Influence of Etchants on Quantitative/Qualitative Evaluations of the γ' Precipitates in a Nickel-Base Superalloy

To evaluate the microstructural characteristics of nickel-base superalloys, the size, number density, volume fraction, and morphology of the gamma' precipitates are frequently analyzed. For accurate microstructural analysis, a proper etching treatment should be applied evenly over the sample. Lack of care in choosing either the etchant or the etching technique can result in significant errors, not only in qualitative observations but also in quantitative analysis of the gamma' precipitates. This research examined the effects of etching technique as well as etchant type on the microstructural characterization of the gamma' precipitates in IN738LC nickel-base superalloy. The heat-treated samples with the same morphology of the gamma' precipitates were independently etched either in matrix phase (c) or in gamma' precipitate etchants. After quantitative analysis by scanning electron microscopy, the results were compared with electrolytically extracted gamma' precipitates for the same samples. The results show clearly that for qualitative and morphological observations of the gamma' precipitates, the matrix phase c etchants are desirable, while for quantitative analysis, the gamma' etchants can provide appropriate results.