Some Investigations on Iron-Cobalt Permanent Magnet Alloys of the Vicalloy II Type

Abstract

A permanent magnet alloy composed of 52% Co, 8% V, 4% Cr,balance Fe was subjected to severe cold forming by drawing or rolling to produce a reduction in cross-sectional area of over 90%,and to subsequent heat treatments at temperatures varying between 20 and 700°C, after which the following properties were measured: magnetic saturation as a function of temperature in the range from −200 to +700°C; coercive force as a function of temperature in the range from −200 to +400°C; remanence;torque in the magnetic field, anisotropy energy, and longitudinal magnetostriction as a function of temperature in the range from+20 to +400°C. Vickers hardness, ultimate tensile strength,Young's modulus, and coefficient of expansion were also determined in some cases and electron micrographs of the structure taken. The permanent magnet alloy tested consists of a fine twophase structure, the body-centered cubic α phase alone being ferromagnetic above room temperature, while the second phase becomes ferromagnetic only at temperatures below room temperature. Magnetizing and demagnetizing of this alloy, too, is brought about chiefly by domain rotation. Basically, permanent magnetcharacteristics are not produced by crystal anisotropy or anisotropy of directional order but by a uniaxial anisotropy which may be either a shape or a strain anisotropy. The measured magnetic properties can be more easily explained by an anisotropy of internal stresses which in one model concept is produced by the process of α-γ transformation because of the clearcut textures of both components. There are also indications that the mutual transformation of the two structure components is coincident with noticeable shifts in chemical composition or order, particularly in the γ phase.

Influence of microstructure and texture evolution on magnetic properties attained by annealing of a cold-rolled Fe-Co-10V semi-hard magnetic alloy

“…With aging at 634 °C, those particles remained stable and ordering transformation took place within them, while the surrounding matrix remained disordered. In another study, the magnetic hardness in Fe-52Co-8V-4Cr alloy was attributed to the formation of fine particles of the magnetic bcc phase with the average size of ~50 nm as a result of uniaxial anisotropy produced by ferrite-austenite transformation [21]. However, our findings concerning the present alloy revealed that ordering transition initiated at temperatures well below 500 °C [24], but no significant enhancement of H c is observed at these temperatures.…”

“…In other words, incipient recrystallization at around 640 °C resulted in high coercive force, while after complete recrystallization at 710 °C, it decreased significantly. Also, Baran et al [21] attributed increasing in the coercive force to the stress anisotropy imposed by transformation of ferrite to austenite in an Fe-52Co-8V-4Cr alloy.…”

Permanent Magnet Materials

“…New results for a Vicalloy-based alloy (Koerzit T, Krupp; 5 2 % Co, 8 % V, 4 % Cr) have been reported by Baran et al [9]. Vicalloy.…”

“…Electron-microscope pictures [9] show a structure with apparently fine particles in the form of "chains of spheres,'' roughly parallel to the direction of cold working and of individual size about 500-1000 Â. This structural anisotropy is most pronounced in the optimum permanent magnet state, reached after annealing for 2 hours at about 500-550° C. However, it seems [103] that this result, which would explain the main magnetic findings in terms of shape or interaction anisotropy, may be largely fortuitous, as similar structures were also observed in alloys which had only been cold worked and had a low coercive force.…”

“…Experiments on dilute alloys include those on AuCo [5], where one of the most comprehensive attempts yet has been made to analyze the experimental data in terms of fine particle theory. Recent work on CuFeNi [8] and FeCoV alloys [9] has helped to evaluate the causes of their magnetic hardness. Recent work on CuFeNi [8] and FeCoV alloys [9] has helped to evaluate the causes of their magnetic hardness.…”