Features of Compacting the Powders of Brittle Materials during Pressing


I. M. Frantsevich Institute for Problems of Materials Science of the NAS of Ukraine, Omeliana Pritsaka str.,3, Kyiv, 03142, Ukraine
Powder Metallurgy - Kiev: Frantsevich Institute for Problems of Materials Science NASU, 2016, #05/06


The compaction of (i) titanium, vanadium, and molybdenum carbide powders, (ii) mixtures of these powders with the addition of nickel, chromium, and niobium carbide powders, and (iii) coarse carbides and diamond powders cladded with cobalt under compressive loading with a constant rate on the powder in a die at room temperature is studied. Based on the variation of the relative density of the samples with pressure, the variation of stresses in the matrix (forming a porous body) with its strain are determined. This enables to reveal the features of compaction, strain hardening,and fracture of brittle powder particles during pressing. It is established that for the molybdenum carbide powder, consisting of powder particles with a pronounced irregular shape, the initial elastic deformation of the matrix abruptly transits into the stage of plastic deformation with almost linear strain hardening. At the late stage of compaction of the powder, the particles are fractured. For titanium and vanadium carbide powders, whose particle shape is close to the round, the elastic deformation does not reveal on the stress–strain curves of the matrix. At an early stage of pressing, an increase in the packing density of the powder particles and in their strain hardening occur. With an increase in density, it is observed the matrix almost linear strain hardening, which changes into decay with decreasing shear stress, when the powder body approaches its non-porous state. As a result of pressing, the size of the coherent X-ray scattering areas decreased to 62 nm and the dislocation density in the titanium carbide particles grew to 8.3 • 1010 cm–2. With increasing the content of metallic particles in a plastic matrix with the particles of brittle materials, it is the metallic particles that predominantly undergo plastic strain with strain hardening. This causes a sharp increase in the total strain hardening, reducing the density of the porous body during pressing. It is established an effective compacting of titanium and tungsten carbide powders with a particle size of 200–600 µm cladded by cobalt. In this case the dependence of the stress upon the strain has a sharp yield point associated with a high elastic limit, significantly exceeding the yield stress, and the time delay in the transition from a purely elastic strain of the body to its plastic flow.