Korngrößenänderung beim Carburieren von Wolfram und Sintern von Wolframcarbid

The change of grain size during each stage of the process was studied, mainly by optical and electron microscopy. The grain size has a marked effect on the properties of cemented carbide. In order to determine the process of formation of WC powder, the cross section and the surface of the carburized powder were examined microscopically. The mechanisms of carburization and grain growth were clarified. The microstructural relationship between the WC powder and the cemented carbide was also determined. The Co from the ball milled WC-Co mixed powder was extracted and the remaining WC powder was studied microscopically and by determining the grain size distribution. Methods of particle crushing and their effect on differences between various forms of WC powder were studied. Grain growth in sintering and the effect of grain size distribution were also studied. It was concluded that grain growth begins long before the appearance of the liquid phase. Finally it was pointed out that the inner structure of WC should be studied at the atomic level.     

去钴深度的测定及其对硬质合金性能的影响

研究了原子吸收光谱法和辉光放电光谱仪定量测定硬质合金去钴深度的新技术。实验得到了腐蚀时间与硬质合金去钴深度、表面显微硬度的关系曲线,分析了去钴深度对硬质合金表面形貌的影响。     

Scanning-Auger-Untersuchungen an Hartmetall-Bruchfl?chen

Zusammenfassung Die Festigkeit von Hartmetallen auf Basis WC-Co hängt u. a. auch von der Kohäsionsenergie der Carbid-Carbid-Korngrenze im Gefüge ab. Geringe Mengen von Spurenelementen, die sich an diesen Stellen anreichern, können die Bruchfestigkeit der Hartmetalle beeinträchtigen. Anhand von Scanning-Auger-Untersuchungen von frischen Hartmetallbruchflächen konnten wir nachweisen, daß zwischen den Carbidkörnern ein monoatomarer Cobaltfilm vorliegt. In titanhaltigen WC-Mischcarbidsystemen wird das Titan auch an den WC-WC-Korngrenzen angereichert. Die Chromverteilung an den Korngrenzen ist heterogen. Kritisch ist jedoch die Phosphorsegregation, die trotz geringer Gesamtkonzentrationen (2 ppm) zu beobachten war. Phosphor wird in den Hartmetallen einen ähnlichen Versprödungseffekt hervorrufen wie bei vielen technischen Stählen und Legierungen. Für die Untersuchung von Carbid-Carbid-Korngrenzen ist es zweckmäßig, den Co-Binder durch HCl-Behandlung herauszulösen. Dadurch wird der Anteil von Carbidkorngrenzen in der Bruchfläche wesentlich erhöht.

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Scanning Auger investigations of cemented carbide fracture surfaces

Summary The mechanical strength of cemented carbide materials depends among other factors on the cohesion energy of the carbide-carbide grain boundaries. Small amounts of trace elements can segregate, reducing the mechanical properties of these materials significantly. We investigated fresh fracture surfaces of cemented carbide samples in a scanning Auger electron spectrometer and determined the chemical composition of the carbide grain boundaries. Based on these data it is confirmed that the binder metal cobalt forms a monolayer at the carbide-carbide grain boundaries. In titanium containing carbides titanium, too, segregates to the grain boundaries. Chromium was also observed, however, with a heterogeneous distribution. In spite of the low phosphorus content of 2 ppm it could be identified at the grain boundaries. It is to be expected that P develops an embrittling effect similar to that in technical steels. For the investigation of carbide-carbide grain boundaries it is advantageous to remove the Co binder by a HCl treatment. The remaining carbide skeleton fractures completely along the grain boundaries.

     

Effect of nano-Al2O3 particles and of the Co concentration on the corrosion behavior of electrodeposited Ni–Co alloys

Incorporation of nano-Al₂O₃ particles into a Ni–Co alloy by electrodeposition influences the corrosion properties, morphology, and structure of the layers. The resistance against corrosion of Ni–Co/Al₂O₃ composite films deposited on stainless steel was investigated in a 0.1-M NaCl solution by potentiodynamic polarization. The presence of nanoparticles improves the corrosion resistance of Ni–Co/nano-Al₂O₃ deposits when compared to pure Ni–Co alloy. Moreover, by increasing the pH of the electrodeposition bath and the content of Co in the alloy, the resistance against corrosion is furthermore improved. The morphology of the deposits before and after their corrosion was analyzed by scanning electron microscopy. The presence of the embedded alumina particles in the Ni–Co alloys was one of the key factors that limited further propagation of corrosion on the metallic surface. Preferential corrosion attack, in the form of a pitting corrosion, was located mainly at the grain boundaries.     

