Effect of a third element on the elastic properties of Mo–Ti and Mo–Cr BCC solid solutions: A first-principles study

This study investigates the effect of a third element X on the formation enthalpy and elastic properties of Mo–Ti- and Mo–Cr-based BCC solid solutions using first-principles calculations. The Mo–Ti- and/or Mo–Cr-based BCC solid solution phase is a major constituent of ultrahigh-temperature MoSiBTiC alloys and plays a crucial role in providing fracture toughness as the ductility phase. To systematically examine the role of the third element, several hypothetical atomic arrangements were modeled. For Mo–Ti– X BCC solid solutions, the valence electron concentration of the third element X (VEC- X ) significantly influences the formation enthalpy, elastic properties, and preferred atomic arrangement. Fourteen elements were identified as improving ductility, with Al being the most desirable. In Mo–Cr- X BCC solid solutions, the formation enthalpy and the elastic properties were also affected by VEC- X ; the ternary alloys energetically favored the same B2-type atomic arrangement as the binary Mo–Cr BCC. However, no third element was found to improve the ductility of Mo–Cr-based BCC. Conversely, several elements were found to increase stiffness in Mo–Cr– X BCC solid solutions, whereas no third element was found to increase stiffness in Mo–Ti– X BCC solid solutions. These findings provide useful guidelines for the design of Mo–Ti- and Mo–Cr-based BCC solid solution phases. • First-principles study on Mo–Ti– X and Mo–Cr– X BCC solid solutions. • Formation enthalpy and elastic properties depend on valence electron concentration. • Fourteen elements improve ductility in Mo–Ti– X BCC solid solutions. • No third element improves the ductility of Mo–Cr– X BCC solid solutions. • A trade–off between ductility and stiffness in Mo–Ti/Cr–based BCC solid solutions.