Inhomogeneous strain and recrystallisation of a shot-peened Inconel 625 alloy after annealing

ABSTRACT

Microstructural variations in a shot-peened nickel superalloy after isothermal annealing were observed via scanning electron microscopy and electron backscatter diffraction analysis. The analysis was based on inverse pole figure and kernel average misorientation maps. Before annealing, a heavily plastically deformed layer was observed under the surface. Post annealing for 1 h, the uppermost 30–40 µm layer was discontinuously recrystallised and inhomogeneous strains remained in the below layers. After a particular duration, the inhomogeneous strains recovered but did not recrystallise. The threshold depth of recrystallisation depended on the inhomogeneous strains. Recrystallisation and growth progressed from the recrystallisation-threshold depth toward the surface. These microstructural variations explain the release mechanism of the residual stress induced by shot peening.

KEYWORDS:

• Nickel alloy
• shot peening
• inhomogeneous strain
• EBSD
• thermal annealing
• recrystallisation

Acknowledgement

We thank Heiwa Technica Co., Ltd. for their support in the preparation of the specimens. We also thank Mr. T. Ito, Y. Okashita and K. Takatsu of Tokyo City University, for their support in the experiments. Furthermore, we are grateful to Mr. Y. Kubo and Y. Kondo of Fuji Manufacturing Co., Ltd. for their discussions that helped shape this article. This work was supported by Tokyo City University Nanotechnology Research Center for the analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Masayoshi Kumagai http://orcid.org/0000-0001-6073-8363