服役条件下材料制备与加工的思路设想

为了获得性能优异的材料,人们采用仿生的方法,制造显微组织与生物系统材料相似的材料,在复合材料制备等方面获得了成功.但可以看出,目前的仿生方法主要是仿制生物系统材料的显微组织和微观结构,还未涉及材料在应用时的具体条件,而这些具体条件对特定的组织形成往往具有极其重要的作用.     

Structure/processing relationships in the fabrication of nanoporous gold

Nanoporous gold made by dealloying silver/gold alloys is a relatively new material finding application in catalysis, sensing, and other areas. Here we discuss the metallurgical processing required to make patterned foils of nanoporous gold with large, flat grains, with which we are exploring an application as substrates for the heterogeneous nucleation of protein crystals.     

Assessment of supply interruption of rhenium,recycling, processing sources and technologies

Rhenium is a unique, valuable and extremely rare chemical element currently used as an alloying element in high-temperature superalloys for aerospace and industrial gas-fired turbines and also as a catalyst in petrochemical industry. Moving towards a more competitive and sustainable economy requires access to this metal in adequate quantities and at competitive costs. However, minerals containing rhenium are generally found in very small quantities and are currently not commercially viable sources. Thus, the method and route for the extraction of primary rhenium is dependent on other metals (copper, molybdenum and uranium) of which rhenium is a by-product. In addition, focusing on alternative sources such as recycling of rhenium from waste alloy scrap and catalysts, is continually gaining attention in the research community. This paper has focused on the assessment of secondary sources of rhenium. The historical cost and also supply interruption indicators of rhenium were examined and assessed. Finally, opportunities of recovering and reusing existing stocks through Industrial Ecology are discussed.     

Powder processing of FeAl sheets by roll compaction

Powder metallurgical processing techniques were employed to manufacture 0.2 mm thickness of iron aluminide sheets based on Fe-40 at% Al using water atomized powder of an FeAl alloy containing C, B, Mo and Zr as alloying elements. Powders of FeAl mixed with polymeric binders were roll compacted using two counter rotating rolls to a thickness of approximately 0.66 mm. Roll compacted sheets were then debindered in nitrogen using a two stage debindering cycle over a period of several hours to effectively decompose the binder. Debindered sheets were vacuum sintered prior to cold rolling them for densification in several different stages. The properties of the FeAl sheets are dependent on the processing temperature, Al content of the final sheet, oxygen content, and the microstructure. Sheets exhibit fine microstructure with a dispersion of Al₂O₃ particles throughout the matrix. Variation of electrical resistivity, specific heat, thermal expansion, and thermal diffusivity with variation of temperature is presented, and compared with 304 steel.     

Effects of ma processing variables on the fabrication of nanocrystalline Al3Nb powders

In the present study, elemental powder mixtures of Al-25at.%Nb were mechanically alloyed in an attritor in order to examine the relationships between MA processing parameters and the structural characteristics of Al₃Nb nanocrystalline powders. Homogeneous Al₃Nb powder particles can be obtained with the following processing variables: The addition of 3wt.% stearic acid as process control agent (PCA), the milling speed of 400 rpm and a MA milling time over 40 hours. The partial formation of Al₃Nb and Nb₂Al intermetallics in the matrix was detected by XRD analysis in as-milled powders. The phase transformation and microstructure evolution in Al₃Nb powders according to heat treatment was analyzed and discussed using TEM analysis.     

Friction stir processing of thixoformed AZ91D magnesium alloy and fabrication of Al-rich surface

The microstructural evolution characteristics of thermo-mechanically affected zone were investigated during friction stir processing (FSP) of the thixoformed AZ91D alloy. Simultaneously, an Al-rich surface layer was prepared by combination of Al powder using FSP method. The results indicate that the dynamic recrystallization and mechanical separation (including splitting and fracture of the primary grains) are the main mechanisms of grain refinement. For the thixoformed alloy, the operation efficiency of these mechanisms is less than that of the permanent mould casting AZ91D alloy, thus its microstructural evolution is relatively slow and the resulting grain size is relatively large. These are attributed to the differences in their original microstructures. The Al-rich surface layer can obviously improve the corrosion resistance in NaCl aqueous solution. A proper solution heat treatment (at 415 ℃ for 1 h) can further increase the corrosion resistance. In order to improve corrosion resistance, increasing the amount and improving the distribution uniformity of the Al-rich phase are more effective than increasing the Al solubility in the matrix.     

