Mechanical properties of ground state structures in substitutional ordered alloys: High strength, high ductility and high thermal stability

We have studied the effect of atom arrangements in the ground state structures of substitutional ordered alloys on their mechanical properties using nickel-molybdenum-based alloys as model systems. Three alloys with nominal compositions of Ni-19.43 at% Mo, Ni-18.53 at% Mo-15.21 at% Cr and Ni-18.72 at% Mo-6.14 at% Nb are included in the study. In agreement with theoretical predictions, the closely related Pt2Mo-type, DO22 and D1a superlattices with similar energies are identified by electron diffraction of ground state structures, which can directly be derived from the parent disordered fcc structure by minor atom rearrangements on {420}fcc planes. The three superlattices are observed to coexist during the disorder-order transformation at 700 °C with the most stable superlattice being determined by the exact chemical composition. Although most of the slip systems in the parent disordered fcc structure are suppressed, many of the twinning systems remain operative in the superlattices favoring deformation by twinning, which leads to considerable strengthening while maintaining high ductility levels. Both the Pt2Mo-type and DO22 superlattices are distinguished by high strength and high ductility due to their nanoscale microstructures, which have high thermal stability. However, the D1a superlattice is found to exhibit poor thermal stability leading to considerable loss of ductility, which has been correlated with self-induced recrystallization by migration of grain boundaries.