Texture of hot-pressed vanadium carbide-carbon materials

Abstract

The physical properties of carbide-carbon materials based on the carbides of groups IV-VI transition metals are determined to a significant degree by their structure, which is formed during production. In [1-3] the relationship between the structural features and the kinetic properties of materials of this class was shown and simple ordered models of conductivity taking into consideration both the anisotropy of the structure and the anisotropy of the properties of the carbon component were proposed. Obviously, the combined use of such models and the results of experimental investigations is at present the only reliable method of predi c tion of the physical properties of real materials. However, the overwhelming number of experimental and theoretical investigations of structurally dependent properties of hot pressed materials have been made with the assumption either that their structure is uniform or that the nonuniformity has a random character. At the same time it is not taken into consideration that the distribution of the preferential orientations of the graphite inclusions in the volume of the material may acquire a quite complex character depending upon the form of temperature--force loading in hot pressing [4] and the geometric form of the part. Even with the simplest and most common form of deformation, uniaxial one-sided pressing of a porous blank in the form of a solid cylinder, friction of the powder on the walls of the die causes a change in texture of the graphite phase along the height and radius of the blank. In this work the texture of the graphite inclusions in the compact of vanadium carbidecarbon material with a i0 vol.% graphite content was investigated by metallographic analysis and measurement of the specific electrical resistance acrossthe volume of the compact. The cylindrical specimen with a relative density of 99.7% and a diameter and h~ight of 40 mm was obtained by uniaxial hot pressing of a powder mixture of VCo.88 and type EUZ-M graphite on a UGP-I press [5] at 2600 K under a pressure of 8.0 MPa. From the axial cross section of the compact according to the plan shown in Fig. i were cut plates, which were then ground and polished on elastic disks with a diamond filler. Portions of the specimens in the horizontal layers with a width of 0.24 mm with coordinates of the axes of I, i0, and 39 mm and an interval of 0.36 mmwere photographed on an MIM-7 metallographic microscope with an MFN-12 plhotomicrographic attachment. The angle ~ between the normal to the plane of an individual graphite inclusion and the axis of pressing was determined with a degree network and also t]ae linear dimension Zj(~) of this inclusion. The portion of the particles of graphite included in each i-th interval of angles may be calculated using the equation