Atomistic mechanism of cyclic phase transitions in Nd–Fe–B based intermetallics

Mechanical milling induced atomic disorder and the mechanism of the cyclic crystal → amorphous → crystal phase transitions in a Nd2Fe14B intermetallic – based Nd10Fe85B5 alloy were investigated. Least square first shell fitting of Extended X-ray Absorption Fine-Structure (EXAFS) data of mechanically milled samples revealed that phase evolution during mechanical milling of Nd10Fe85B5 alloy occurred in two stages as a result of two dynamical effects: milling induced forced atomic mixing and enhanced diffusion due to continuous production of point defects. In the first stage of milling, atomic mixing of Nd and Fe atoms dominated, resulting in extensive chemical and structural disorder in the initially ordered intermetallic, followed by amorphization. The second stage of milling was characterized by a remarkable increase in point defect density. Enhanced diffusion resulted in redistribution of alloy atoms and annihilation of these point defects leading to the precipitation of α-Fe nanocrystals in the amorphous matrix. Thus the time evolution of defect structures formed during mechanical milling and their atomic scale interactions led to the cyclic phase transitions in the Nd10Fe85B5 alloy. These milling induced changes in atomic order were found to strongly influence magnetic properties. Crystallization of mechanically milled alloy and effect of annealing parameters on magnetic properties was also analyzed.

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► Mechanism of cyclic crystal → amorphous → crystal phase transitions during mechanical milling of Nd2Fe14B intermetallic studied.
► Time evolution of atomic scale defect structures studied by EXAFS.
► 1st stage of milling: forced atomic mixing resulting in local disorder and amorphization.
► 2nd stage of milling: enhanced diffusion leading to redistribution of alloy atoms and precipitation of nanocrystals.
► Milling induced atomic disorder has a strong effect on magnetic properties.