The purpose of this paper was to study the regularities of formation of ultrafine structure in alumina by magnetic pulse compaction (MPC) and spark plasma sintering, and the producing of nanostructured compacts having high density and microhardness. The combined application of two technologies magnetic pulse compaction and spark plasma sintering in the practice of compacting powders is very rare and unique. We have studied the microstructures of consolidated alumina samples. The anomalous zones present in volume of magnetic pulse compacted and spark plasma sintered samples of both types α and δ phases of alumina. The microstructure of the fracture surface between anomalous zones depends on the phase state of the particles of the initial powder. MPC of δ-alumina leads to a more uniform distribution of anomalous zones along diameter compact after SPS. MPC of α-alumina leads to an increase of the microhardness on the surface of compacts.
Author: Olevsky E.
It is known, nanocrystalline metals are characterized by high strength and hardness, but low ductility. One way to increase the ductility is based on the creation materials with bimodal grain structure. General principle of this method is the nano- or ultrafine matrix provides a high strength, and evenly dispersed therein coarse grains contribute acceptable ductility. An important condition for preparing the bimodal material is to control and retention of the bimodal structure during processing. SPS method is promising for the development of materials, where need the collaborative and rapid consolidation of micro and nanosized powders. In this paper as the initial materials used copper powder of different fractional composition – a particle size of 40-90 μm and 50-70 nm. Nanopowder has consisted of micro agglomerates which have been divided during the preparation of the powder mixture. Initial powders were mixed in the mechanical mixer. Experiments were conducted a spark-plasma sintering system model Labox-625. The increasing of sintering temperature results in the density increasing. The pressure and the sintering time can also cause density increasing, but not as much as the temperature. The density of the sintered sample was measured by the Archimedes method. The mechanical property was tested using the hardness testing instrument (FM-800). In order to compare the mechanical properties of mono- and bimodal copper prepared by SPS, samples with a diameter of 10 mm were prepared. As a result, the microhardness of bimodal sample (180 HV) is higher than that of the micro- (63 HV) and nanosamples (159 HV) obtained by SPS. Fabricated by SPS the bimodal copper has a higher microhardness, compared with micro- and nanocrystalline samples. The microstructure of the bulk compact was observed by JEOL 6610LV scanning electron microscopy. The final grain size of the sintered nanomaterial varies depending on the sintering temperature ranging from 200 to 500 nm.