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: _b = ⁰ _b(1 – 3.53) and K _1c = K ⁰ _1c (1 – 3.44). Prolonged solid-phase annealing of hot-pressed specimens improves their mechanical properties owing to a decrease in porosity.     

Role of polymorphic transformation in the hot pressing of alumina

Conclusions At temperatures of up to 1400°C hot pressing under isothermal conditions enables higher (by 5%) densities to be obtained in compacts from-Al₂O₃ powders than in compacts from-Al₂O₃ powders. With rise in temperature the difference between the densification rates of- and -Al₂O₃ powders diminshes. The difference in density between- and-Al₂O₃ compacts is preserved up to a temperature of 1600°C in hot pressing at a constant heating rate of 5 deg C/min. In the temperature range 1300–18QO°C the energy of activation for hot pressing Al₂O₃ varies from 50 to 75 kcal/g· form. Estimated values of viscosity and effective diffusion coefficient are in accord with values determined in creep and sintering experiments.Translated from Poroshkovaya Metallurgiya, No. 9(165), pp. 76–80, September, 1976.     

Quality control of hot-pressed silicon nitride dielectric materials

Conclusions Exceeding an optimum temperature in the pressing of silicon nitride parts leads to the appearance of conducting inclusions (SiC and free Si) in specimens, while pressing at temperatures below the optimum temperature increases the porosity of specimens and hence the amount of water absorbed by them. With rising conducting inclusion content the electrical conductivity, , and tan of a material grow and its E_br falls. Increasing porosity has, by increasing water absorption, a similar effect. The coefficient of losses at a frequency of 1 kHz can be employed as a criterion for assessment of the quality of dielectric materials based on silicon nitride.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 53–59, March, 1982.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 1. Effect of Subsurface Layer on Process of Phase Redistribution in System WSi2 ― W

The effect of a subsurface layer of metal silicide on the phase composition stability of high-temperature MeSi₂-type coatings on refractory metals was investigated. Using the system WSi₂ W as a prototype it was experimentally determined that a subsurface layer of the tungsten disilicide, and the distribution profile of silicon in the diffusion zone upon high-temperature heating have a substantial effect on the formation of a barrier layer of lower silicide which determines the stability of the system as a whole. It is proposed that the search for more stable silicide coatings on refractory metals should be directed toward the creation of diffusion barriers on not only the inner interface MeSi₂ Me, but also the external surface of the coating. The process of coating formation must be accompanied by the formation of a metal silicide on the external surface with the highest possible concentration of silicon.     

Surface and internal structures of titanium alloy granules

Conclusions A characteristic feature of VT3-1 alloy granules is that, as their size decreases, their structure changes from dendritic to acicular and the -phase appears in them. The surface condition of these granules is characterized by the presence of films of the lower titanium oxides Ti₂O₃, Ti₃O₅, and Ti₄O₇. Hightemperature annealing in a vacuum dissolves the surface films. Effective consolidation of starting granules with films less than 350 Å thick is observed at temperatures of 900° C and higher. The presence of thicker surface oxide films on these granules has a marked adverse effect on the quality of resultant dense compacts. The ductility of compacts can be improved by subjecting them to cyclic heat treatment in the +/ transformation temperature range. Isostatic pressing of granules can be employed as a means of obtaining dense compacts having dimensions close to those of finished parts.Deceased.Translated from Poroshkovaya Metallurgiya, No. 12(204), pp. 1–7, December, 1979.     

Effective area of contact of reinforced plasma-sprayed coatings

Conclusions An expression relating H to and d has been derived, with the aid of which it is possible to select an optimum interfiber spacing in the production of monolayer coatings. The deposit layer height has been found which is necessary for obtaining, after pressing, material of minimum porosity. An angle of inclination of the nozzle has been selected which enables good-quality deposits to be applied to substrates.Translated from Poroshkovaya Metallurgiya, No. 8(224), pp. 39–43, August, 1981.     

