Том 59, № 6 (2018)

Predicting the misrun formation in thin-walled castings of magnesium alloys is a critical task for foundry. The computer simulation of casting processes can be used to solve this problem. Adequate results of simulation can be attained in the presence of the correct thermal properties of the alloy and a mold in a wide temperature range, interface heat-transfer coefficient between the casting and a mold, and the critical solid fraction (at which the melt flow in a mold is stopped). In this work, the interface heat-transfer coefficient between the ML5 (AZ91) magnesium alloy and a no-bake sand mold is found by comparing simulation spiral test lengths with experimental spiral test lengths under the same pouring conditions. Its values above the liquidus temperature are h_L = 1500 W/(m² K) at pouring temperatures of 670 and 740°C and h_L = 1800 W/(m² K) at 810°C. Below the solidus temperature, h_S = 600 W/(m² K). The critical solid fraction for the ML5 (AZ91) magnesium alloy was also determined for no-bake mold casting (with a cooling rate of ~2 K/s)—its value was 0.1–0.15. The critical solid fraction is refined by comparing the position of misruns by the results of simulation and in an actual “Protective cap” ML5 (AZ91) alloy casting poured into the no-bake mold. Castings are poured at temperatures of 630 and 670°C, and the critical solid fraction is 0.1 in both cases.

The main problem in the production of bimetals (BMs) is the necessity to ensure adhesive interaction at the contact boundary of the layers, preventing their delamination during the operation. Hot forging of porous preforms (HFPP) offers the possibility of fabricating high-density powder BMs with a minimal amount of pores both in the bulk of the layer material and at the boundary between the layers, which promotes an increase in the adhesion strength. When fabricating hot-forged powder BMs, it is possible to mix materials of charges of a working layer and a substrate, which can lead to uncontrollable interface “spread.” In this work, to fabricate porous BM performs of the structural steel–high-speed steel type, the previously proposed method foreseeing the preliminary prepressing of the powder of the hard-to-deform material is used. In order to determine mechanical properties and perform a structural analysis, bilayer cylindrical samples 20 mm in diameter and 30 mm in length are prepared. The BM base material is steel PK40 and that of the working layer is atomised high-speed steel M2 powder with satisfactory compressibility characteristics. Porous preforms of BM samples are pressed in a specially designed mold using a hydraulic press, enabling one to perform two-side pressing of bilayer powder moldings with the specified distribution of density and strength of layers. Cold-pressed BM preforms were sintered in a protective atmosphere and then subjected to hot repressing using a laboratory drop hammer. Some preforms are examined as sintered. In addition, hot repressing of cold-pressed green preforms is performed. The satisfactory process strength of the working layer material is observed at its porosity of 34% < P_wl < 45%. The powder is not molded at P_wl > 45% and delaminates at P_wl < 34%. It is established that the maximal layer bonding strength and BM thermal-shock resistance is provided by the application of a flowsheet foreseeing the preliminary sintering of cold-pressed preforms and subsequent hot forging. The optimal repressing pressure of the working layer is 145 MPa.

The structure, phase composition, and electrical conductivity of TiB₂–AlN–BN-based ceramics fabricated by self-propagating high-temperature synthesis (SHS) are investigated. The temperature dependence of the specific electrical conductivity was measured in range T = 300–1300 K in vacuum of 2 × 10^–3 Pa according to the standard four-probe dc-current procedure. It is established that the TiN and BN contents in the synthesis products increase while those of TiB₂ and Al decrease with an increase in the TiB₂ content in the initial mixture from 60 to 80 wt % and a decrease in the Al concentration from 40 to 20 wt % because of the reaction of TiB₂ with nitrogen. A decrease in the Al concentration in the initial mixture leads to a decrease in the AlN content in the synthesis products. The results showed the mismatch of electrical resistance curves ρ(T) during the heating–cooling cycle for all ceramics compositions, which is associated with the variation in the length of the contact zone of conducting phases in range T = 800–1200 K. Three characteristic temperature regions are found: (I) from 300 to 800 K, when ρ monotonically increases with an increase in temperature; herewith, heating and cooling ρ(T) curves coincide completely; (II) the behavior of electrical resistance varies at T = 800–1200 K—its values depend strongly on the heat treatment mode of the sample; and (III) heating–cooling curves coincide completely at T > 1200 K.

The influence of alloying TiC_0.5N_0.5 carbonitride by transition metals of Group V (V, Nb, and Ta) on the contact interaction mechanism with the Ni–25%Mo melt (T = 1450°C, τ = 1 h, rarefaction is 5 × 10^–2 Pa) is systematically studied by X-ray spectral microanalysis and scanning electron microscopy for the first time. It is established that the dissolution of single-type Ti_1 –n\({\text{Me}}_{n}^{{\text{V}}}\)C_0.5N_0.5 carbonitrides (n = 0.05) is an incongruent process (alloying metal and carbon preferentially transfer into the melt), and the relative rate and degree of incongruence of the dissolution process of carbonitrides in a series of alloying metals V–Nb–Ta vary nonmonotonically. An explanation for the discovered effects is proposed. The causal-effect relation between the initial composition of Ti_0.95\({\text{Me}}_{{0.05}}^{{\text{V}}}\)C_0.5N_0.5 carbonitride (the grade of the alloying metal) and composition of the Ti_{1 – n – m}Mo_n\({\text{Me}}_{m}^{{\text{V}}}\)C_x K-phase that is precipitated from the melt upon system cooling is analyzed. It is shown that the factor determining the composition of the forming K-phase is the ΔT factor (the degree of exceeding crystallization temperatures of carbide eutectics Ni/Me^VC over the crystallization temperature of the Ni/Mo₂C eutectic that is the lowest melting in these systems). The conclusion is argued that the interrelation between the initial carbonitride composition and the composition of the K-phase is a consequence of a microinhomogeneous structure of metallic alloys. It is shown that this interrelation is rather common and manifests itself in all studied systems irrespective of the type of alloying Group V metal and presence or absence of molybdenum in the melt.

