A. V. Laptev

Structural Features and Properties of Alloy 84% WC ― 16% Co, Obtained by Hot Pressing in the Solid and Liquid Phases. Part 2. Influence of the Temperature at which the Specimens are Made on Their Physicomechanical Properties

We have studied the changes that occur in the resistivity, the transverse bending strength, and the fracture toughness (cracking resistance) of a hard alloy obtained by hot pressing at about 500 MPa and sintering over a wide range of temperatures (950-1450°C) as well as the how those parameters are affected by solid-phase and liquid-phase annealing. The porosity dependence of the resistivity is shown not to be single-valued. Other factors affect the resistivity, e.g., the degree of particle interaction and the state of the structural components, which vary with the porosity. The resistivity curve for hot-pressed specimens has an inflection in the region of 1200°C. The resistivity increases at a faster rate at lower temperatures. In the temperature range studied the porosity dependence of the transverse bending strength and the fracture toughness is linear: σb = σ0b(1 − 3.53Θ) and K1c = K01c(1 − 3.44Θ). Prolonged solid-phase annealing of hot-pressed specimens improves their mechanical properties owing to a decrease in porosity.

Relationship between Erosion and Mechanical Characteristics of Fine-Grained Hard Alloy WC – 16% Co Obtained by Solid-Phase and Liquid-Phase Consolidation. Part 1. Erosion Resistance

We have studied the erosion characteristics of hard alloys under electrospark alloying conditions, and the relationship of such characteristics with the physical and mechanical properties. WC – 16% Co alloys were obtained by conventional sintering under vacuum in the temperature range 950-1350°C, and by sintering followed by high-energy pressure treatment at the same temperatures. We have shown that the erosion resistance depends ambiguously on the porosity and bending strength but is quite clearly connected with the crack resistance and the electrical resistivity which in turn is determined by the quality of the interphase and intergrain boundaries. We have refined the structural dependence of the erosion resistance of hard-alloy electrodes. This dependence is manifested to the same extent as the structural dependence of the electrical resistance.

Hard Alloy WC – 24% Ni Obtained in the Solid Phase from Ultrafine WC, NiO, and C Powders. Part 1. Density and Structure of Specimens

We have studied the density and structure of specimens of the alloy WC – 24 mass% Ni, obtained by combining into one step the processes of synthesis of the metallic phase and compaction of the ultrafine mixture of WC – Ni powders by high-energy pressing and sintering. We have established that reduction of nickel monoxide by carbon occurs at temperatures of 650-750°C and does not affect the shrinkage process which in the case of sintering begins only at a temperature of 1050°C. High-energy pressing of briquettes sintered at the indicated temperature reduces their porosity from 30-25% down to 8-4%. Specimens of porosity <1% can be obtained by pressing at 1150°C or 1050°C in the case of triple pressing. Raising the temperature at which the briquettes are heated is accompanied by enlargement of the pores together with a decrease in the total porosity, but at temperatures of 1300°C (sintering) and 1250°C (pressing), the pore dimensions are sharply reduced. The high density of the specimens pressed at low temperature does not provide low electrical resistance, which suggests the presence of weakly connected boundaries. When the specimens are sintered and pressed in the solid phase, we observe the growth of tungsten carbide particles. It is most rapid at 1150-1250°C, while at 1050°C the particle growth process slows down. Reduction of the metal oxide when the powders are heated promotes formation of structure in the higher temperature range.

Potential of the High-Energy Hot Compaction in a Vacuum for Creating Materials with an Ultrafine Structure and High Strength

The typical form of specimens prepared by high-energy hot pressing in a vacuum using different deformation schemes and also the results of studying compaction, structure formation, and the physicomechanical properties of different materials in relation to billet heating temperature are presented. The structure and properties of specimens prepared by vacuum sintering are given for comparison. The efficiency of high-energy pressing of single-phase materials (nickel and molybdenum) and two-phase composites (dispersion-strengthened nickel Ni ― NiO and WC ― Co hard alloys) is demonstrated. The method makes it possible to prepare compact billets at quite low temperatures that provides retention of a fine-grained structure within them. As a result of this the materials have high mechanical properties that exceed those of specimens prepared by traditional methods.

