Structure of Alloys for (Sm,Zr)(Co,Cu,Fe)z Permanent Magnets: III. Matrix and Phases of the High-Coercivity State.

Andrey G Dormidontov,Mark V Zheleznyi,Aleksandr S Andreenko,Natalia B Kolchugina,Pavel A Prokofev,Nikolay A Dormidontov,Anna S Bakulina,Yury V Milov,Nikolay N Sysoev

doi:10.3390/ma14247762

Abstract

Observations of the surface domain structure (Kerr-effect), optical metallography, scanning electron microscopy (SEM-SE), and electron microprobe analysis (EPMA-SEM), measurements of major and minor magnetic hysteretic loops were used to study pseudo-single-crystal samples of (Sm,Zr)(Co,Cu,Fe)z alloys subjected to heat treatments to the high-coercivity state, which are used in fabricating sintered permanent magnets. Correlations between the chemical composition, hysteretic properties, structural components, domain structure, and phase state were determined for the concentration ranges that ensure wide variations of 4f-/4d-/3d-element ratio in the studied samples. The phase state formed by collinear and coherent phase components determines the high coercive force and ultimate magnetic hysteresis loops of the pseudo-single crystals. It was found that the 1:5 phase with the hexagonal structure (P6/mmm) is the matrix of the alloys for (Sm,Zr)(Co,Cu,Fe)z permanent magnets; the matrix undergoes phase transformations in the course of all heat treatments for the high-coercivity state. The heterogeneity observed with optical magnifications, namely, the observation of main structural components A and B, is due to the alternation, within the common matrix, of regions with modulated quasi-spherical precipitates and regions with hexagonal bipyramids (cellular phase) although, traditionally, many investigators consider the cellular phase as the matrix. It is shown that the relationship of volume fractions of structural components A and B that account for more than 0.9 volume fraction of the total, which is due to the integral chemical composition of the alloys, determines the main hysteretic performances of the samples. The Zr-rich phases, such as 5:19, 2:7, and 6:23, and a structural component with the variable stoichiometry (Sm(Co,Cu,Fe)3.5–5) that is almost free of Zr and contains up to 33 at% Cu, were found only within structural component A in quantities sufficient for EPMA analysis.

Highlights

Introduction iationsOwing to the high both remanence and coercive force, samarium-cobalt- and neodymiumiron-boron-based permanent magnets draw the attention of numerous professionals.In particular, considerable attention is paid to high-coercivity materials prepared fromSm–Co alloys containing copper, iron, and zirconium, the permanent-magnet properties of which are reached at the expense of the formation of nano-sized structure in the course of complex heat treatments, namely, high-temperature homogenizing annealing for supersaturated solid solution and subsequent long-term isothermal and stepped aging
Despite numerous studies related to the formation of the structure and properties of (Sm,Zr)(Co,Cu,Fe)z alloys and permanent magnets based on them, the phase composition, and sequence of the phase-formation and the description and sequence of individual phase reactions, which occur in the course of the complex heat treatment for the high-coercivity state, up to now are a matter of controversy of research groups of various scientific schools
We showed that samples prepared from grains separated from coarse-grained ingots of (Sm,Zr)(Co,Cu,Fe)z alloys with the optimum chemical composition, which were subjected to traditional heat treatment for the high-coercivity state, demonstrate ultimate hysteresis loops that are free of steps in the magnetization-reversal curves, which usually are observed for samples having heterogeneous morphology [9,10,11]

Summary

Introduction

Introduction iationsOwing to the high both remanence and coercive force, samarium-cobalt- and neodymiumiron-boron-based permanent magnets draw the attention of numerous professionals.In particular, considerable attention is paid to high-coercivity materials prepared fromSm–Co alloys containing copper, iron, and zirconium, the permanent-magnet properties of which are reached at the expense of the formation of nano-sized structure in the course of complex heat treatments, namely, high-temperature homogenizing annealing for supersaturated solid solution and subsequent long-term isothermal and stepped aging. Owing to the high both remanence and coercive force, samarium-cobalt- and neodymiumiron-boron-based permanent magnets draw the attention of numerous professionals. Considerable attention is paid to high-coercivity materials prepared from. As a result of purposeful alloying and heat treatments, the effective coercivity mechanism, namely, the domain-wall pinning at structural inhomogeneities and, the unique high time–temperature stability of magnetic properties of the (Sm,Zr)(Co,Cu,Fe)z -based permanent magnets were realized. Despite numerous studies related to the formation of the structure and properties of (Sm,Zr)(Co,Cu,Fe)z alloys and permanent magnets based on them, the phase composition, and sequence of the phase-formation and the description and sequence of individual phase reactions, which occur in the course of the complex heat treatment for the high-coercivity state, up to now are a matter of controversy of research groups of various scientific schools

Methods

Results

Discussion

Conclusion