Oxidation Behavior of ODS Alloy MA 6000

MA 6000 is a mechanically alloyed, Ni-base ODSalloy. Excellent high-temperature strength makes it astrong candidate for application in high-temperatureindustrial processes. In order to assess its usefulness for high-temperature structural components,in-depth knowledge of its oxidation behavior,particularly long-term exposure, is necessary. Thepresent work deals with studies of the cyclic andisothermal oxidation of MA 6000 in the temperature range900-1050°C, with emphasis at 1050°C. A fewcomplementary studies have been carried out on the oxideintegrity under creep conditions for exposure times of up to 11,000 hr. The results have shown thatoxidation of MA 6000 involves rather complex mechanismsand alterations of the oxidation behavior still occurafter long-term exposure. Excellent oxidation resistance is based on the formation of an internalcontinuous Al₂O₃ layer.     

Monte Carlo simulation of grain boundary segregation and decohesion in NiAl

Grand canonical Monte Carlo simulations are performed to study grain boundary segregation in the ordered intermetallic compound NiAl. The embedded atom method is applied to model atomic interactions in NiAl. The structure and chemical composition of Σ=5 (210) [001] and Σ=5 (310) [001] symmetrical tilt grain boundaries are studied as functions of the bulk composition at 1200 K. The grain boundaries tend to be enriched in Ni. Deviations of the bulk composition from the stoichiometry towards Ni-rich compositions increase local disorder and enhance Ni segregation at the grain boundaries. In one of the boundaries, the Ni segregation induces a structural transformation to a new metastable grain boundary structure. The effect of grain boundary disorder and segregation on grain boundary decohesion is evaluated by simulated tensile tests.     

Grain boundary grooving with simultaneous grain boundary sliding in Ni-rich NiAl

Thermal grooving at grain boundaries in Ni-rich NiAl was studied by atomic force microscopy technique. The determined average ratio of grain boundary to surface energy for large-angle grain boundaries at 1400°C is 0.45, which is in a good agreement with the results of computer simulations. It has been found that in most cases thermal grooving at the grain boundaries is accompanied by relative shift of the adjacent grains. This shift is associated with the grain boundary sliding caused by the relaxation of internal substructure of the specimen. A model of grain boundary grooving with the simultaneous sliding is developed. The calculated grain boundary groove profiles are in a good agreement with the experimentally measured ones.     

Many-body central force potentials and properties of grain boundaries in NiAl

Empirical many-body central force potentials of the Finnis-Sinclair type have been constructed for B2 NiAl by fitting a number of equilibrium properties of this alloy and reproducing the asymmetric behaviour of constitutional point defects in off-stoichiometric NiAl. At the same time these potentials ensure the structural and mechanical stability of the B2 lattice and reproduce quite adequately the equilibrium properties of Ni₃Al. Using these potentials, grain boundaries in NiAl have been studied by computer simulation. It was found that in stoichiometric NiAl alloy boundaries with a surplus of aluminium have appreciably lower cohesive strength than the stoichiometric boundaries or boundaries with a surplus of nickel. From the structural point of view, boundaries with a surplus of aluminium possess the largest expansions and large ‘holes’ usually occur in the boundary regions. On the other hand, boundaries with the stoichiometric configuration or with a surplus of nickel have more compact structures. The interaction of the antisite defects and vacancies with grain boundaries was also studied and segregation of nickel and aluminium in off-stoichiometric alloys discussed with the help of these results.     

An Evaluation of Creep Behaviour in Friction Stir Welded MA754 Alloy

Effect of friction stir welding (FSW) on microstructure and creep properties of oxide dispersion strengthened (ODS) alloy MA754 were investigated. Fine-grained microstructure developed in the weld zone. TEM results showed some degree of particle agglomeration as a result of intense material flow. Creep tests of the FSW material were carried out at 973 and 1073 K. Power law creep behaviour was observed with stress exponent values of 6.9 and 6.3 at 973 and 1073 K, respectively. The results were compared to those of the as-received material. Creep resistance of FSW material was lower than that of as-received material associated with significantly reduced threshold stress. Post-weld annealing was carried out at 1598 K for 1 h. The heat treatment resulted in a coarse-grained microstructure and enhanced the creep resistance of the welded material. The creep data were compared with those of ODS Ni-Cr alloys in literature. The analysis shows the threshold stress of ODS alloys to be grain size- and temperature-dependent.     

