High-throughput assessment of local mechanical properties of a selective-laser-melted non-weldable Ni-based superalloy by spherical nanoindentation

Abstract

Additive manufacturing (AM) enables complex localized microstructure tailoring by layer-wise printing, which potentially offers exceptional mechanical performance. However, conventional mechanical testing methods are not optimal for interrogating the corresponding heterogeneous microstructure, limiting the in-depth understanding and exploitation of the structure-property linkages. Here, we leverage a spherical nanoindentation method to efficiently measure indentation stress-strain (ISS) curves at the microscale. This was demonstrated on a selective-laser-melted Ni-based superalloy (IN738LC) which has high economic value but suffers from hot-cracking. The measured average elastic modulus (193 ± 9 GPa) agrees with the literature data. The ISS curve is comparable with micro-pillar compression results with a similar size (stressed volume) and crystallographic orientation; however, it is higher than the bulk tensile test results because of the size effect. Furthermore, combined with surface morphology, elemental distribution, and crystallographic orientation analyses, abundant microstructure-mechanical property linkages were explored. The indentation yield stress weakly depends on the indentation direction but is sensitive to the local precipitation distribution. Grain boundaries (GBs) mediate the strain-hardening behavior when the indents are close to the GBs. The ISS curve is accessible even when the indent contains microcracks because the hydrostatic stress underneath the indenter suppresses crack propagation. Overall, the demonstrated spherical nanoindentation method is suitable for the high-throughput assessment of local mechanical properties of additively manufactured materials.