Various Metallography Techniques for Microstructure Analysis of Ni-Based Superalloys

The Ni-based superalloys are known for the number of phases presented in alloy microstructure. Depending on alloying elements, phases such as austenitic FCC gamma matrix, L₁₂ gamma prime, DO₂₂ gamma double prime, BCT delta phase, and MC or M₂₃C₆ (or M₆C) carbides have occurred. With these microstructure components’ variety, the usual optical microscopy analysis is not satisfactory many times. For a better understanding of how single structures affect the lifetime of Ni-based superalloys components (turbine blades or turbine discs) and how they are changing during their operation load is SEM a very powerful tool. Two different types of superalloys were used for experimental evaluation. The first were cast superalloys ZhS6K and IN 738. Both cast superalloys in bar form were subjected to additional heat treatment at 800°C for 10 and 15 hours followed by air-cooling. The influence of gamma prime phase morphology changes on alloy lifetime and Vickers hardness were evaluated by SEM coherent testing grid methods. The second was wrought superalloy IN 718 and Nimonic 80A in bar form to present the various grain size evaluation techniques according to ASTM E112 standards. Various analyses were employed (SEM fractography, BSE observation, EDS mapping, etc.) for microstructure evaluation. The main goal is to show how effective light microscopy and SEM on Ni-based superalloy analysis are when microstructure change evaluation is needed.