Unexpected δ-Phase Formation in Additive-Manufactured Ni-Based Superalloy
An as-built and solutionized Ni-based superalloy built by additive manufacturing through a direct metal laser sintering technique is characterized to understand the microstructural differences as compared to the as-wrought alloy. Initially, each layer undergoes rapid solidification as it is melted by the laser; however, as the part is built, the underlying layers experience a variety of heating and cooling cycles that produce significant microsegregation of niobium which allows for the formation of the deleterious δ-phase. The as-built microstructure was characterized through Vickers hardness, optical microscopy, scanning and transmission electron microscopy, electron back-scattering diffraction, x-ray diffraction, and synchrotron x-ray microLaue diffraction. The isothermal formation and growth of the δ-phase were characterized using synchrotron-based in situ small angle and wide angle x-ray scattering experiments. These experimental results are compared with multicomponent diffusion simulations that predict the phase fraction and composition. The high residual stresses and unexpected formation of the δ-phase will require further annealing treatments to be designed so as to remove these deficiencies and obtain an optimized microstructure.
The use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CG11357. Research performed in part at the NIST Center for Nanoscale Science and Technology. This material is based upon work supported by the Defense Advanced Research Projects Agency under Contract No. HROO 11-12-C-0037. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Defense Advanced Research Projects Agency.
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