel surface were ineffective in the inhibition of oxidation and were spread throughout the whole oxide. Coatings with small ceria particles markedly improved the oxidation resistance. After oxidation such particles were present in the surface region of nickel oxide, acting as the sources of cerium ions segregated at the nickeloxide grain boundaries. The stereological analysis of oxide microstructure as well as microchemical examination supported the predominant role of grain-boundary segregation of cerium ions decreasing the oxidation rate. The results are discussed in terms of reactive-element effect on the development of microstructure of nickel oxide film during initial stages of oxidation.     

The cyclic-oxidation behavior of several nickel-base single-crystal superalloys without and with coatings

Several commerical single-crystal superalloys (CMSX-2, CMSX-3, CMSX-4, CMSX-6, SRR 99) and some laboratory versions of one of them (CMSX-4) with various Y additions were investigated concerning their cyclic-oxidation resistance in air at 1000 and 1150°C. The investigations also included two materials (CMSX-6, SRR 99) with an RT-22 coating. Weight changes and acoustic emission were recorded up to 1000 cycles and scales, coatings, and substrates were characterized by metallography, SEM, and microprobe in postexperimental investigations. The best cyclic-oxidation behavior and excellent resistance to spalling even at 1150°C were shown by a laboratory version of CMSX-4 containing between 10 and 60 ppm Y. While at 1000°C interdiffusion can be taken as tolerable for the coated alloys, there is rapid degradation of the coating by interdiffusion at 1150°C.     

Influence of CeO2-coating on the high-temperature oxidation of chromium

The reactive-element effect on high-temperature oxidation of chromium metal was studied at oxygen pressures of 10⁵ Pa (1 atm) and 0.67 Pa and at temperatures of 800 and 1050°C under both isothermal and cyclic conditions. The reactive element, cerium, was applied as a cerium-oxide coating by sol-gel deposition. Growth and adherence of the chromia scale on the metallic substrate were investigated by kinetic weight-gain measurements, and the microstructure was characterized by scanning electron microscopy, transmission electron microscopy, EDX-analysis, X-ray diffraction, and Auger analysis. The study of the oxidation kinetics under isothermal conditions at 800°C clearly showed a reduction in the growth rate of the oxide film when cerium oxide was present. Improvement of the oxide-scale adhesion was also observed when ceria-coated chromium was subjected to thermal cycling. The applicability of a number of models which have been used to explain the reactive-element effect is discussed.     

Mechanical behaviors of alumina ceramics doped with rare-earth oxides

The effects of three types of additives Y₂O₃, La₂O₃, and Sm₂O₃ on the sintering and mechanical behaviors of alumina ceramics were investigated. The bending strengths of alumina ceramics with Sm₂O₃ and Y₂O₃ additions were 455 and 439 MPa, respectively, higher than that with La₂O₃ addition. The fracture toughness of the ceramics with Sm₂O₃ and Y₂O₃ were also higher than that with La₂O₃ addition. The fracture mode of rare earth oxides doped alumina ceramics exhibited obvious transgranular fractures as well as intergranular fracture. The results of research show that the improvement of bending strength and fracture toughness of alumina ceramics with rare earth oxides was achieved by refining the grain size and strengthening the grain boundary.     

Mechanical aspects of high-temperature oxidation

Under service conditions it is most often not high temperature corrosion alone which causes failure of components but the interaction with simultaneous mechanical stresses. A key role is played in this case by the protective surface oxide scales which may crack and heal. This role is discussed in the form of model considerations for the beginning of scale failure under tensile, compressive and more complex stress situations. Discussion leads to the formulation of rules for a protective scale with optimum behaviour under the effect of stresses.     

Mechanical aspects of multichip module reliability

Stress in thin film multichip-module interconnect layers is often a consequence of the mismatch in the coefficients of thermal expansion among the substrate, polymer dielectric, and metal used as interconnect lines and power/ground planes. This article considers the origin of both biaxial in-plane stress and the more complex multiaxial stress state at the edge of the film for several representative multichip-module materials systems. Single-layer films are considered in detail, and then the implications for multilayer films are discussed. Adhesion of the film is a concern since delamination is a failure mode. The role of residual stress in reducing the energy to cause decohesion is considered for various polyimide systems.     

稀土对高效铁粉焊条焊缝组织及性能的影响

通过对普通铁粉焊条药皮成分进行调整,并加入稀土添加剂研制出了一种新型含稀土高效铁粉焊条.对试件冲击断口的扫描电镜分析,采用IA3001型图像分析仪对焊缝金属夹杂物分析,以及通过低温冲击试验,证实了适量的稀土可起到细化晶粒,净化焊缝的作用.所研制的稀土高效铁粉焊条工艺性能良好,焊缝金属低温韧性好,可用于重要结构的焊接.     

Prediction of the location of delamination in the drilling of composite laminates

An analytical approach for the determination of the position of the onset of delamination during the drilling of composite laminates based on linear elastic fracture mechanics is presented in this paper. The critical thrust force calculated from the predicted position of the onset of delamination has a reasonable agreement with experimental results obtained in the drilling of T300/5208 graphite epoxy composite laminates reported in the literature. Thus, the analytical method developed in the present study may be applied to the drilling of composite laminates drilling with the avoidance of delamination.     

Carburization Behavior of Electrodeposited Ni3 Al–CeO2-Base Coatings on Fe–Ni–Cr Alloys

The cyclic carburization of electrodeposited pure and CeO₂-dispersed Ni₃Al intermetallic coatings on Fe–Ni–Cr alloys has been investigated at 850 and 1050°C for periods up to 500 h in a reducing 2%CH₄–H₂ atmosphere. At 850°C, all Ni₃Al-base-coating samples showed excellent carburization resistance and slow mass increases due to the formation of a thin γ-Al₂O₃ scale and a low carbon activity (a _c = 0.73). At 1050°C and a high carbon activity (a _c = 3.21), all coatings are superior to the uncoated Fe–Ni–Cr alloy in terms of carburization resistance. A thin α-Al₂O₃ scale slowly formed on all Ni₃Al coatings effectively blocked the carbon attack. The addition of CeO₂ particles in the Ni₃Al coatings significantly mitigated the cracking of the α-Al₂O₃ scale and the resultant internal oxidation and carburization. For all coatings, Ni-rich particles were found to be formed on the α-Al₂O₃ scale during oxidation, which had led to the deposition of catalytic coke.     

Material characterization for the prediction of ductile fracture occurrence: An inverse approach

The most advanced approach to the prediction of ductile fracture occurrence in metal forming processes is represented by a damage mechanics formulation. Such an approach requires a suitable evaluation of the parameters which appear in the yield condition for the damaging material and in the model which governs the evolution of microvoids (nucleation, growth and coalescence), which allow to calibrate the model with respect to the actual state and properties of the material.

In the paper this aim is pursued employing an inverse identification approach based on an optimization technique which permits to adjust the material variables so that the numerical response matches the experimental one. In particular the load vs. displacement curve during a tensile test has been used to optimise the comparison between the numerical and the experimental results and to develop a full damage characterisation of the material.     

CVD金刚石自支撑膜的力学性能

CVD金刚石自支撑膜已经作为一种崭新的工程材料出现,其应用日益广泛,急切需要对其力学行为进行深入了解。为此对有关金刚石膜的断裂强度和断裂韧度的研究,以及金刚石膜的断裂机制进行了详尽的论述。简略介绍了金刚石膜的断裂强度和断裂韧度的测试方法,给出了典型的测试数据。最后就如何提高金刚石膜的力学性能和更好地应用这种新型工程材料提出了建议。