Microstructure and mechanical characteristics of bulk polycrystalline Ni consolidated from blends of powders with different particle size

Near fully dense polycrystalline nickel with a random crystallographic texture was consolidated by hot isostatic pressing of blends of nanocrystalline and conventional microcrystalline powders with different volume fractions. The transformation process resulted in a composite-like microstructure constituted by clusters of soft coarse grains regularly distributed in a hard ultrafine-grained matrix. It was found that the ultrafine-grained matrix hinders the coalescence of the coarse grains component during sintering resulting in a smaller grain size than in the fully coarse-grained counterpart in a sort of “barrier effect”. This effect was found to depend on the volume fraction of the ultrafine-grained matrix. Conversely, during the hot isostatic pressing, the plastic deformation of the coarse-grained fraction is preferred to that of the ultrafine-grained fraction, because of greater dislocation activity in the former type of grains, resulting in lower defect densities in the ultrafine-grained matrix (“shielding effect”). It is shown that as a result of the interplay between the coarse-grained and ultrafine-grained components during sintering, the mechanical behavior of the composite materials cannot be obtained by a linear interpolation between the characteristics of fully ultrafine-grained and coarse-grained counterparts.