Influence of vibrational entropy on structural stability of Nb–Si and Mo–Si systems at elevated temperatures

✍ Scribed by Yue Chen; T. Hammerschmidt; D.G. Pettifor; Jia-Xiang Shang; Yue Zhang

• Publisher: Elsevier Science
• Year: 2009
• Tongue: English
• Weight: 764 KB
• Volume: 57
• Category: Article
• ISSN: 1359-6454
• DOI: 10.1016/j.actamat.2009.02.014

✦ Synopsis

The heats of formation of stable and metastable phases of the Nb-Si and Mo-Si systems were studied using density functional theory (DFT). The high-temperature behavior of the competing phases was studied by performing additional phonon calculations. Our theoretical results rationalize the major differences observed in the behavior of the Nb-Si and Mo-Si systems: Nb 3 Si is only stable at temperatures above 2043 K, whereas Mo 3 Si is always stable; Nb 5 Si 3 and MoSi 2 undergo phase changes at elevated temperatures, in contrast to Mo 5 Si 3 and NbSi 2 . These differences are qualitatively explained by including the vibrational entropy to the free energies within the harmonic approximation. In particular, the softer shear moduli of the Nb 5 Si 3 and MoSi 2 b phases cause their stabilities over the a phases at elevated temperature.