Preparation Of Ni Nano-powders And Deformation Behavior Of SPS Sintering Bulk After HPT

Pure Ni nanopowders are prepared by the direct current arc plasma evaporationmethod. The morphologies of the prepared Ni nanopowders are observed by X-raydiffraction (XRD) and scanning electron microscope (SEM). The XRD shows that theprepared powders are high pured and they belong to the face centered cubic (FCC)structure; the average grain size is26nm. At the same time we find that the powders showlattice contraction and the smaller the grain size, the more serious the lattice contraction.In this paper, we prove the lattice contraction by the Bragg equation, spacing equation,surface tension and surface energy equation. The results show that the calculated resultsare consistent with the experimental results. SEM results show that most of thenanopowders are spherical particles whose sizes are uniform, with tiny aggregationgenerated.The Ni nanopowders are sintered by spark plasma sintering (SPS) equipment, andhow the sintering temperature influences the grain size, density, hardness andmicrostructure of the sintered bulks is discussed. XRD shows that the sintered bulks areFCC structure. SEM and transmission electron microscope (TEM) show that the densityand grain size of the sintered bulks vary. The grain size and density increase at first, andthen drop with temperature increasing. The grain size and density reach a maximum at600°C and650°C respectively. Hardness testing result shows that both the density of thebulks and density of the dislocation influence the hardness. When the sintered temperatureis under450°C, the density of bulks is the main factor and when the sintered temperatureover450°C, the density of the dislocation is the main factor.Use high pressure torsion (HPT) to dispose the sintered bulks and discuss theinfluence. XRD shows that after HPT the bulks are FCC structure also, and the grain tendsto rotate from (111) to (200). SEM and TEM show that after HPT, the deformation is smallat the center of the samples and is large at the edge of the samples, and the number oftwinning decrease, but the dislocation density increase dramatically. Grain refinement isobvious with semicycle HPT, continue to reverse the grain growth. Density testing shows that the density of the bulks sintered at low temperature increases significantly, and that ofthe bulks sintered at high temperature are almost the same or even has a slightly decline.The hardness testing shows that the density of dislocation is the main factor influencingthe hardness.