Characterization of Superfine Sr2CeO4 Powder Prepared by Microemulsion-Heating Method

Powder phosphor of Sr2CeO4 is prepared by microemulsion-heating method and a film of the phosphor on ITO glass is formed by electrophoretic deposition. Field emission scanning electron microscopy (FE-SEM) images show that the powder fired at 850 ℃ for 4 h has a spherical shape with an average diameter of 70 ～ 80 nm whereas the powder sintered at 900 ℃ for 4 h and 1000 ℃ for 4 h have shuttle-like and spherical shapes, respectively, with both sizes less than 1 μm.X-ray diffraction (XRD) patterns indicate that the superfine Sr2CeO4 exhibits an orthorhombic crystal structure. Roomtemperature photoluminescence (PL) measurements show that there are three excitation peaks located at around 262, 280and 341 nm, and all the Sr2CeO4 samples display an intense blue emission at 470 nm with CIE coordinate of (x, y) =(0.176, 0.283). The quantum yield of phosphor is high up to 0.47 ± 0.04. Compared with Sr2CeO4 samples prepared with traditional high-temperature heating, the phosphor synthesized with this method has a smaller size, lower calcination temperature, and shorter calcination time, and the main excitation and emission bands are blue shifted about 30 and 12 nm respectively. The startup voltage for Sr2CeO4 film on ITO glass shifts from 2700 to 4000 V with increasing thickness of the film.     

Enthalpies of Mixing in Liquid Alloys of Copper with Hafnium

Enthalpies of mixing of liquid alloys of copper with hafnium were measured at 1650 K in the concentration range 0 < x _Hf < 0.50 by high-temperature isoperibolic calorimetry. The experimental data obtained indicates exothermic effects upon alloy formation, which confirm a strong interaction of the components in copper hafnium melts.     

Bulk high hardness Al90Ce2Mn8 alloy prepared by powder metallurgy

Abstract

Using powder metallurgy, bulk high strength Al₉₀Ce₂Mn₈ alloy 25 mm in diameter by 10 mm with near nil porosity has been obtained under certain pressing and heating conditions. The conditions for the best mechanical properties are a pressing temperature of 753-793 K, a pressing time of 30 min, and pressing stress of 1·2 GPa. The compression strength reaches 895 MPa with a hardness of 26 HRC when the alloy is pressed at 753 K. The strength increase is attributed to second phase strengthening and fine grain strengthening.     

Special features of the ductile-brittle transition in iron-based powder materials. II. Physicomechanical model of the ductile-brittle transition in cracking resistance tests

Conclusions The presence of pores in the material leads to the redistribution of stresses in the material and to localization of strains in small volumes between the pores. The susceptibility to ductile failure increases with increasing porosity.At –196°C, the failure mechanism of the porous materials based on iron changes from ductile to brittle (cleavage) with decreasing porosity. The dependence of cracking resistance on the porosity of these materials is nonmonotonic and the maximum cracking resistance is recorded at porosity values at which the failure mechanism changes. The nonmonotonic nature of cracking resistance reflects the high sensitivity of this characteristic to the change of the stress state in the material. The change of the stress state in the ductile-brittle transition temperature range can be described by the condition _f = _T. In the porous materials examined this condition is valid because of the specific form of the _f- and _T- dependences. The model of the ductile-brittle transition proposed in this work can be used to determine the analogy between the cold brittleness temperature T_br and the critical value of porosity _c corresponding to the point of intersection of the _f- and _T- curves for the porous materials.Translated from Poroshkovaya Metallurgiya, No. 3(303), pp. 39–42, March, 1988.     