High-resolution Auger electron spectroscopy of phase boundaries in TiC/TiB2 materials

Summary Multi-phase hard materials free from binder metals are used to meet the requirements of high wear resistance and high temperature stability. Investigations of properties and wear behavior showed extremely high values in carbide/boride combinations, particularly a TiC/TiB₂ mixture of a 11 molar ratio. The pseudobinary phase diagram shows low intersolubilities. High-resolution Auger Electron Spectroscopy was used to examine mixed two-phase TiC/ TiB₂ hard materials and compare them to pure TiC and TiB₂. The AES analyses were carried out on cross sections and fresh fracture surfaces. At the grain boundaries in pure TiC as well as in pure TiB₂, some amounts of Fe, Cr, Co and P can be detected. These elements are already in the powders as received. In addition, an O-rich phase has been identified in hot pressed TiB₂. The same impurity elements as in the pure materials can be found at a few phase boundaries and triple points in two-phase TiC/TiB₂ with a thickness of a few monolayers. However, large areas of the fracture surface were found to carry a TiB_xC_y phase on the grains. The considerable increase in wear resistance and fracture toughness of TiC/TiB₂ compared to pure TiC and pure TiB₂ can be explained by the presence of this TiB_xC_y phase.

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Grenzflächenuntersuchungen an TiC/TiB₂-Hartstoffen mit hochauflösender Auger-Elektronenspektroskopie

     

EIS investigation of a Ce-based posttreatment step on the corrosion behaviour of Alclad AA2024 anodized in TSA

In the aircraft industry, anodizing and posttreatment steps use Cr (VI) compounds, which, despite offering good corrosion resistance and self-healing properties, are highly toxic and carcinogenic. Ce compounds are recognized as efficient corrosion inhibitors for Al alloys, and several works report self-healing ability for these chemicals. In this investigation, the corrosion resistance of Alclad AA2024-T3 alloy anodized in tartaric-sulphuric acid (TSA) bath and posttreated in a solution comprising cerium nitrate without and with hydrogen peroxide was evaluated. The purpose is to investigate the potentiality of using hydrothermal treatment in Ce nitrate solution as candidate to replace Cr (VI) posttreatment. The aim is to provide a posttreatment step which, while improving the corrosion resistance, does not plug the mouths of the pores maintaining the adhesion properties of the porous anodic layer. Microstructural characterization was carried out by SEM-EDS whereas corrosion resistance was evaluated by EIS. The surface analysis showed that the posttreatments, all performed at 50°C, kept the open structure of the pores. EIS analysis showed that the posttreatments performed in the H₂O₂ solution for short immersion times were the most effective in improving the corrosion resistance of the samples, whereas electrical equivalent circuit (EEC) fitting of the data indicated sealing of the porous layer during the immersion of the different samples in the test solution. SEM-EDS analysis of the samples posttreated in the H₂O₂ containing solution, prior and after the corrosion test, showed the presence of Ce oxy-hydroxide randomly deposited on the sample surface, indicating that Ce could be incorporated/stored in the anodic layer.     

