Scanning electron microscopy and atom probe tomography characterization of laser powder bed fusion precipitation strengthening nickel-based superalloy

Atom probe tomography (APT) was utilized to supplement scanning electron microscopy (SEM) characterization of a precipitation strengthening nickel-based superalloy, Alloy 247LC, processed by laser powder bed fusion (L-PBF). It was observed that the material in the as-built condition had a relatively high strength. Using both SEM and APT, it was concluded that the high strength was not attributed to the typical precipitation strengthening effect of gamma'. In the absence of gamma' it could be reasonably inferred that the numerous black dots observed in the cells/grains with SEM were dislocations and as such should be contributing significantly to the strengthening. Thus, the current investigation demonstrated that relatively high strengthening can be attained in L-PBF even in the absence of precipitated gamma'. Even though gamma' was not precipitated, the APT analysis displayed a nanometer scale partitioning of Cr that could be contributing to the strengthening. After heat-treatment, gamma' was precipitated and it demonstrated the expected high strengthening behavior. Al, Ta and Ti partitioned to gamma'. The strong partitioning of Ta in gamma' is indicative that the element, together with Al and Ti, was contributing to the strain-age cracking occurring during heat-treatment. Cr, Mo and Co partitioned to the matrix gamma phase. Hf, Ta, Ti and W were found in the carbides corroborating previous reports that they are MC.