Substitution du chrome par de l'aluminium et du silicium dans des aciers a 13 % de chrome et 0,1 % de carbone

Aluminium and silicon were substituted to chromium in iron chromium containing initially 13 wt % of chromium and 0.1 wt % of carbon. The structures were studied after different thermal treatments: in every case the precipitation of carbides was observed. The main result is the decrease in the hardness of aluminium-chromium alloys when maintained at 1300°C, whereas it seems that alloys containing silicon showed a hardness maximum.     

Rheological behaviour and modelling of semi-solid Sn-15% Pb alloy

The rheological behaviour and modelling of a semi-solid, Sn-15% Pb alloy characterized by a special coaxial-cylinder rheometer over a wide range of process conditions is reported. In particular, the effect of shear rate , volume fraction of solid (f _s), and cooling rate on the apparent viscosity () of the semi-solid Sn-15% Pb alloy under isothermal and various cooling conditions was studied. Based on the experimental data, the shear rate used in preparing the semi-solid alloy as well as the volume fraction of solid have the most dominant effects on the rheological properties of the semi-solid Sn-15% Pb alloy. A viscosity model expressed as is proposed in which f _s ^* is the critical solid fraction at which the apparent viscosity goes to infinity, (F _s) corresponds to the asymptotic viscosity at infinity shear rate, and characterizes the transition shear rate between the power-law and Newtonian regions. finally, measurements with a differential scanning calorimeter were made and used to correlate the temperature and volume fraction of solid which, in turn, was corroborated with available data from the literature.     

High-temperature deformation characteristics of Ti-15at.% Al alloy

The deformation characteristics of Ti-15 at.% Al alloy have been investigated by compression tests in the temperature range 873 to 1273 K (0.44 to 0.64T _m) and by extensive transmission electron microscopy. Two types of deformation patterns were identified depending on the temperature: at lower temperatures below about 1073 K, the yield stress of the sample showed inverse temperature dependence, and serrations were found on the flow curves, whereas the normal dependences of the yield stress on temperature and strain rate were found at higher temperatures above about 1073 K. Corresponding dislocation substructures were composed of coarse bands of localized slip at 1023 K, and of rather uniformly distributed dislocations at 1123 K, and sub-boundaries as well as free dislocations at 1273 K. The main operating mechanisms in these temperature regimes were assumed to be the co-operative movement of numerous dislocations under the condition of the dynamic strain ageing, viscous glide of dislocations and dynamic recovery, respectively.     

Mechanical alloying and microstructure of a Nb–20% V–15% Al alloy

A niobium-based alloy with 20% V–15% Al (at.%) was synthesized by mechanical alloying of elemental powders. During milling, the splitting of Nb X-ray peaks into two components was observed. Each component was found to correspond to a niobium solid solution (Nb_I and Nb_II) with a different lattice parameter. The intensities of Nb_I peaks on X-ray diffraction patterns decreased with the milling time and disappeared completely after 180 h of milling while the intensities of Nb_II peaks gradually increased. The powders were hot pressed and microstructural and phase analyses of the consolidated material were carried out. The microstructure consisted of Nb solid solution, Nb₃Al-base intermetallic with the A15 crystal structure and dispersoid Al₂O₃. Also an unexpected, detrimental, Nb₂Al-base σ phase was found. The volume fraction of the σ phase depended on the temperature of consolidation.     

纳米复合WC-Co粉末的烧结研究进展

纳米复合WC-Co粉末的烧结是纳米晶WC-Co硬质合金制末的烧结研究进展,包括纳米复合粉末的烧结特性、烧结机理和常用的烧结方法.     

Indentation fracture of WC-Co cermets

Indentation fracture of a series of well-characterized WC-Co cermets was studied with a Vickers diamond pyramid indenter. The resulting crack length-indentation load data were analysed in terms of relations characteristic of radial (Palmqvist) and fully developed radial/median (half-penny) crack geometries. The radial crack model gave a better fit to the data on all the alloys studied. Crack shapes determined by repeated surface polishing confirmed the radial nature of the cracks. An indentation fracture mechanics analysis based on the assumption of a wedge-loaded crack is shown to be consistent with the observed linear relation between the radial crack length and the indentation load. The analysis also predicts a simple relation among the fracture toughness (K _lc), the Palmqvist toughness (W) and the hardness (H) of the WC-Co alloys.     

