Ab Initio Investigation of Planar Defects in Immm-Ni2(Cr,Mo,W) Strengthened HAYNES 244 Alloy

The Immm-Ni-2(Cr,Mo,W) body centered orthorhombic (BCO) intermetallic phase, denoted as gamma ''', is the primary strengthening phase in commercial Ni-base HAYNES (R) 242 (R) and 244 (R) alloys. Due to the relatively low symmetry and six crystallographic variants precipitating in the face-centered cubic solid solution gamma matrix, the deformation mechanisms are expected to be complex, and a myriad of planar defects are predicted to be observed on the close-packed {013} and {110} planes, which are nearly co-planar to the matrix {111} plane. We find that these defects include those analogous to the gamma '-Ni3Al phase: superlattice intrinsic stacking faults, antiphase boundaries, and complex stacking faults. We determined these planar defect energies and generalized stacking fault energy surfaces utilizing ab initio density functional theory calculations. The gamma ''' {013} and {110} planes exhibited comparable planar defect energies but showed drastically different dislocation shear pathways due to the lower symmetry of the orthorhombic phase. We observed that the addition of W increased the fault energies significantly, which could correspond to the observed increase in yield strength of 244 alloy over that of 242 alloy.