Some Characteristics of Automotive Exhaust Valve Alloys

The PH stainless steels and the nickel-base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys. The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel-base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.     

Oxidation of chromia forming molybdenum-tungsten based alloys

The oxidation behaviour of tungsten and molybdenum based, chromia-forming alloys prepared by powder sintering activated with group VIII metals has been investigated. The influences of the alloy composition, nature of the sintering agent and oxidation temperature have been studied. A good oxidation resistance is observed with palladium as sintering agent. This metal is rejected at the grain boundaries and allows a fast diffusion of chromium to the metal-oxide interface. Contrary to palladium, nickel leads to a catastrophic oxidation of the sample. The formation of a two-phase interface enriched in nickel leads to a non-protective oxide layer constituted with Cr₂O₃ and NiWO₄.Catastrophic oxidation is observed when the refractory metals are oxidised into volatile oxides, i.e. in the case of the alloys with a high molybdenum content. Contrary to molybdenum, a high tungsten level leads to high oxidation resistance, even at temperature as high as 1300°C. In this latter case, alloys are two-phase: this result has led to the investigation of the ternary section of the Cr-Mo-W system at 1300°C.     

The oxidation of binary alloys of chromium with metals of the first long period

Binary alloys of chromium with titanium, vanadium, manganese, iron, cobalt and nickel respectively have been oxidation-tested in air up to 1100° C. From the results the temperature corresponding to a weight change of 1 mg cm^–2 over 4 h was determined by interpolation, and this temperature has been plotted against composition.With the exception of a 50% cobalt alloy, alloys of chromium with either iron, cobalt or nickel withstand air oxidation at much higher temperatures than the constituent metals. Oxidation resistance in the other systems is generally lower.An additional note describes the determination, during oxidation, of comparative electrical resistance of scales on iron, chromium, and an iron-20% chromium alloy.     

Mikroanalytische Untersuchungen des Ausscheidungsverhaltens in kommerziellen Alitier- und Chromalitierschutzschichten auf Nickelbasislegierungen und austenitischen Stählen

Microanalytical Investigations of the Precipitation Behaviour of Commercial Aluminized and Chromium-Aluminized Coatings on Ni-Base Alloys and Austenitic Steels The precipitation and diffusion behaviour of commercial aluminized and chromiumaluminized coatings on nickel-base alloys and austenitic steels was studied by aid of an electron probe microanalyzer. It is shown, that the protected nickel-base alloys all have a similar layer structure: an external NiAl-layer, which is delimited by an intermediate CrNi-(Mo)-layer to a small Al- containing mixed crystal zone. In the case of aluminized austenitic steels only a very small NiAl-layer can be detected, followed by an extended Al-containing mixed crystal. These layers on Fe-base alloys are predamaged by a perforated zone, due to the Kirkendall-effect. A previous chromium diffusion prevents this effect, but the diffusion behavior was not influenced during an extended heat treatment. The protected nickel-base alloys showed the best durability during long time oxidation tests.     

Optical modeling of nickel-base alloys oxidized in pressurized water reactor

The knowledge of the aging process involved in the primary water of pressurized water reactor entails investigating a mixed growth mechanism in the corrosion of nickel-base alloys. A mixed growth induces an anionic inner oxide and a cationic diffusion parallel to a dissolution-precipitation process forms the outer zone. The in situ monitoring of the oxidation kinetics requires the modeling of the oxide layer stratification with the full knowledge of the optical constants related to each component. Here, we report the dielectric constants of the alloys 600 and 690 measured by spectroscopic ellipsometry and fitted to a Drude-Lorentz model. A robust optical stratification model was determined using focused ion beam cross-section of thin foils examined by transmission electron microscopy. Dielectric constants of the inner oxide layer depleted in chromium were assimilated to those of the nickel thin film. The optical constants of both the spinels and extern layer were determined.     

