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.     

Kinetics of pore size increase in the activated sintering of powdered tungsten

Conclusions An increase in the size of pores determined by the Barus-Bechgold method in porous specimens from fine tungsten and tungsten-nickel powders takes place during heating to the isothermal sintering temperature. The addition of nickel to tungsten activates the pore size growth process. The size of the increased pore channels in porous solids from W and W-0.46% Ni powders in the temperature range 1000–1300°C depends on the particle size and sintering temperature. A correlation has been found between the integral shrinkage during isothermal sintering and the capillary stresses acting on the attainment of the isothermal sintering temperature in compacts from W-0.46% Ni powders of various particle sizes. The rates of isothermal shrinkage are the same, being independent of the previous history of the powders.Translated from Poroshkovaya Metallurgiya, No. 9(249), pp. 18–23, September, 1983.     

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.     

Analysis of structure parameters for compacted porous materials. II. Integrated and local compaction during sintering of porous bodies

Integrated and local compaction of porous bodies during sintering are analyzed and an algorithm is developed for calculating the structure parameters of sintered materials. Accurate quantitative estimates are obtained for the efficiency of porous material sintering in different structural models. The most precise procedure is based on a model developed by the author for the structure of porous materials and an approximation according to which Voronoi polyhedra are considered to be unchanged with a decrease in system porosity, and pore volume decreases as a result of an apparent increase in particle diameter. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 106–111, July–August, 1999     

Structuring on sintering in the presence of liquid phase in Cr-Cu-iron group metal systems. I. Cr-Cu system

Structuring has been examined for Cr-Cu composites under conditions of impregnation and subsequent liquid-phase sintering at 1200°C in a vacuum of (2–4) · 10⁻³ Pa with reduced and electrolytic chromium powders. The size distribution for the particles of the refractory component in the microstructure containing the reduced chromium on liquid-phase sintering for 60 min corresponds to a logarithmic normal distribution; the distribution parameters are sensitive to the volume fraction of refractory particles. The calculated values for the dihedral angle are close to one of the modes of the distribution for the dihedral angles in the microstructure for specimens made of electrolytic chromium (115°). At 1200°C, the equilibrium Cr_s-Cu_l system obeys the condition . This indicates the probability of formation or preservation of framework structure elements during the liquid-phase sintering, which are observed by experiment in specimens containing reduced chromium. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 3–9, May–June, 2006.     

Pore filling process in liquid phase sintering

Models for liquid flow into isolated pores during liquid phase sintering are described qualitatively. The grains are assumed to maintain an equilibrium shape determined by a balance between their tendency to become spherical and a negative capillary pressure in the liquid due to menisci at the specimen surface and the pore. With an increase of grain size, the grain sphering force decreases while the radius of liquid menisci increases to maintain the force equilibrium. When grain growth reaches a critical point, the liquid menisci around a pore become spherical and the driving force for filling the pore rapidly increases as liquid flows into it. The critical grain size required for filling a pore increases linearly with pore size. Experimentally, filling of isolated pores has been investigated in Fe-Cu powder mixture after liquid phase sintering treatment and after dipping into a molten matrix alloy. The observed pore filling behaviors agree with the qualitative predictions based on the models. In Fe-Cu alloy, pore filling is terminated by gas bubbles formed in liquid pockets. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME.     

Activated sintering of W-HfC composite materials

The features of consolidation of the particles during the activated sintering of tungsten powders with different values of dispersity (d _av = 2–3 and 0.8–1.0 μm) are investigated. Sintering was activated by introducing nickel additives (up to 0.5 wt %), tungsten nanoparticles (up to 30 wt %), and finely dispersed hafnium carbide (5–30 vol %) with subsequent milling in a vibrating mill. The uniaxial compaction of the samples has been performed under pressures from 50 to 1000 MPa, and sintering was performed in vacuum at 1850°C with holding for 1 h. It is shown that the additives of tungsten carbide increase the density of sintered billets and, in combination with dispersed hafnium carbide, tungsten-based composite materials with a grain size up to 2 μm can be obtained.     

