Mechanics of sintering materials with bimodal pore distribution. II. Mean-square strain rate for the solid phase of a biporous body and instability of the monomodal pore structure during constrained sintering

Sintering of materials with a bimodal porous structure under conditions of external kinematic limitations that make their macroscopic deformation impossible is considered. It is established that in contrast to materials that contain pores of the same size, with sintering of materials that have a bimodal pore distribution under the conditions indicated above there is deformation within the limits of the matrix material. It is demonstrated that a monomodal pore structure under the same conditions is not stable, in the sense that small disturbance of the uniform pore distribution with respect to size may lead to marked stratification during sintering. Tangential stresses that arise in the solid phase as a result of the existence of two types of pores are evaluated. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 36–44, January–February, 2006.     

Manufacturing porous materials from metalorganic mixtures: Report 1

The possibility of manufacturing the highly porous materials from refractory metals (Mo, Mo-Ni, and Ti-Mo) while combining the techniques of powder metallurgy with chemical-metallurgical processes is considered. The formation of metallic nanostructures at the surface of powder particles immediately in the course of sintering the moldings is found. These nanostructures are capable of activating the sintering process for compositions based on Mo, Mo-Ni (50/50), and Ti-Mo (85/15) and provide the formation of highly porous (70–80%) materials at 1000°C.     

Influence of the structure on the elastic modulus of highly porous copper-based materials

The effect of the structure on the elastic modulus of two groups of highly porous sintered copper-based materials (foam and fiber metal) with 40 to 90% porosity is investigated. The elastic modulus of the materials is described within the sphere-rod model developed for foam plastic with an open porosity; the model uses an equation containing a structure parameter. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 79–83, March–April, 2000.     

Mechanics of sintering materials with bimodal pore distribution. III. Kinetics of sintering combined with various forms of loading and adhesion conditions

A study has been made on the effects of loading scheme and conditions restricting macroscopic strain on the work-hardening kinetics and strain accumulation in the solid state in sintering materials with bimodal pore size distributions. Active loading intensifies the reduction in the small pores. The greatest effect comes from combining sintering with hydrostatic compression. At the same time, kinematic constraints (partial or complete adhesion in surfaces) substantially retards the shrinkage of large pores, which means that the porous structure can be controlled. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 10–15, May–June, 2006.     

Effect of compressive stresses on deformation and contact phenomena in pressed fibrous materials during sintering

A study is made of the processes that control deformation and contact phenomena in porous fibrous metallic materials. It was established that sintering of fibrous materials which are subjected to small compressive stresses in the pressing direction makes it possible to completely suppress the volumetric growth of specimens, which is seen during pressing and sintering. Due to the reestablishment of interparticle contacts, the resulting specimens have a significantly higher level of mechanical properties. For example, the ultimate tensile strength of the specimens is increased by a factor of 1.5–2 compared to control specimens of the same porosity. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 29–34, January–February, 2000.     

Determination of the ultimate plastic capacity for powder materials based on the plastic flow model for porous solids. II. Influence of porosity and the stress-strain properties of the solid phase on the ultimate plastic capacity of a porous sintered material

A procedure for theoretical construction of the limiting surface for porous materials is described. The procedure requires the porosity, the stress-strain properties of the solid phase, and decohesion to be predefined. The limiting surface is defined for proportional loading paths. The obtained limiting states are also represented as plasticity diagrams. A satisfactory correspondence of the theoretical results with experimental data is shown for a number of both ferrous and non-ferrous powder materials. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 9–13. January–February, 2000.     

Rapid rate sintering of dispersed systems: Theory, processing, and problems

The process of rapid rate sintering of dispersed powders is studied. It is shown that rapid rate sintering results in high density and uniform fine-grained microstructure of the sintered materials. Structure evolution of porous powder packing is analyzed from the viewpoint of the statistics of randomly packed particles with varying average relative density. A new mechanism of rotational rearrangement controlled through grain boundary diffusion and providing uniform densification along with reduction in inter-particle distance is proposed. The advanctages of non-isothermal and rapid heating sintering are discussed. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 30–39, July–August, 1999.     

