Characterization of Microstructures Containing Abnormal Grain Growth Zones in Alloy 718

Abstract

A well known issue encountered by forgers transforming Alloy 718 consists in the appearance of coarse grain zones during the thermo-mechanical process. Although some industrial solutions have been implemented in order to get rid of this type of microstructural defect, the consequences of the presence of such coarse grained areas on the material's mechanical properties are of great interest.In this work, some zones that have undergone abnormal grain growth (AGG) have primarily been characterized from a microstructural point of view by means of both optical and EBSD observations. Special attention has been paid to grain orientation, grain boundaries and grain size distributions. In addition the effects of these microstructures on 350°C deformation controlled fatigue crack initiation mechanisms and the subsequent lifetime have been assessed as well as on 350°C tensile properties have also been evaluated.The results will be discussed with regard to microstructure/mechanical behavior relationships.

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.…”

“…Abnormally coarse grains are problematic in disc alloys as they can negatively impact the fatigue performance of components made from them. This was shown by Flageolet et al in an investigation into the effects of sporadic abnormally coarse grains on Low Cycle Fatigue (LCF) performance on Alloy 718 at 350°C [2]. When these microstructures occur the usual practice is to scrap the component and retool the dies to change the shape that the part is deformed into, leading to reduced yield and increased production times and costs.…”

Strain Induced Abnormal Grain Growth in Nickel Base Superalloys

“…Controlling grain size is necessary for nickel base superalloys, designed for high temperature applications. Under certain circumstances abnormal grain growth (AGG) occurs during the forming process [1], leading to bimodal grain size distributions that are detrimental for the in-use properties [1][2][3]. In the present paper, two examples of such a heterogeneous microstructural evolution are depicted and analyzed in order to identify the underlying mechanisms.…”

Characterization of abnormal grain coarsening in Alloy 718

“…Flageolet et al studied the effect of abnormally coarsened grains on the LCF (low cycle fatigue) and tensile properties of Alloy 718 [7]. The work showed little effect of the abnormal microstructure on the tensile properties at 350…”