High-resolution electron microscopy analysis of the structure of copper precipitates in a martensitic stainless steel of type PH 15-5

The copper-rich precipitates of size 4-8 nm which are found in a PH 15-5 martensitic stainless steel aged at 480°C for 2h are found to have the same twinned 9R structure as copper-rich precipitates of similar size in thermally aged and in electron-irradiated model Fe-Cu and Fe-Cu-Ni alloys. Larger precipitates produced by ageing for the same time at 500°C appear to have transformed to a distorted fcc structure without first untwinning by a mechanism also found in the model alloys, which suggests that the precipitation sequences in the steel and the model alloys are essentially similar.