Interrelationship between the properties of powders,the pressing and sintering conditions,and the structure of porous niobium III. The porous structure of material of sintered niobium powders

Conclusions In the initial stage of sintering of high-porosity compacts of niobium powder the average pore size increases, the fine pores disappear, and coarser pores than in the original compacts appear.There is a linear relationship between average pore radius and the total volume of pores for similar sintering conditions.The relationship between open and closed porosity after sintering is determined completely by the total porosity and does not depend upon sintering conditions.In sintering of high porosity specimens of dispersed niobium powders zonal isolation of the particles occurs.Translated from Poroshkovaya Metallurgiya, No. 7(331), pp. 23–28, July, 1990.     

烧结矿孔洞结构对烧结强度的影响

通过微型烧结研究了3种精矿粉造成烧结矿粘结相中孔洞结构形成差异的原因,测试并分析了孔洞的尺寸和分布对烧结矿强度的影响。结果表明:烧结矿强度与孔洞形态、数量和尺寸有关;液相扩展能力过大或太差的矿粉都会在烧结过程中引起烧结矿内孔洞尺寸和数量的增加,从而影响其强度。     

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.     

Determination of pore mobility during sintering

The mobility of pores being dragged by grain boundaries has been modeled in terms of shape-independent microstructural parameters measurable by stereological means. The derived mobility is expressed with respect to the amount of porosity per unit area of grain boundary and the driving pressure for grain boundary and pore motion,i.e., as velocity per unit driving pressure. The resultant mobility for pore surface diffusion-controlled motion is found to be inversely proportional to the mean-pore intercept, regardless of pore shape and volume fraction. Solutions for lattice and vapor transport yield mobilities independent of pore size but inversely proportional to pore volume fraction. Computed pore mobilities, assuming surface diffusion-controlled motion, during final-stage sintering of copper and tungsten are shown to decrease with densification. In one case, mobility was found to increase during early densification, prior to the final stage. These results are explained with reference to the simultaneous changes occurring in grain boundary and pore surface areas during densification.     

Allowance for gas solubility and the finite strength of vacancy sinks in the sintering of a ceramic

The process of diffusional evolution (shrinkage or growth) of gas-filled pores in a hot-pressed ceramic is analyzed, assuming that the gas is soluble in the matrix. It is additionally assumed that the strength of vacancy sources and sinks is finite. An equation is derived for the porosity of the ceramic at a late stage of sintering. N. N. Ts. “Kharkov Physicotechnical Institute.” Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 24–32. May–June, 1998.     

A theoretical study of grain growth in porous solids during sintering

The processes of grain boundary migration, pore drag and pore/boundary separation are described on the basis of the phenomenological equations for boundary migration and surface diffusion. Cylindrical pores on triple grain junctions are assumed to represent the open porosity during intermediate-stage sintering. It is found that cylindrical pores can hardly detach from migrating boundaries. Three-dimensional closed pores, however, which predominate during final stage sintering, can separate from migrating grain junctions. The separation process is modelled numerically and the conditions for separation are formulated. Analytical approximations for the pore mobility are shown to describe the numerical results well. They serve to establish effective mobilities of grain boundaries bearing pores in various configurations. Classical theories of grain coarsening are modified by using these effective mobilities. Mechanical constitutive models of sintering contain the grain size as an internal variable. The present analysis leads to an evolution equation for the average grain size, which depends on the volume fraction of the pores and on their configuration.     

Experimental investigation of the kinetics of regrouping and zonal isolation of particles in the laser sintering of a monolayer of one-component metallic powder

The laws of particle regrouping in the laser sintering of a one-component metallic powder were experimentally investigated. The main features of zonal isolation were studied. Spherical particles arranged in a monolayer were subjected to laser sintering. The variation of porosity and average number of interparticle contacts in the monolayer with time were investigated under different conditions of initial particle distribution. Belarus Academy of Sciences, Institute of Technical Acoustics, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos 3–4(406), pp. 52–58, March–April, 1999.     

