We have analyzed a set of minor hysteresis loops of Fe alloys with 0.3 wt.% Cu in the isothermal aging at 773 K to elucidate the influence of copper precipitation on magnetic property. We found that the magnetic coefficients of minor loops take a maximum at the same aging time of ∼100 min as that of Vickers hardness. This result indicates that the minor loops are sensitive to a structural transition of copper precipitates from coherent to noncoherent structure. The present analysis of minor loops shows that minor loops are very useful for nondestructive evaluation of degradation due to the copper precipitation in view of amount of information and very small magnetic field to be required.
