Magnetic hardening in Co-Al Alloys

The decomposition of a β-CoAl (B2) alloy containing nominally 13 weight percent aluminum has been studied using transmission electron microscopy and diffraction. The microstructure has been correlated with changes in magnetic properties (coercivity and magnetization) during isothermal aging at 500° and 550°C. The magnetic hardening of the resultant two-phase mixtures derives from the formation of an elongated, Co-rich phase involving a displacive controlled/diffusion-assisted structural transformation. The magnetization reversal in the fine-particle assemblies occurs by a Stoner-Wohlfarth coherent rotation in the ESD particles. The coercivity can be quantitatively described by a simple model in which the crystal and shape anisotropies of the elongated precipitates are in opposition.