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The occupancy preferences of transition metal elements and its effects on ductility and brittleness of γ/β phases in TiAl alloys: a first-principles investigation

ABSTRACT The occupancy tendency of M elements in γ/β-TiAl and the bonding strength, electronic structure, and tough-brittle transition of γ/β-TiAl-M were systematically investigated by first-principles calculations. According to the point defect anti-site-formation energy, charge density difference and electron localization function simulations, in the fourth and sixth cycles, M = Fe, Co, Os, Ir, Pt and Au (Ru in β-TiAl) elements preferentially occupy Al sites and lead to the formation of strong covalent-like directional bonds between themselves and surrounding Ti or Al elements. The electronic structure and chemical bonding features are analyzed. γ/β-TiAl-M compounds consist of covalent and metallic bonds, and M elements in the intermediate region give rise to strong covalent bonding features. In addition, transition metal elements that form metallic bonds with Ti or Al will contribute to the ductility of TiAl-based alloys. The diagram of B/G reflects this very well.