Electrochemical and microgravimetric characterization of magnetron-sputtered Fe–Cr–Ni–Ta and Fe–Cr–Ni alloy films in neutral and strongly acidic media

The Fe–Cr–Ni and Fe–Cr–Ni–Ta alloy films were deposited on quartz substrates by magnetron-sputtering using targets of AISI 316 stainless steel and in combination with pure tantalum. The conventional melting of the Fe–Cr–Ni–Ta alloy formed is virtually impossible because the melting point of tantalum is higher than the boiling points of the other components. Elemental content of the films was determined by XPS analysis. Corrosion behaviour of both alloy films was studied in 5% NaCl and 10 M HCl by electrochemical quartz crystal microgravimetry (EQCM), electrochemical impedance spectroscopy (EIS) and dc-voltammetry. The corrosion resistance of Fe–Cr–Ni–Ta appeared to be significantly higher than that of Fe–Cr–Ni in both neutral (5% NaCl) and strongly acidic (10 M HCl) media. The Fe–Cr–Ni–Ta specimen exhibited an extremely high corrosion resistance in 10 M HCl, where the corrosion rates were about one order of magnitude lower than those of Fe–Cr–Ni in neutral solution. EQCM measurements in NaCl solution indicated accumulation of corrosion products on the Fe–Cr–Ni–Ta surface, which was evident from a distinctive increase in electrode mass. By contrast, the mass of the tantalum-free alloy film decreased with a constant rate, which indicated alloy dissolution to prevail. The corrosion current calculated from the mass decrease was in good agreement with that derived from voltammetric measurements. The EQCM data showed that the corrosion resistance of the Fe–Cr–Ni–Ta alloy film in 10 M HCl was about two orders of magnitude higher than that of the Fe–Cr–Ni.     

Exploiting carbon‐based (Cr,Ce)/a‐C:H nanocomposite film on cemented WC

Cemented tungsten carbide (WC) has widely served in modern industry because of its outstanding characteristics, while it could suffer from severely wear both under ambient air and water environments. To exploit a novel carbon‐based film should be a feasible way to modify the surface of cemented WC and overcome these shortcomings. In the present study, the Cr/Ce co‐incorporated (Cr,Ce)/a‐C:H carbon‐based film was successfully deposited on cemented WC. The microstructure and mechanical properties of films were systematically characterized, and their tribological behaviors were tested in ambient air and deionized water environment. The results showed that (Cr,Ce)/a‐C:H film dominated by the typical amorphous structure and the doping Cr existed with the metallic Cr nanocrystallites as well as Ce formed CeO₂. The (Cr,Ce)/a‐C:H film could possess good mechanical performances, which could own higher hardness, elastic module, low internal stress, and better adhesive strength. Especially, the as‐prepared (Cr,Ce)/a‐C:H film could present relatively lower friction coefficient and wear rate compared to uncoated cemented WC both under ambient air and deionized water environment, indicating that the Cr/Ce co‐doped (Cr,Ce)/a‐C:H film could be an effective method to modify the surface of cemented WC so as to improve the friction and wear performances of cemented WC materials.     

核壳结构碳化钨/碳化钨铁复合材料的制备与电催化活性

以铁黄为载体,偏钨酸铵为钨源,将直接包覆与原位还原碳化技术相结合制备了碳化钨/碳化钨铁复合材料.经X射线衍射(XRD)分析和透射电子显微镜(TEM)观察,复合材料的主要物相为碳化钨铁(Fe3W3C)、碳化钨(WC)和碳化二钨(W2C),且构成了以Fe3W3C为核、WC和W2C为壳的核壳结构.采用三电极体系循环伏安法测试了复合材料在酸性、中性和碱性体系中对甲醇的电催化氧化活性.结果表明,与颗粒状碳化钨和介孔空心球状碳化钨相比,复合材料的电催化活性有了明显的提高;进一步研究发现,复合材料的电催化活性不仅受到体系性质的影响,还与其物相组成和微结构相关.上述结果说明,通过控制复合材料的物相组成及微结构,以及反应体系的性质可实现对其电催化活性的调控;同时表明,核壳结构是提高碳化钨催化材料活性的有效途径之一.     

Chemical analysis of the submicron binder phase in hard metals

Summary Chemical Analysis of the Submicron Binder Phase in Hard Metals The chemical analysis of the cobalt binder phase in sintered hard metals poses severe problems if performed by means of physical methods of surface analysis, such as SIMS, EMPA or AES. Compared with their limit of lateral resolution the average mean free path of the binder phase between the carbide particles, especially in micrograin hard metals, is less than 500 nm and thus too small. An electrochemical technique has been developed by which the carbide phase can be selectively etched away from the surface of a polished section, leaving the cobalt binder phase chemically and morphologically unchanged. The carbide-free cobalt binder phase can now be separated from the bulk hard metal using a technique similar to the well-known extraction replica technique commonly applied in transmission electron microscopy. The extracted binder phase can then be analyzed by X-ray fluorescence or other methods.     

