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

Dirras, G. and Gubicza, Jenő and Ramtani, S. and Bui, H. Q. and Szilágyi, T. (2010) Microstructure and mechanical characteristics of bulk polycrystalline Ni consolidated from blends of powders with different particle size. MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCURE AND PROCESSING, 527. pp. 1206-1214. ISSN 0921-5093

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Abstract

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.

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