Microstructure and yield strength of SLM-fabricated CM247LC Ni-Superalloy

Specimens of Selectively Laser Melted (SLM) CM247LC powder have been characterised using analytical scanning and transmission electron microscopy. In specimens from the bulk, it has been found that longitudinal sections consist mainly of columnar γ grains, containing virtually identically oriented cells, approximately 700 nm in width and length up to hundreds of microns. These cells are separated from adjacent cells and from adjacent grains by γ′/γ eutectic, by high densities of Hf/Ti/Ta/W-rich precipitates and high densities of dislocations. The eutectic γ′ is up to about 50 nm in diameter but up to 10 nm within the cells. The microstructure in the top layer is similar to that taken from the bulk, but single-track samples are heterogeneous along the track length. The microstructures are interpreted in terms of the precipitation sequence, the volume fraction of eutectic and partitioning of the solute elements during solidification and the influence of subsequent laser-tracks. The cooling rate during solidification, calculated from the observed cell diameters is about 10⁶ K/s, but the value obtained from the size of γ′ within the bulk is about 10⁴ K/s. It is suggested that the discrepancy is due to the limited accuracy of this approach. Tensile tests on as-fabricated samples show that the yield strength is comparable with that of cast samples after standard heat treatments. The high strength of the as-fabricated samples is interpreted in terms the high densities of precipitates and dislocations in cell boundaries, the fine cell structure and the γ′ within cells.