A study of time-resolved high-temperature structural order-disorder transformations in rare earth-transition metal intermetallics with 2-17 stoichiometry.

摘要

The metastable phases that can form during order-disorder transformations in rare earth-transition metal R2T17 intermetallics have the potential to produce novel materials with enhanced permanent magnetic properties. The transition is not straightforward and the understanding of its mechanism is critical.; The order-disorder transformation involves the arrangement of transition metal "dumbbell" pairs in the structure. In the disordered state, obtained via rapid solidification, the TbCu7-type structure forms where the dumbbells are randomly distributed throughout the lattice. The ordered state with specific dumbbell positions offers two polymorphs: rhombohedral Th2Zn17 and hexagonal Th2Ni17. This work investigates the order-disorder transformations, where the immediate goal is to examine the role of light and heavy rare earth elements in combination with different transition metals and the influence of additives on the formation of long range order.; Experiments conducted at the Advanced Photon Source utilized time-resolved x-ray diffraction. Synchrotron radiation provides very intense, highly penetrating x-rays and is ideal to study phase transformations. Formation of ordered R 2T17 structures, obtained during isothermal and temperature scanning mode of the high-temperature experiments, was derived from the disordered precursors. Experimental setup utilized the Debye-Scherrer geometry and the full-profile diffraction patterns were collected every ten seconds. The diffraction patterns were subsequently refined by Rietveld method for structural phase determination and analysis.; It has been found that the ability to create disorder during rapid solidification strongly depends on the rare earth element, with light rare earth systems possessing more disorder, and the transition metal, where cobalt favored the formation of disordered structures. Additions of Nb-C and Zr-C have also greatly influenced the order formation. Sharp changes observed in evolution of the lattice parameters during the transformations suggested that ordering occurred via "nucleation and growth" mechanism. Kinetics of ordering transformation and alloy thermal expansion have also been studied. Kinetic parameter n=1--1.5 was obtained for the Sm-Co(Fe)-based intermetallics. The linear coefficients of thermal expansion in the range of (4--30)˙10 -6 K-1 were consistent with the reported values for similar intermetallic structures.