Non-equilibrium processing of magnetic nanocomposites

• Main Author: Shukla, Shashwat.
• Other Authors: Raju Vijayaraghavan Ramanujan
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Materials::Magnetic materials
• Online Access: http://hdl.handle.net/10356/52047
• Institution: Nanyang Technological University

Exchange coupled magnetic nanocomposites provide a novel methodology for producing high performance permanent magnets. These materials are comprised of two phases; one phase is a magnetically hard material, such as Nd2Fe14B with high coercivity; the second phase is magnetically soft, e.g., α-Fe with large magnetization. However, the magnetic properties of these materials are far less than theoretical predictions because of challenges in understanding the processing mechanisms to achieve enhanced exchange coupling between the hard and the soft grains and thereby enhance the magnetic properties. Hence, in this work, the time evolution of the microstructure of Nd-Fe-B based nanocomposites synthesized by annealing of mechanically milled powders was investigated by Rietveld refinement XRD analysis, TEM, EXAFS and in-situ neutron diffraction. This study revealed how processing variables influence magnetic properties. Melt-spun amorphous Nd-Fe-B ribbons and arc melted Nd-Fe-B cast buttons were employed as starting materials. Milling led to precipitation of α-Fe nanocrystals in initially amorphous ribbons. EXAFS studies revealed that deformation induced the formation of vacancy–interstitial pair type free volume – anti free volume (FV–AFV) defects, which migrated during milling, facilitating the atomic redistribution required to precipitate α-Fe nanocrystals in the amorphous phase. Milling of the cast button resulted in crystal → amorphous → crystal transformations. Milling induced ballistic atomic mixing led to chemical and structural disorder; increase in the enthalpy of disorder induced amorphization. As in the case of milled ribbons, further milling of the button resulted in FV–AFV pair defect formation and nanocrystal precipitation occurred due to enhanced diffusion. Milling induced changes in local structure strongly influenced the magnetic properties. A kinetic model was developed to explain the steady state crystal size. This model predicted that steady state crystal size was a result of dynamic equilibrium between defect enhanced diffusional crystal growth and crystal attrition, resulting from impact induced ballistic jumps. An increase in number density and a decrease in steady state precipitate size of -Fe nanocrystals with increasing milling intensity were also predicted; which was substantiated by the experimental data. In-situ neutron diffraction study of crystallization kinetics revealed that Nd2Fe14B phase formed by diffusion of Fe atoms in -Fe nanocrystals to the Fe-lean amorphous matrix. Good magnetic properties were achieved when the annealing temperature was in a temperature window that was adequate to induce complete crystallization and microstructural uniformity, but not too high to facilitate crystal growth. -Fe nanocrystals acted as nucleation centers. Milling at higher intensities produced a higher number density of smaller -Fe nanocrystals, which induced considerable impingement of diffusion fields around randomly distributed precipitates. This led to smaller average crystal sizes in the post-anneal microstructure, resulting in enhanced exchange interactions and good magnetic properties. Thus, this work has revealed the atomistic mechanisms that control the processing of magnetic nanocomposites. These results help to understand which microstructure will yield good magnetic properties and what processing conditions should be employed to obtain such a microstructure.