卷 60, 编号 5 (2019)

A low-grade zinc-oxide sample (6–8% Zn) from Mehdiabad mine in Iran, with a high amount of slime and iron (17–19% Fe) and manganese (5–7% Mn) impurities was studied. Pre-treatment processes, such as desliming and wet high-intensity magnetic separation were applied for impurity removal. Petrographic studies showed that the zinc minerals were hemimorphite, hetaerolite, smithsonite, sphalerite and a low amount of zincite; lead was present in galena and manganese was in hetaerolite and pyrolusite. The gangue minerals were dolomite, calcite, hematite, goethite, pyrite and quartz. From a processing perspective, the sample can be characterized as a low-grade zinc-oxide sample. Experimental data and scanning electron microscopy/energy-dispersive X-ray spectroscopy analysis showed that desliming prior to magnetic separation is required, and the optimum concentration ratio and recovery in the absorbed part can be determined by applying one stage of wet high-intensity magnetic separation with grinding and a magnetic field intensity for 16 min and 14 000 G. By considering the abovementioned conditions, the concentration ratio and recovery of iron and manganese in the absorbed part have been obtained as 1.42 and 1.54 and 73.28% and 79.79%, respectively, and the non-absorbed part of the pre-treatment stage with 10.56% Fe, 3.29% Mn and 6.83% Zn can be considered as flotation feed.

Micro-nanometer sized MgO products have unique properties and hence are widely used. In the process of preparing titanium sponge by Kroll method, a large number of by-products such as molten MgCl₂ is produced. In China, it is treated as a waste, as it wastes energy and causes pollution. In this study, a new process for the preparation of micro-nanometer sized MgO by the direct pyrolysis of molten MgCl₂ is proposed. Also, the followed pyrolysis process is numerically simulated. The results from this investigation show that the fluid flows very precisely and smoothly in orbits in the reactor, and the flow velocity of fluid was maximum at the mouth of the throat. Under the gravitational action, MgO was concentrated at the middle and lower parts of the reactor. A reaction temperature of 1473 K ensured both the highest yield and consuming a reasonable amount of energy. The standard velocity guaranteed the highest reaction efficiency while preventing the accumulation of product in the reactor.

La–Ni based alloys have been established as suitable materials for reversible hydrogen storage. Low-temperature metal hydrides (LT MH) facilitate hydrogen storage at ambient temperatures and pressure of 1–1.5 MPa. Nevertheless, further research and modifications of these alloys are needed. In this paper, the parent LaNi₅ alloy was modified by partially replacing La with Ce and Ni with Fe. The alloys were prepared in two different ways: conventional melting of pure powder metals, and thermochemical reaction using corresponding metal chlorides. Steps were taken to optimize the time-temperature parameters of alloy synthesis. A comparative analysis of the obtained alloy samples followed. Hydrogen sorption isotherms were obtained for the unmodified LaNi₅ and modified (La_0.5Ce_0.5)Ni₅ samples made from pure powder metals. Due to strong oxidation of the alloys prepared from metal chlorides, sorption isotherms for these alloys were not obtained.

The dependence of porosity θ of the spongy titanium-based powder material on stress state coefficient k during the plastic deformation with the prevailing effect of the uniform compression is investigated. Based on the results found in previous works, the family of yield curves with a variable porosity is plotted on the σ–T plane. The yield criterion of the powder material is based on the Modified Drucker–Prager Cap Model. Straight lines corresponding to various values of stress-state coefficient k = σ/T, where σ is the average hydrostatic stress and T is the intensity of tangential stresses, are represented in the graph of the geometric interpretation of the accepted plasticity model. To formulate the relation of porosity (θ, %), average normal stress (\(\bar {\sigma }\)), expressed in the dimensionless form, and stress-state coefficient k, intersection points of the family of curves corresponding to generatrices of yield surfaces on the σ–T plane and radial straight lines, are used. This results in the derivation of the equation of the θ = θ (\(\bar {\sigma }\), k) form. To verify the adequacy of this relationship, the experimental part of the investigation is fulfilled. Powder billets preliminarily compacted at a pressure of 1000 MPa and temperature 325°C are subjected to electroerosion cutting along the axial section to form planar samples (templates). Several characteristic segments are selected on the template surface to determine the local surface porosity by quantitative metallography. The stress state was additionally determined in representative segments by the numerical simulation. Values of bulk plastic strain (\(\varepsilon _{{v}}^{{{\text{pl}}}}\)), intensities of tangential stresses (T), and hydrostatic normal stress (σ) are calculated in axial section zones corresponding to the regions under study. It is shown that the stress state coefficient insignificantly affects the porosity upon its variation in a rather broad range (k = –10 to –0.86).

