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

• Published in: JETP letters 2020-07, Vol.112 (1), p.37-44
• Main Authors: Straumal, B. B., Mazilkin, A. A., Protasova, S. G., Kilmametov, A. R., Druzhinin, A. V., Baretzky, B.
• Format: Article
• Language: English
• Subjects: Alloys; Atomic; Biological and Medical Physics; Biophysics; Cobalt; Condensed Matter; Crystal defects; Equivalence; Inclusions; Iron; Molecular; Optical and Plasma Physics; Particle and Nuclear Physics; Permanent magnets; Phase diagrams; Phase transitions; Physics; Physics and Astronomy; Quantum Information Technology; Room temperature; Solid State Physics; Spintronics; Temperature; Thermal relaxation
• ISSN: 0021-3640; 1090-6487
• DOI: 10.1134/S0021364020130020

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 τ 1 (Nd, Dy) 2 (Fe, Co, Cu) 14 B. After HPT at room temperature ( T HPT = 30°C), a mixture of an amorphous phase with nanocrystalline inclusions of the τ 1 phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the τ 1 phase with the melt at the temperature T eff = ∼1100°C. The thus determined T eff value is called the effective temperature. When the T HPT temperature of the HPT treatment increases to 300 and 400°C, the amorphous phase disappears, and the Fe 2 B and γ-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases τ 1 + Fe 2 B + 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 T 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 eff in the equilibrium phase diagram. The previously predicted decrease in T eff with an increase in T HPT is observed for the first time.