Dense nanostructured nickel produced by SPS: Influence of the ball milling conditions on tensile properties

The aim of the present research work was to fabricate dense nanostructured nickel in order to take advantage of the improvement of mechanical properties with reduced grain size. One of the challenges was to limit the grain coarsening even in significant size specimens, through the control of the SPS processing parameters (such as heating rate, sintering temperature, holding time, and pressure). However, it is also crucial to control the production of nanopowders in order to get powders having a nanometric size and a size distribution as narrow as possible. Such a control is not obvious due to the large reactivity of nanoparticles. That is why asmilled powders have been produced using high-energy ball milling. Indeed, the milling process leads to both the reduction in the crystallite size and the accumulation of defects in powder particles. The other interest of ball milling is the possibility to form agglomerates with a micrometer size (size usually available with commercial ones) containing nano-crystallites. However, ball milling conditions have also an influence on the purity of milled powders by a possible contamination from milling tools. Therefore, the objective of this work is to study the influence of the microstructure (crystallite size and crystallite deformation rate) and the purity (milling contamination) of the Ni milled powders on the mechanical properties of Ni sintered by SPS.