Structural Features and Properties of Alloy 84% WC ― 16% Co, Obtained by Hot Pressing in the Solid and Liquid Phases. Part 1. Effect of the Temperature at which the Specimens are Prepared on Their Density and Structure

Studies are made of changes in the porosity and structure of a hard alloy obtained by hot pressing and sintering within a broad range of temperatures (950-1450°C). The effect of solid- and liquid-phase annealing on their porosity and structure is also examined. It is shown that the hot-pressed specimens are denser than the sintered specimens within the solid-phase region, other conditions being equal. Solid-phase annealing of hot-pressed and sintered specimens increases the density of both types of specimens, but the annealing operation is more effective in the former case. It is established that the open pore channels are 2-3 times smaller in the hot-pressed specimens than in the sintered specimens and are no larger than the average size of the tungsten carbide particles. Active growth of the carbide particles and redistribution of the ductile phase are observed even in the solid phase. The carbide particles grow considerably faster (by a factor of four) at sintering temperatures above 1200°C.

Structure and Properties of Kh20N80 Alloy Powders Produced by Impact Sintering at Different Temperatures

The compaction, structure, and mechanical properties of Kh20N80 powder samples are studied. Most of the powder particles were 25 μm in size. The powders were compacted by impact sintering in vacuum at 1100, 1150, 1200, 1250, and 1300°C at an impact energy of 1200 J/cm3. Isothermal holding at these temperatures lasted for 20 min, vacuum during heating and compaction was maintained at 0.013 Pa, and initial strain rate was 6.5 m/sec. The rectangular bars cut out of cylindrical disks were used to determine the density, electrical resistivity, tensile strength, fracture toughness or fracture work of a notched sample, Brinell hardness, compression strength, and plasticity of the samples in various test conditions. It is shown that the mechanical properties of Kh20N80 samples subjected to impact sintering at 1100–1300°C correspond to those of standard Kh20N80 alloy obtained by conventional melting and forging techniques. In particular, the average tensile strength varies from 630 to 740 MPa and compression strength from 800 to 830 MPa. The plasticity of the samples evaluated from necking changes from 30 to 54% with compaction temperature increasing from 1100 to 1300°C. The average plastic strain energy at break of a notched sample increases from 19.5 to 36.0 J/cm2.

Shock-Wave Sintering of the 70 vol.% Kh13M2–30 vol.% Cr3C2 Carbonized Steel in a Wide Temperature Range

The compaction of the 70 vol.% Kh13M2–30 vol.% Cr3C2 carbonized steel is carried out by the shock-wave sintering under 1200 MPa in the 950–1200°C temperature range and the free sintering is performed at 1250°C in vacuum (0.13 Pa). The effect of the compacting temperature and annealing at 1150°C on the structure and physical and mechanical properties of the composite is studied. It is shown that an active interaction between carbide phase and steel matrix occurs in the structure of the material during heating. As a result, the amount of carbide component in the composite increases by 10 vol.% and by 30 vol.% during annealing, compared to the starting carbonized steel. High strength and plasticity properties of the composite are obtained at a shockwave sintering temperature of ≥1100°C. The carbonized steel possesses the following properties: bending strength 1165 MPa, compression strength 2665 MPa, compressive plastic strain 8.2%, fracture toughness 20.4 MPa · m1/2, and the Rockwell hardness 78 HRA.

Structure and Properties of Ni3Al Intermetallic Under Vacuum Impact Sintering

The compaction, structure, and mechanical properties of Ni3Al intermetallic, corresponding to PN85Yu15 commercial powder and mainly consisting of 50–100 μm particles, are studied. The preforms were subjected to impact sintering in 0.013 Pa vacuum at 1100, 1150, 1200, 1250, and 1300°C. Isothermal holding at these temperatures lasted for 20 min. The samples were compacted at an impact energy of 1200 J/cm3 and an initial impact velocity of 6.5 m/sec. The disk samples were used to cut out rectangular bars to determine their density, resistivity, bending, tensile, and compression strength, conditional fracture toughness, and fracture energy (for notched samples). The Vickers hardness and plasticity of the samples were evaluated in different types of tests. The mechanical properties of Ni3Al intermetallic powder samples compacted at 1250°C and higher temperatures are consistent with those of the standard conventionally melted intermetallic. In particular, the average bending strength is 650–675 MPa, tensile strength 385–400 MPa, fracture toughness 14.6–18.2 MPa · m1/2, compression strength 1650 MPa, and Vickers hardness 2500–2600 MPa.

The Structure and Properties of Powder Copper Hardened by Fine Tungsten Particles

The Cu–W pseudoalloys with 2–10 vol.% of nanosized tungsten particles (30–40 nm) are studied. The bulk samples with relative density up to 99.1–99.6% are produced by shock compaction. The introduction of nanosized tungsten particles increases the pseudoalloy strength to 2.7HCu with insignificant reduction in plasticity (δ = 13%) and conductivity (to δ = 0.875% IACS for the pseudoalloy with 10 vol.% W). The microstructures of the Cu–W pseudoalloys are analyzed.