Identification of interdendritic phases in Ni-30Cr weld metal with additions of tantalum and molybdenum using electron diffraction pattern and high-resolution scanning transmission electron microscopy image analysis

摘要

Ta-rich gamma/MC eutectic is the predominant interdendritic constituent that forms upon solidification in Ni-30Cr weld metal with Ta and Mo additions. In addition, a very small amount of sigma phase (sigma) forms on weld solidification. These terminal solidification constituents were investigated in detail by a combination of various methods in transmission electron microscopy (TEM): selected area electron diffraction (SAED) analysis and associated SAED pattern simulation, high-angle annular dark-field (HAADF) STEM imaging and associated image simulation, as well as energy-dispersive X-ray spectroscopy (STEM-EDS). Ta-rich gamma/MC and tetragonal sigma (sigma) phase were observed based on acquisition, analysis and simulation of six SAED patterns by specimen tilt into six defined zone axis orientations. The presence of sigma (o) phase was further validated and its atomic configuration was revealed by high-resolution HAADF-STEM imaging in combination with HAADF-STEM image simulation experiments.