Density functional theory study of ω phase in steel with varied alloying elements

Abstract

The presence of a long-abandoned hexagonal omega (ω) phase in steel samples is recently gaining momentum owing to the advances in transmission electron microscopy (TEM) measurements, even though it is already reported in other transition-metal alloys.The stabilization of this metastable phase is mainly investigated in presence of C, even though the formation of the ω phase is attributed to the combined effect of many factors, one among which is the enrichment of solute elements such as Al, Mn, Si, C, and Cr in the nanometer-sized regimes. The present study investigates the effect of the above alloying elements in ω-Fe using density functional theory (DFT) calculations. It is seen that the magnetic states of the atoms play a major role in the stability of ω-Fe. Cohesive energy calculations show that the alloying elements affect the energetics and stabilization of ω-Fe. Further, density of states calculations reveal the variation in d-band occupancy in the presence of alloying elements, which in turn affects the cohesive energy. Phonon band structure calculations show that only ω-Fe with substitutional C shows positive frequencies and hence possess thermodynamic stability. Finally, we confirm the existence of ω-Fe using TEM measurements of a steel sample containing the same alloying elements. Our results can shed light on the stabilization of the ω in other transition-metal alloys as well, in the presence of minor alloying elements. K E Y W O R D S ω-Fe, density functional theory, Fe alloys, TEM, ultrahigh-strength steel, Al Mn Si C Cr alloying elements

HRTEM Investigations on the Substructures of the Quenched Pearlite and Martensite

“…α-Fe is a stable phase, and ω-Fe is an unstable phase. Furthermore, carbon atoms are believed to contribute to the stability of the ω-Fe phase. − In real carbon steels, large ω-Fe 3 C particles cannot be observed, which is simply due to their thermodynamic instability. The following transformation process shown in Figure helps us to understand the mechanism of this instability.…”

Density functional theory study of ω-phase in steel with varied alloying elements

“…The Coulomb repulsion U = 1.8 eV and Hund's exchange J = 0.9 eV is used, from existing reports, as these values have given accurate results. [25][26][27][28] We have compared the DOS of a 1 × 1 × 1 ω-Fe cell containing interstitial C atom, calculated with F I G U R E 5 Density of states of ω-Fe with alloying elements GGA + U and GGA, and shown in Figure 4A,B, respectively. It is evident from the DOS that the spin polarization and metallic character are retained with the inclusion of U and J parameters as well.…”