Quantifying the impact of oxidation on the mechanical properties of Alloy 718 using local mechanical testing techniques

Despite excellent oxidative properties of the Alloy 718 Ni-based superalloy, long-term exposure to oxidative environments in service creates a chemical gradient in the sub-surface affected by oxidation. Its characterization is key to assessing the evolving mechanical behavior of such affected materials. The present study focuses on the γ'-γ” precipitation depletion induced by the chemical gradient and benchmarks micro-mechanical testing techniques to assess local mechanical properties. Local techniques such as nanoindentation and micro-pillar compression were used to measure both elastic and plastic properties of a pre-oxidized Alloy 718 Ni-based superalloy, having a chemical gradient. These results were compared to a global approach by tensile testing and high resolution-digital image correlation (HR-DIC) on model materials corresponding to regions of the chemical gradient: the solid-solution and the precipitation-hardened Alloy 718. The plastic behavior was investigated in terms of macroscopic yield strength and slip activity. Results obtained by the local and global techniques were found to be different but complementary. The relevance of the association of multiple micro-mechanical tests and sample preparation techniques to probe chemical gradients is discussed and technique advantages and drawbacks are exposed based on the single crystalline or polycrystalline nature of the micro-mechanical testing.