Some characteristics of the reverse polymorphic transformations of sintered iron and cobalt

Conclusions The permanent compression exhibited by sintered iron during the reverse, transformation depends more on the particle size of the powder than on the porosity of the material. Filling the pores with copper at a residual porosity of 1.5% does not prevent this phenomenon, and enables internal stresses in the material to manifest themselves during the volume change accompanying the transformation. Thermal cycling in the region of the transformation temperatures, which leads to a substantial permanent compression buildup from the very first cycles, can be employed for improving the structure-sensitive mechanical properties of the material. The porous structure of sintered cobalt creates conditions facilitating the retention of its high-temperature phase at room temperature. The increase in the relative amount of the fcc phase brought about by an increase in porosity and hence in excess surface area and the sharp fall in the amount of the high-temperature phase induced by repressing the material with a very small reduction enable the inhibition of the reverse transformation to be attributed to the influence of the excess surface and stresses set up by dislocation pile-ups in the contact regions.Translated from Poroshkovaya Metallurgiya, No. 7 (259), pp. 61–65, July, 1984.     

Sintering of very fine molybdenum and tungsten powders

Conclusions Vacuum annealing at a temperature above 900°K enables the specific surfaces of very fine loose tungsten and molybdenum powders to be varied in a wide range. The vacuum sintering of compacts pressed from very fine (particle size less than 0.05m tungsten and molybdenum powders is accompanied by severe cracking. In the hot pressing of very fine Mo and W powders produced by the pyrolysis of carbonyls in a stream of high-temperature plasma, a specimen density close to theoretical is reached at 1600°K i.e., at a temperature not less than 400°K lower than the sintering temperatures of powders of particle size more than 1 m. Sintering lowers the amounts of carbon and oxygen in Mo and W by more than half compared with the starting condition.Translated from Poroshkovaya Metallurgiya, No. 1(229), pp. 47–51, January, 1982.     

Effect of zirconium,cerium, and nitrogen on the properties of a cast chromium-base dispersion-strengthened alloy

Conclusions An analysis of the chemical inertness and wettability of various nonmetallic compounds has shown zirconium nitride to be the most suitable strengthening phase for a chromium-base alloy. Endogeneously strengthened alloys have been produced having values of high-temperature strength _t ⁹⁰⁰ of between 270 and 340 MPa and of room-temperature ductility ²⁰ of between 5 and 44%. The properties of such an alloy are determined by the size and uniformity of distribution of its strengthening particles, while these in turn depend on the zirconium-tonitrogen ratio in the alloy.Translated from Poroshkovaya Metallurgiya, No. 11(227), pp. 81–84, November, 1981.     

Influence of hot isostatic pressing on the structure and properties of an innovative low-alloy high-strength aluminum cast alloy based on the Al–Zn–Mg–Cu–Ni–Fe system

Hot isostatic pressing (HIP) is applied for treatment of castings of innovative low-ally high-strength aluminum alloy, nikalin ATs6N0.5Zh based on the Al–Zn–Mg–Cu–Ni–Fe system. The influence of HIP on the structure and properties of castings is studied by means of three regimes of barometric treatment with different temperatures of isometric holding: t ₁ = 505 ± 2°C, p ₁ = 100 MPa, τ₁ = 3 h (HIP1); t ₂ = 525 ± 2°C, p ₂ = 100 MPa, τ₂ = 3 h (HIP2); and t ₃ = 545 ± 2°C, p ₃ = 100 MPa, τ₃ = 3 h (HIP3). It is established that high-temperature HIP leads to actually complete elimination of porosity and additional improvement of the morphology of second phases. Improved structure after HIP provides improvement properties, especially of plasticity. In particular, after heat treatment according of regime HIP2 + T4 (T4 is natural aging), the alloy plasticity is improved by about two times in comparison with the initial state (from ~6 to 12%). While applying regime HIP3 + T6 (T6 is artificial aging for reaching the maximum strength), the plasticity has improved by more than three times in comparison with the initial state, as after treatment according to regimes HIP1 + T6 and HIP2 + T6 (from ~1.2 to ~5.0%), which are characterized by a lower HIP temperature.     