Laws of capillary transport in porous materials prepared from titanium alloy VT6 fibers

A study was made of the kinetics of capillary absorption of ethanol in highly porous (70–75%) materials prepared from discrete fibers of titanium alloy VT6 (specimen dimensions 330×20×(0.4–1.3) mm). The fibers, 3 mm in length and 20–140 μm in diameter, were obtained by rapid solidification from the melt. Capillary transport against the force of gravity up to the equilibrium height of capillary rise was investigated in an atmosphere of saturated ethanol vapor. Experimental data on the rate of absorption were analyzed with reference to the properties of the pore space structure—effective pore size, tortuosity of the pore channels, and free surface area. The laws of capillary transport of ethanol in porous materials composed of titanium alloy VT6 fibers, discrete copper fibers, and grade VTEM-2 titanium powder were compared. It was shown that, with regard to the speed of absorption, the advantage of fiber materials over those made from powder is attributable to the less convoluted pore channels in the former. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 67–74, January–February. 1997.     

Manufacturing porous materials from metalorganic mixtures: Report 2

The morphology of the surface and fracture of highly porous materials obtained during the thermal destruction of metalorganic mixtures (MOM) was investigated. The size and shape of pore channels in the finished product were shown to be determined through the fractional composition and the particle shape in MOMs. The effect of the porosity on the permeability and strength of sintered materials was considered. The permeability was demonstrated to depend not only on the value, but also on the shape of pore channels. Combining the techniques of powder metallurgy with chemical-metallurgical processes allows us to obtain materials with a porosity of 70–80% and a strength of no less than 5–25 MPa. If the pore size was from 10 to 100 μm, the permeability of materials was (in 10⁻¹² m²) 1–3 for Mo, 0.5–6 for Mo-Ni, and 2–6 for Ti-Mo.     

Obtaining Fe—Ni—Co—Ti alloys having a thermoelastic martensite transformation

Using powder metallurgy methods, we have produced Fe—Ni—Co—Ti alloys that have a thermoelastic martensive transformation, which is the basis of the shape-memory effect manifested by such materials. Since pores are believed to improve the shape memory, specifically the reversible nature of the strain, attention was focused on development of the technology and investigation of the characteristics of porous Fe—Ni—Co—Ti alloys. The problems that arise during sintering of such alloys from sputtered powders are due to the chemical inhomogeneity of the initial structure and of the structure formed when the liquid phase appears. Various forms of activation, such as cyclic sintering and a stepped increase in temperature, were used to prevent the liquid phase from appearing. The properties of Fe—Ni—Co—Ti alloys with a shape memory effect can be improved if the porosity is increased by obtaining larger powder grains with a more complicated shape. Materials Science Institute, Ukrainian Academy of Sciences Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 79–85, January–February. 1997.     

Consolidation of a porous material in hot pressing in a closed mold fitted with a compensator

A method has been developed for calculating the compaction of a porous material in hot pressing in a closed mold fitted with a compensation slot. Simulation is reported for various initial porosities and compensating slot heights. The results from pressing in a closed mold with compensator and in an open mold are compared. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 14–18, May–June, 1998.     

Hardening of compact and porous titanium in asymmetric rolling

The hardening of materials deformed by shear rolling between rolls of different diameters is examined. The effect of the rolling conditions on structure-formation and hardening mechanisms is studied. The process of consolidation within porous titanium billets subjected to conventional and asymmetric rolling is examined. It is established that asymmetric rolling increases the strength of compact and porous titanium. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 15–22, 2007.     

Peculiarities of use of the method of visioplasticity to determine the stress state of porous semifinished products

An experimental-computational method is developed to study the stress state of a material on the basis of its experimental kinematic diagram by using the techniques of visioplasticity. The method of R-functions and the method of stress functions are used in calculating the stress state. Volumetric plastic deformation is also taken into account. The stress field for the initial (nonsteady) stage of extrusion of an axisymmetric porous semifinished product is obtained. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 10–21, March–April, 2000.     

Features of skeletal composite compaction and sintering

Skeletal composites are prepared using highly porous cellular nickel as a volumetrically-bonded framework and features of their compaction and sintering are studied. The skeleton improves composite compactability and may slow down volumetric changes during low-temperature sintering, but it does not affect them during high-temperature sintering. In compacts of skeletal composites at skeleton-ceramic boundaries there is formation of pores that by growing during sintering form a volumetric network of microcracks. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 10–18, July–August, 2006.     

Method of producing large porous pipes

Conclusions The technological features of sintering large porous cermet articles are described. Technological conditions are proposed for producing large porous pipes. The structure of the butt joint is shown. The use of the sintering method permits producing large porous pipes up to 2 m long with a filtration area up to 0.7 m².Translated from Poroshkovaya Metallurgiya, No. 11(47), pp. 89–92, November, 1966.     

