Coupling computational thermodynamics with density-function-theory based calculations to design L12 precipitates in Fe-Ni based alloys

Achieving a high-volume fraction of thermodynamically stable L1(2)-type precipitates that are resistant to coarsening is of great importance for the development of low-cost Fe-Ni based austenitic steels. With the aid of computational thermodynamics, this work designed two model alloys: Fe-37.4Ni-6.1Al-2.9Ti (FNAT) and Fe-45.2Ni-5.9Al-8.5Si (FNAS). Both alloys were designed to contain a similar amount of L1(2) precipitate in Fe-Ni matrix with-out forming other precipitates. Density-Function-Theory (DFT) calculation was coupled with computational thermodynamics to predict the critical radius at which the precipitates change shape from spherical to cuboidal. The calculation results suggest that critical radius for the FNAT alloy is about two orders of magnitude larger than that for the Fe-Ni-Al-Si alloy. Phase stability and morphology of the L1(2) precipitates in these two alloys were experimentally investigated through X-ray diffraction, atom probe tomography, and scanning and transmission electron microscopy. The L1(2) precipitates in the Fe-Ni-Al-Si system were found to be cuboidal and rod shaped, with much larger size than the spherical ones in the Fe-Ni-Al-Ti system, agreeing with the calculation results.