Crystallographic analysis and observations of true twinning modes in the Ni2Mo superlattice phase contained in a Ni-25Mo-8Cr alloy

Deformation twinning in a two phase Ni25Mo8Cr (in wt%) alloy containing Ni2(Mo, Cr) domains/precipitates in a face centred cubic (f.c.c.) matrix has been studied by detailed crystallographic analysis, coupled with transmission electron microscopy and experimental and computer simulated electron diffraction. Correlation of crystallography with diffraction evidence indicates that all the twinning modes that lead to the formation of true twins (combined, types I and II) in the Ni2Mo superlattice are in operation. Long range order within the domains/precipitates of the ordered phase is preserved during deformation by the reorientation of the sheared regions into anti-parallel or perpendicular twin variants, and the rationale for this, from crystallographic considerations, is provided. Defect analysis, through weak-beam dark field imaging, revealed that the twins in the Ni2Mo domains form as a consequence of shearing by 16〈112〉 Shockley partial dislocations with the same Burgers vector on adjacent {111} planes and eventual overlapping of intrinsic stacking faults. Twin nuclei are composed of three-layer or nine-layer stacking faults, depending on the geometry of the twinning plane within the Ni2Mo domains. A new dislocation model, based on the dissociation of a single set of 12〈011〉 unit dislocations—in slips bands at the precipitate/matrix interface—to release the twinning Shockley partial dislocations, is able to satisfactorily account for twin formation within the Ni2Mo precipitates.