Session Sheet - Superalloys 2024: General Session 1: Alloy Design/Development I

Superalloys 2024: General Session 1: Alloy Design/Development I
Program Organizers: Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346

Monday 8:30 AM
September 9, 2024
Room: Exhibit Hall
Location: Seven Springs Mountain Resort

Session Chair: Katerina Christofidou, University of Sheffield; Paraskevas Kontis, Norwegian University of Science and Technology

8:30 AM
“Microstructure Informatics” of Polycrystalline Ni-base Superalloys Using Computer Vision Techniques to Understand Properties and Performance: Pascal Thome¹; Luis Arciniaga¹; Sammy Tin¹; ¹University of Arizona
    Recent advances in hardware technology as well as sophisticated methods for post-processing of Electron Backscatter Diffraction (EBSD) and Energy Dispersive X-Ray Spectroscopy (EDS) data have opened up new possibilities for detailed quantitative microstructure characterization of polycrystalline Ni-based superalloys. However, combining EBSD and EDS scans to reconstruct the true morphology of primary ă’ particles remains challenging, as some important microstructural features exist at a scale below the EDS method's lateral resolution limit, which leads to undesired artifacts at ă/ă’ interfaces. We present an automated computer vision architecture capable of resolving the meso-scale features of polycrystalline ă/ă’ microstructures with a level of detail that has not previously been demonstrated. Our methodology involves the following steps: 1. The combination of multiple elemental EDS maps. 2. Edge-preserving filtering of EDS maps using a non-local-means algorithm. 3. Unsupervised machine learning phase segmentation based on k-means clustering and 4. An automated artifact correction for the combination of EDS and EBSD information based on morphological conditions. In this manner, digital micrographs are reconstructed in a way that allows for quantitative determination of meaningful numeric metrics by utilizing methods from the field of algorithmic geometry. Various microstructural entities, such as discrete primary ă’ particles, mixed ă/ă’ grains, or ă grains can be characterized separately, including properties of related boundaries. Geometric characteristics can be quantified in terms of the local arrangement and cluster behavior of particle groups, as well as their spacings. The present work contributes to the development of digital workflows for precise and automatic microstructure characterization.

8:55 AM
Optimizing Local Phase Transformation in Ni-based Superalloys Utilizing Thermodynamically Driven Design Framework and Multiscale Characterization: Ashton Egan¹; Longsheng Feng¹; Timothy Smith²; Yunzhi Wang¹; Michael Mills¹; ¹The Ohio State University; ²NASA Glenn Research Center
    Superalloys are inherently complex alloys to design due to their multicomponent nature; designing alloys to take advantage of the newly discovered Local Phase Transformation (LPT) strengthening creates constraints on alloy composition beyond the conventional considerations for polycrystalline, precipitate-strengthened microstructures. The basis for design is precipitation of ÷/ç on superlattice stacking faults and microtwins while remaining thermodynamically inaccessible to form in bulk. This approach to LPT strengthening has now been demonstrated by optimizing ç-LPT in an empirically designed alloy, NA1, the performance of which is shown here by testing [001] oriented single crystals at several conditions. Computationally designed alloy, NA6, in polycrystalline form, was shown to perform similarly to single crystalline NA1 at 760 °C 552 MPa and outperform single crystal CMSX-4 as well as all LPT-strengthened polycrystalline alloys at this temperature. The deformation substructure of NA6 was investigated via HR-STEM, showing both ç-LPT at SESF as well as ÷-LPT at microtwins. Compositions of these LPT were elucidated using atomic resolution energy dispersive X-ray spectroscopy and compared to LPT compositions in relevant alloys and discussed considering recent studies on fault propagation velocities.

9:20 AM
Unambiguous Stacking Fault Analysis for Unraveling Shearing Mechanisms and Shear-based Transformations in the L1₂-ordered γ′ Phase: Nicolas Karpstein¹; Malte Lenz¹; Andreas Bezold¹; R Zehl²; M Wu¹; Guillaume Laplanche²; Steffen Neumeier¹; Erdmann Spiecker¹; ¹Friedrich-Alexander-Universität Erlangen-Nürnberg; ²Ruhr-Universitat Bochum
    With its precipitation strengthening effect, the L12-ordered ă′ phase contributes substantially to the mechanical properties of superalloys; therefore, understanding the microscopic mechanisms by which it can be sheared is of key importance. A commonly used method to study these mechanisms involves high-resolution imaging in the transmission electron microscope in projection which enables straightforward discrimination between intrinsic and extrinsic stacking faults as well as microtwins. However, the complex or superlattice nature of these stacking fault structures, which provides key information on their formation mechanism, is not necessarily revealed in this projection. In the present work, an experimental approach is presented to resolve this ambiguity and reliably determine the complex or superlattice nature of a stacking fault in the L12 structure by additionally imaging the fault in a nearby projection, which is achieved by tilting the specimen by 30°. The method is demonstrated using two different application examples in single-crystalline Co-base superalloys. In the first example, the approach enabled the direct experimental verification of two key aspects of the well-known Kolbe mechanism for superlattice extrinsic stacking fault formation, namely the complex nature of the leading intrinsic stacking fault segment and the occurrence of diffusion-mediated re-ordering. In the second example, microscopic details of the shear-based transformation from the cubic L12-ă′ to the hexagonal D019-÷ phase are revealed, accounting for additional complexities – again including a re-ordering process – arising from the superlattice ordering of both phases.

9:45 AM
Effects of Alloying Elements on Twinning in Ni-based Superalloys: Valery Borovikov¹; Mikhail Mendelev¹; Timothy Smith¹; John Lawson¹; ¹NASA
    Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 C. Recent experiments suggest that superlattice stacking faults in g phase may serve as the precursors to twin formation. Segregation of alloying elements to these precursors may have a significant effect on formation and extension of micro-twins. Using atomistic modeling we investigate and explain the effects of Nb and Cr alloying additions on these processes. The simulation shows that Nb increases the creep resistance which is mostly associated with impeding the reordering of the high energy double complex stacking fault. Cr, on the other hand, promotes twin growth, degrading the high temperature creep properties. These results can help to understand the effects of elemental composition of the alloy on creep resistance.