Design of low mass density γ/γ′ CoNi-based superalloys with promising high-temperature mechanical properties

Abstract

We report new compositions of CoNi-based superalloys that have low mass densities (7.84--8.20 g/cc), negative $\ensuremath{\gamma}/{\ensuremath{\gamma}}^{\ensuremath{'}}$ lattice misfit, and high ${\ensuremath{\gamma}}^{\ensuremath{'}}$ solvus temperature up to 1094 \ifmmode^\circ\else\textdegree\fi{}C with promising high-temperature mechanical properties. The $\ensuremath{\gamma}/{\ensuremath{\gamma}}^{\ensuremath{'}}$ in the base Co-30Ni-10Al-5Cr alloy was stabilized with Nb and further developed by varying Ti and Cr. The \ensuremath{\gamma}\ensuremath{'} precipitates in these alloys are richer in Ni than the \ensuremath{\gamma} matrix phase, unlike for ${\ensuremath{\gamma}}^{\ensuremath{'}}$ in other CoNi-based superalloys. The atomic-scale compositional analysis indicates that the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ has a stoichiometry of ${(\mathrm{Ni},\mathrm{Co})}_{3}(\mathrm{Al},\mathrm{Nb},\mathrm{Ti},\mathrm{Cr})$. In these alloys, the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ microstructure is stable at 900 \ifmmode^\circ\else\textdegree\fi{}C up to 1000 h. In an alloy with a ${\ensuremath{\gamma}}^{\ensuremath{'}}$ solvus of 1089 \ifmmode^\circ\else\textdegree\fi{}C, the coarsening kinetics of \ensuremath{\gamma}\ensuremath{'} at 900 \ifmmode^\circ\else\textdegree\fi{}C follows the Lifshitz-Slyozov-Wagner model with a rate constant of $6.6\ifmmode\pm\else\textpm\fi{}0.7\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{3}/\mathrm{s}$, comparable with high-density Co-Al-W-based superalloys. The alloys also show an anomalous increase in strength, with temperature having a peak of 0.2% proof stress (PS) and specific PS (SPS) of \ensuremath{\sim}710 MPa and $\ensuremath{\sim}91\phantom{\rule{0.16em}{0ex}}\mathrm{MPa}\phantom{\rule{0.16em}{0ex}}{\mathrm{g}}^{--1}\mathrm{cc}$, respectively, at 770 \ifmmode^\circ\else\textdegree\fi{}C. The low density and high strength make this class of alloys a potential and cost-effective alternative to the other Co-based superalloys and comparable with Ni-based superalloys.