The proposed experiment aims at obtaining a systematic characterization of the microstructural evolution during long-term ageing with or without external stress of the aerospace alloy 15-5PH martensitic steel. This long term ageing involves both spinodal decomposition linked to the high Cr content of the steel and evolution of the Cu nanoscale precipitates. These two contributions will be individually determined by using the difference in the magnetic and nuclear contrasts of these two contributions.