Ir-Nb-Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir-Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L12/silicide structure. Typical Ir-Nb binary alloys, including a hypoeutectic Ir-10Nb, an eutectic Ir-16Nb, and a hypereutectic Ir-25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir-(10-25)Nb-5Si ternary alloys contained three phases: fcc, L12, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir-10Nb-5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir-25Nb-5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir-Nb-Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L12 and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.