Structure-Property Correlation in Additively Manufactured High-Temperature Materials: Insights from Nickel-Based Superalloy IN718 & Nickel-Based Eutectic High Entropy Alloy AlCoCrFeNi2.1

Abstract

High-temperature alloys are technically very important materials that possess higher resistance toward mechanical and chemical degradation at 0.4-0.6 TM for a constant/cyclic load. These attributes make them candidate materials for aerospace and energy generation systems that require harsh operating conditions. The most popularly used high-temperature alloys are the Nickel-base superalloys, and among various Ni-base alloys, Inconel 718 (IN 718) is the most used material. The currently used superalloys are already exploited to very high operating temperatures near their solidus or the precipitate solvus temperatures. Any further increase in Carnot efficiency is only possible through design improvements and enhanced turbine cooling efficiency, which the Additive Manufacturing (AM) technique can address. However, the adoption of AM processes for aerospace/mission-critical components is impeded due to microstructural heterogeneities and spontaneous defects observed in AM-produced parts. It is, therefore, necessary to develop processes that head to minimum porosity and design suitable heat treatments, and to evaluate the relevant mechanical properties post-heat treatment. A further approach could be to develop some new alloys that could be additively manufactured and exhibit the desirable properties. The work carried out in the present thesis is aimed at addressing both these issues. In the first part of the thesis, attempts have been made to develop suitable heat treatments that would lead to desirable microstructures for tailoring relevant optimal mechanical properties that could render the material for the desired applications. Both the most commonly used routes of powder bed additive manufacturing have been used, namely the Electron Beam Melting (EBM) and the Selective Laser Melting (SLM). Heat treatment schedules have been designed for both the so-obtained materials, and the microstructure, texture, and mechanical behavior of the as-built and heat-treated AM materials have been investigated. In the case of IN718, the specially designed heat treatment was aimed at optimizing the volume fraction of coherent and incoherent precipitates, and their role in the mechanical response has been explored in greater depth. In the second part, the present work will be substantiated with another class of high-temperature material, Ni-base eutectic high entropy alloy (EHEA) AlCoCrFeNi2.1 has been produced by additive manufacturing which has shown better mechanical response than the conventional Ni-Al system. A comprehensive understanding of room and high temperature mechanical properties has been developed.