Development of chromia forming Mo-W-Cr alloys: synthesis and characterization

The sintering process and microstructural characteristics of tungsten-based and molybdenum-based alloys containing chromium and group VIII metals as sintering agents have been investigated. The influences of the alloy composition, the nature of the sintering agent and the synthesis process on the microstructure and microhardness of these materials have been studied. Homogeneous alloys can be obtained with palladium or nickel as the sintering agent. The mechanisms are totally different with these two metals. In the case of nickel, diffusion of the refractory metals through a nickel layer is responsible for the densification of the alloys, whereas with palladium, a CrPd liquid phase at the grain boundaries leads to homogeneous sintering. Formation of the CrPd phase is directly dependent on the chromium content wich influences the solubility of palladium in the MoW matrix. Consequently, a low chromium content leads to a high CrPd content, and to an increase in the grain size. On the contrary, with a nickel sintering agent, ahigh Ni content leads to an increase in thickness of the interdiffusion layer and thus a decrease in the grain size. Sresses generated by uniaxial sintering and mechanical alloying are not released during the annealing sequence and contribute to increase the microhardness of the alloys. Microhardness is also a strong function of the tungsten content.     

Spectroscopic studies of the chemical properties of thin metal films on poly(ether imide)

X-ray photoelectron spectroscopy studies were conducted on thin films of copper and nichrome (80:20 Ni:Cr) on neat poly(ether imide) (PEI) substrates. Evaporated copper thin films reacted mildly with PEI and formed metal carbides and oxides at the copper - PEI interface. In contrast, the chromium component of the nichrome alloy reacted strongly with PEI to generate a distribution of chromium oxides, nitrides, and carbides. However, the nickel component of nichrome exhibited little reactivity with the resin. Differences in the reactivities of metal thin films on PEI can be correlated to the heats of formation of the corresponding metal carbides, oxides, and nitrides. This information provides key insights into metals for use as adhesion promoters on printed circuit boards and molded circuits containing PEI.     

Effect of metal diffusion barrier on thermal stability of metal-coated carbon fibers

The thermal stability of carbon fibers coated with different metals was investigated. The fracture strength of metal-coated carbon fibers was measured at room temperature as a function of heat-treatment temperature. It was demonstrated that the fracture strength of the copper-coated carbon fibers (C/Cu) was not affected by heat-treatment at temperatures up to 900°C because of the inert of copper. However, the fracture strength of the carbon fibers coated with an active metal (nickel, iron and chromium) were found to decrease significantly after heat-treatment at temperatures >700°C, owing to graphitization (induced by nickel) of the carbon fibers or a chemical reaction (between fibers and iron or chromium) at the interface. A diffusion barrier layer between the active metal coating and the fibers can reduce the strength loss of the carbon fibers, but the effectiveness of a metal diffusion barrier differed depending significantly on the nature of the introduced metals. It was found that if the diffusing active metal had an adequate solubility in a metal, the latter can effectively hinder the diffusion of the former; if not, the latter can not effectively hinder the diffusion of the former.     

Compatibility of chromium carbide coated graphite fibres with metallic matrices

Chromium carbide coating on graphite fibres was obtained by diffusing chromium through a coating of copper on a graphite fibre. Coated fibres, then, were infiltrated with liquid copper, aluminium and nickel alloys in a vacuum furnace. Chromium carbide proved to be reasonably wetted by those alloys, although the wettability is affected by the alloying elements.     

Haftfestigkeitsuntersuchungen an galvanischen Niederschlägen

Investigations on the Strength of Galvanic Deposits Pure nickel, nickel alloys, austenitic steel, copper and aluminium alloys were coated with adhesive and heat resisting layers of nickel or copper by electroplating. Production and examination of these metal connections was performed by specially developed methods. The investigations demonstrated, that high quality metal connections can be produced by electroplating. These connections will be of great interest for construction and fabrication.     

Reduction of titanium and other metal oxides using electrodeoxidation

Abstract

Many reactive and refractory metals are currently produced industrially by reducing their compounds, including oxides, using a more reactive metal. In some cases, where there is substantial oxygen solubility in the metal, the oxygen is first removed by carbochlorination followed by reduction. Titanium and zirconium are made by reduction of the volatile tetrachlorides by magnesium. The processes consist essentially of two reduction steps: reducing magnesium chloride to magnesium metal and then reduction of the metal compound; this makes the overall reduction process relatively expensive. Electrodeoxidation is very simple in that the oxide to be reduced is rendered cathodic in molten alkaline earth chloride. By applying a voltage below the decomposition potential of the salt, it has been found that ionisation of oxygen is the dominant cathode reaction, rather than alkaline earth metal deposition. In the laboratory, this technique has been applied to reduce a large number of metal oxides to the metals, including titanium, zirconium, chromium, niobium, tantalum, uranium and nickel. Furthermore, when mixed oxides are used as the cathode, alloys or intermetallic compounds of uniform composition are obtained. This may offer advantages over conventional technology for those alloys that are difficult to prepare at present, owing to differences in either density or vapour pressure.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

