Phase transformations in Nd–Fe–B-based alloys under high pressure torsion at different temperatures

In this work, we studied the behavior of the Nd–Dy–Fe–Co–Cu–B alloy for permanent magnets under high pressure torsion (HPT). In the initial state of the studied alloy, it mainly contained the crystalline phase $\tau_1$ (Nd,Dy)$_2$(Fe,Co,Cu)$_{14}$B. After HPT at room temperature ($T_{\text{HPT}} = 30^\circ$C), a mixture of an amorphous phase with nanocrystalline inclusions of the $\tau_1$ phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the $\tau_1$ phase with the melt at the temperature $T_{\text{eff}}= -1100^\circ$C. The thus determined Teff value is called the effective temperature. When the $T_{\text{HPT}}$ temperature of the HPT treatment increases to $300$ and $400^\circ$C, the amorphous phase disappears, and the Fe$_2$B and $\gamma$-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases $\tau_1+\text{FeB}_2+\gamma$-Fe, which is observed in the temperature range from $\sim950$ to $\sim1050^\circ$C. We explain this phenomenon by the fact that with an increase in the HPT temperature $T_{\text{HPT}}$, the rate of formation of defects during deformation remains constant, but the rate of their thermal relaxation (annihilation) increases. This is equivalent to decrease in the effective temperature $T_{\text{eff}}$ in the equilibrium phase diagram. The previously predicted decrease in $T_{\text{eff}}$ with an increase in $T_{\text{HPT}}$ is observed for the first time.