  A novel y-y'-y" dual-superlattice superalloy, with promising mechanical
properties up to elevated temperatures was recently reported. The present work
employs state of the art chemical and spatial characterization techniques to
study the effect systematic additions of Mo, W and Fe and variations in Nb and
Al contents have on the phase fraction, thermal stability, elemental
partitioning and mechanical properties. Alloys were produced through arc
melting followed by heat treatment. Multi-scale characterization techniques and
hardness testing were employed to characterize their microstructure, thermal
stability and mechanical properties. Alterations in such properties or in
elemental partitioning behaviour were then explained through thermodynamic
modelling.
  A modest addition of 1.8 at.% Mo had a strong effect on the microstructure
and thermal stability: it minimized microstructural coarsening during heat
treatments while not significantly decreasing the y' solvus temperature. A
reduction of Nb by 0.6 at.%, strongly reduced the y" volume fraction, without
affecting the y' volume fraction. The reduced precipitate fraction led to a
significant reduction in alloy hardness. Fe, added to achieve better
processability and reduced material cost, decreased the y' solvus temperature
and caused rapid microstructural coarsening during heat treatments, without
affecting alloy hardness. A reduction of Al by 0.4 at.%, reduced the y' volume
fraction and the y' solvus temperature, also without affecting alloy hardness.
The addition of 0.9 at.% W decreased the y' solvus temperature but increased
both precipitate volume fractions. These data will be invaluable to optimize
current alloy design and to inform future alloy design efforts.
