Microstructural evolution during mechanical milling of Nd-Fe-B nanocomposites

Significant work has been done to synthesize high performance Nd2Fe14B/α-Fe magnetic nanocomposites by mechanical milling. However, the progress in this area has been limited and the processing remains empirical, based on trial and error. Hence in this project the correlation between processing variables and microstructural evolution was investigated. The arc melted Nd12Fe82B6 alloy was subjected to various milling intensities and the kinetics of milling induced changes in phase composition, crystal size and strain were examined by Rietveld refinement X-ray Diffraction (XRD) analysis. The rate of chemical disorder as a function of milling intensity was studied by Extended X-ray Absorption Fine Structure (EXAFS) technique. XRD results revealed that higher milling intensity increased the rate of phase transformation and the rate of change in crystal sizes and strain but did not affect the steady state phase composition. A two step phase transformation was observed: the amorphization of the initially crystalline ingot, followed by the precipitation of α-Fe nanocrystals in the amorphous matrix. EXAFS results showed that milling resulted in extensive bond breaking and formation of non equilibrium bonds; the steady state defect structure was found to change with milling intensity. The energy stored due to disorder and grain refinement resulted in amorphization. The thermal behavior of milled powder for different milling conditions was also studied using Differential Scanning Calorimetry (DSC) analysis. Changes in milling time and rest time were found to significantly affect the structural relaxation and crystallization.Bachelor of Engineering (Materials Engineering