Influence of grain size on the oxidation behaviour of MA 956

The influence of grain size of alloy MA 956 on the oxidation kinetics and scale morphology is investigated at 900 and 1100°C for up to 1000 h exposure. Specimens with grain sizes of about 3, 150, and 300 μm mean diameter were studied. Grain growth during oxidation was not observed. The results of this work at 1100°C show that grain size of the alloy does not significantly influence the oxidation kinetics. At 900°C, however, mass gain values for the small‐ and medium‐grain size material are somewhat lower than those for the large‐grain size material, especially for exposures below 100 h. This feature indicates that grain boundaries could have played a role for the supply of Al to the scale.     

双相NiAl金属间化合物超塑性

研究双相Ni-31Al金属间化合物的高温变形行为。结果表明,该合金在950~1075℃温度范围,1.25×10-4~8×10-3s-1应变速率范围内呈超塑性变形。在温度为1000℃、应变速率为5×10-4s-1时,最大延伸率可达281.3%。显微结构分析表明,超塑性变形过程中两相具有很好的协调变形能力,超塑性变形后原始组织拉长、细化。双相Ni-31Al金属间化合物超塑性变形机制可能为连续动态回复与再结晶。     

Creep and stress relaxation in free-standing thin metal films controlled by coupled surface and grain boundary diffusion

An analytical solution is presented that interprets the effects of grain size, surface and grain boundary diffusivities, surface and grain boundary free energies, as well as grain boundary grooving on the creep rate in free-standing polycrystalline thin metal films. The Coble creep in the film plane is also taken into account; this has a significant effect on the creep rate of the film. The effects of diffusion ratio and free energy ratio between surface and grain boundary on film agglomeration are illustrated as well. A closed-form expression for stress relaxation in films under constrained strain conditions is derived from this solution. An exponential decay in stress is found as a function of the film microstructure. Results predicted by the solution are shown to be in agreement with the experiments.     

氧化物弥散强化铁粉在金刚石工具中的应用

本文采用共沉淀法制得一种氧化物弥散强化铁粉(Fe - Y2 O3粉),通过热压烧结方法测定了纯铁粉、纯钴粉、ODS铁粉烧结致密度、硬度、抗弯强度.结果表明:ODS铁粉显示出了优良的机械性能,硬度达HRB98,抗弯强度达1240 MPa.以ODS铁代替钴作为金属结合剂不仅成本低,而且性能优异,具有相当大的应用潜力.     

Effect of differences in grain size on the mechanical properties of steel 18Kh2N4MA

Conclusions A difference in grain sizes has a negative effect on the ductile characteristics of steel. The ductile characteristics (T₅₀,ap) of the steel with differing grain sizes (grades 1 and 8) are slightly better than those of the coarse-grained steel (grade 1). A difference in grain size mainly lowers the resistance to ductile (ap) and brittle (T₅₀) fracture. The strength characteristics (b, _0.2), hardness (HRC), and ductility (, ) of the quenched and tempered steel do not depend greatly on the original austenite grain size. With ductile fracture the negative effect of a difference in grain size is more evident after high-temperature tempering. A difference in grain size has the same negative effect on the ductile-brittle transition temperature (resistance to brittle fracture) after tempering at both low and high temperatures.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallodenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 2–5, April, 1977.     

Solidification microstructure maps in NiAl alloys

Laser surface resolidification experiments have been performed on NiAl alloys with 70–89 at. % Ni in the velocity range of 5·10⁻⁴–5·10⁻¹ m/s. Microstructures have been studied by optical and transmission electron microscopy. Growth of primary γ-Ni, β-NiAl and metastable β-γ eutectic occurred whereas no primary γ′-Ni₃Al was observed. A solidification microstructure selection map has been drawn, showing the coupled zone of metastable eutectic and the limit of plane front growth of γ-Ni. The microstructure map has also been calculated on the basis of plane front, cellular/dendritic and eutectic growth models. There was a good agreement between experimental and calculated results, showing the potential of this approach for prediction of solidification microstructure.     

