Ab initio Study of Alloying Impact on the Stability of Cementite in TRIP-assisted AHSS Steels

Abstract

Transformation-induced-plasticity (TRIP) steels from the third generation of advanced high-strength steels (AHSS) contain Si additions to prevent the formation of carbides. Cementite (Fe3C) is a prototype among the carbides, and despite the importance of the influence of alloying elements on its stability, mechanisms by which the elements act has not been clarified so far. In this work, ab initio calculations were employed using the projected augmented wave (PAW) method, as incorporated in the Vienna Ab-initio Simulation Package (VASP), to study the impact of several alloying elements, including Al, Cr, Mg, Mn, and Si, on the stability of cementite. Partitioning energies were calculated to determine the segregation tendency of alloying elements between the phases ferrite, austenite, and cementite. The change in formation energy between the alloyed cementite and the pure cementite was then used to quantify the phase (de)stabilization. Mg and Si were found to reduce cementite stability. Si, however, preferentially segregates into ferrite and therefore has no effect on the stability under equilibrium conditions. In addition, the effects of the technically most important elements Al and Si on the mechanical properties of cementite were calculated using the stress-strain method. Both elements were found to increase the mechanical stability of cementite.