High performance materials used in the design of aeronautical and terrestrial turbines are subjected to increasingly high temperatures and mechanical loads to improve their energy efficiency and their environmental imprint. These increasingly demanding service conditions promote early surface damage of the material due to the concomitant action of deformation and oxidation (Figure 1). Despite very small oxidation-affected thicknesses (0.1 to 100 micrometers), the mechanical behavior of the material affected by oxidation is critical to the integrity of parts in service. Therefore, understanding and modeling how the material deforms at the microstructure scale at high temperature in these harsh environments is a major objective of the present project to clarify these thermo-mechanical-chemical interactions. This project, entitled “High Temperature-Small Scale Sub-Surface Deformation assisted by Oxidation”, aims at developing new advanced micromechanical characterization techniques at high temperature, such as high-resolution digital image correlation (Figure 2), instrumented indentation (Figure 3), synchrotron topotomography - a three-dimensional crystal characterization technique based on the combination of X-ray diffraction topography and computer microtomography - to access local mechanical properties. These data, in conjunction with various physico-chemical analyses, will be used to feed mechanical behavior models and numerical simulations necessary to predict the durability of these materials in service.
The scientific and technical objectives through HT-S4DefOx are detailed below:
• To assess microscale deformation-assisted oxidation events at the metal/oxide interface resulting from thermo-mechano-chemical coupling, i.e. tracking kinematic field of a time-evolving surface concealed by the oxide layer.
• To discriminate the effect of strain localisation on “sub-surface” microstructure affected by stresses AND surface reactivity.
• To identify the mechanical behaviour of “surface grains” versus “core grains” in the presence of a free-surface or a metal/oxide adherent interface.
• To develop oxidation models for metallic alloys having limited volume (« Reservoir » effect).
• To model and simulate time-evolving gradient of microstructure due to localised stresses AND surface reactivity and predict the mechanical properties within the gradient.
• To develop unique and complementary mesomechanical flexural testing with real-time 3D observation up to 1000°C. Such a development coupled with inverse identification methods will bridge the gap between micro- and macroscale mechanical characterisations.
