Designing L21-Co2TiAl precipitate-strengthened ferritic alloys with excellent high-temperature mechanical properties

Recently, Fe-Cr-Ni-Al-Ti ferritic alloys strengthened by L21-type Ni2TiAl precipitates have demonstrated superior high-temperature strength compared to conventional ferritic steels. However, it still shows a lower hightemperature strength than that of Ni-based superalloys. Therefore, in the current study, we designed new ferritic alloys to improve the high-temperature strength by introducing L21-Co2TiAl precipitates, known as having better high-temperature properties than the Ni2TiAl structure. Specifically, the Ni element has been replaced with the Co element. In addition, the Ti element has been adjusted because the structural evolutions of precipitates, such as composition, lattice misfit and size rely on the Ti content. For instance, the addition of 2 wt% Ti forms a semicoherent Co2TiAl precipitate with a relatively low density of misfit dislocations. More Ti addition up to 6 wt % leads to an increase in the lattice parameter of the precipitate, while that of the Fe matrix remains unchanged, which results in an increased lattice misfit. The increased lattice misfit causes a higher amount of misfit dislocations, which leads to a reduction of the elastic strain field around the precipitates. It was found that the semicoherent interface with a high level of the elastic strain field (2 wt % Ti), as compared to the precipitates with a high amount of misfit dislocations (4 and 6 wt % Ti), plays a crucial role in enhancing the mechanical properties at 973 K. In addition, the designed ferritic alloys show superior strength, compared to the previously-reported ferritic alloys.