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

## TL;DR

This paper introduces tailored etching methods to reliably reveal nanoscale precipitates and carbides in Ni-based superalloys for accurate microstructural analysis.

## Contribution

The study presents composition-specific etching protocols for different Ni-based superalloys to achieve stable and phase-selective SEM imaging.

## Key findings

- Nitric-acid etchant effectively reveals γ′ precipitates in Haynes® 282 and Model alloy 1 but not in Model alloy 2.
- An HF-containing etchant provides stable γ′ contrast in Model alloy 2 with finer precipitates.
- Chloride-based etchants are necessary to reveal carbides in Nb/Ta-containing model alloys.

## Abstract

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.

The online version contains supplementary material available at 10.1186/s42649-026-00125-x.