Surface characteristics of Ti‐10Ta‐10Nb alloy modified by hydrogen peroxide treatment for dental implants

The use of titanium‐based alloys as biomaterials is becoming more common because they have a reduced elastic modulus, superior biocompatibility, specific strength, good corrosion resistance, superior strain control, and fatigue resistance compared to conventional stainless steel and Co? Cr alloys. However, when implanted into the human body these metals are problematic because they do not directly bond with living bone. Surface treatments play an important role in nucleating calcium phosphate deposition on a surgical titanium alloy implant. The purpose of this study is to examine whether the precipitation of apatite on Ti? 10Ta? 10Nb alloy is affected by surface modification in H₂O₂ solution. Specimens were chemically treated with a solution containing 30 wt% H₂O₂ at 80 °C for 1 h, and subsequently heat treated at 400 °C for 1 h. All specimens were immersed in SBF (Simulated Body Fluid) with a pH of 7.4 at 36.5 °C for seven days, and the surfaces were examined with XRD, SEM, EDX and in vitro testing. The microstructure analysis of the Ti? 10Ta? 10Nb alloy after etching with Keller's etchant showed a Widmanstatten pattern. The micro‐Vickers hardness number was 236.44 ± 4.99, and surface roughness was increased by the surface treatment. The wettability after surface treatment was better than on the nontreated surface. Resistance to cytotoxicity was decreased by the chemical surface treatment (P < 0.05). Copyright © 2011 John Wiley & Sons, Ltd.     

Electrochemical study on corrosion process characteristics of the high-strength low-alloy steels in NaHSO3 solution

Electrochemical methods were used to study the characteristics of corrosion process for the high-strength low-alloy steel and carbon steel used as a huge oil storage tank in NaHSO₃ solution. The polarization curve results show that both steel samples take place in active solution, and the high-strength low-alloy (HSLA) steel has higher i _corr value than carbon steel, which is due to the small grain size that provides high density of active sites for preferential attack. The electrochemical impedance spectroscopy (EIS) results make known that the corrosion process presents two stages. In the first 136 h, one-time constant in EIS diagrams can be shown. Both steels have similar corrosion resistance due to the combination effects of the grain size and microstructure. After 240 h of immersion, a complete passive film forms on the specimen surface, and two-time constants can be shown in EIS diagram. The HSLA steel exhibited improved corrosion resistance when compared with the carbon steel, which is due to the effect of the shape Fe₃C in microstructure and the deposition of FeSO₄ on the electrode surface. The scanning electrode microscopy analyses show that both steels take place in homogenous corrosion, and the carbon steel shows higher surface roughness and many Fe₃C residues. XRD results show that both steels have similar phase constitutes of corrosion products.     

Study of the cobalt diffusion in CVD-layers by scanning Auger microscopy (SAM)

Summary Multilayers of TiC/Al₂O₃/TiC were deposited by CVD (Chemical Vapor Deposition) on pure cobalt, cemented carbides (WC-Co) and Al₂O₃ substrates. By a low angle cut produced with a ball cratering technique, depth profile measurements were determined by SAM.In the case of the cobalt substrate, the cobalt concentration in the first TiC layer was greater than 10 at %. For the cemented carbide substrate, an enrichment at both interfaces of the first TiC layers was clearly detected. This occurred by diffusion of Co during the cleaning of the hot (1050 ° C) CVD reactor with hydrogen while no CVD deposition occurred. A thin Al₂O₃ layer (0.5 m) proved to be a very efficient diffusion barrier for cobalt.The cobalt diffusing from the substrate acted as accelerator and grain refining agent for the TiC CVD deposition.

