Precipitate Evolution and Creep Behavior of a W-Free Co-based Superalloy

The morphological and temporal evolution of \(\gamma ^{\prime }\) (L1\(_2\))-precipitates is studied in a polycrystalline Co-based superalloy (Co-30Ni-9.9Al-5.1Mo-1.9Nb at. pct) free of tungsten, aged at 1173 K (900 °C). Over a \(1000\,{{\rm{hours}}}\) heat-treatment, the \(\gamma ^{\prime }\) morphology evolves due to precipitate coalescence. The particles grow in size and the volume fraction decreases, while there is no significant change in the microhardness value. Compressional creep tests at 1123 K (850 °C) on a specimen aged at 1173 K (900 °C) demonstrate that the creep resistance is comparable to the original, W-containing, higher-density Co-based superalloy (Co-9Al-9.8W at. pct). This represents the first creep study of the Co-Al-Mo-Nb-based superalloy system. The W-free alloy exhibits directional coarsening of the \(\gamma ^{\prime }\) precipitates in the direction perpendicular to the applied compressive stress, which indicates a positive misfit. This is consistent with neutron diffraction results.