Soviet Powder Metallurgy and Metal Ceramics

This paper reviews work carried out at the Institute for Problems of Materials Science, Ukraine National Academy of Sciences, in developing physical bases for shock-wave synthesis of superhard phases of carbon (diamond, lonsdaleite) and boron nitride (wurtzite and sphalerite modifications). The effect of phase transformation mechanisms on structure features of the phases that are obtained under shock compression conditions is considered.

A new multilayer polytype of aluminum nitride has been discovered, forming during the sintering of powdered aluminum nitride under conditions ensuring oxygen diffusion into its lattice. It has been found by the electron diffraction technique, using double diffraction effects, that the multilayer polytype has the 165R structure. An x-ray diffraction study has revealed the presence in the wurtzite AIN modification of interstitial stacking faults, whose concentration falls with rise to sintering temperature. This is attributable to the expenditure of the stacking faults on the formation of multilayer polytypes.

The optimum combination of frictional characteristics and wear resistance under unlubricated friction conditions is exhibited by materials containing 11–14 wt.% Al in their bronze matrix. The structure of the matrix of these materials consists of a heterogeneous mixture of a ductile (anα solid solution of aluminum in copper) and a brittle (the intermetallic compound Cu2Al) component, the concentration of the latter being sufficiently low to produce no marked decrease in strength. The presence in the structure of a second phase reduces the plastic deformation of the surface layer of the material during its sliding on the mating part, results in lower temperatures being generated on the frictional surfaces, prevents the components of the frictional pair from seizing with each other and, hence, increases the wear resistance of these frictional materials.

A plant has been constructed, and a method developed, for the production of nickel alloy powders by centrifugal atomization of molten metal. A high-quality powder produced by this method was virtually free from gas pores in its <630-μm fraction and contained not more than 0.008 wt. % of oxygen.

The formation of liquid-phase interlayers between particles of rapidly cooled powder of the alloy Al-5% Cu during sintering was studied by the methods of complex thermal analysis and electron-scanning metallography. It was found that fluctuations of the chemical potential which arise as a result of the decomposition of the solid solution and excess grain-boundary energy drive the sintering process. Contact bridges between the particles are formed as a result of extrusion of melt to the particle surface during grain growth.