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Untersuchung der Kobaltdiffusion in CVD-Schichten mit Scanning Auger Mikroskopie (SAM)

     

V2.1TiNi0.4Zr0.06Cu0.03M0.10 (M=Cr,Co, Fe,Nb, Ta)储氢合金的微结构及电化学性能

采用磁悬浮感应熔炼方法制备了V2.1TiNi0.4Zr0.06Cu0.03M0.10(M=Cr, Co, Fe, Nb, Ta)储氢电极合金, 通过X射线衍射(XRD)、扫描电子显微镜(SEM)、电子衍射能谱(EDS)分析和电化学测试等手段系统研究了添加元素M对合金微结构与电化学性能的影响. 结果表明, 所有合金均由BCC结构的V基固溶体主相和C14型Laves第二相组成, 且第二相沿主相晶界形成三维网状分布; Cr、Nb 和Ta元素主要分布在合金主相中, 而Co和Fe元素主要分布在第二相中. 电化学性能测试表明, 在V2.1TiNi0.4Zr0.06Cu0.03合金中掺加Cr、Co、Fe、Nb或Ta元素后, 虽然会降低最大放电容量, 但能有效抑制合金中V和Ti的腐蚀溶出, 提高电极充放电循环稳定性; 同时还能明显改善合金的高倍率放电性能. 相比之下, V2.1TiNi0.4Zr0.06Cu0.03Cr0.10合金具有最佳的综合电化学性能.     

Corrosion resistances of passive films on low‐Cr steel and carbon steel in simulated concrete pore solution

The passive ranges of carbon steel rebar and 3Cr steel rebar in saturated Ca(OH)₂‐simulated concrete pore solution with pH 12.6 were determined by means of cyclic voltammetry and potentiodynamic polarization curves. Chronopotentiometry was used to obtain steady‐state conditions for the formation of passive films on rebar samples at different anodic potentials. Electrochemical impedance spectroscopy, Mott–Schottky and X‐ray photoelectron spectrometer curves were employed to compare the formed passive films at different potentials. Additionally, cyclic polarization curves were used to compare the corrosion resistances of formed passive films on the two rebars in saturated Ca(OH)₂‐simulated concrete pore solution with different concentration of Cl^?. The results show that the passive ranges of the two rebars are all between ?0.15 and +0.6 V, and more stable passive films can be formed on both rebars at the anodic potential of +0.3 V. In the absence of Cl^?, the stability and corrosion resistance of the passive film formed on the 3Cr rebar are better than those of CS rebar. The passive film of 3Cr steel has the relatively better pitting corrosion resistance than carbon steel in saturated Ca(OH)₂‐simulated concrete pore solution that contains different concentration of Cl^?. Copyright © 2016 John Wiley & Sons, Ltd.     

Corrosion product build-up on stainless steel and its influence on hydrazine decomposition.60Co deposition in loop tests using borated water at 300°C

The corrosion rate of stainless steel decreases with time of operation under conditions characteristic of the primary circuit of pressurized-water reactors. Due to this the concentration of corrosion products in solution rapidly increases during heating-up and decreases after reaching the operation product levels is up to about three orders of magnitude. By a preoxidation treatment using potassium dichromate, the corrosion product release can be essentially diminished. In order to describe the influence of corrosion products in solution we investigate the thermal decomposition of hydrazine as well as the deposition behavior of⁶⁰Co. the half life of the thermal N₂H₄ decomposition at 300°C in borated water increases from about 1 to about 60 minutes, starting with glossy loop surfaces. Without adding boric acid to the water, half lives between 1 and 120 minutes were found; with an activation energy for the decomposition being estimated in the temperature region 270–310°C. The deposition behavior of⁶⁰Co is characterized by a very fast primary step, followed by retarded secondary deposition steps. The primary deposition step can be described by half lives of about 5 seconds. The influence of preoxidation treatment and subsequent decrease of corrosion product concentration on decomposition of N₂H₄ and deposition of⁶⁰Co is discussed.Presented at the Second International Symposium on Chemistry in High Temperature Water, Provo, UT, August 1991.Deceased (April 23, 1991).     