Results of an investigation into the development of the composition and fabrication technology of compact billets of the aluminum powder composite material based on the Al–Si–Ni system for space-rocket hardware are presented. The composite production was performed as follows: initially the powder of the matrix alloy is prepared by gas sputtering, and then a mixture of the matrix alloy powder and alloying dispersed additives is subjected to mechanical alloying in high-energy apparatuses. The method of degassing the mechanically alloyed composition in a thin layer (in order to exclude material ejection from a container when degassing a larger volume of powder) and process regimes of composition compaction are developed using the unique equipment available at OAO Kompozit (Korolev, Moscow oblast)—a vacuum press. Using this technology, cylindrical briquettes up to 100 mm in diameter and up to 120 mm in height are fabricated. Newly developed and patented Kompal-301 composite material has substantial advantages over SAS-1-50 power alloy applied for similar purposes. Its thermal linear expansion coefficient is lower by a factor of 1.5, while the precision limit of elasticity is higher by a factor of 2–3 upon similar strength characteristics. The final structure of a compact briquette is a matrix in which dispersed particles of excess silicon are distributed rather uniformly against the background of the aluminum solid solution. Coarser isolated silicon particles are met in separate regions of the structure. Unfortunately, they are the cause of low plasticity of briquettes, which prevents the formation of semifinished products by plastic deformation; however, such low plasticity does not immediately negatively affect the fabrication of the briquettes themselves.

Comparative studies of peculiarities of the formation, thermal stability of the structure, and mechanical properties of heat-resistant alloys based on iron and nickel and fabricated using additive technologies (ATs) by laser metal deposition and selective laser melting are performed. It is established that a cellular structure is formed in alloys fabricated by the laser metal deposition and small pores up to 200 nm in size are present. The structure of alloys fabricated by selective laser melting contains elements with a globular and lamellar morphology and incompletely melted regions, as well as large pores on the order of 5 μm in size. The possibility of manifestation of the nanophase hardening effect due to the presence of nanodimensional particles of chromium silicides is shown. A comparative analysis of mechanical properties of materials under study is performed. It is shown that iron-based alloys possess higher strength and lower ductility when compared with nickel alloys. All studied samples fabricated by selective laser melting have higher strength characteristics when compared with alloys fabricated by laser metal deposition. Short-term annealing at 900–1000°C for 1 h noticeably decreases both strength and plasticity in tensile and compression tests at room and elevated temperatures. Alloys based on iron and nickel fabricated by laser metal deposition and subjected to compression tests at t = 900°C have similar strength characteristics. In contrast with iron-based alloys, additional annealing of the nickel-based AT alloy almost does not decrease its strength characteristics.

The radiation resistance of a composite material filled with finely dispersed tungsten powder with a particle size of 200–500 nm is investigated. A new composite is intended to provide the radiation protection for electronic radio equipment. The sample with the material under study is irradiated by continuous-spectrum X-ray radiation to the absorbed dose of 3 MGy. The variation in the sample microhardness before and after X-ray irradiation serves as the radiation-resistance characteristic. The microstructure of the transverse sample cleavage after irradiation is investigated by scanning electron microscopy and the absence of visible structural defects is established. This result can be explained by uniform energy scattering from local stresses due to a high degree of composite filling with the tungsten powder possessing a high heat-conductivity coefficient. A 10% increase in microhardness of the irradiated sample is revealed during the investigation, which can be explained by the radiation strengthening effect, when the simultaneous rise in microhardness occurs with an increase in strength. It is established experimentally that this effect manifests itself with an increase in the absorbed radiation dose.

Single-layer MoSi₂, MoSiB, and multilayer MoSiB/SiBC coatings are fabricated by magnetron sputtering. Coating structures are investigated using X-ray diffraction, a scanning electron microscopy, and glow-discharge optical emission spectroscopy. Mechanical properties of coatings are determined by nanoindentation. The thermal stability of coatings is studied in a temperature range of 600–1200°C and oxidation resistance is studied upon heating to 1500°C. It is established that single-layer MoSiB coatings possess a hardness of 27 GPa, elasticity modulus of 390 GPa, and elastic recovery of 48%. They can also resist oxidation up to 1500°C inclusively, which is caused by the formation of the SiO₂-based protective film on their surface. The MoSi₂ coatings can have hardness comparable to the hardness of MoSiB coatings, but they are somewhat worse than them in regards to oxidation resistance. Multilayer MoSiB/SiBC coatings have hardness 23–27 GPa and oxidation resistance restricted by 1500°C, but they herewith have higher elastoplastic properties when compared with MoSiB.