Powder Metallurgy and Metal Ceramics最新文献

The influence of pore structure evolution in compacts sintered from nickel carbonyl powder with an average particle size of 1.4 μm in the temperature range 200–1000°C on local and bulk shrinkage was analyzed. The pore structure of the samples was characterized by the maximum and average diameters of pore channel constrictions determined by the Barus–Bechhold method. To minimize local (incoherent) shrinkage in the sintering of fine nickel powders, a criterion for pore structure homogeneity in compacts, α ≤ 0.03, was selected. The criterion was determined by the difference between the maximum and average diameters of pore channel constrictions. The influence of pore structure evolution on local and bulk shrinkage during sintering of compacts produced from nickel carbonyl powder with an average particle size of 1.4 and 4 μm was experimentally confirmed. The local shrinkage was due to the three-level structure and wide particle size distribution of the nickel carbonyl powders. A method was proposed to determine the average diameter of particles (agglomerates) in nickel carbonyl powders using the Kozeny equation, establishing a relationship between the particle diameter, the maximum diameter of pore channel constrictions, and the porosity of the compacts, varying from 0.25 to 0.45.

A Discrete Element Method (DEM) was applied to establish a model that simulates a cross-shaped powder system under hot compaction. The average stress, force chains, principal stress angles, and coordination numbers were recorded and studied. The experimental results show that the stresses in the vertical part of the cross-shaped powder system are higher than in the lateral part, and the highest stress value is always concentrated in the upper zone of the system. This is also consistent with the strength of the force chains in the vertical part being stronger than that in the lateral part. The angle of the principal stress is consistent with the direction of the external load and shows anisotropy and irregular distribution during the compaction process. The vertical section of the cross-shaped powder system tends to be 90°, except for the area close to the lateral section, which tends to be 70°. However, the principal stress angle of the lateral part tends to be 0° during the compaction process. The coordination numbers of the measurement circles have a series of sudden changes and increase with the pressing, the changes of which correspond to the stress distribution.

As the aerospace industry continues to grow, so does the demand for new materials that can withstand high temperatures and corrosive environments. In this paper, materials from the Ti–Al–C system that thrives in the aforementioned environments are studied. The method of measuring the grain size was described according to the relevant standards. The geometrical parameters of titanium carbide and its volume fraction have been determined under the ASTM E112 and ASTM E562 standards, respectively, for two series of specimens that were produced with different parameters and methods. The grain sizes determined are G12 and G12.5 according to ASTM E112. The volume fractions determined for the two series of samples are 20.22 and 17.65%, respectively. Using the above parameters, elastic and shear modulus, and Poisson’s ratio were determined for the specimens tested using RVE modeling. RVE results showed that materials with higher volume fractions and larger average grain size resulted in stiffer materials. Specimens with higher TiC content exhibited higher elastic and shear modules, which were 153.6 and 58.3 GPa, respectively. Poisson’s ratio was the lowest at 0.315. However, the difference was not significant between the specimens, the elasticity and shear modulus, of a specimen with a lower concentration of TiC, are 145 and 55.2 GPa, respectively. Poisson’s ratio was higher and equal to 0.319. Comparing the above properties with the popular aerospace alloy Ti–6Al–4V, both specimens are much stiffer.

Metallic reinforcing titanium is added to the magnesium matrix to improve the mechanical properties without losing ductility. Titanium has negligible solid solubility in magnesium below 500°C therefore it does not form a tertiary hard phase with Mg. Therefore, when titanium is added to magnesium, both strength and ductility are improved. However, due to the low solid solubility of Ti in Mg, the bonding between matrix and reinforcement is poor. Therefore, a small amount of metallic reinforcement Cu is added to fabricate Mg/Ti/Cu hybrid composites by powder metallurgy technique to enhance the bonding between Mg and Ti. Cu is selected as a binding agent because it has significant solid solubility with Ti and Mg. In the present work, the effect of Cu on the physical, mechanical, and thermal properties of Mg/Ti/Cu composites has been investigated. The addition of Cu was found to decrease the strength, hardness, and wear rate. On the other hand, the thermal conductivity increased. The strength, wear resistance and thermal stability of the prepared Mg- based hybrid composites are sufficient enough to replace some components of cast iron and aluminum in automotive special seat frames, door panels, brake disks of light-duty vehicles, etc. Thus, the prepared material is recommended for use in automotive and other industries.

