Phase Transformations in Nd–Fe–B-Based Alloys under High Pressure Torsion at Different Temperatures

Abstract

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 τ1 (Nd, Dy)2(Fe, Co, Cu) 14B. After HPT at room temperature (THPT = 30°C), a mixture of an amorphous phase with nanocrystalline inclusions of the τ1 phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the τ1 phase with the melt at the temperature Teff= ∼1100°C. The thus determined Teff value is called the effective temperature. When the THPT temperature of the HPT treatment increases to 300 and 400°C, the amorphous phase disappears, and the Fe2B and γ-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases τ1+ Fe2B + γ-Fe, which is observed in the temperature range from ∼950 to ∼1050°C. We explain this phenomenon by the fact that with an increase in the HPT temperature THPT, 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 Teff in the equilibrium phase diagram. The previously predicted decrease in Teff with an increase in THPT is observed for the first time.