A 3D Analysis of Dendritic Solidification and Mosaicity in Ni-Based Single Crystal Superalloys

Abstract

Ni-based single crystal superalloys contain microstructural regions that are separated by low-angle grain boundaries. This gives rise to the phenomenon of mosaicity. In the literature, this type of defect has been associated with the deformation of dendrites during Bridgman solidification. The present study introduces a novel serial sectioning method that allows to rationalize mosaicity on the basis of spatial dendrite growth. Optical wide-field micrographs were taken from a series of cross sections and evaluated using quantitative image analysis. This allowed to explore the growth directions of close to 2500 dendrites in a large specimen volume of approximately 450 mm3. The application of tomography in combination with the rotation vector base-line electron back-scatter diffraction method allowed to analyze how small angular differences evolve in the early stages of solidification. It was found that the microstructure consists of dendrites with individual growth directions that deviate up to ≈4° from the average growth direction of all dendrites. Generally, individual dendrite growth directions coincide with crystallographic <001> directions. The quantitative evaluation of the rich data sets obtained with the present method aims at contributing to a better understanding of elementary processes that govern competitive dendrite growth and crystal mosaicity.

Evolution of local misorientations in the γ/γ’‐microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method

“…In the present work, three new additional EBSD results were obtained. First, the misorientations at the γ/γ 0 -interface associated with the formation of interface dislocation networks (arrow 2 in respectively) are significantly smaller than those reported for dendritic crystal mosaicity (≈ 5 (Hallensleben et al, 2017;Hallensleben et al, 2019;Scholz et al, 2021;Thome et al, 2019)). At the same time, they are significantly larger than the misorientations measured across γ/γ 0 -phase boundaries (0.25 , present work).…”

“…In recent years advanced imaging technology and indexing algorithms have helped to increase the angular resolution of the EBSD method by more than 1 order of magnitude (from about 1° to below 0.1°) (Chen et al, 2015; Nolze et al, 2016; Thome et al, 2019; Wilkinson et al, 2006). This allows to explore microstructural phenomena which are associated with small processing or deformation induced lattice rotations in single crystalline materials, for example, single crystal mosaicity and crystal plasticity, for example (Pantleon, 2008; Scholz et al, 2021). In the present work we apply the rotation vector baseline EBSD (RVB‐EBSD) technique to study small misorientations in Ni‐base superalloy single crystals, with microstructures which feature small ordered γ′‐cubes which have coherently precipitated in a FCC γ‐matrix.…”

“…The RVB‐EBSD method (Thome et al, 2019) was developed to study crystal mosaicity associated with the growth of dendrites during solidification (Hallensleben et al, 2017; Hallensleben et al, 2019; Scholz et al, 2021). It was found that the nature of dendritic solidification resulted in orientation spreads of up the order of 5° (Scholz et al, 2021; Thome et al, 2019). In the present work, three new additional EBSD results were obtained.…”

“…Only in the final stages of creep, where necking occurred during final rupture, large lattice rotations (> 20 ) were reported (Ardakani et al, 2000;Basoalto et al, 2002;Cao et al, 2020). The RVB-EBSD method (Thome et al, 2019) was developed to study crystal mosaicity associated with the growth of dendrites during solidification (Hallensleben et al, 2017;Hallensleben et al, 2019;Scholz et al, 2021). It was found that the nature of dendritic solidification resulted in orientation spreads of up the order of 5 (Scholz et al, 2021;Thome et al, 2019).…”

Correlation between the Dendritic Structure and Lattice Parameter of γ′-Phase in Single-Crystalline Turbine Blades Made of Superalloys

“…However, some methods give information about the changes of the nanoscale structure for the points of the macroscopic areas. These include, for example, the positron annihilation spectroscopy (PAS) method [20,32] or the Mössbauer spectroscopy (MS) method [33], as well as X-ray diffraction methods [20,[34][35][36][37]. In the PAS and MS methods, data for each measuring point are collected from a sample area of several tens of mm 2 , while the X-ray diffraction methods allow for collecting data from the area above one mm 2 .…”