Influence of Post-Heat Treatments on the Microstructure, Mechanical Responses of IN738LC Superalloy Manufactured via Laser Powder Bed Fusion

초록

This study investigates the influence of post-heat treatment on the microstructure, mechanical properties, and strengthening mechanisms of the IN738LC fabricated via laser powder bed fusion (LPBF). The alloy is subjected to fully hardening (FH) and direct aging (DA) post-heat treatments, respectively. The supersaturated matrix of the as-built (AB) IN738LC has a fine cell and columnar structure that is aligned almost parallel to the building direction. After heat treatment, the microstructure of the FH alloy presents a partially recovered matrix. However, the DA alloy indicates a similar matrix structure to that of the AB material. Additionally, bimodal Ni-3(Al, Ti) precipitates with average sizes of 176.8 nm in the FH alloy and 112.2 nm in the DA alloy are formed. The ultimate tensile strength (UTS) and elongation at fracture (El.) of AB IN738LC are 1089 MPa and 17%, respectively. The UTS of the DA alloy increases to as much as 1295 MPa and the El. decreases drastically to 3.1%. Analytical calculations for the yield strength prediction were also performed using quantitively measured microstructural parameters. The dominant strengthening contributions for the AB IN738LC are suggested to be the solid solution and dislocation strengthening mechanisms. However, the FH and DA alloys are hardened by solid-solution and precipitate strengthening.

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