Structure of Alloys for (Sm,Zr)(Co,Cu,Fe)z Permanent Magnets: II. Composition, Magnetization Reversal, and Magnetic Hardening of Main Structural Components.

Andrey G. Dormidontov,Yury V. Milov,Nikolay A. Dormidontov,Natalia B. Kolchugina,Alexander S. Andreenko

doi:10.3390/ma13235426

Abstract

Experimental series of alloys for (Sm,Zr)(Co,Cu,Fe)Z permanent magnets are presented in the concentration ranges that provide wide variations of (4f)/(4d)/(3d) ratios of comprising elements. Optical metallographic analysis, observation of the surface domain structure upon magnetization reversal (Kerr effect), electron microprobe analysis, and measuring the major hysteresis loops of samples at different stages of heat treatment are used to study processes related to the development of the highly coercive state of these samples. It was found that the volume fractions of two main structural components A and B, which comprise 90% of the total sample volume, rigorously control the coercivity at all stages of thermal aging. At the same time, structural components A and B themselves in samples being in the high-coercivity state differ qualitatively and quantitatively in the chemical composition, domain structure and its development in external magnetic fields and, therefore, are characterized by different morphologies of the phases comprising the structural components. Two stages of phase transformations in the sample structure are observed. During isothermal annealing, the cellular structure develops within the B component, whereas, during stepwise (slow) cooling or quenching from the isothermal aging temperature to 400 °C, a phase structure evolves within both the cell boundaries in B and the structural component A. The degree of completion of the phase transformations within micro- and nano-volumes of the components determines the ultimate hysteretic characteristics of the material.

Highlights

The record temperature stability of characteristics of (Sm,Zr)(Co,Cu,Fe)z permanent magnets determines their traditional applications in a wide range of products
The present study was performed to widen the objective picture about the formation of the structure and magnetic properties of (Sm,Zr)(Co,Cu,Fe)z alloys with the compositions that show promise for manufacturing high-coercivity permanent magnets; the present paper is the logical continuation of work [5]
The heat treatment for the high-coercivity state leads to the substantial separation of two main structural components (A and B) in the chemical composition; the separation is caused by different temperature dependencies of the solubility of 3d elements in the phases being the basis for these structural components (1:5 and 2:17, respectively)

Summary

Introduction

The record temperature stability of characteristics of (Sm,Zr)(Co,Cu,Fe)z permanent magnets determines their traditional applications in a wide range of products. The high magnetization and the high internal coercive force in the operation temperature range are two the most important characteristics of permanent magnets of any type [6]. Techniques of the powder metallurgy (refinement of an alloy to form magnetically-uniaxial micron-sized particles, alignment of powder in a uniform magnetic field, mechanical compacting in compression molds, and subsequent thermal consolidation or sintering of blanks), which allow one to obtain the maximally possible crystallographic anisotropy of blanks, traditionally are used for Materials 2020, 13, 5426; doi:10.3390/ma13235426 www.mdpi.com/journal/materials. The obtained anisotropic massive with the easy magnetization axes of all grains (crystallites) oriented along single preset direction allows the magnetization potential to be most completely realized [6]. The periodic phase nanostructure results from the sequential phase transformations, and ensures the coercivity mechanism, namely, the efficient domain-wall pinning at interfaces of the formed phases [6]

Objectives

Methods

Results

Discussion

Conclusion