Materials Engineering Department
BS in Materials Engineering
Forged components must pass a grain size specification (grain size, distribution) for acceptance in an application. The varying amounts of plastic deformation during forging can lead to abnormally large recrystallized grain sizes in certain regions of the part, which will not pass specification. The question exists whether these abnormally coarse grains are truly comprised of poly crystalline fine grains with grain boundaries resistant to etching techniques. To investigate this abnormal grain size effect, a cross section of a forged René 41 nickel-based superalloy aircraft engine ring was cut and sectioned into six segments. Those segments were then prepared for microstructural analysis using a 95% HCl and 5% H2O2 etch typical of metallographic testing companies. The results indicated an apparent segregation of the grains with multiple regions showing a few abnormally large grains surrounded by much smaller ones. A fine grain size dominated some samples (ASTM grain size 5 to 7), these would pass specification. Other regions showed intermittent large grains (ASTM grain size 1 to 2). The presence of annealing twins within the large grains proved that these larger grains were single grains, but their true size is masked by discontinuous boundaries throughout. The abnormally large grains followed some flow pattern suggesting their manifestation may be a function of how much strain was applied to that section of the alloy during forging. Scanning electron microscopy (SEM) in conjunction with electron backscatter diffraction (EBSD), determined the orientation and size of each grain on a map and identified grains independently of etching grain boundaries. As a result, the discontinuous grain boundaries in question were determined to be low angle grain boundaries that were resistant to chemical attack by the etchant.