Characterization of abnormal grain coarsening in Alloy 718

Abstract

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.

The Effect of Strain and Temperature Profiles on Static Recrystallization during Solution Heat Treatment After Hot Deformation of Alloy 718

“…The abnormal microstructures appear to contain a high proportion of annealing twin boundaries [1], similar to those found in strain annealed steels [2][3][4]. However there are some fundamental differences between the conditions under which these microstructures are formed.…”

“…During a trial forging of an Alloy 718 component Otto Fuchs AG observed an abnormally coarse microstructure that was not predicted by current recrystallization models [1]. These abnormal grains appeared after the solution heat treatment (SHT).…”

“…In contrast, Alloy 718 contains second phase particles when solution heat treated below the 5 solvus that should prevent grain coarsening. Some characterisation of the phenomenon in Alloy 718 showed that abnormally coarse regions contained 66% twin boundaries compared with 24% for the normal regions, and that the abnormally coarse region is characterized by low levels of stored energy [1].…”

Emergence and Progression of Abnormal Grain Growth in Minimally Strained Nickel-200

“…The exact mechanisms underpinning the occurrence of AGG are still unclear and remain a subject of much interest and research. Though debate on specific mechanisms exists, most authors agree that the formation of AGG is related to low-energy, high-mobility grain boundaries such as ∑3 and its variants (e.g., ∑9 and ∑27) [8][9][10][11][12][13][14][15][16]. This is of note as Grain Boundary Engineering (GBE) is a specific type of thermomechanical process used to improve material properties by altering their grain boundary network and often resulting in increased frequency of ∑3 boundaries [17,18].…”

An Examination of Abnormal Grain Growth in Low Strain Nickel-200

“…Gabb et al reported a similar reduction in fatigue life for a low solvus high refractory (LSHR) nickel-based superalloy when abnormally large grains were present within the microstructure [5]. While there still exists much debate regarding the specific mechanisms for the onset of AGG, most authors agree that the formation of abnormal grains is commonly related to the selective migration of low-energy or high mobility grain boundaries [6][7][8][9][10].…”