When producing castings of wide-interval magnesium alloys, their structure is the factor that most decisively affects a complex of mechanical, manufacturing, and operational properties. The specified structure of alloys of the Mg–Al–Zn system is impossible without using the melt modification operation in the smelting manufacturing process. In this work, the results of studying the modification of the ML5 magnesium alloy by different substances are presented. The influence of introducing magnesite into the melt in an amount of 0.4–0.45 wt % at 720–740°C, as well as the influence of melt blowing by oxygen-free carbon-bearing gases at the same temperature on the structure of the alloy and conservation duration of the modification effect is investigated. The latter is especially important in the large-scale and mass production of small castings made of alloys of the Mg–Al–Zn–Mn system when the melt pouring process is prolonged. It is shown that the use of oxygen-free carbon-bearing gases to modify the ML5 alloy provides the attainment of the level of mechanical properties of castings elevated by the 15–20% level of mechanical properties of castings when compared with the standard one according to GOST (State Standard) 2856–79. The conservation efficiency of the duration of the modifying effect by the conventional method (magnesite) is compared with using oxygen-free carbon-bearing gases. It is shown that the modification effect by magnesite is retained no longer than 30–40 min, while, when using oxygen-free carbon-bearing gas, it is no shorter than 4 h, which makes it possible to perform the prolonged pouring of a melt over molds.

High-temperature (t = 800°С) ion nitriding of carbide disposable inserts of the T15K6 brand is performed taking into account the formation of structures, phase composition, and thickness of the surface coating, ensuring an increase in their durability during cutting tests. It is revealed that hardness and microhardness increase up to 15% after such treatment, but gradually decrease to the initial values with an increase in temperature above 600°C. Bending strength increases by 27% after ion nitriding. Fractographs of cleavages of surface layers of the T15K6 hard alloy after ion nitriding for 1 and 2 h at various temperatures evidence that the cleavage over the edges is characterized by a strongly branched linear structure, while the brittle fracture pattern is observed inside the material. Areas of segments of intergrain fracture increase with an increase in the ion nitriding duration, while those of the intragrain fracture decrease. The results of an analysis of microstructures of the surface layer of the T15K6 alloy after ion nitriding show that the sizes of conglomerate carbides in the surface layer decrease with an increase in the ion nitriding temperature. The depth of the nitriding layer of the T15K6 alloy is from 1 to 7 μm. The regularities of the influence of various temporal and temperature ion nitriding modes on operational characteristics of wares made of titanium–tungsten hard alloys of the TK group are determined. An increase in hardness, microhardness, and ultimate strength with a decrease in wear when cutting carbide disposable inserts of the T15K6 brand is established at ion nitriding temperatures of 600, 700, and 800°C under isothermal holding from 1 to 8 h. It is established that the areas of intergrain fracture segments increase with an increase in the ion nitriding duration, while those of intragrain fracture decrease. It is shown that the (Ti_xW_x)(C_{1 – y}N_y) and (Co_{1 – x}W_x)(C_{1 – y}N_y) tungsten-supersaturated solid solutions are formed during ion nitriding and ternary and quaternary compounds are isolated in the surface layer.

