Achieving superior strength and low mass density in a novel γ′ strengthened CoNi-based superalloy

The present paper reports the design of a new class of γ/γ′ containing high-strength CoNi-based superalloys in the Co–Ni–Al system by the addition of Ta and Ti. The designed alloys with compositions Co–30Ni–10Al–2Ta–xTi (where x = 0, 2, 4 at.%) exhibit a γ/γ′ microstructure suitable for moderate temperature applications (≤ 800 °C) requiring exceptional specific strength. The 3D APT compositional analysis of the γ and γ′ phases indicates a Co-rich matrix, while the ordered γ′ precipitates are Ni-rich with a stoichiometry of (Ni,Co)₃(Al,Ta). The subsequent addition of Ti up to 4 at.% increases the Ni partitioning to the γ′ precipitates, enhancing their stability. Furthermore, the addition of 4 at.% Ti increases the γ′ solvus temperature by 170 °C, from 950 to 1120 °C, and concurrently decreases the mass density from 8.50 to 8.34 g/cm³. A positive correlation between yield strength and temperature is observed in the Ti-containing alloys, with maximum yield strengths and specific yield strengths of 1025 ± 25 MPa and 122 MPa·g⁻¹·cc, respectively, in the 2Ta2Ti composition, and 1030 ± 20 MPa and 123 MPa·g⁻¹·cc, respectively, in the 2Ta4Ti composition. Long-term stability studies, including microhardness measurements and quantitative assessments of microstructural features, demonstrate excellent microstructural stability and resistance to coarsening in the 2Ta4Ti alloy at 900 °C. This work provides the opportunity to further design low-mass-density CoNi-based alloys with superior performance in the medium-temperature domain required for Advanced Ultra-Super-Critical (AUSC) power plants. This paper is part of a special volume in Journal of Materials Science in honor of Professor Kamanio Chattopadhyay, renowned for his research in our field and his significant contributions to the development of novel alloys for various engineering applications. In his early 60 s, Professor Chattopadhyay’s group pioneered the discovery of low-density Co-based superalloys with a γ/γ' microstructure closely resembling that of Ni-based superalloys. This work involves some of the alloy compositions we conceived during our doctoral thesis work with him.