Effect of annealing on the structure and properties of hot-pressed boron carbide base cermets

Conclusions During the annealing of a hot-pressed boron carbide-aluminum materials at a temperature exceeding the melting point of aluminum intense formation of-AlB₁₂ takes place. This has a deleterious effect on the strength properties (transverse rupture strength _tr, fracture toughness k_Ic, and impact strengtha) of the material and increases its hardness and electrical resistivity. It follows therefore that, to obtain boron car-bide-aluminum materials of maximum strength, it is necessary to perform their liquid-phase pressing under high pressures (above 0.5 GPa) and dispense with subsequent annealing. Annealing at a temperature below the melting point of aluminum brings about only very small changes in the structure of the material, and the resultant fall in hardness is less than the increase in hardness. Consequently, when it is necessary to increase the hardness of the material and a slight loss of strength is acceptable, recourse may be had to low-temperature annealing (below 660°C).Translated from Poroshkovaya Metallurgiya, No. 9(273), pp. 51–54, September, 1985.     

Creep lifetime prediction of oxide-dispersion-strengthened nickel-base superalloys: A micromechanically based approach

The high-temperature creep behavior of the oxide-dispersion-strengthened (ODS) nickel-base superalloys MA 754 and MA 6000 has been investigated at temperatures up to 1273 K and lifetimes of approximately 4000 hours using monotonic creep tests at constant true stressσ, as well as true constant extension rate tests (CERTs) at . The derivation of creep rupture-lifetime diagrams is usually performed with conventional engineering parametric methods, according to Sherby and Dorn or Larson and Miller. In contrast, an alternative method is presented that is based on a more microstructural approach. In order to describe creep, the effective stress model takes into account the hardening contributionσ _p caused by the presence of second-phase particles, as well as the classical Taylor back-stressσ _p caused by dislocations. The modeled strain rate-stress dependence can be transferred directly into creep-rupture stress-lifetime diagrams using a modified Monkman-Grant (MG) relationship, which adequately describes the interrelation between representing dislocation motion, and lifetimet _f representing creep failure. The comparison with measured creep-rupture data proves the validity of the proposed micromechanical concept.     

Kinetics of formation and dissociation of Na2SiF6

Spontaneous fluoride emissions from high-temperature processes can result in an increased atmospheric fluorine content and environmental contamination. Slags containing SiO₂, Na₂O, and CaF₂ tend to be unstable at high temperatures, and gaseous species such as NaF and SiF₄ evolve simultaneously. Furthermore, a reaction between NaF and SiF₄ can occur to produce Na₂SiF₆ (sodium hexafluorosilicate), the behavior and properties of which are not well established. In a previous study, the diffusivity of NaF in argon, nitrogen, and helium was measured. In this study, the rate of NaF vaporization in a SiF₄-Ar atmosphere was investigated and the rates of formation and dissociation of Na₂SiF₆ were measured. Kinetic analyses were carried out and the rate constants of the formation and dissociation of Na₂SiF₆ were obtained. The rate equation for Na₂SiF₆ formation was expressed as follows:

Effect of composition and porosity on the hardenability and hardness penetration of structural powder steels

Conclusions The effect of porosity ( > 10%) on the hardenability of the powder steels is stronger than the effect of carbon content. The role of these factors changes with the reduction of porosity ( < 10%) and the carbon content exerts a stronger effect on the hardenability of the steel. After hot pressing, the hardenability of the powder steels approaches the hard-enability of the dense steels of a similar chemical composition.The hardenability of the alloy powder steels at a constant carbon content and porosity is independent of the degree of alloying of the steel.The strength of the effect of complex alloying on the hardness penetration of the powder steels increases with decreasing porosity, especially after hot pressing and annealing as a result of the more efficient homogenizing of the structure with respect to carbon and alloying elements.Translated from Poroshkovaya Metallurgiya, No. 1(277), pp. 82–86, January, 1986.