一种多孔镍基合金在850~1000°C下的氧化行为和机理（英文）

以一种多孔Ni-Cr-Al-Fe合金为研究对象,分别利用金相显微镜、扫描电子显微镜(SEM)及能谱仪(EDS)、X射线衍射(XRD)、X射线光电子能谱仪(XPS)等分析手段,研究其在850~1000°C温度范围内的氧化行为及机理。研究表明,该多孔合金在950°C和1000°C时呈伪抛物线型的氧化动力学曲线特征,其表面形成由外层Cr_2O_3/NiCr_2O_4和内层α-Al_2O_3构成的复杂氧化物结构。γ相的存在能促进氧化初期NiO/Cr_2O_3/NiCr_2O_4复合氧化物的形成,而多孔合金中众多快速扩散的通道有利于其氧化膜的发展。随着氧化时间的延长和氧化温度的升高,多孔合金的开孔率和渗透率均有所减小,但可控制在一定的范围内。

Influence of powder oxidation on powder properties and formability in H13 hot-work steels processed by electron beam melting

The oxygen content of metal powder is decisive for the recyclability of powder.The research on the effect of oxygen content on powder properties and material formability has practical significance for economical production with additive manufacturing while preventing the waste of resources.Here,we deliberately oxidized the powder by baking at high temperature to increase the oxygen content in the powder and gave the calculation method of the oxygen content in the powder oxidation film.The majority of oxygen element was found in the oxide particles in the powder and the oxide flm on the powder surface,which did affect the flowability of the powder.It is worth noting that the increase in the oxygen content does not change the phase of H13 steel,but it can promote the molten pool flow and obtain a smoother surface.The increase in the oxygen content in the powder is not the decisive factor for the formability and defects of the printed samples.It is the combined effect of the powder deformation,the increase in the oxygen content,and the impurity pollution after repeated use,which leads to the limitation of repeated utilization of the powder.

Hot deformation behavior and constitutive model of TC18 alloy during compression

The hot deformation behavior of TC18 alloy at strain rates ranging from 1 × 10-4 to 1 x 10-2 s-1 and temperatures ranging from 25 to 800 ℃ was studied using a WDW-300 electronic universal testing machine. The relationships between true flow stress decreases with stress and true strain show that the increase of temperature and increases as strain rate increases. The effect of strain rate on the flow stress becomes pronounced at higher temper- atures. At room temperature, the river pattern characteristic of brittle fracture and the dimple pattern typical of ductile fracture are found to exist in different regions of fracture surfaces of the samples. An improved constitutive rela- tionship is proposed to accurately describe the flow stress of TC18 by considering the effect of strain. And a micro- scopic model is also deduced which can link the physical mechanisms to the macroscopic experimental results. A good agreement is obtained between the predictions of the microscopic model and the results of the macroscopic experiment.

Subgrain microstructures and tensile properties of 316L stainless steel manufactured by selective laser melting

316L stainless steel samples were manufactured by selective laser melting(SLM).The microstructure of SLM-made 316L stainless steel and the room temperature tensile properties both perpendicular and along the building direction were studied and characterized.The static temperature field during the molten pool formation was simulated by finite element simulation.It indicates that the nonlinear asymmetrical inclined temperature gradient in SLM process produces a large surface tension gradient.The melt forms a Marangoni flow with different convection modes under the action of surface tension as well as a micro-molten pool morphology with subgrain structures such as strip,hexagonal and elongated cellular structures.In addition,there are also epitaxially grown columnar grains.The growth of columnar crystals is not affected by the boundary of the molten pool.Subgrain structures and low-angle grain boundaries make the tensile strength and the elongation of SLM-made 316L sample higher as compared to those of the cast and wrought samples.The room temperature tensile strength of the sample perpendicular to the building direction is higher than that of the sample along the building direction,while the elongation is lower than that of the sample along the building direction.

Compressive Behavior of Porous Titanium Fiber Materials

Porous titanium fiber materials with the fiber sizes of 70--120 μm in diameter were prepared by vacuum sintering technology. The morphology and compressive properties of porous titanium fiber materials were investigated by using a scanning electron microscope （SEM） and an MST 858 compression testing machine in quasi-static condition. The results show that porous titanium fibers form complex micro-networks. The stress-strain curves of por- ous titanium fiber materials exhibit elastic region, platform region and densification region and no collapse during platform region. The yield strength of porous titanium fiber materials decreases with increasing the porosity and increasing the fiber diameter.

Sintering Behavior of Porous Titanium Fiber Materials

The porous titanium fiber materials with open porosity were successfully prepared by the vacuum sintering technology. The morphology characteristics of sintering neck of porous titanium fiber materials were investigated by scanning electron microscopy （SEM）. The results show that the formation and growth of sintering neck of porous ti- tanium fiber material approximately follow the rule that the primary mechanism is grain boundary diffusion and sub- sidiary mechanisms are other diffusion mechanisms during the sintering process. The formation and growth of the sintering neck depend mainly on the sintering temperature and slightly on the soaking time. The sintering system of porous titanium fiber material was determined and the equation of the sintering neck＇s length was established.

Fabrication of multilayer Nb_2O_5 nanoporous film by anodization of niobium foils

Multilayer Nb2O5 nanoporous films were successfully synthesized on Nb surfaces by the control anodization process in ethylene glycol containing 4 vol% HF and 2 vol% H2O2 electrolyte. The nanoporous films are characterized in detail by field-emission scanning electron microscopy（FESEM）, transmission electron microscopy（TEM）, and X-ray diffraction（XRD）. The Nb_2O_5 nanoporous films have a multilayer morphology with the side wall thickness of ～5 nm, irregular pores with a diameter of ～25 nm, and a length of up to 7.39 lm, depending on the anodization time. A mechanism for the multilayer Nb2O5 nanoporous formation was also discussed. These nanoporous materials can be very useful in the fields of solar cells, gas sensors, catalysts, optical filters, and capacitors.

Modeling sintering behavior of metal fibers with different fiber angles

The formation of sintering necks between two metal fibers was investigated using the oval-oval model with respect to the fiber angle range of 0°-90°. Surface diffusion was assumed to be the predominant mechanism in every section of the junction of two metal fibers in this model, which was addressed numerically using the level- set method. The growth rates of the sintering necks in the direction of the bisector of obtuse angle, the bisector of acute angle and the fiber axis were discussed in detail. It is found that the growth rate of the sintering necks decreases with fiber angle increasing in the direction of the fiber axis and the bisector of acute angle. However, an opposite variation in growth rate of sintering necks can be found in the direction of the bisector of obtuse angle. The numerical simulation results show that the growth rate of the sintering necks is significantly affected by the initial local geomet- rical structure which is determined by the fiber angle.