Preparation and Property Evalution of Ultrafinegrained Materials by Ultra-High Pressure Sintering

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The hot-pressing condition for complete densification of Co+Ni and Fe+Co+Ni mixed ultrafine powders (0.02 ; mum), the average crystal grain size and hardness (Hv) of the sintered compacts were investigated (Research I) and the effect of test temperature on the hardness (Hv) of ultrafinegrained sintered compacts with the average crystal grain size of 0.10 0.35mum was investigated for Fe, Co, Ni, Fe-Co, Fe-Ni, Go-Ni and Fe-Co-Ni systems (Research II). The results obtained were as follows. Research I ; (1) The complete densification temperatures for Fe+Co+Ni mixed powders, for example at 400 MPa, was about 200-300K higher than those (500-800K) of Fe, Co and Ni powders, and about 100K higher than those of Co+Ni, Fe+Co and Fe+Ni powders. (2) The minimum crystal grain size of the Co-Ni and Fe-Co-Ni sintered compacts were 0.18 and 0.12 um respectively ; the crystal grain size under the same hot-pressing condition tended to decrease with the number of components, in accordance with the tendencies of both the homogenization temperature of the components and the densification temperature. (3) The maximum hardness of Co-Ni and Fe-Co-Ni sintered compacts were about 600 and 800 Hv. Research II ; . (1) The order of Hv observed at room temperature among the alloy systems did not change in general at the higher test temperatures. (2) The Hv of Fe fine-grained compacts decreased sharply at about 500K, which was much lower than that (about 800K) for coarse-grained compacts of 3.5um. This was attributed to the fact that creep deformation easily occurs even at such low temperatures for fine-grained compacts.