Novel Creep-resistant Ferritic Alloy Designed by the Concept of Precipitation Hardening with Two Types of Precipitates

Based on the concept of precipitation hardening, novel ferritic alloys FBB8 + 2 wt.% Hf, FBB8 + 2 wt.% Zr, and FBB13 + 3.5 wt.% W alloys were developed and investigated, in order to introduce additional particles for further strengthening of the current alloy FBB8. Scanning electron microscopy (SEM), atom probe tomography (APT), neutron/synchrotron diffraction were employed in order to characterize the microstructural features, combined with mechanical tests on strength, as well as creep tests, for a better understanding on the microstructure-mechanical properties connection. The results show that although these alloys possess two or more kinds of precipitates, the major strengthener is still B2-NiAl. Other types of particles either have too low volume fraction, or are too large to provide sufficient strengthening. Results reveal that volume fraction of secondary phases is proportional to the particle size, if the volume fraction of certain phase goes higher, its particle size goes larger as well. Therefore, in order to optimize the strengthening effect, the amount of the additive elements is needed to be carefully adjusted, in order to reach a balance between volume fraction and particle size. The calculation for the strength of the alloys supports the microstructural discoveries from the FBB8 + 2 wt.% Hf, FBB8 + 2 wt.% Zr, and FBB13 + 3.5 wt.% W alloys, where the FBB13 + 3.5 wt.% W alloy has the worst strength among these three alloys, majorly due to low volume fraction of particles and smaller particle size. Minor discrepancies between the experimental data and the calculation might be due to inaccurate measurement on the microstructural parameters. The creep results and the modeling for the FBB13 + 3.5 wt.% W alloy shows that the extremely small B2-NiAl precipitate leads to the major discrepancy between the experimental data and the modeling results, where the actual creep rate vi is much faster than the modeling results. Such discrepancy lies on the fact of utilizing the wrong strengthening mechanism for the back stress calculation, and a better equation that scales with creep strain is required to reflect the actual back stress contributed from the small, coherent precipitates.