The influence of post heat treatments on the evolution of microstructure and mechanical properties of EP741NP nickel alloy produced by laser powder bed fusion

Abstract

EP741NP superalloy was fabricated by laser powder bed fusion (LPBF) and subsequently subjected to different heat treatments including hot isostatic pressing (HIP), solution + aging, HIP + solution + aging. The structural features of the alloy in the as-built condition as well as after heat treatment were studied and compared to its mechanical characteristics such as hardness, tensile strength, impact strength, and compression strength at elevated temperatures. It was found that LPBF samples have a fine-grained columnar-cellular microstructure formed from colonies of primary dendrites and poor mechanical properties caused by structural defects (pores and isolated microcracks). The Laves phases of different compositions (Cr2Nb, Co2Nb and Cr2Hf) were detected in the interdendritic space. HIP contributes to elimination of microcracks, precipitation of the γ′ phase (0.5–7 μm in size), dissolution of the Laves phases, and formation of MeC carbide phases. A combination of these events increases strength and ductility of the alloy. Solution followed by aging without HIP results in formation of the fine-grained γ′ phase with size ranging from 150 nm to 2 μm, elimination of the Laves phases, and precipitation of MeC carbides (Cr23C6), which enhances strength. Ductility, however, remains low as the alloy structure still contains microcracks. The LPBF samples subjected to combined post-treatment (HIP + solution + aging) exhibit the maximum mechanical properties due to the elimination of structural defects during HIP and formation of the fine-grained γ/γ’ microstructure during solution and aging.