On the microstructural instability of an experimental nickel-based single-crystal superalloy

The susceptibility of an experimental nickel-based single-crystal superalloy to the precipitation of topologically close-packed phases (TCPs) is considered. Its composition has been chosen to be enriched with regard to molybdenum with no tungsten being present, in order to promote microstructural instability and to allow this to be studied. Two conditions are examined: (1) as-cast and (2) as-cast with a solutioning and aging treatment. In the as-cast condition, it is shown that the interdendritic region is already prone to TCP formation, and that further heat treatment in the vicinity of 1000 °C increases the extent of this severely. This is attributed to the partitioning of Ta, which causes an increase in the γ′ volume fraction in the interdendritic regions and a concomitant enrichment of the γ matrix with respect to Mo and particularly Cr. In the solutioned and aged state, TCPs form after heat treatment in the range from 800 °C to 1100 °C and form preferentially at the dendrite cores; this is due to the presence of residual Re, which does not diffuse as quickly as Ta in the opposite direction. The different TCP particles exhibit very different morphologies. At 1000 °C, the P phase is prevalent; around 850 °C, the P phase is still found, but μ is predominant and is found in association with σ. The experimental data are compared with the predictions of thermodynamic software and a database of thermodynamic parameters; the predictions are reasonable, although some discrepancies are noted.