Investigation of physicochemical properties of tungsten — Rhenium thermoelectrodes

Summary The authors studied some physicochemical properties of tungsten rhenium thermoelectrodes. It is shown that the instability of the work of a thermocouple with these electrodes and the increase in their brittleness are due to the change in the chemical composition during operation.The authors thank E. I. Gladyshevskii and P. I. Kripyakevich for discussing the problems touched upon in this study.     

Fabrication of W–20%Cu composite powder from copper and tungsten salts

Abstract

An investigation has been made to prepare homogeneous powders of CuWO₄ and WO₃ from ammonium paratungstate and copper nitrate to prepare nanosized W–Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water; while the content of the beaker was being stirred at a certain speed to reach the desired composition of W–20 wt-%Cu. Mixture was heated to 80–100°C for 6 h. Also, pH range was adjusted at about 3–4. The mixture was then evaporated and dried in the air. To reach W–Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W–Cu oxide powder and then were ball milled and reduced in H₂ atmosphere to convert it into W–Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W–Cu with particle size of ～100 nm and a nearly spherical shape could be obtained by this process.     

Investigation on compressibility of Al–SiC composite powders

Abstract

The consolidation behaviour of particulate reinforced metal matrix composite powders during cold uniaxial compaction in a rigid die was studied. Al–SiC powder mixtures with varying SiC particle size, ranging from nanoscale (50 nm) to microscale (40 µm), at different volume fractions up to 30% were used. Based on the experimental results, the effect of the reinforcement particles on the densification mechanisms, i.e. particle rearrangement and plastic deformation, was studied using modified Cooper–Eaton equation. It was found that by increasing the reinforcement volume fraction or decreasing its size, the contribution of particle rearrangement on the densification increases while the plastic deformation becomes restricted. In fact, when percolation network of the ultrafine reinforcement particles is formed, the rearrangement could be the dominant mechanism of consolidation. It was also shown that at tap condition and at the early stage of compaction where the particle rearrangement is dominant, the highest density is achieved when the reinforcement particle size is properly lower than the matrix (0˙3<the size ratio<0˙5) and the fraction of hard particles is relatively low (<10%). At high compaction pressures, the reinforcement particles significantly influence the yield pressure of composite powders, thereby retarding the densification.     

Structure and thermal properties of the matte of the autogenous smelting of copper–zinc concentrates

The phase composition and the structure of the solidified mattes of the autogenous smelting of copper–zinc concentrates containing 49.8 and 61.0% Cu in Vanyukov’s furnace are studied. The forms of the main elements (Cu, Ni, Fe, S, O) and the accompanying impurities (Zn, Pb, As, Sb, Co, Sn, Au, Ag) in the rich matte are determined. The phase-transformation temperatures in cooling of the mattes in an inert or oxidizing gas atmosphere are found.     

Production and properties of AlN–BN composite

Powder Metallurgy and Metal Ceramics - The nitridation of an AlB2 precursor in a controlled nitrogen flow is examined with the aim of obtaining AlN–BN composite powders. It is shown that...     

Effects of Nd–Ga intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

In this study, we propose an approach of grain boundary modification that can significantly increase the coercivity of the B-lean Nd-Fe-B sintered magnets by intergranular addition of Nd–Ga. The coercivity is substantially enhanced from 1.51 to 2.04 T through optimizing the microstructure and adjusting the phase composition for the grain boundary phase in the annealed magnets. The matrix grains are covered by a continuous thin grain boundary phase accompanying the formation of intermetallic Nd₆Fe₁₃Ga phases. The analysis of magnetic behaviors above Curie temperature confirms that the grain boundary phase of annealed Nd–Ga doped magnets appears to be non-ferromagnetic, facilitating the intergrain exchange decoupling. Microstructure observation in grain boundary area indicates that some surface of the matrix grain is dissolved in the formation process of the Nd₆Fe₁₃Ga phase. It gives rise to a decrease in the proportion of matrix grains and saturation magnetization of the magnet. The detailed relationship between magnetic properties and microstructure is discussed based on these results.     

Effect of gadolinium on the properties of Pr–Dy–Fe–Co–B materials

Sintered (Pr_1–x–y Dy _x Gd _y )_13–14(Fe_1–z Co _z )_balB_6–7 materials (x = 0.18–0.58, y = 0.05–0.33, z = 0.2–0.36) have been studied. The magnetic moments of gadolinium ions and those of the sublattice formed by Fe and Co ions are shown to be ordered antiferromagnetically. It is noted that an increase in the content of gadolinium, which substitutes for dysprosium, leads to an increase in residual induction B _r , a decrease in coercive force H _cJ , and an increase in the absolute value of the temperature coefficient of induction. The opposite effect takes place in the case of substitution of gadolinium for praseodymium in materials with a fixed dysprosium content.     

