Effects of Alloying Elements M (M = Fe, Mo) on Phase Stability of Cr23C6 Carbides from First-principles

Abstract

Abstract. The structural and electronic properties of (Cr, M)23C6 (M = Fe, Mo) with a full composition range of Cr/M ratio were carried out by first-principles calculations. The phase stability was investigated by calculating the reaction energy. Results reveal that Cr22FeC6, Cr21Fe2C6, Cr20Fe3C6, Cr21Mo2C6 and Cr20Mo3C6 are the stable phases. Alloying element Fe and Mo prefer to occupy the 4a and 8c sites of Cr23C6 carbides, respectively. Then the stabilized mechanism by alloying elements in Cr23C6 carbides was explored by employing the charge density difference. It provides the explanation that the high stability of Cr22FeC6 and Cr21Mo2C6 is mainly attributed to the strengthening of covalent bonds between the 8c and 32f sites. IntroductionNi-based superalloys are widely used in hot section parts of aircraft engines because of their superior creep strength and thermo mechanical fatigue properties [1,2]. At high temperatures, owing to alternating stress, fatigue related failure becomes one of the primary failure modes for Ni-based superalloys in the hot section structures [3]. According to experimental observations, M23C6-type carbides are principle sites for fatigue crack initiations [4][5][6]. The previous study proposed that the precipitation of M23C6 carbides result in the formation of grain bound serrations [5]. It was also found by scanning electron microscopy that the fatigue cracks followed the continuous film of M23C6 carbides at the grain boundaries [6]. These facts indicate that M23C6 carbides have crucial effects on the formation of fatigue crack. Therefore, the information on the stability and structure of M23C6 carbides, as well as the corresponding mechanical and thermodynamic properties, is very helpful to deeply understand the mechanism of fatigue crack initiation. Since the observation of γ-M23C6 carbides [7], the properties of γ-M23C6 binary phases had been systemically studied [8][9][10][11][12][13][14][15][16][17][18][19][20][21]. The structural, mechanical and electronic properties of Cr23C6 were reported by Jiang [10]. The phase stability of M23C6(M = V, Cr, Mn, Fe, Co, Ni) was discussed by Medvedeva [22]. However, M23C6-type carbides in multi-component alloys contain more than one metallic element. Generally, Cr23C6 can dissolve Fe, Mo, or W atoms and form complex γ-(Cr, M)23C6 phases [23]. Xie et al. [8,9] explored several compounds of γ-(Cr, M)23C6 (M = Fe, W, Ni) phases and found that Fe atoms preferred to substitute for Cr at 4a site and then 8c site of Cr23C6 phase. To simulate more substitution situations, Han et al.[24] studied γ-Cr23C6 compounds with each of the four metal Wyckoff sites being occupied in turns by Fe atoms. The obtained substitution sequence was 4a, 8c, 48h and 32f. Recently, the site preference of γ-(Cr, M)23C6 (M = Mo, W; x = 0-3) phases was investigated and Cr21M2C6 (M = Mo, W) was found to be the most stable phase [23]. However, the affecting status of higher concentration of typical alloying elements in Cr23C6 carbides (e.g., Fe, Mo.) is still scarcely studied...

Formation of chromium-iron carbide by carbon diffusion in AlXCoCrFeNiCu high-entropy alloys

“…The chromiumiron carbide phase formed in HEAs was Cr 23−x Fe x C 6 (x = 1, 2, 3) type. These three carbide phases have the same structures (Fm-3 m) with very similar lattice constants, a = 10.516 Å for Cr 22 FeC 6 [29], a = 10.507 Å for Cr 21 Fe 2 C 6 [29], and a = 10.505 Å for Cr 20 Fe 3 C 6 [25]. The chromium-iron carbide phase is named (Cr,Fe) 23 C 6 in the current work.…”

“…It is known that the formation of chromium-carbide compounds improves the hardness and the strength of metallic alloys [24]. However, these compounds, specifically M 23 C 6 -type carbides, are the primary sites for fatigue crack initiations [25].…”

“…The chromium-iron carbide phase is named (Cr,Fe) 23 C 6 in the current work. It should be noted that Cr 22 FeC 6 is known to be the most stable chromium-iron carbide structure due to its more negative formation and reaction energies with respect to Cr 21 Fe 2 C 6 and Cr 20 Fe 3 C 6 [25,29].…”