Temperature hysteresis and time-dependent effect in the initial susceptibility of rapidly solidified Monel (abstract)

Abstract

The temperature dependence of the ac susceptibility for a Cu-Ni alloy near the Monel composition (28 at. % Cu) was previously reported.1 A careful recording of this temperature dependence revealed the existence of temperature hysteresis for both the as-spun condition and after various isothermal anneals. The hysteresis is exhibited by higher susceptibility values during the cooling-down period compared to the warming up. The difference exists over the entire temperature range, though it is most pronounced around the peak. The peak for the as-spun sample is broader and therefore the effect is more clearly observed there. The time dependence decrease of the ac susceptibility was followed at room temperature for both the as-spun and the annealed samples and was found to have a long time constant. The decrease could be abruptly generated by ‘‘shaking’’ the sample with an external disturbing magnetic field. The hysteresis is attributed to the fact that above the peak and just below the Curie temperature, the sample is magnetically soft and is magnetized parallel or antiparallel to the alternating small magnetic field. Due to relaxation effects below the ac susceptibility peak (and the corresponding increase of the anisotropy), the direction of the magnetization tends to rotate to the local easy direction, but since the relaxation time constant is long compared to the measuring period, it partially retains its previous direction and thus the susceptibility remains higher during the cooling duration. The anisotropy keeps it at the lower value when the temperature is increased again.2