Abstract
Small-angle neutron scattering experiments with an applied external magnetic field have been performed at 20°C upon Mn and Ni maraging alloys aged in the temperature range 400–500°C. In the Mn alloy aged at 425°C, it has been shown that ageing occurred in two distinct stages as a function of time. The contour plots of constant intensity recorded from aged specimens are always anisotropic. In both ageing stages, magnetic inhomogeneities have been revealed. Taking into account the transverse contribution to the magnetic response, the occurrence of superparamagnetic clusters has been suggested. In stage I, it was not possible to identify the chemical composition of the clusters, while in stage II, magnetic inhomogeneities have been related to Fe2 Mo precipitates revealed by prior transmission electron microscopy (TEM) observations. An increase in Mo content in the Mn maraging alloy also leads to the observation, in stage II of ageing, of a transverse contribution which has been related to superparamagnetic X precipitates revealed in conjunction with TEM.
The substitution of Ni for Mn in the former Mn maraging alloy gives rise to a magnetic response only of longitudinal type. The magnetic inhomogeneities have been associated with the diamagnetic precipitates of ordered w isothermal phase which are responsible for the maximum observed on the scattering curves.