Nano-scale characterization of white layer in broached Inconel 718
Journal article, 2017

The formation mechanism of white layers during broaching and their mechanical properties are not well investigated and understood to date. In the present study, multiple advanced characterization techniques with nano-scale resolution, including transmission electron microscopy (TEM), transmission Kikuchi diffraction (TKD), atom probe tomography (APT) as well as nano-indentation, have been used to systematically examine the microstructural evolution and corresponding mechanical properties of a surface white layer formed when broaching the nickel-based superalloy Inconel 718. TEM observations showed that the broached white layer consists of nano-sized grains, mostly in the range of 20–50 nm. The crystallographic texture detected by TKD further revealed that the refined microstructure is primarily caused by strong shear deformation. Co-located Al-rich and Nb-rich fine clusters have been identified by APT, which are most likely to be γ′ and γ′′ clusters in a form of co-precipitates, where the clusters showed elongated and aligned appearance associated with the severe shearing history. The microstructural characteristics and crystallography of the broached white layer suggest that it was essentially formed by adiabatic shear localization in which the dominant metallurgical process is rotational dynamic recrystallization based on mechanically-driven subgrain rotations. The grain refinement within the white layer led to an increase of the surface nano-hardness by 14% and a reduction in elastic modulus by nearly 10% compared to that of the bulk material. This is primarily due to the greatly increased volume fraction of grain boundaries, when the grain size was reduced down to the nanoscale.

Transmission electron microscopy

Crystal microstructure

Grain refinement


Dynamic recrystallization

Shear bands

Grain size and shape

Mechanical properties

Grain boundaries

Cobalt compounds

High resolution transmission electron microscopy

Engineering controlled terms: Cobalt


Zhe Chen

Linköping University

Magnus Hörnqvist Colliander

Chalmers, Physics, Materials Microstructure

Gustav Sundell

Chalmers, Physics, Materials Microstructure

Ru Lin Peng

Linköping University

J. Zhou

Lund University

Sten Johansson

Linköping University

JJ Moverare

Linköping University

Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing

0921-5093 (ISSN)

Vol. 684 373-384

Subject Categories

Other Engineering and Technologies

Other Engineering and Technologies not elsewhere specified

Metallurgy and Metallic Materials


Chalmers Materials Analysis Laboratory

Areas of Advance

Materials Science



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