The relationship between structure and properties in a magnetic precipitation hardened Co-Fe-Ti alloy

The development of magnetic properties during isothermal aging of a solution treated and quenched 82 Co-12 Fe -6 Ti alloy and the relationship of these magnetic properties to the microstructure of the alloy were studied. Specifically, the intrinsic coercivity, initial susceptibility, magnetization, and hysteresis loop shape were correlated with the microstructure observed by transmission electron microscopy (TEM). During aging at 750°C, a periodic, coherent γ' precipitate (L12superlatliee phase) aligned along the <100> directions of the ferromagnetic fcc matrix was observed. Coarsening of this γ' precipitate results in coercivity increases directly proportional to the precipitate size and in an initial susceptibility that is essentially independent of particle size. These results are explained by a model which considers the interaction of the non-magnetic precipitates with the energy density of a domain wall. The model predicts the coercivity will be directly proportional to the precipitate size, a, fora < \delta, the Bloch wall width. The model also predicts an initial susceptibility that is independent of particle size for\delta_{o}< a < \deltawhere δois a wall width parameter. In this study, the maximum coercivity occurred when the microstructure consisted of a complex mixture of coarsened coherent and semi-coherent γ' precipitates. Loss of coherency accelerates the kinetics of coarsening and leads to a coalescence of the individual γ' precipitates into rods along the <100> directions. This coalescence resulted in the magnetic overaging observed for long aging times.