Study on corrosion resistance of electroplating zinc–nickel alloy coatings

Electrodeposited zinc–nickel alloy coatings have been widely adopted for surface treatment of automobile body steel sheet for high corrosion resistance. The corrosion behavior of the coatings has been related with the components of nickel, and the zinc–nickel alloy passive coatings have much higher corrosion resistance than that of zinc–nickel alloy coatings. In the present paper, the corrosion resistance behavior of the zinc–nickel alloy coatings obtained by new process and formulation has been studied by means of the electrochemistry test and neutral salt spray test. And it is discovered that the properties of corrosion resistance of zinc–nickel alloy passive coatings were better than that of zinc passive coatings, Cadmium passive coatings and alloys of electrodeposited cadmium–titanium. The components of corrosion productions, in terms of X‐ray diffraction (XRD), are mainly ZnO, ZnCl₂ · 4Zn(OH)₂ and small quantity of 2ZnCO₃· 3Zn(OH)₂. The component of zinc–nickel alloy coatings has been investigated with Glow Discharge Optical Emission Spectrometry (GDA‐750). And it is found that as the thickness of zinc–nickel alloy coatings increases, the component of zinc increases from beginning to end, but the peak value of nickel appears and an enrichment of nickel in the coatings comes into being. Because the electrodeposited zinc–nickel alloy coatings exhibit different alloy phases as a function of their alloy composition, in this paper, the crystal structure changing with the different component of nickel has been studied in terms of XRD. The result shows that electrodeposited zinc–nickel alloy has different phases: α‐phase, a solid solution of zinc in nickel with an equilibrium solubility of about more than 79% nickel; γ‐phase, an intermediate phase with a composition Ni₅Zn₂₁; η‐phase, a solid solution of nickel in zinc with less than 5% nickel; and δ‐phase (Ni₃Zn₂₂) appeared from η‐phase to α‐phase with increasing content of nickel. Copyright © 2009 John Wiley & Sons, Ltd.     

碳化钨/碳化二钨核壳结构纳米复合材料的制备及电化学性能

以偏钨酸铵为钨源, 铁黄(FeOOH)为载体, 将表面包覆法与原位还原碳化技术相结合, 制备出了具有核壳结构的碳化钨(WC)/碳化二钨(W₂C)纳米复合材料; 应用X射线衍射(XRD)分析、透射电子显微镜(TEM)和X射线能量散射谱(EDS)等手段对不同阶段样品的晶相、形貌、微结构和化学组成等特征进行了表征. 结果表明, 负载体经煅烧后, 载体及包裹层的物相均发生了变化, 形貌也相应地发生了改变; 经盐酸处理及还原碳化后, 样品由WC和W₂C纳米颗粒构成, 并构成了以W₂C为壳, 以WC为核的典型核壳结构; 结合表征结果对核壳结构的形成机理进行了探讨. 采用三电极体系循环伏安法测试了样品在酸性、中性和碱性溶液中对甲醇的电催化氧化活性. 结果表明, 与颗粒状碳化钨和介孔空心球状碳化钨相比, 样品的电催化活性有了明显的提高. 这说明W₂C与WC构成核壳结构纳米复合材料后, 其电化学性能有了明显的提升, 核壳结构纳米复合材料是提高碳化钨催化材料活性的有效途径之一.     

Microstructure, surface and electrochemical studies of electroless Cr–P coatings tailored for the methanol oxidative fuel cell

Cr–P is a new material of great importance as a decorative coating with nickel in automobile industries. Electroless plating of Cr–P alloy has been carried out using a suitable plating bath solution and working conditions. The deposit is characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray diffraction, X-ray photoelectron spectroscopy and polarization techniques. New phases appear on heat treatment of the coating. The composition (Cr/P) of the coating and the oxidation states of alloying elements vary from the surface to the bulk of the material. The coatings acted as a novel electrode material with good electrocatalytic activity (low overvoltage) and good corrosion resistance for anodic oxidation of methanol in H₂SO₄ at normal working temperature. The good corrosion resistance of the Cr–P film is accounted for by the existence of a double oxyhydroxide passive film on the surface. The electrocatalytic activity of Cr–P is very high when compared with chromium alone.