Microstructure and high temperature mechanical properties of ferritic superalloys
Ferritic superalloys are an attractive alternative to Cr-rich martensitic steels or Ni-based superalloys for high-temperature applications in thermal power plants due to their excellent mechanical properties, oxidation resistance and low density. Strengthening of the Fe-matrix by coherent B2-NiAl precipitates leads to an increase in creep resistance up to temperatures of 700 ºC and stresses of 100 MPa.
This research project presented a comprehensive microstructural study of a B2-NiAl strengthened ferritic superalloy and the resulting creep properties in the temperature range between 600–700 ºC and stresses up to 300 MPa. The coherent B2-(Ni,Fe)Al precipitates adopted a spherical shape with an average precipitate radius of 62 nm and volume fraction of 13 % as illustrated in Fig. 1a. In addition, diamond like Zr-rich precipitates and nano-scale secondary precipitates were embedded in the Fe-matrix and at the matrix-precipitate interface.
The creep mechanism was rationalized in terms of dislocation creep and lattice diffusion in the Fe-matrix obtained from tension creep tests. These findings were in agreement with the most commonly observed dislocation climb, where a strong interaction of the dislocations with the matrix-precipitate interface could be established (Fig. 1b).
As demonstrated in Fig. 1c a temperature dependent threshold stress was determined, below which no measurable creep strains were observed.