Deformation-induced point defects in NiAl single crystals

NiAl single crystals, oriented for single slip, were deformed at room temperature to a strain of 2%, and were subsequently annealed in the temperature range of 673–873 K (T/T_m=0.35–0.45). In as-deformed samples, dislocation substructures consist of jogged edge and screw dislocations, and prismatic loops. Densities of vacancy- and interstitial-type loops are about equal. Annealing causes shrinkage and disappearance of the interstitial loops and significant growth of the vacancy loops. These observations suggest that excess vacancies are present after room temperature deformation. These non-equilibrium point defects may result from non-conservative motion of jogged screw dislocations.     

Structure factors and bonding in ΒNiAl

A combination of powder x-ray and critical voltage electron diffraction was used to determine the lattice parameter and low-angle structure factors of stoichiometric (ΒNiAI. A weighted least squares fit method was used to obtain an accurate lattice parameter from the x-ray line position data, and the integrated intensities were carefully analyzed to determine the Debye-Waller factors; the latter were found to be in good agreement with those obtained from the electron diffraction experiments. Highly accurate (errors ≅ 0.1%) low-angle structure factors were also obtained from the electron diffraction measurements, which were found to agree closely with first principles calculations in the local density approximation. This information reveals that bonding in NiAl is mostly ionic with some covalent contribution.     

Carburization of Cr-based ODS alloys in SOFC relevant environments

Chromium based ODS alloys are being discussed as interconnector materials in solid oxide fuel cells (SOFC's). One of the major requirements for the interconnect material is sufficient corrosion resistance in the anode and cathode side gases at the operating temperature of around 950°C. In the present study the corrosion behaviour of a number of chromium based ODS alloys has been investigated in carbon containing simulated anode gas of an SOFC. Under these experimental conditions all alloys studied are prone to carburization. The carbon uptake, which mainly occurs in the early stages of exposure, leads to the formation of a mixed carbide/oxide layer beneath the external chromia scale. The carburization resistance of the alloys depends on the type and concentration of the oxide dispersion but even to a larger extent on its distribution. Therefore the careful control of the alloy manufacturing process is of great significance for obtaining sufficient carburization resistance under SOFC conditions.     

Divacancy reorientation in Al-rich B2 NiAl compounds

High-purity NiAl single crystals with different composition and different crystallographic orientation have been prepared by crucible-free inductive zone melting under Ar atmosphere. Internal friction measurements of undeformed Al-rich single crystals show a relaxation maximum with an activation enthalpy of (2.44 ± 0.05) eV and a very small pre-exponential factor of 3.3·10^−16±1 s. The relaxation strength of this maximum increases quadratically with the deviation from stoichiometry and shows an orientation dependence which is typical for atomic defects with tetragonal symmetry. In combination with the knowledge that structural defects in Al-rich NiAl are vacancies on the Ni-sublattice, these experimental results allow evidently to assign the relaxation maximum to the reorientation of divacancies on the Ni-sublattice. From the orientation dependence of the relaxation strength it is possible to determine the anisotropy of the elastic dipole tensor of a single divacancy to 0.2 ± 0.05.     

定向凝固NiAl合金的超塑性行为

研究了定向凝固NiAl合金NiAl-25Cr、NiAl-15Cr、NiAl-27Fe-3Nb和NiAl-5Mo-0.5Hf的超塑性行为及其变形机制.结果表明: 合金在一定的变形条件下均表现出超塑性行为, 位错的滑移和攀移行为提供了主要的应变量; 超塑性变形过程中的稳态流变源于变形过程中所发生动态回复或再结晶产生的软化与应变硬化的平衡; 动态回复或再结晶过程一方面在晶粒内部削弱了位错滑动的阻力, 另一方面协调了晶界之间的变形, 使合金以超塑性方式变形.     

Observations of dynamic strain aging in polycrystalline NiAl

Dynamic strain aging has been investigated at temperatures between 77 and 1100 K in eight polycrystalline NiAl alloys. The 0.2% offset yield stress and work hardening rates for these alloys generally decreased with increasing temperature. However, local plateaus or maxima were observed in conventional purity and carbon doped alloys at intermediate temperatures (600–900 K). This anomalous behavior was not observed in low interstitial high-purity, nitrogen doped, or in titanium doped materials. Low or negative strain rate sensitivities (SRS) were also observed in all eight alloys in this intermediate temperature range. Coincident with the occurrence of negative SRS was the occurrence of serrated flow in conventional purity alloys containing high concentrations of Si in addition to C. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main species causing strain aging in polycrystalline NiAl is C but indicate that residual Si impurities can enhance the strain aging effect.