Structural changes accompanying the sintering of ultrafine aluminum nitride powders

Conclusions An electron-microscopical investigation has established that the sintering of compacts from an ultrafine plasma-chemical AlN powder is determined by the following processes: coagulation, with the formation of strong polycrystalline aggregates (1000–1400°C); coalescence, made possible by a geometric grain boundary match inside the aggregates, and formation of high-angle boundaries between aggregates (1200–1600°C); and collective recrystallization (above 1600°C). In an x-ray diffraction study annealing at temperatures above 1700°C was found to decrease the parameter c of AlN by 0.002 Å as a result of dissolution of oxygen in the lattice without affecting its parametera. The results of both electron-microscopical and x-ray diffraction studies show that annealing at temperatures above 1600°C increases the defectiveness of the wurtzite structure of AlN, brings about twinning and the formation of polytypal interlayers in recrystallized grains, and raises the concentration of other defects causing a broadening of lines in x-ray diffraction photographs. As a result of these phenomena the material experiences a strengthening at sintering temperatures of 1700–1800°C.Translated from Poroshkovaya Metallurgiya, No. 10(226), pp. 35–40, October, 1981.The authors wish to thank A. N. Pilyankevich for helpful discussion.     

Volume changes accompanying the sintering of compacts from mixtures of titanium and iron powders

Conclusions The sintering of compacts from mixtures of titanium and iron powders at temperatures below the eutectic point is accompanied by their shrinkage, the sintering process being mainly determined by the particle size of the starting powders. During the sintering of compacts from mixtures of titanium and iron powders at temperatures exceeding the eutectic point the compacts grow in size, the extent of the growth depending on the particle size of the powders. This phenomenon may be attributed to the effect of heterodiffusion on sintering processes and to the crystallization pressure generated during the formation of intermetallic compounds.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 17–21, May, 1982.     

Cyclic sintering of cermet powders

Summary It is shown that after eleven cycles of sintering under conditions of 1200–800 °C a stage of porosity stability of the sample sets in, and a stage of exhaustion of shrinkage.Cyclic sintering has no advantages over isothermal sintering: with equal temperatures and summary durations during sintering, the magnitude of porosity of the samples is the same.     

Supersolidus liquid-phase sintering of prealloyed powders

A model is derived for the sintering densification of prealloyed particles that form internal liquids when heated over the solidus temperature. The model considers the powder size, composition, and microstructure, as well as the processing conditions of green density, heating rate, maximum temperature, hold time, and atmosphere. Internal liquid forms and spreads to create an interparticle capillary bond that induces densification during sintering. Densification is delayed until the particles achieve a mushy state due to grain boundary wetting by the internal liquid. This loss of rigidity and concomitant densification of the semisolid particles depends on the grain size and liquid quantity. Viscous flow is the assumed densification mechanism, where both viscosity and yield strength vary with the liquid content and particle microstructure. Densification predictions are compared to experimental data, giving agreement with previously reported rapid changes in sintered density over narrow temperature ranges. The model is tested using data from steels and tool steels of varying carbon contents, as well as boron-doped stainless steel, bronze, and two nickel-based alloys.     

Selective laser sintering of compacted powders

The selective laser sintering of preliminarily compacted powders was investigated. Depths of sintering lower than those with freely poured powders were attained. This has a negative effect on the production of multilayered articles. The main disadvantage of the process, however, was the difficulty in separating the article from the unsintered compacted powder. Institute of Technical Acoustics, Byelorussian Academy of Sciences, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 27–31, July–August, 1998.