Influence of clinoptilolite rock on chemical speciation of selected heavy metals in sewage sludge

The chemical speciation of Cd, Cu, Pb, Cr and Ni in Torun municipal sewage sludge is investigated with addition of a natural sorbent (clinoptilolite rock). The total contents of the heavy metals in the sludge are substantially lower than the corresponding limits established by European or Polish legislation excepting nickel only. But the metals concentrations excepting lead exceed significantly the natural background (average contents in soils and in the Earth's crust) in dozens. Application of the sequential chemical extraction indicated that the metals in the sewage sludge are bound mainly (over 50%) in the residual fraction. The metals form the following order by parts of the mobile form: Ni> Cd> Cr> Cu> Pb. Addition of the clinoptilolite to the sludge leads to the metals contents fall in all four fractions of the sequential procedure. Concentrations of mobile forms of cadmium, chromium, copper and nickel decrease by 87, 64, 35 and 24%, respectively, as a result of addition of 9.09% of the clinoptilolite. The total decreases of the metals amount after 9.09% clinoptilolite addition to the sludge are around 11, 15, 25, 41 and 51% for copper, nickel, chromium, cadmium and lead, respectively.     

The wetting of silicon nitride by chromium-containing alloys

The wetting of ceramic materials by metallic melts is the most important characteristic of brazing alloys. The effect of chromium additions to copper-base alloys on the wetting of silicon nitride was investigated. Wetting experiments were carried out on Si₃N₄ using liquid Cu-Cr, Cu-Ni-Cr, Cu-Si-Cr and Cu-Ni-Si-Cr alloys. The addition of chromium to liquid copper up to its solubility limit promoted wetting on Si₃N₄. Improved wetting with a higher chromium content was achieved by the addition of nickel to the Cu-Cr alloys. The formation of an interfacial reaction layer, which is detrimental for brazing ceramics, was suppressed by the addition of silicon to the chromium-containing brazing alloys.     

Methodology for analysing the degradation of Mg-PSZ

For expanded applications and ease of manufacture, joining ceramics to other ceramic and metal components is a subject of intense interest, especially for heat engine applications. Magnesia partially stabilized zirconia (Mg-PSZ) is one possible material for various desired applications, due to its toughness and thermal and mechanical shock resistance. During processing of the join and during the lifetime of the ceramic component, thermal and chemical potential gradients are expected to cause complex reactions at the zirconia-metal interface. Particularly important reactions are the oxidation of the metal-joining agents and their diffusion/migration into the ceramic. Because of the small spatial scales of both the complex reactions and the interface, identifying mechanisms of degradation due to particular metals or metal oxides would be difficult. This research focuses on a methodology to identify whether the reaction of the metal oxides with Mg-PSZ would cause degradation. The methodology for investigating these reactions of Mg-PSZ to oxidized metals was developed by adapting a conventional metallurgical technique known as the temperature-time-transformation diagram. The metals selected for investigation were copper, tin and zinc (typical brazing agents), titanium and aluminium (reactive metals), and cobalt and nickel (super alloys and typical interlayer metals). To model the reaction at the interface layer, oxides of the metals were mixed with Mg-PSZ powder and its effect on precipitation analysed. All metal oxides accelerated the precipitation rate of the tetragonal phase, thereby shifting the nose of the temperature-time-transformation diagram to shorter times as compared to undoped Mg-PSZ. Additionally, these oxides enhanced growth of the monoclinic phase with increasing time at temperature.     