Investigation into regularities of leaching sulfated cinders of roasting copper–zinc middlings

Results of an analysis of thermodynamic characteristic and the investigation into kinetic regularities of leaching a new type of sulfated copper–zinc middlings formed with the removal of sulfide materials that cannot be effectively selected by flotation are presented. When analyzing the kinetic curves, it is established that the reaction rate abruptly differs between the initial time period (30 min) and the subsequent period at any acid concentration in a range of 0–20 g/dm³. In the initial instant, the rate is controlled only by the external diffusion, while after 30 min of the process, the kinetic obstacles start to manifest themselves. Reaction orders of cinder leaching are determined to be as follows: 1.0 ± 0.1 for copper, 0.87 ± 0.1 for zinc, and 0.56 ± 0.1 for iron. A calculation of the apparent rate constant of the leaching reaction showed its weak temperature dependence, which correlates well with the thermodynamic analysis data.     

Structure of the molybdenum–carbon–copper composite coatings produced by electroexplosive spraying followed by electron-beam treatment

Electroexplosive Mo–C–Cu composite coatings are modified by a high-energy electron beam for the first time. Our studies demonstrate that the electron-beam treatment of the electroexplosive spraying layer that is performed under melting conditions leads to the formation of a structurally and concentration homogeneous surface layer.     

Influence of titanium microalloying on the properties of low-alloy Cr–Mo–V steel

The influence of titanium, which readily forms nitrides, on the structure and the long- and short-term mechanical properties of Cr–Mo–V steel is investigated. Increase in the Ti content to 0.075% increases the thermal stability of the steel. With increase in Ti content to 0.13%, the thermal stability of the steel declines sharply, on account of structural changes.     

Impact properties of sintered and wrought 17–4 PH stainless steel

Abstract

Charpy V notch (CVN) impact testing was conducted on full size and subsize specimens of sintered and wrought 17–4 PH stainless steel (17–4 PH SS) in the as sintered and H900 heat treated conditions. Test geometries correspond to the American Society for Testing and Materials (ASTM) and Metal Powder Industries Federation (MPIF) impact testing standards. Merits of a notched specimen compared with an unnotched specimen were analysed for both the wrought and sintered materials. The notched ASTM standard bars had a lower coefficient of variance for impact energy than the unnotched MPIF standard bars and displayed greater toughness. Porosity and grain size have a detrimental synergistic effect on impact toughness for the sintered material. Following a discussion about the differences in the wrought and sintered microstructures, it is recommended that impact testing of the injection moulded and sintered specimens should be evaluated according to the ASTM test specifications.     

Thermodynamic consideration and phase evolution during oxidation of tungsten–copper electrical contactor scraps

Present studies account the feasibility studies on isothermal oxidation of W–Cu hard metal electrical contractor scraps for recycling by roasting-leaching method. This was investigated as one of main processes to produce friable oxidized product amenable to subsequent leaching process. Oxidation is facilitated using scrap turnings instead of solid W–Cu contactor rods. The alloy oxidizes to Cu_xO, WO_x and CuWO₄ upon heating in the temperature range 400–1000°C under oxygen flow. Apart from oxidized W and Cu phases, significant amount of Cu nuggets formed initially at 450°C that rose gravimetrically up to 750°C, and then disappeared at higher temperature. The maximum weight gain was about 22% compared to initial weight when oxidation was carried out at 750°C. Oxidation beyond the temperature of 750°C corresponded to significant loss of tungsten by evaporation of WO₂ · (OH)₂ gaseous product. Thermal oxidation of W–Cu metal electrical contractor tip scraps produced porous and friable oxidized product of W and Cu.     

Tribological properties of the babbit B83–based composite materials fabricated by powder metallurgy

Technological processes are developed to fabricate composite materials based on B83 babbit using hot pressing of a mixture of powders in the presence of a liquid phase. As a result, the structure of the matrix B83 alloy is dispersed, the morphology of intermetallic phases is changed, and reinforcing micro- and nanosized fillers are introduced and uniformly distributed in the matrix. The tribological properties of the synthesized materials are studied. The friction of the B83 babbit + 0.5 wt % MSR + 3 wt % SiC (MSR is modified schungite rock) composite material at high loads is characterized by an increase in the stability coefficient, and the wear resistance of the material increases by a factor of 1.8 as compared to the as-cast alloy at comparable friction coefficients.