Periodic Reporting for period 3 - HT-S4DefOx (High Temperature – Small-Scale Sub-Surface Deformation assisted by Oxidation)
Reporting period: 2024-01-01 to 2025-06-30
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.
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.
We have developed different experimental techniques to sound how material deforms and behaves at the microstructure scale at various temperature from room to high temperature using either high-resolution digital image correlation (HR-DIC) techniques at high temperature, high speed nanoindentation mapping, stress-drop analyses from either nanoindentation and micro-pillar compression. High-resolution digital image correlation techniques were used to assess intra and intergranular strain localization depending on the loading conditions and the temperature. We have conducted large nanoindentation mapping to image the mechanical properties of titanium- and nickel-based superalloys at the microstructure scale and more particularly at the oxidation-affected material scale (Figure 2). Additional chemical and crystal orientation analyses aimed to conducted multi-modal correlative analyses to dissociate the contribution of the grain orientation from the local chemical composition on the local mechanical response. Numerical models and simulation were developped in order to better assess the mechanical properties as a function of the crystal orientation and chemical composition using inverse methods. We developed micromechanical testing on ultrathin specimens with various thicknesses to identify the mechanical behavior of surface grains, i.e. grains directly affected due to high temperature oxidation, from the one of core grains using inverse methods combined with crystal plasticity models. These micromechanical characterizations also aimed to quantify surface effects in the presence of a free surface leading to surface softening due to dislocation escape. We also developed the microthermoreflectometry techniques to identify very local change in oxidation products leading to premature cracking from fast growing oxides at the surface samples due to the combined contribution of the mechanical loading and the oxidation. This latter technique is of high importance since optical techniques are spatially resolved to detect onset of events that conventional thermobalances can not capture for the investigation of intrinsic chemical failure but also mechanically induced chemical failure.
Progress beyond the existing state of the art was achieved on several aspects:
- to “image” the mechanical properties and local mechanical behavior of materials at the microstructure scale, pushing to the limits nano- and micro-mechanical testing up to high temperature (both nanoindentation techniques and HR-DIC techniques);
- to decorrelate the contribution of the orientation and chemical composition on the local mechanical response of the materials within the oxidation-affected region;
- to identify the tensile mechanical behavior of surface grain using inverse methods combined with crystal plasticity models;
- to develop oxidation models for metallic alloys having limited volume: the reservoir effect;
- to screen very local oxidation events in short oxidation time using microthermoreflectometry;
- to develop unique and complementary mesomechanical flexural testing with real-time 3D observation up to 800°C. Such a development coupled with inverse identification methods will bridge the gap between micro- and macroscale mechanical characterizations.
The evaluation and simulation of the micromechanical behavior of time-evolving graded materials at high temperature is the final and target point of this pluridisciplinary project.
- to “image” the mechanical properties and local mechanical behavior of materials at the microstructure scale, pushing to the limits nano- and micro-mechanical testing up to high temperature (both nanoindentation techniques and HR-DIC techniques);
- to decorrelate the contribution of the orientation and chemical composition on the local mechanical response of the materials within the oxidation-affected region;
- to identify the tensile mechanical behavior of surface grain using inverse methods combined with crystal plasticity models;
- to develop oxidation models for metallic alloys having limited volume: the reservoir effect;
- to screen very local oxidation events in short oxidation time using microthermoreflectometry;
- to develop unique and complementary mesomechanical flexural testing with real-time 3D observation up to 800°C. Such a development coupled with inverse identification methods will bridge the gap between micro- and macroscale mechanical characterizations.
The evaluation and simulation of the micromechanical behavior of time-evolving graded materials at high temperature is the final and target point of this pluridisciplinary project.