Anisotropy in microstructure and mechanical properties of additively manufactured Ni-based GH4099 alloy

Abstract

Additively manufactured Ni-based superalloys exhibit strong anisotropy due to microstructural differences resulting from their unique fabrication method. This study systematically investagiated the microstructure and mechanical properties of Ni-based GH4099 alloy fabricated by Selective Laser Melting (SLM) with subsequent heat treatment (HT). The causes of anisotropy in the deformation mechanism of alloys at room temperature and high temperatures are discussed. The results indicate that epitaxial grain growth occurs along the building direction, and after SLM, entangled dislocations gather at the cell boundaries. After HT, high-density dislocations are eliminated, while most grains remain in the as-deposited form. At room temperature, the deformation mechanism of the alloy remains consistent. Cracks generate and propagate inside grains, while grain boundaries provide a more substantial barrier for dislocations. Therefore, the difference in mechanical anisotropy depends on the difference in grain morphology in the build direction and perpendicular to the build direction. At high temperatures, fracture failure occurs at grain boundaries. The growth of carbides at high temperatures leads to the weakening of grain boundaries. The alloy exhibits different deformation mechanisms when loaded along the short and long axes of the grain. Only dislocations are activated when loaded along the short axis of the grain. Stacking faults and deformation twins provide higher plastic-deformation ability when loaded along the long axis of the grains. Moreover, dislocations also activate, however the dislocation density is lower compared to loaded along the short axis.