Understanding stress-oxidation interaction on grain boundary failure: in situ microscale crack propagation experiments

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Oxidation-induced failure and stress corrosion cracking (SCC) of engineering materials has been a long-standing issue in material science and engineering application, for instance, energy conversion at elevated temperatures. This project aims to understand the interplay of thermal, mechanical, and environmental factors on the grain boundaries (GBs) in a NiFe binary alloy (Ni-40at.%Fe alloy, fcc, gamma-phase).To achieve this goal, I plan to deform representative GBs in bi-crystal NiFe alloy via micro-cantilever bending and fracture experiments. Various cantilever sizes (size effect), loading rates (diffusion time scale), loading conditions (stress effect), and environments (vacuum and oxidation for comparison) will be considered to investigate the stress-oxidation interaction and the microstructure evolution:1) I study the stress effect on the oxide growth rate by producing micro-cantilevers by quantifying the oxide intrusion length along the GBs with and without mechanical loading in different environments at fixed temperature. I compare the effects of tension and compression within the micro-cantilevers on diffusion and oxide formation, and thus on the micro-cantilever failure.2) I study the diffusion and oxide formation at the evolving crack tip along the GBs by using multiple loading rates and different sample sizes during in situ micromechanical fracture experiments at fixed temperature. New method will further the understanding of crack propagation with oxide formation. 3) I quantify the GB fracture toughness via fracture tests of micro-cantilevers. This fracture toughness is studied in function of the oxide intrusion length that relates to geometry, loading rates, stress states and GB type. Then I determine the most robust GB type with the highest resistance to oxidation and fracture.4) By examining the fractured sample using high resolution scanning electron microscopy and transmission electron microscopy, I can evaluate the mechanisms which govern the oxide-induced fracture: i) oxide induced dynamic embrittlement; ii) fracture induced by stress-aided grain boundary oxidation.Finite element modelling will be used to calculate the oxide intrusion length, stress states, and geometry effect. Experimental results of the fracture toughness are compared to the modeling to validate the simulations and quantify the influence of the oxide. The developed models can be used to improve the material design of oxidation resistant metals.