Revealing sulfur- and phosphorus-induced embrittlement and local structural phase transformation of superlattice intrinsic stacking faults in L12-Ni3Al

In the present work, the effects of S and P atoms on the local phase transformation and embrittlement mechanism of superlattice intrinsic stacking faults (SISF) in L12-Ni3Al are studied via the first-principles calculations. It is presented that both S and P atoms are energetically preferable to stay away from fault layers, leading to the degraded stability and lattice expansion in L12-Ni3Al with SISF. In the view of bonding charge density, it is captured that the impurity atoms form strong covalent bonds with their nearest nickel atoms and impair the normal Ni–Ni bonding strength, which reveals the impurities-induced embrittlement in L12-Ni3Al. Moreover, the local structural phase transformation with and without impurities are also characterized as the transformation from the tetrahedra-type bonds of FCC lattice to the rod-type bonds of HCP ones. These impurity atoms improve the stacking fault energies attributing to the increased bonding charge density of adjacent close-packed layers, which indicates their unfavorable segregation behavior at the fault layers of SISF. This work provides the atomic and electronic insights into the site occupations and the embrittlement mechanism of S and P atoms in L12-Ni3Al, paving a path to reduce the impacts of S and P atoms on the cohesive strength and steady creep rates of nickel-based superalloys.