Abstract
Even though the phenomenon of abnormal grain coarsening (AGC) or “exploded grains” has been known to occur in Alloy 718 industrial forgings there is still no satisfactory explanation for it. For this reason, detailed microstructure analysis has been carried out in normal and abnormal regions. Electron Backscatter Diffraction (EBSD) was employed to determine grain size, boundary distribution and measure stored energy, while backscattered imagining in a FEGSEM was used to measure δ precipitate size and morphology. It was found that abnormal regions show almost 3 times as many twin boundaries compared to a normal region. In addition, the δ phase morphologies differ very significantly when comparing these two different regions. Normal regions display δ phase with a plate like nature, whereas in abnormal regions, δ particles appear to be more spherical. Furthermore, there are clear indications of differences in δ volume fractions between the two regions. Whilst in normal regions the δ phase is found predominantly at grain boundaries, in abnormal regions the δ is also found within grains. Both backscatter images and EBSD scans indicate that there are higher levels of stored energy within the normal regions, compared to the abnormal regions. These observations suggest that AGC occurs in regions where dynamic recrystallization does not happen and where recrystallization during solution heat treatment is affected by the local particle distribution.
Highlights
Due to its good weldability, corrosion resistance, high strength and good fatigue life, Alloy 718 has found uses in a variety of applications in Aerospace and oil and gas
This study showed through the use of Electron Backscatter Diffraction (EBSD) that the a Corresponding author: richard.watson-2@postgrad. manchester.ac.uk abnormal regions contained a higher proportion of twins than the normal regions
The present paper aims to characterise a more extreme form of undesired microstructure observed in an Alloy 718 trail forging
Summary
Due to its good weldability, corrosion resistance, high strength and good fatigue life, Alloy 718 has found uses in a variety of applications in Aerospace and oil and gas. A few papers do exist on the effect of abnormal grain growth (AGG) in Superalloys. Bozzolo et al showed that in low strain torsion tests Alloy 718 produced abnormally large grains that grew unhindered despite the presence of δ phase particles [5]. The large grains seen in this study had roughly twice the amount of twin boundaries than normalsized grains, appearing scattered throughout a low strained part of the microstructure. This phenomenon is not just limited to Alloy 718 but has been shown to occur in Udimet 720 derivatives [6]
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