Microstructure characterization of ball-milled Ni 50Co 50 alloy by Rietveld method

Abstract

A nanostructured Ni 50Co 50 mixture was prepared by high-energy ball milling in a planetary ball mill (Frisch P7) under an argon atmosphere. Scanning electron microscopy and X-ray diffraction (XRD) have been used to study the morphology of the powder particles, microstructure, and structure evolution. Detailed analysis of the XRD patterns was performed by the MAUD program, which is based on the Rietveld method. It reveals the complete vanishing of the Co peaks after 3h of milling, indicating the allotropic transformation of Co from hcp to fcc structure. The reciprocal dissolution of the elemental Ni and Co powders leads to the formation of a heterogeneous solid solution with two structures: fcc-Ni(Co) and fcc-Co(Ni) with relative fractions of about 76 and 20%, respectively, after 24h of milling. Microstructural parameters such as lattice parameter, crystallite size, microstrains, dislocation density, and stacking faults have been deduced from the best Rietveld refinements. The formation kinetics of nanostructured Ni 50Co 50 mixture can be described by a lower Avrami parameter, n, close to 0.34. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.