Self-sustained oscillations of the torque under high-pressure torsion in an NdFeB alloy

The behavior of an NdFeB-based multicomponent alloy subjected to high-pressure torsion has been studied. The high-pressure torsion results in the partial amorphization of the alloy, where the fractions of the amorphous and crystal phases vary in the process of torsion. The torque increases smoothly with the angle of rotation of anvils, and self-sustained oscillations of the torque appear beginning with a certain torsion ($\sim$ 1000$^\circ$). The torque varies from 500 to 600 N m with a period of about 1.5 s. The torsion of the alloy in the regime of self-sustained oscillations is accompanied by intense acoustic emission at a frequency of $\sim$ 1–2 s$^{-1}$. This phenomenon can be explained by the periodic change in the mechanism of the high-pressure torsion from the dislocation one (characteristic of the crystal phase) to the dislocation-free mechanism (typical of the amorphous state).