Microstructural design of Ni-base alloys for high-temperature applications: impact of heat treatment on microstructure and mechanical properties after selective laser melting

Abstract

Optimising the microstructure and mechanical performance of Ni-base alloys is vital to achieve higher efficiencies in many high-temperature applications. With the recent emergence of the additive manufacturing technologies, new possibilities with respect to freedom of design and microstructure manipulation are opened up. This study aims at adjusting a microstructure suited for high-temperature applications by employing high-power selective laser melting. The parts are subjected to diverse post-process heat treatments to examine if the desired microstructural features can be preserved and proper precipitations can be formed. Results obtained by electron backscatter diffraction show a high resistance against recrystallization in the temperature range contemplated. Still, process-induced formation of undesired brittle Laves phase particles was observed via transmission electron microscopy and backscattered electron imaging. Mechanical tests in the form of hardness measurements, tensile tests, and high-temperature compression creep tests proved a high dependency of the performance on the post-processing treatment conducted. Hardness, yield strength and elongation at failure at room temperature are adequate in comparison with conventionally processed materials. Hightemperature compression creep tests emphasised the importance of solution annealing to enable for proper precipitation of strengthening phases during subsequent ageing.

Solution Treatment Study of Inconel 718 Produced by SLM Additive Technique in View of the Oxidation Resistance

“…This kind of as built microstructure with the presence of Laves phases and carbides at different locations is also reported in other studies . Brenne et al also identified Laves phases in the as built state of SLM Inconel 718 using EDS and TEM techniques …”

Effect of Compositional Variation Induced by EBM Processing on Deformation Behavior and Phase Stability of Austenitic Cr-Mn-Ni TRIP Steel

“…16), have been reported many times. This microstructure evolution has been reported for various alloys, e.g., Ni-based superalloy IN 718, [17][18][19][20][21][22] aluminum alloys, 23 the primary bcc b-phase in Ti-6Al-4V, 24,25 and several austenitic steels processed by L-PBF 26 and EBM. 27 Evolution is mainly attributed to the principal heat flux alongside BD and the favorable h100i growth direction in cubic crystal systems, due to an anisotropy in the solid-liquid interface energy.…”

Damage Tolerance Evaluation of E-PBF-Manufactured Inconel 718 Strut Geometries by Advanced Characterization Techniques

“…It is important to note that both the Laves phase and the hardening phases γ and γ" consume niobium for their formation. The dissolution temperature of the Laves phase is between 982 and 1093 • C [43], while γ and γ" precipitates form during the cooling phase at about 900 • C [44]. As already detailed above, it can be concluded that during the build, the temperatures within the strut deviated significantly from the bulk, eventually leading to the different appearance of the microstructure within the struts.…”