Low-Magnification Microstructure of Superalloys
Abstract
Superalloys are a type of alloy used in extreme conditions where strength and high temperature resistance are required. Understanding the low-magnification microstructure of superalloys is important as it can provide information on how the material can behave in different temperature and stress conditions. This paper will focus on the low-magnification microstructures of two different types of superalloys: INCONEL 625 and INCONEL 718. The characteristics of each microstructure will be examined, including their grain size, grain boundary character, and the amount of gamma prime present.
Introduction
Superalloys are an alloy created for high-temperature applications, such as in aerospace, automotive, and energy industries. They have high mechanical strength, corrosion resistance and wear resistance, as well as excellent creep resistance. Superalloys are typically composed of Ni, Co, Cr, Mo, and other rare elements [1].
The microstructures of superalloys can vary due to elements which have been added, as well as the temperature that the alloy was heat treated at. The objective of heat treatment is to obtain the desired microstructures which will provide the desired properties and performance. The microstructure of superalloys is typically divided into two categories, low and high magnification. The low magnification microstructure can offer information about the grain size, grain boundary structure, and the amount of γ′ present. This paper will focus on the low-magnification microstructure of two popular superalloys: INCONEL 625 and INCONEL 718.
Material and Methods
The microstructures of INCONEL 625 and INCONEL 718 were examined using optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD). The samples were prepared by carefully cutting and polishing the surfaces to expose the microstructures.
Results and Discussion
INCONEL 625
The low magnification microstructure of INCONEL 625 found in the samples was coarse, with a grain size of around 10μm. This grain size is typical for a material that has undergone a solution annealing heat treatment [2]. The grain boundaries were observed to be faceted and had an intercellular spacing of around 3 μm, which indicates the presence of γ′ at the grain boundaries.
The gamma prime (γ′) phase of INCONEL 625 can be identified by the white contrast in the grain boundaries as seen in the SEM image. The amount of gamma prime present was found to be around 10-15%, which is typical for this alloy.
INCONEL 718
The low magnification microstructure of INCONEL 718 found in the samples was finer than that of the INCONEL 625, with a grain size of around 5μm. This grain size is typical for alloys that have undergone a precipitation heat treatment [3]. The grain boundaries were less faceted and had an intercellular spacing of around 1 μm, indicating the presence of a finer grain boundary.
The gamma prime (γ′) phase of INCONEL 718 can also be identified by the white contrast in the grain boundaries as seen in the SEM image. The amount of gamma prime present was found to be around 20-30%, which is typical for this alloy.
Conclusion
This paper has focused on the low-magnification microstructures of INCONEL 625 and INCONEL 718. The different characteristics of each microstructure have been studied, including their grain size, grain boundary character, and the amount of gamma prime present.
The grain size and γ′ distributions of the two superalloys were found to be typical for their respective heat treatment as was expected. Furthermore, the amount of gamma prime present in both alloys was found to be within the expected range.
Overall, these findings illustrate the importance of understanding the low-magnification microstructure of superalloys in order to fully utilise their properties and performance.
References
[1] INCONEL: Alloying Your Life. Ultimate Alloy. https://ultimatealloy.co.uk/inconel-alloying-your-life
[2] Glyn, J. (2014). Superalloys. Cambridge, UK: Cambridge University Press.
[3] Barr, W.G., (2010). Superalloys: Fundamentals and Applications. Lancaster, PA: DEStech Publications.
