Microstructural variables that affect the fatigue crack initiation location in a nickel-base superalloy at 800°C

Fatigue tests under high cycle fatigue loading conditions were performed for Haynes 282® superalloy at 800°C. Fractographic examination of failed specimens indicated that, under the same stressing parameters, the longer life specimens formed fatigue cracks that initiated at a subsurface location, whereas the shorter life specimens formed cracks that initiated near the specimen surface. The microstructural source of variability in the fatigue crack initiation location will be described in this presentation. Larger grains or grain clusters with small misorientation between neighboring grains were identified to be crack initiation locations. It was concluded through electron backscattered diffraction characterization of the material microstructure that the probability of surface crack initiation could be predicated upon the probability of locating a critical microstructural neighborhood in the surface of the specimen. Implications of the analysis for the calculated variability of fatigue lives will be discussed by comparing the predictions with experimental fatigue lives. Research sponsored by the U.S. Department of Energy, Office of Fossil Energy, Crosscutting Research Program

Effect of H2O and CO2 on the Oxidation Behavior and Durability at High Temperature of ODS-FeCrAl

International audienceCyclic oxidation testing was conducted on alloy MA956 and two different batches of alloy PM2000 at 1,100 and 1,200 °C in different atmospheres rich in O2, H2O and CO2. Compared to 1 h cycles in dry O2, exposure in air + 10 vol.% H2O resulted in an increase of the oxidation rate and a decrease of the time to breakaway for all alloys at 1,200 °C, and a faster consumption of Al in the MA956 alloy. One hour cyclic testing in 49.25 % CO2 + 50 % H2O + 0.75 % O2 had a smaller effect on the oxidation rate but led to increased formation of voids in alloy MA956, which had an impact on the alloy creep resistance. At 1,100 °C, exposure in 50 % CO2 + 50 % H2O resulted in significant oxide spallation compared with oxidation in air, but this was not the case when 0.75 % O2 was added to the CO2/H2O mixture as a buffer. The control of impurity levels drastically improved the oxidation resistance of PM2000

Insights from Microstructure and Mechanical Property Comparisons of Three Pilgered Ferritic ODS Tubes

International audienceThree oxide dispersion strengthened alloys were fabricated into thin-walled (~500 µm wall thickness) tubes and characterized using x-ray, electron microscopy, and atom probe tomography methods. The three iron-based alloys included the 14%Cr alloy 14WYT, the 12%Cr alloy OFRAC, and a 10%Cr-6%Al alloy CrAZY. Each tube was subjected to a different thermal history during the pilgering process, which allowed for a detailed comparison between varying grain structures and alloy compositions. Atom probe tomography and energy-filtered transmission electron microscopy (TEM) comparisons showed good agreement in precipitate distributions, which matched predicted values using state-of-the-art nanoprecipitate coarsening models. The grain size, precipitate dispersion characteristics, and dislocation densities were then used to estimate yield strengths that were compared against room temperature axial and ring-pull tensile test data. For all three alloys, axial tensile specimens exhibited high tensile strength (>1 GPa) and reasonable plastic strains (10-17%). Ring tensile specimens, conversely, showed limited ductility (~1%) with similar strengths to those measured in the axial orientation. The strengthening models showed mixed agreement with experimentally measured values due to the highly anisotropic microstructures of all three ODS tubes. These results illustrate the need for future model optimization to accommodate non-isotropic microstructures associated with components processed using rolling/pilgering approaches