Residence Time Distribution of Segregating Sand Particles in a Rotary Drum

Leaching, which is described as the extraction of soluble constituents from a solid by means of a solvent, is an important separation technique in the refining of precious metals from their matte. It is, therefore, important to investigate the extraction behavior of metals from the matte, which is the focus of this study. This study reports the influence of concentration of the solvent (ammonia), leaching temperature, leaching time, and pH on the recovery of nickel and copper from the matte. The elemental composition analysis of the matte indicated that it contains 23% copper, 37% nickel and 1.1% ferrous compound. The analysis also showed that the major mineral phases present in the matte were heazlewoodite (Ni₃S₂), chalcocite (Cu₂S), djurleite (Cu1.9S), and nickel alloy. The leaching parameters studied were concentration of ammonia (1.5, 2.0, 2.5, and 3.0 M), leaching time (0–270 min, at 15 min sampling interval), leaching temperatures (50°C, 60°C, and 70°C), and pH (9.3–11.2). The results obtained revealed that the recovery of nickel and copper from the matte was greatly influenced by the concentration of ammonia, leaching time, leaching temperature, and pH. It was established from this study that the highest dissolution of nickel and copper was obtained at 3 M and 2 M ammonia concentration, respectively. The results also revealed that a decrease in the pH of the solution resulted in a decrease in both nickel and copper recovery, with maximum leaching time of 270 min. It was observed that less than 50% of both nickel and copper was leachable due to the presence of metal alloys. The analyses of the results also showed that as the leaching temperature increased from 50°C to 60°C, the amount of nickel and copper that was recovered from the matte significantly increased. However, there was reduction in the amount of nickel and copper recovered from the matte as the temperature was increased from 60°C to 70°C, due to loss of ammonia by evaporation. The shrinking core model was used to explain the behavior of the recovery of these metals at different temperatures, and both metals were found to be favored by diffusion controlled reaction.     

Improvement of mechanical properties of chromium–nickel sintered compacts by multiple rolling

The present paper provides details on the thermomechanical treatment of chromium–nickel alloys with high content of chromium, which leads to considerable improvement of their mechanical properties. The investigated alloys were prepared from Cr–Ni powder mixtures containing 50% and 80% of chromium in mass. They were formed by a particular sequence of cold and hot isostatic pressing. The as-sintered compacts were essentially brittle at room temperature, while they have exhibited considerable ductility after the special thermomechanical treatments. The treatment consisted of a sequence of repeated rolling and annealing operations, which is relatively simple and applicable in the industrial production. It is found that the treatments led to a more homogeneous, better alloyed structure and enhanced interdiffusion of the component metals.     

Modifizierte Nickelbasisstandardlote für korrosiv belastete Bauteile

Modified nickel based standard brazing filler metals for units under corrosive loads Nickel‐based alloys are presently used as brazing filler metals for components which undergo mechanical stress in corrosive conditions, f. e. heat exchangers. When soldering chrome containing steel parts with nickel based brazing filler metals additionally containing boron and silicon a reaction of chrome and boron can occur. This evolution of chromium borides, depositing on grain boundaries, causes a lack of chrome in the steel part. A drop of the chrome content in the parts below 13 % leads to a loss of corrosion resistance. It is possible to change the microstructure of brazing joints by modification with chromium and molybdenum. Continuous brittle phases could be successfully avoided with this modification. Furthermore it could be shown that the choice of additives, the heating respectively cooling rate and the brazing temperature have important influence on the microstructure evolution and therefore on the mechanical and corrosive properties of joints.     

Solid-liquid-solid extraction of heavy metals (Cr, Cu, Cd, Ni and Pb) in aqueous systems of zeolite-sewage sludge

This paper presents results of kinetic and equilibrium studies of the removal of heavy metals (Cr, Cu, Cd, Ni and Pb) from the sewage sludge using a new technique of solid-liquid-solid extraction with the adsorption-diffusion column filled by the zeolite. The metal extraction onto the zeolite from aqueous solution of the clinoptilolite and the sludge composition is characterized by three stages: intensive extraction, inversion and stabilization with the moderate extraction increase. Addition of 25% of the zeolite provides extraction efficiency of cadmium and lead of about 84%, chromium, copper and nickel of 66%, 61% and 50%, respectively. The estimated values of Gibbs free energy change DeltaG show that the metals extraction by the clinoptilolite may be considered as a physical adsorption. The negative values of DeltaG testify to an exothermic nature of the process. The difference between energetic potentials of the components is a driving force of the metal redistribution in the system "clinoptilolite-water-sludge".     

The structures of fully eutectic aluminium-silicon alloy castings

Strontium-modified aluminium alloys containing 14 to 15 wt% silicon were cast with fully eutectic structures by using heated moulds and high-purity materials. In alloys containing the additional elements magnesium, copper or nickel, a distinct eutectic colony structure was evident outlined by intermetallic compounds. At the edges of the castings the eutectic colony structures and the aluminium grains (revealed by anodizing) were of a similar scale. Further from the mould walls the eutectic colonies contained several aluminium grains. This is believed to result from the blocking of growth of the aluminium phase by the silicon phase.