Abstract:DZ40M is a newly developed directionally solidified Co-based superalloy. Its microstructural characteristics and mechanical properties at room and elevated temperatures after ageing for 100~1 000 h at 850℃were studied. The results showed that 100h ageing at 850℃results in a substantial increase of tensile strength at room temperature and stress-rupture life at elevated temperatures, serious decrease of ductility at room temperature and improvement for stress-rupture ductility. With increasing ageing time, stress-rupture life tends to decrease and the other properties keep at essential constant. During ageing, primary carbides M₇C₃and MC decompose sluggishly, fine M₂₃C₆precipitates copiously around them and M₆C forms on the sur-
face of some M₇C₃. The increased strength of the aged alloy is attributed to a strong interaction between M₂₃C₆and dislocations.

Key words: directional solidification　Co-based superalloy　long-term ageing　mechanical property