Strengthening mechanism of a Ni-based superalloy prepared by laser powder bed fusion: The role of cellular structure

The unique cellular structure has been reported to be the dominant reason for the remarkable mechanical properties in laser powder-bed fusion (LPBF) superalloys. However, the specific strengthening mechanism of the cellular structure needs to be further researched. In this study, the microstructure and tensile properties of a composition-optimized Haynes 230 alloy fabricated by the LPBF method were systematically investigated. The post-heated HT1 and HT2 samples with different cellular and grain structures served as the reference. The results show that the micron-scale grain and submicron-scale cellular structures are present in the as-built (AB) sample. The cellular boundaries are characterized by solute Nb segregation and high-density dislocations. The tensile tests show that the AB sample illustrates excellent tensile properties with ultimate tensile strength (UTS) of 1180.2 MPa, yield strength (YS) of 842.5 MPa and elongation (EL) of 29.3 %. The high YS mainly results from the strengthening effects of the cellular structure, which can be considered as a combination of dislocation and elemental segregation strengthening. The contribution of cellular structure to YS is much higher than that of GB strengthening. Furthermore, the deformation incompatibility of heterogeneous grain and cellular structures leads to high hetero-deformation induced (HDI) strengthening, which is also the primary reason for the abnormal upturn of strain hardening rate (SHR) in the AB sample.