Alloy 718 - Electrolytic etching

Alloy 718 - Electrolytic etching

Material Name: Alloy 718
Recipe No.: 12074
Primary Chemical Element in Material: Ni
Sample Type: Bulk
Uses: Microstructure, gamma prime particles
Etchant Name: No data
Type (Macro/Micro): Micro
Etching Method: Electrolytic etching
Etchant (Electrolyte) Composition: 170 cc H3PO4, 10 cc H2SO4, and 15 grams of CrO3
Procedure (Condition): The electropolished sample is etched at 5 volts for 5-10 seconds in a solution of 170 cc H3PO4, 10 cc H2SO4, and 15 grams of CrO3, and rinsed as in the electropolishing step. The sigma, gamma”, and gamma’ phases should be in relief while alpha-Cr and sigma phases are etched out.
Note: If the sample is in a solution conditioned where the sigma phase is absent, the grain boundaries will not be delineated by the electro-etch. In such cases, to gain contrast at the grain boundaries, the electropolished sample can be immersion etched in a solution of 50 cc H2O, 25 cc HCl, 5 cc HF and 2 cc H2O2.
Because the electropolish and etch reveals all the structures produced by different thermal treatments, high contrast can be obtained in the SEM due to the high yield of secondary electron from the precipitation morphologies. In addition, those phases which have low atomic number elements will produce fewer secondaries compared to precipitates composed of high atomic number elements; thus, different phases or areas will show up due to differences of secondaries.
Reference: John F. Radavich, Electron Metallography of Alloy 718, Superalloys 718, 625, 706 and Various Derivatives, Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997, pp. 17-26.