Nanoscale Orientation Mapping in Deformed Ni-Based Superalloys

Deformation in superalloys initiates at the atomic level through dislocation motion, lattice rotation, and slip activation. Yet conventional TEM/STEM imaging is limited to morphology visualization based on the scattering contrast, offering no direct information on crystal orientation or rotation gradients.

To understand how materials respond to applied force, researchers need methods that link mechanical features like slip planes and indentations to precise crystallographic changes. This is especially relevant in γ/γ′ superalloys, where the interaction between phases influences strength, creep resistance, and failure mechanisms.

A single-crystal Ni-based superalloy with complex alloying elements was subjected to Vickers indentation along the [001] axis. FIB-prepared lamellae were extracted from beneath the indent for STEM and 4D-STEM analysis.

STEM imaging was performed using a 6.5 mrad convergence angle and a 100 pA probe current. For 4D-STEM orientation mapping, the convergence angle was reduced to 2 mrad with a 50 pA probe. A 14 mrad precession angle was applied, and data were collected at 1000 frames per second with 1 ms dwell time.

Two 4D-STEM scan regimes were used:

• Overview scan: 55 nm pixel size to survey the deformed region

• High-resolution scan: 6.5 nm pixel size to examine the zone of interest

Diffraction patterns were matched against kinematic templates of Ni₃Al (a = 3.56 Å, space group Pm-3m), using TENSOR’s automated orientation analysis workflow.

STEM images revealed prominent slip planes beneath the indentation, consistent with heavy plastic deformation. The γ and γ′ phases were clearly distinguishable in the lamella, appearing as channels and deformation cells respectively. These regions maintained a cube–cube orientation relationship before deformation.

The 4D-STEM overview map showed increased reorientation of the original cubic lattice beneath the nanoindentation trough. These areas with a high gradient of lattice reorientation correspond to dense slip plane intersections and were selected for high-resolution mapping.

In the detailed scan, a distinct zone displayed lattice rotation from [001] toward [103], with high indexing confidence across most of the region. Smaller subzones near the indent tip showed larger orientation shifts and slightly reduced pattern quality, indicating severe local deformation.

These findings confirm that kink bands and lattice rotations form within deformation cells bounded by slip planes and demonstrate the ability of TENSOR to resolve these features with the nanometer precision required for the elucidation of plastic deformation in metals and alloys.