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.     

The role of pore evolution during supersolidus liquid phase sintering of prealloyed brass powder

ABSTRACT

The aim of this research is to study the pore structure as well as to assess the liquid phase sintering behaviour of Cu-28Zn powder specimens at different green density levels and temperatures. For this purpose, samples were compacted to obtain six different green densities and then sintered at 870°C, 890°C and in part at 930°C for 30?min. The results revealed that the spherical pores which are formed inside the grains can be swept by grain boundaries due to grain growth and join to primary pores so that secondary intragranular pores are eliminated and intergranular pores enlarged at higher temperatures. Also, the pores move upwards to the top of sample due to buoyancy forces. The role of pore structure in distortion is more tangible at higher temperatures (930°C) so that O-shape and X-shape distortions were observed at high and low green density samples, respectively.     

Effect of entrapped inert gas on pore filling during liquid phase sintering

The effect of an inert gas entrapped in isolated pores on liquid flow into them during liquid phase sintering has been studied. An analysis of the balance between the capillary pressure of the liquid menisci and the gas pressure shows that the entrapped gas delays the pore filling and produces bubbles. If the gas pressure exceeds a critical level, the pores remain intact and the critical point for their filling will never be reached. These predictions are confirmed by experimental observations on large spherical pores produced artificially in an Fe-Cu alloy. Argon gas is trapped in the pore by first sintering in Ar-H₂ mixture gas and then in H₂ after the isolated pores are formed. The entrapped inert gas of even low pressure is thus shown to cause a substantial porosity in liquid phase sintered specimens. Formerly a Doctoral Student at the Korea Advanced Institute of Science and Technology.     

Ni-Cr-Co-W-Mo-Al-Ti合金制备多孔材料的组织与性能研究

用粉末烧结法制备了孔结构为球形中空孔和线型中空孔的镍基多孔高温合金材料.对试样进行显微组织观察和力学性能测试.结果表明:制备的多孔高温合金材料的孔隙分布均匀,孔径大小一致.通过高温烧结,多孔合金骨架处的金属颗粒之间形成了烧结颈,发生了烧结结合.生成孔的孔隙度随造孔剂(尿素)的添加量增加而增加,当造孔剂的质量分数为40%时,可得到孔隙度为81.62%的球形多孔材料.多孔材料具有优良的能量吸收性能,其压缩性能随孔隙度和孔径的增加而下降.     

Synthesis and characterization of porous Fe–25 wt.% Al alloy with controllable pore structure

Porous Fe–25 wt.% Al alloy is synthesized by reactive synthesis with Fe and Al powders as the raw materials. Various processing parameters such as the final sintering temperature, pressure during cold pressing, and powder size are investigated to control the pore structure of the alloy. It has been shown that the optimal final sintering temperature is around 950–1050°C. When the pressure is higher than 110 MPa, the variation of the pore structure is mainly caused by the change of the pore nature in the as-pressed compact. In addition, the pore structure of porous Fe–25 wt.% Al alloys depends on the size of the raw powders and the weight fraction of different-sized raw powders.     

Structure formation during sintering in the presence of a liquid phase in Cr-Cu-metal of the iron family systems. II. Systems Cr-Cu-Ni(Co, Fe)

Features of structure and phase formation in Cr-Cu-Ni(Co, Fe) systems during liquid-phase sintering in a vacuum of (2–4) · 10⁻³ Pa at 1200 and 1270°C are studied. It is shown that with introduction of elements of the iron family into molten copper there is dispersion of refractory particles and formation at the boundary with the readily melting component of a transition layer whose structure and composition are determined by the nature of the addition. Growth of refractory particles during liquid-phase sintering of composites of the system Cr-Cu-Ni is controlled by diffusion of elements into the solid phase of the transition layer. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 19–25, July–August, 2006.