Effect of Milling Time on the Microstructure and Tensile Properties of Ultrafine Grained Ni–SiC Composites at Room Temperature

Bulk metallic nickel–silicon carbide nano-particle(Ni–Si CNP) composites, with milling time ranged from8 to 48 h, were prepared in a planetary ball mill and sintered using a spark plasma sintering(SPS)furnace. The microstructure of the Ni–Si CNP composites was characterized by transmission electron microscopy(TEM) and their mechanical properties were investigated by tensile measurements. The TEM results showed well-dispersed Si CNP particles, either within the matrix, between twins or along grain boundaries(GB), as well as the presence of stacking faults and twin structures, characteristics of materials with low stacking fault energy. Dislocation lines were also observed to interact with the Si CNP which were plastically nondeformable. A synergistic relationship existed between Hall–Petch strengthening and dispersion strengthening mechanisms, which was shown to greatly influence the mechanical properties of the Ni–Si CNP composites. Both the maximum yield and tensile strengths were found in the Ni–Si CNP composite with a milling time of 48 h, whereas the increased rate of strengths drastically decreased in material milled above 8 h due to the significant Si CNP agglomeration. The ball milling process resulted in the formation of nano-scale, ultra-fine grained(UFG) Ni–Si CNP composites when the milling time was extended for longer periods, greatly strengthening these materials. The sharp decrease in elongation percentages, however, should be comprehensively considered before irreversible inelastic deformation.