© Max-Planck-Institut für Eisenforschung GmbH

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.

Fig 1: Annular dark-field scanning transmission electron microscopy (STEM) images of a) the alloy in the as-heat-treated stage and b) after tensile creep deformation at 700 ºC and 107 MPa.

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.

Fig 1c) Double logarithmic plot of the steady-state creep rate vs. applied stress for 600, 650 and 700 ºC
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