Sintervorgänge in Karbid-Kobalt-Legierungen mit 0,2 bis 15% Kobalt

Sintering processes in carbide-cobalt-alloys containing 0,2 to 15% Co. The sintering processes in alloys consisting of 0,2, 0,5, 3, 6 and 15% cobalt, the remainder being either WC, TiC, WC/TiC or WC/TiC/TaC, were observed after annealing at 400°C to 1500°C by measuring the bending strength at rupture, the magnetic saturation, the specific electrical resistance and the coercive force at room temperature. As early as the grinding stage the WC-Co-compounds can be clearly distinguished from all other blendings. Thin, not necessarily coherent layers of Co form on the tungsten carbides, a fact which is indicated by the higher coercive force and bending strength at rupture of the WC-Co-green compacts. The actual sintering processes can be divided in to four temperature ranges:

• (I) T ≤ 400°C
• (II) 400°C < T ≤ 800°C
• (III) 800°C < T < liquid Phase
• (IV) T ≥ liquid Phase

The sintering process within these ranges are disscussed in detail and should greatly contribute to an understanding of the structural composition of carbide metals.     

爆炸喷涂纳米WC-Co涂层的研究

为促进爆炸喷涂法替代常规喷涂工艺在工业上的应用,采用爆炸喷涂法制备了纳米和普通WC-12Co涂层.采用金相显微镜、扫描电镜分析比较了两种涂层的显微组织,用显微硬度计和磨料磨损试验机测试了两种涂层的显微硬度及耐磨料磨损性能,并利用扫描电镜对磨损形貌进行了分析.结果显示,纳米涂层具有比普通涂层更高的致密度和显微硬度,纳米涂层中WC颗粒的分布更均匀;纳米涂层的磨损机理为微观切削机制,其耐磨料磨损性能比普通涂层差.     

Achieving Over 15% Efficiency in Organic Photovoltaic Cells via Copolymer Design

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron‐withdrawing unit, ester‐substituted thiophene, is incorporated into a PBDB‐TF polymer to downshift the molecular energy and broaden the absorption. Copolymer‐based solar cells suitable for large‐area devices can be fabricated by a blade‐coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment‐friendly processing conditions and show relatively low photostability compared to copolymer‐based devices. These results not only demonstrate high‐efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.     

The shear strength of copper and brass soldered with Sn-40% Pb containing 0 to 10% Sb and/or 0 to 15% Zn

The single lap shear strength of copper and brass soldered with Sn-40% Pb containing 0 to 10% Sb and/or 0 to 15% Zn has been determined and the microstructure examined using metallographic techniques. For any solder composition, brass joints were stronger than copper joints. The strength of copper joints decreased monotonically with the increase of antimony in Sn-Pb solder, and the strength of brass joints increased to a peak with about 3% Sb in the solder and thereafter decreased on further additions of antimony. With less than a few per cent antimony in the solder, 1 % Zn in the solder decreased the strength of both copper and brass joints; with more than 1 % Zn in the solder the strengths of both copper and brass joints were increased substantially. Fracture occurs mainly in the Cu₆Sn₅. The microstructure and the presence of zinc in the intermetallic compounds were determined, and the results are discussed.     

Cobalt intergranular segregation in WC-Co composites

Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.     

MACHINING OF THERMALLY SPRAYED WC-Co COATINGS

This paper reports the research results obtained from machining of thermally sprayed WC-Co coatings. WC-Co coatings are used in marine applications. The machined coatings were obtained by using two thermal spraying processes: arc spraying and high-velocity oxy-fuel (HVOF) spraying. Different techniques were tried to optimize the surface finishing of the thermally sprayed coatings based on surface finish and time required. The machining processes tried for WC-Co coatings were diamond grinding, cubic boron nitride (CBN) grinding, diamond turning, and diamond polishing. Diamond turning had the advantage of speed, however, the surface finish was not as good as that with fine grinding. Polishing should have given the best surface finish, however, this process was slow. It is suggested to reduce the machining time, first by rough grinding or diamond turning to near the final dimensions, and then by fine grinding or polishing to get the required surface finish and dimensions.     

Isothermal crystallization on the surface of Te-15 at % Ge alloy glass

Isothermal surface crystallization is investigated for Te-15 at % Ge alloy glass. This surface-crystalline pure tellurium nucleates by ageing at low temperature below 451 K. Since germanium-rich phases and GeTe chemical compound prevent this growth, the maximum thickness of the surface layer is below 0.005 mm.