Effect of Silicon Additions on Structural and Microstructural Properties of Nanostructured FeCoNi Alloy Prepared by High-Energy Ball Milling

Full use of experimental measurements obtained by the mean of the X-ray diffraction technique and the Rietveld refinement has been implemented to study the effects of the addition of Si on the microstructural evolution of the Fe-Co-Ni alloy prepared by using a planetary ball mill (Fritsch P7). For this purpose, the mixtures of powders prepared have the general formula Fe60-xCo25Ni15Six (x = 0, 5, 10, 20, and 30 at.%). The related structures and microstructures were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Microstructural parameters were determined by the Rietveld fitting of the X-ray Diffraction patterns. The systematical discussions about microstructures, including morphology, phase structure, lattice parameter, crystallite size, and lattice strain, were carried out. It was found that for the sample without Si, the allotropic transition hcp-Co to fcc-Co was identified, while the presence of Si metalloid may stabilize the hcp-Co phase during milling. Several solid solutions as well as bcc-Fe(Co), bcc-Fe(Co, Ni,Si), and fcc-Ni(Si) were refined. As a consequence of the Si additions, intense refinement of the microstructure (D similar to 10 nm; epsilon = 1.25%) and an amorphous phase were obtained for 30%Si after 100 h milling, namely that the Si has segregated in the grain boundaries and may increase the structural hardening and brittleness of the particles. The resulting nanostructure state is related to the concept of dislocations.