Powder processing and properties of zircon-reinforced Al-13.5Si-2.5Mg alloy composites

Zircon, ZrSiO₄, is a thermally stable mineral requiring expensive and energy-intensive process to reduce. Owing to its abundance, high hardness, excellent abrasion/wear resistance, and low coefficient of thermal expansion, a low-cost alternative use of the mineral for medium-strength tribology was investigated. The present study has developed a conventional low-cost, double-compaction powder metallurgy route in the synthesis of Al-13.5Si-2.5Mg alloy reinforced with zircon. The mechanical and physical properties were determined following the development of optimum conditions of cold pressing and reactionsintering. Reinforcing the hypereutectic Al-Si alloy with 15 vol% zircon particles (size < 200 μm) and cold pressing at 350 MPa to near-net shape, followed by liquid-phase reaction sintering at 615 °C in vacuum for 20 min, improved the ultimate tensile strength, 0.2 % yield strength, and hardness of the alloy by 4,12.8, and 88%, respectively. At values of more than 9 vol% zircon, percent elongation and the dimensional changes of the sintered composites remained virtually unchanged. At a critical volume fraction of zircon, between 0.03 and 0.05, a sharp rise in hardness was observed. Microstructural and mechanical property analysis showed that the improvement in the mechanical properties is attributable largely to the load-bearing ability and intrinsic hardness of zircon, rather than to particulate dispersion effects. A good distribution of the dispersed zircon particulates in the matrix alloy was achieved.     

采用CVD法还原挥发性铼的氧化物制备超细铼粉(英文)

研究了一种以高铼酸铵为原料,采用化学气相沉积(CVD)制备超细铼粉的新方法。通过控制氧分压,使得NH4ReO7分解为具有挥发性的ReO4、Re2O7,再采用载气将其输运至还原区,经氢气还原生成超细铼粉。热力学计算表明,在NH4ReO7分解过程中,控制氧分压高于101.248Pa时,Re2O7将不会分解为低价氧化物,DSCTGA分析结果也证实了这一点。采用该方法制备的铼粉,粒度为100~800nm,D50为308nm,比表面积为4.37m2/g,氧含量为0.45%。     

Welding fabrication and the environment

Summary

An initial attempt, from a management point of view, to correlate welding fabrication with environmental issues.

Following a brief outline of standard UNI EN ISO 14001, the principal operational phases which may be involved in welded structures and/or fabrications are considered, in conjunction with their relationship with the environment.     

聚四氟乙烯对搅拌摩擦加工制备Al-Ni复合材料均匀性的影响

采用搅拌摩擦加工技术制备了Al-Ni系金属间化合物增强铝基复合材料,通过SEM、XRD对复合材料的微观结构及相组成进行了分析,研究在原材料中添加聚四氟乙烯对搅拌摩擦加工制备复合材料中增强相含量及分布均匀性的影响,并对增强相形成的物理模型进行了探讨。结果表明:添加聚四氟乙烯可以显著改善Al-Ni复合材料的均匀性;加入聚四氟乙烯后会阻隔Ni粉的团聚,致使Ni与Al的接触面增大,从而增大了Ni与Al的反应界面,导致反应程度增大,Al_3Ni金属间化合物生成量增多。     

Powder processing and densification behaviour of alumina–high zirconia nanocomposites using chloride precursors

Al₂O₃–ZrO₂ composites having nominal equal volume fraction of Al₂O₃ and ZrO₂ were prepared from gel-precipitated powder, precipitated powder and washed precipitated powder. These different processing routes affected the crystallization temperature of the amorphous powder as well as the phase evolution of Al₂O₃ and ZrO₂ during calcination. The agglomerate size was largest for gel-precipitated powder (30 μm) and it was smallest for washed precipitated powder (19 μm). While gel-precipitated powder produce hard agglomerated powder (P_j = 110 MPa), washed precipitated powder produce soft agglomerates with low agglomeration strength (P_j = 70 MPa). Thus, washed precipitated powder could sinter to a high density at lower sintering temperature. The bending strength exhibits a semi logarithmic relationship with porosity. The hardness shows an increasing trend with sintering temperature.     

