Mechanical properties of white layers formed by different machining processes on nickel-based superalloy

Nickel-based superalloys are widely used in the aerospace industry in the production of turbine discs and blades because of their good mechanical properties and great corrosion resistance at high temperature.

Although very useful, these alloys are hard to machine. Their structure is responsible for rapid wear of cutting tools. Moreover, under certain machining conditions, near-surface regions of the material undergo a phase transformation resulting in the formation of a thin layer called "white etching layer" at the surface of the machined workpiece. Because turbine discs are safety critical components, no defects can be tolerated on the workpiece. Therefore, efforts should be made to ensure that this white etching layer can't influence the operating life of the workpiece and make its operation unsafe.

Even if the existence of the white etching layer is well known, its mechanical properties have never been assessed in detail. In this thesis, we present a study of the mechanical (hardness and Young's modulus) and microstructural properties of white etching layers formed at the surface of nickel-based superalloy IN100 turbine discs fabricated by different machining processes. This work aims at evaluating the impact of the machining process and of fatigue on the properties of the white etching layers under study.

The originality of this study primarily lies in the employed characterization technique. Using nanoindentation has allowed us to very precisely assess the variations of both the hardness and the Young's modulus along the white etching layers. Also, the use of a sophisticated indentation system has enabled the acquisition of very precise surface images of the samples and therefore to study the microstructure of the white etching layers.

This research has demonstrated that the mechanical and microstructural properties of the white etching layers are closely linked to the machining conditions of the material. Therefore, our study will help researchers gain a better understanding of white etching layer formation mechanisms on IN100 nickel-based superalloy and find ways to prevent their formation: a point of crucial importance for the aerospace industry.