Small-angle neutron scattering analysis of the precipitation behaviour in a maraging steel

Maraging steels are widely used in many technological sectors such as aerospace and military as well as for tools and dies because of their high strength and toughness. For further improvement of these mechanical properties, which are mainly controlled by intermetallic precipitates in the nanometer size range, fundamental understanding of the influence of composition and heat treatment on the microstructural development is necessary. Quantitative characterisation of the precipitate dispersion based on transmission electron microscopy (TEM) is complicated because of the small size of the precipitates. Therefore, small-angle neutron scattering (SANS) was used in addition to determine precipitate size distributions and volume fractions. SANS and TEM measurements were carried out on a cobalt-free corrosion resistant maraging steel after different ageing treatments. The SANS results showed that after ageing for 15 min a large number of precipitates with radii less than 1 nm have been formed. An analysis of the chemical composition of the nanometer-sized precipitates by direct methods is rather difficult. Therefore, the ratio of magnetic to nuclear scattering intensity was additionally analysed in order to gain information on the composition of the precipitates. From the results it is concluded that the precipitates consist of the intermetallic Ti₆Si₇Ni₁₆ phase.