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.