铼元素分离富集研究进展

铼元素是一种可广泛应用于国防、航空航天以及新型科技领域的重要战略资源。铼元素在地壳中含量极低,是一种稀散金属,需要通过高效的分离富集才能获取。介绍近年来铼金属的冶炼/回收工艺流程及进展,综述近年来铼金属的冶炼/回收工艺流程及其进展,重点介绍可应用于铼富集分离的块体材料、纳米材料、生物质材料、介孔硅材料等新型吸附材料及其在铼一次资源与二次资源的开发与回收中的应用,并对铼富集与分离技术的发展进行展望。     

Primary creep: Secondary gamma prime and the rhenium effect

Creep at low temperatures and high stresses in some second-generation nickel-based superalloys can result in large primary creep strains exceeding 5% in fewer than 5 hours. Several possibilities have been given for the appearance of large primary creep strains including the presence of secondary γ′ and the addition of rhenium to these alloys. In light of the recent surge in cost of rhenium, this article explores secondary γ′ and the rhenium effect in the first-and second-generation alloys PWA 1480 and PWA 1484, respectively. In addition, it will be shown that selecting higher aging temperatures can aid in the reduction of primary creep strains in PWA 1484.     

On the possibility of rhenium clustering in nickel-based superalloys

In order to elucidate the role of this element in superalloy metallurgy, the binding energy of Re–Re pairs and the stability of small Re clusters in the nickel face-centred cubic (fcc) lattice is investigated using ab initio density functional theory. It is shown that the formation of Re–Re nearest neighbour pairs is energetically unfavourable, and that this repulsive energy is dramatically reduced as soon as the solute atoms move further apart from one another. Furthermore, small nearest neighbour and second neighbour Re clusters are found to be unstable. The calculations are repeated for W and Ta, which lie beside Re in the periodic table; the results are essentially the same, except that some Ta–Ta higher order pairs have a positive binding energy, consistent with the Ni–Ta binary phase diagram exhibiting several ordered intermetallics. The predictions show that Re clusters are unstable in fcc Ni and it is unlikely that clustering has a role in improving creep and fatigue properties (the rhenium-effect) in Ni-based superalloys.     

Effect of friction stir processing tool probe on fabrication of SiC particle reinforced composite on aluminium surface

Abstract

An investigation has been carried out of the effects of tool probe shape and size on the formation of surface composite by uniformly distributing SiC particles into a surface layer of an A1050-H24 aluminium plate through friction stir processing (FSP). Tool probes of three different diameters (3, 5 and 7 mm) and four different shapes (circular with threads, circular without threads, square and triangular) have been used to fabricate the surface layers at rotation speeds of 1500–2250 rev min^?1 and a travelling speed of 1·66 mm s^?1. The SiC particles were packed into a groove of 3 mm width and 1·5 mm depth cut on the aluminium plate and covered by an aluminium sheet of 2 mm thickness. A rotating tool was plunged into the plate through the cover sheet so that the tip of the probe reached beyond the bottom of the groove. As a result, it was found that the square probe dispersed the SiC particles homogeneously in the nugget zone compared with other probe shapes regardless of the rotation speeds. Furthermore, the distributed particles and also the aluminium matrix grain size became finer by the use of square probe than those of the other shapes. On the other hand, the wear rates of the square and triangular probes were higher than that of circular shape. The worn iron debris from the tool reacted with aluminium matrix and form fine iron aluminides compound dispersed in the nugget zone. The probe size had limited effects on the homogeneity of the SiC particles distribution in the nugget zone; the distribution of SiC particles obtained by triple FSP passes was less homogeneous when the probe size was smaller. Microhardness of the nugget zone was homogeneously increased to a level as high as 60 HV with tool of square probe shape after three passes to be compared with 23 HV of the aluminium matrix beside the nugget zone.