Improvement of room and high temperature tensile properties of NiAl-strengthened ferritic heat-resistant steels through Mo addition

In order to improve the room and high temperature strength-ductility balance of ferritic Fe-Al-Ni based alloys strengthened by the B2-type NiAl precipitates, Fe-Al-Ni-Cr-Mo alloys was developed. The microstructures and tensile properties of the alloys were examined focusing on Mo content. We found that the 2 at% or 4 at% Mo addition is effective in the improvement of the elongation of the Fe-Al-Ni based alloys at room temperature (RT) while maintaining high strength, which reflects the suppression of intergranular fracture by the strengthening of the bcc matrix by Mo addition. We also found that the RT tensile properties and deformation behavior of the alloys depend strongly on the volume fraction and the size of the NiAl precipitates. For instance, the yield stress decreases by the change in the primary slip direction from < 111 > to < 001 > and the formation of the Orowan loops with increasing the volume fraction and the mean diameter of the NiAl precipitates. In addition, the alloy with the appropriate Mo content exhibits a high yield stress above 660 MPa up to 923 K, owing to the solid-solution hardening by Mo addition and the low growth rate of the NiAl precipitates even at high temperatures. These findings indicate that the alloys have a great potential for steam turbines of advanced ultra-supercritical thermal power plants.