The evolution of the microstructure and microhardness of the Al–0.05 vol % nAl₂O₃ nanocomposite (where nAl₂O₃ are aluminum nanoparticles) and aluminum without nanoparticles fabricated by accumulative roll bonding with annealing in a temperature range of 373–573 K is investigated. Ball-shaped Al₂O₃ nanoparticles with an average diameter of 50 nm are introduced between rolled plates of technically pure aluminum from the first to fourth rolling cycles, while the fifth to the tenth rolling cycles are performed without nanoparticles. The average grain size and aspect ratio of elements of the material grain–subgrain structure in the initial state and after annealing at 473 K are measured using transmission electron microscopy. It is shown that the nanocomposite microhardness is 5–13% higher than the corresponding value of HV for aluminum over the entire studied annealing temperature range. The main factor of the higher nanocomposite microhardness is precipitation hardening due to Al₂O₃ nanoparticles. The contribution of the substructural and grain-boundary hardening is identical in both materials. The thermal stability of the nanocomposite microhardness is only ~25 K higher than that of aluminum, which is caused by the nonuniform distribution of nanoparticles in the matrix and their low volume fraction. The high thermal stability of the fine-grain structure itself, formed by accumulative roll bonding, when compared with other methods of intense plastic deformation, also plays a role. It is established that most Al₂O₃ nanoparticles remain at the grain boundaries of nanocomposite after annealing at 473 K; therefore, the ability to fasten the boundary by Al₂O₃ nanoparticles under the conditions under study is retained at least to 473 K.

The structure and properties of coarse-grained WC–6% Co carbides with carbon deficiency from 0.11 to 1.31% relative to the stoichiometric ration prepared from narrow-fraction tungsten carbide powder with a grain size of 5–15 μm are studied. It is established by the results of metallographic analysis that sintering temperatures in a range of 1390–1420°C provide the pore-free state of the alloy with the normal carbon content, but the samples have considerable porosity at its lowered concentrations. It is revealed that sintering temperatures of 1450–1475°C, irrespective of the carbon content, make it possible to prepare carbides with residual porosity lower than 0.02%. It is shown that alloys with a carbon deficit of 0.11–0.91% had a two-phase structure, while the alloy with a carbon deficit of 1.31% contained inclusions of the η phase in addition to WC and γ phase. It is established that the retardation of the growth of tungsten carbide grains is observed with a decrease in the carbon content during liquid-phase sintering. The concentration of dissolved tungsten is established by electron probe microanalysis. It was 10, 12, 15, and 19 wt % for carbides with a normal, low, mid, and high carbon deficit, respectively. The use of narrow-fraction tungsten carbide powders makes it possible to fabricate carbides with rounded grains having a shape factor of about 0.77. The alloy with a carbon deficit of 0.91% relative to the stoichiometric ratio had the best combination of hardness and crack resistance:11.1 GPa and 16.0 mPa m^1/2.

In this study, the micro-mechanisms involved in fatigue crack propagation are investigated qualitatively in a Al/Al₂O₃/SiC hybrid metal matrix composite (MMC) and the results are compared with Al₂O₃ fibre reinforced MMC and monolithic Al alloy. The three-point bending fatigue test was carried out in a rectangular notched specimen and crack propagation was monitored until the fracture of the specimen. The crack profile on the surface of the specimen was analyzed via optical microscope. The fracture surface and the crack-path profile in the fracture surface were analyzed by scanning electron microscopy (SEM) and three dimensional (3D) surface analysis respectively. The hybrid MMC shows higher crack propagation resistance than that of fibre reinforced MMC and Al alloy in the low ∆K region. In the threshold region, the crack in hybrid MMC is directed by the reinforcement–matrix debonding, followed by void nucleation in the Al alloy. Additionally, the crack propagation in the stable-crack-growth region is controlled by reinforcement-matrix interface debonding caused by the cycle-by-cycle crack growth along the interface, as well as by the transgranular fracture of particles and fibres. The presence of large volumes of inclusions and the microstructural inhomogeneity reduces the area of striation in hybrid MMC, leading to unstable fracture.