Changes in the dielectric properties of two-dimensional (2D) microsized molybdenum disulfide powders in response to ambient air humidity at room temperature were studied (impedance spectroscopy, 1 Hz–20 MHz). The microsized 2H-MoS₂ powders were found to absorb significant amounts of moisture (0.43–2.88 wt.%, 3.5 h, relative air humidity of 45–100%). According to impedance spectroscopy data, reversible water intercalation/deintercalation processes led to significant changes in the dielectric properties (total, active, and reactive (capacitive) resistance, capacitance, loss tangent, and real component of relative permittivity) of 2H-MoS₂ powders until equilibrium was reached. In equilibrium, the dielectric properties depended on humidity and frequencies. The dielectric properties of microsized 2H-MoS₂ powders are dynamic functional characteristics that can be effectively controlled over wide ranges by varying the humidity and frequency levels. It is assumed that changes in the dielectric properties of microsized 2H-MoS₂ powders are due to the formation of 2D nanosized MoO_3–x/MoO₃/H⁺_x(H₂O)_yMoS₂ heterostructures on the surface of the intercalated H⁺_x(H₂O)_yMoS₂ phase particles. These findings can be used to improve nanotechnologies that use aqueous environments, optimize the semiconductor, tribological, and catalytic properties of 2H-MoS₂, and develop multifunctional 2D nanomaterials (humidity sensors, sorbents, and photocatalysts for water purification and electro(photo)catalysts for hydrogen production by water electrolysis).

The TiN–20% TiB₂ and TiN–20% Si₃N₄ nanocomposites sintered in a microwave field with a frequency of 2.45 GHz were applied to a steel substrate by electrospark deposition in the temperature range 1400–1500°C in a nitrogen atmosphere. In deposition modes with an energy of isolated pulses ranging from 0.2 to 0.75 J, changed surface layers consisting of a coating 50–90 μm thick and a heat-affected zone of increased hardness 40–60 μm thick on the substrate were produced. A part of the samples was subjected to additional surface laser processing to increase the density and homogeneity of the deposited layers. Substantial influence of electrospark mass transfer on the phase composition of the transferred material was established. According to XRD data, the TiN–TiB₂ composite, with all its components being present in the coating, was more stable. In the case of the TiN–Si₃N₄ composite, silicon nitride completely dissociated to form Ti₅Si₃ and Ti₂N compounds. For both compositions, iron, penetrating into the coating from the substrate, was found in the deposited layer. The TiN–TiB₂ and TiN–Si₃N₄ coatings had a hardness of 14–15 GPa and 11–12 GPa, respectively. Comparative tribotechnical tests of the coatings with a spherical VK6 hardmetal counterface in quasistatic and dynamic modes revealed that the electrospark deposition of the TiN–TiB₂ composite combined with subsequent laser processing was highly efficient. In tribotechnical tests, the linear wear of this coating was 0.5 μm, corresponding to a twelvefold increase in the wear resistance as compared to that of the TiN–Si₃N₄ coating for dynamic friction tests. The deposition of the TiN–Si₃N₄ composite enabled a double increase in the wear resistance of the substrate in dynamic testing mode. In this case, additional laser processing of the coating turned out to be inefficient.

Aluminum matrix nanocomposites (AMNCs) are a distinct category of advanced materials that incorporate nanoscale reinforcement in a ductile material matrix. Various nanomaterial reinforcements for AMNCs have been reported in the literature, including multi-walled carbon nanotubes (MWCNT), graphene nanoplatelets, silicon carbide, and boron nitride. These classes of materials have been described to exhibit both improvements and reductions in mechanical properties. The interfacial material phases result in low-strength materials. Improvements in mechanical properties are attributed by refined grain size and shape for both the matrix material and the reinforcement agent. These materials demonstrate higher hardness, yield strength, and wear corrosion compared to conventionally prepared aluminum composites. Spark plasma sintering (SPS) is one of the non-conventional sintering methods used to prepare metal matrix composites, resulting in fully dense composite materials. The SPS-produced metal matrix composite can be manufactured rapidly and finds its applications in the automotive, aerospace, and defense industries. This review provides an overview and current status of metal matrix composites regarding matrix and reinforcing materials and the SPS process for producing metal matrix composites.

Data on the mixing enthalpies of liquid alloys in ternary Me–Ti–Zr (Me = Fe, Co, Ni, Cu) systems and boundary binary systems are summarized. The partial mixing enthalpies of titanium and zirconium and the integral mixing enthalpy of liquid Co–Ti–Zr alloys were investigated for the first time by high-temperature calorimetry at 1873 K along the x_Co/x_Ti =3 section at x_Zr = 0–0.57 and x_Co/x_Zr = 3 section at x_Ti = 0–0.54. It was shown that the investigated partial and integral functions were characterized by significant negative values. The isotherms of the integral mixing enthalpy of liquid Fe–Ti–Zr alloys at 2173 K and liquid Co–Ti–Zr alloys at 1873 K are described using the Redlich–Kister–Muggianu polynomial. A new description for the liquid Cu–Ti–Zr alloys at 1873 K is also presented. The negative values and composition dependence of the ∆_mH function for liquid alloys of each ternary system are determined by the predominant influence of MeTi and MeZr pair interactions, in which iron, cobalt, nickel, and copper are electron acceptors, while titanium and zirconium are donors. In the considered series of the binary Me–Ti and Me–Zr systems and ternary Me–Ti–Zr systems, the absolute values of the integral mixing enthalpy of liquid alloys increase in the transition from the iron systems to the nickel systems and are minimal in the systems with copper.