Nanometric grain formation in ductile powders by low-energy ball milling

Based on microstructural observations by TEM and in particle size distribution done by sedimentation-photometry, a new grain size refinement mechanism for ductile powders in mechanical alloying is proposed. A 90-95% of the particle population was of submicrometric fragmented particles. These were detected from the beginning of the milling process up to 90 ks. It seems that the fragmentation of the original particles occurred under dynamic conditions to generate those submicrometric ones. Under these conditions the original grain size (100 nm to 350 nm) was preserved and a low level of dislocations was observed at these submicrometric particles. Once these submicrometric particles were deformed, grains smaller than 20 nm were observed. It seems from TEM results that the submicrometric fragmented particles were also deformed under dynamic conditions. This could be a new grain size refinement mechanism present in ductile metallic powder systems where the fragmentation is the dominant stage from the beginning of the milling up to some intermediate milling time. In the Cu-20at%Ni, Cu and Ni systems where the particle coalescence process was the dominant stage during all the milling process, a derivation of the mechanism proposed by Hellstern [3] was identified. In our case, powders were mainly deformed by slip and not by shear. It recognizes that the way to refine the grain size in milled powders is influenced at least by the metallic system used as well as by equipment and the process conditions employed.