Stacking fault segregation imaging with analytical field ion microscopy

TL;DR

This study employs analytical field ion microscopy combined with density-functional theory to directly image and identify tungsten segregation at stacking faults in a Ni-W alloy, providing new insights into defect chemistry and its impact on alloy strength.

Contribution

It introduces a novel application of aFIM for atomic-scale imaging of stacking fault segregation, including quantitative analysis of trajectory aberration and atom identification.

Findings

W segregates at stacking faults in Ni-W alloy.

Trajectory aberration causes ~0.4 nm deviation, limiting resolution.

W prefers the center of stacking faults, affecting dislocation behavior.

Abstract

Stacking faults (SF) are important structural defects that play an essential role in the deformation of engineering alloys. However, direct observation of stacking faults at the atomic scale can be challenging. Here, we use the analytical field ion microscopy (aFIM), including density-functional theory informed contrast estimation, to image local elemental segregation at SFs in a creep-deformed solid solution single crystal alloy of Ni-2 at.% W. The segregated atoms are imaged brightly, and time-of-flight spectrometry allows for their identification as W. We also provide the first quantitative analysis of trajectory aberration, with a deviation of approximately 0.4 nm, explaining why atom probe tomography could not resolve these segregations. Atomistic simulations of substitutional W atoms at an edge dislocation in fcc Ni using an analytic bond-order potential indicate that the experimentally observed segregation is due to the energetic preference of W for the center of the stacking fault, contrasting with e.g., Re segregating to partial dislocations. Solute segregation to SF can hinder dislocation motion, increasing the strength of Ni-based superalloys. Yet direct substitution of Re by W envisaged to lower superalloys' costs, requires extra consideration in alloy design since these two solutes do not have comparable interactions with structural defects during deformation.