Effect of a shell on the change in shape of a porous billet during isostatic pressing. 1. stress deviator with isostatic pressing

Isostatic compaction of a porous billet in an incompressible container is considered. The behavior of the container as well as the matrix phase of a porous billet is assumed to be described by power law equations. Macroscopic deformation of a billet is controlled by flow theory for a compressible body with a smooth potential. It is established that the stressed state in a billet is not isostatic. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 36–42, January–February, 1997.     

放电等离子烧结金属多孔材料研究现状

金属多孔材料是一种新兴功能材料,具有良好的渗透性、规则的孔道结构、独特的力学、吸附及光电性能等诸多优点。利用放电等离子烧结技术(spark plasma sintering,SPS)制备金属多孔材料具有升温速度快,高效干洁等优点。本文简述了放电等离子烧结技术在材料制备中的应用,讨论了放电等离子烧结参数对金属多孔材料的影响,并对放电等离子烧结制备金属多孔材料的应用现状及前景做出了展望。     

Densification and structure formation in the sintering of composite materials based on nitrides of the IV–V group transition metals. III. Sintering of composite materials in the (VN, TaN)—Cr systems

An investigation was made of densification and structure formation in composite materials in the systems (VN, TaN)—Cr during sintering in argon. It was shown that the shrinkage of these materials during liquidphase sintering is insufficient to provide dense composites (residual porosity was 35–40%). This is attributed to the low thermodynamic stability of VN and TaN, and the rapid evolution from these of nitrogen which accumulates in closed pores. Processes of heterodiffusion and alloy formation also have a negative effect on densification. Exchange reactions between chromium and the nitride-forming metals lead to the formation of a large quantities of intermetallics which embrittle the composite materials. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 13–18, January–February, 1999.     

Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

We used the continuous weighing method to study the oxidation kinetics in air for TiN specimens pressed and sintered from nanocrystalline powders with particle size ≤55 nm. Oxidation was carried out at 500–1000 °C for 240 min. By comparing with the oxidizability of compact titanium, we estimated the total reaction surface S of the porous specimens as a function of their oxidation conditions. The mass of absorbed oxygen Δm was calculated from the mass gain ΔP, taking into account the volatile component N₂. We have shown that the maximum mass gain Δm at 600 °C is due to reaction of oxygen with the largest reaction surface. Within 120 min, external pores close up, S decreases, and then a continuous oxide layer forms in which diffusion of oxygen is slowed down. At 700–800 °C, the process of closing up of the pores is activated, and S decreases by an order of magnitude compared to 600 °C. After the first 40–50 min, a continuous oxide film forms and virtually no further mass gain occurs. As the temperature increases, the oxidation rate increases. At 900 °C, the reaction surface becomes equal to the external surface of the specimen, but the thickness of the scale increases linearly. We hypothesize that for T > 850 °C, counterdiffusion of titanium ions is superimposed on diffusion of oxygen. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 98–103, January–February, 2006.     

Oxidation of porous nanocrystalline titanium nitride. III. Oxidation mechanism

The air oxidation mechanism of nanocrystalline TiN at 500 to 900 °C is examined. It is shown that at t ≤ 800 °C the oxidation of titanium nitride is controlled by the diffusion of oxygen and at t > 800 °C the interdiffusion of titanium ions is observed. The oxidation properties of porous TiN are determined by the chemical interaction of oxygen and the reaction surface, which includes the external surface of samples and the internal surface of the pores into which oxygen penetrates. The time and temperature dependence of the weight increment complies with the porous material oxidation model. Active initial oxidation is due to the interaction of oxygen and large internal surface. Short-term self-heating of porous samples is also possible. At t ≤ 800 °C, the pores are obliterated with oxides with time, the internal reaction surface reduces, an external oxide film is formed, the oxygen diffusion and weight increment slow down, and the process stabilizes. With temperature increase, these processes are activated and lead to a smaller weight increment at the final stage (2 to 4 h) at 800 °C as compared with 600 °C. At t > 800 °C the pore obliteration rate increases, but due to the interaction of oxygen and titanium ions that diffuse into the external scale surface, weight increment continuously increases with both time and oxidation temperature. The phase composition of the scale also affects the oxidation mechanism of porous TiN. Oxynitride of terminal composition plays a protective role; the transformation of anatase into rutile is accompanied by a decrease in the oxygen diffusion rate; Ti₂O₃ formed in pores accelerates their obliteration. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 95–104, 2007.