Microstructure evolution, phase transformation and mechanical properties of IN738 superalloy fabricated by selective laser melting under different heat treatments

Selective laser melting (SLM) is a new method for manufacturing IN738 superalloy hot-end components. This study evaluated the microstructure evolution, phase transformation, and mechanical properties of IN738, fabricated using SLM before and after different heat treatments. The results showed that the microstructure of the as-SLMed sample was mainly columnar, with cell grains aligned in the building direction (BD) due to the high cooling rate. Many Laves phases and nanoscale Ti-, Nb-, and Ta-rich MC carbides precipitated in the sub-grain boundaries. The grains exhibited full recrystallization and serious coarsening at a solution temperature of 1260 °C. The residual Laves phases completely dissolved when the solution temperature rose above 1140 °C. After solution heat treatment (SHT), the primary MC carbides obviously grew up and were distributed as grains on the grain boundaries, and a large number of fine and homogeneous γ′ phases precipitated from the γ matrix. After aging heat treatment (AHT), hardness, ultimate tensile strength (UTS), and elongation (EL) decreased due to an increase in γ′ size and a reduction in uniformity. With increasing aging temperatures, this situation worsened. However, the yield strength (YS) increased under AHT at 750 °C, which was attributable to the formation of chain-like Cr-rich M23C6 and MC carbides on the grain boundaries. Finally, the overall performance of SLMed IN738 samples improved under solution treatment at 1200 °C for 2 h, followed by aging at 750 °C for 24 h, with increases of UTS and YS from 1205 MPa to 1,351, and from 850 MPa to 1319 MPa, respectively.