Etching methods for revealing nanoscale precipitates and carbides in Ni-based superalloys

Gamma prime (γ') precipitates and grain boundary (GB) carbides govern the high-temperature performance of Ni-based superalloys, and their reliable quantification is essential for microstructural evaluation and alloy development. However, conventional etching procedures are often transferred between alloys without considering composition-dependent changes in γ' size and fraction or carbide population, which can cause unstable contrast, γ-matrix damage, and unreliable image-based interpretation. Here, we establish composition-tailored etching conditions for Haynes^® 282 (γ' ~23 nm) and two model alloys with modified Al-Ti and Nb-Ta contents, and evaluate their suitability for phase-selective Scanning electron microscopy (SEM) imaging. After identical mechanical preparation, γ' precipitates in Haynes^® 282 and Model alloy 1 are clearly revealed using a nitric-acid etchant, whereas the same condition fails in Model alloy 2 with reduced Al and Ti, where much finer γ' precipitates form. An HF-containing mixed-acid etchant is introduced to obtain stable γ' contrast in Model alloy 2 without excessive surface relief. GB carbides also show composition-dependent responses. In Haynes^® 282, Cr-rich M₂₃C₆-decorated boundaries are revealed by nitric acid, whereas Nb/Ta-containing model alloys require chloride-based etchants to expose both MC and M₂₃C₆ carbides. These protocols provide reproducible, phase-selective SEM contrast for robust image-based quantification.