Microstructures and properties of DZ40M Co-base superalloy after long-term ageing at 950°C

DZ40M, a newly developed directionally solidified Co-base superalloy was aged at 950°C for long term. Its microstructural charateristics and mechanical properties at room and elevated temperatures were studied. Long-term ageing results in substantial increase in tensile strength and sharp reduction in ductility at room temperature. Its effect on mechanical properties at elevated temperatures is dependent on experimental parameters. At 980°C , 83 MPa, stress-rupture life is prolonged and at 816°C, 215 MPa, it is shortened. However, in all of the cases the alloy possesses a high stress-rupture ductility. During ageing, primary carbide M₇C₃ and MC decomposed sluggishly, fine M₂₃C₆ precipitated copiously around them and M₆C formed on the surface of some M₇C₃. Decomposition of the primary carbides promoted the precipitation of M₂₃C₆ by supplying carbon atoms and segregation of tungsten caused the precipitation of M₆C. It is thought that the change of mechanical properties of the alloy is attributed to the M₂₃C₆ precipitation.