Nimonic Etch - Alloy 720Li

Nimonic Etch - Alloy 720Li

Material Name: Alloy 720Li
Recipe No.: 12076
Primary Chemical Element in Material: Ni
Sample Type: Bulk
Uses: Microstructure
Etchant Name: Nimonic Etch
Type (Macro/Micro): Micro
Etching Method: Chemical
Etchant (Electrolyte) Composition: 40 ml H2O, 10 ml HNO3, 50 ml HCl, 2.5g CuCl2.
Procedure (Condition): The microstructure was assessed to characterise grain size and gamma'. distributions in the material. The mounted samples were ground with 600 and 1200 grit SiC paper and then polished with a 9 µm diamond suspension, followed by a 3 µm diamond suspension and finally a 0.25 µm colloidal silica polish. Etching was carried out on the polished samples with Nimonic Etch (40 ml H2O, 10 ml HNO3, 50 ml HCl, 2.5 g CuCl2). Typical etching times were 10 to 15 seconds.
Note: To overcome the poor contrast, a binary image of the grain structure and the primary gamma' was produced manually by carefully tracing over enlarged SEM micrographs. To produce the gamma grain size information, the primary gamma' were omitted and grain boundaries interpolated in those regions. For secondary gamma', the precipitates were outlined from SEM micrographs of the microstructure where the gamma' had been etched out. The binary images were then scanned and archived for further analysis. To measure grain and precipitate size and distribution, an automatic image analysis software, ImageJ, was used. The pictures were scaled and parameters such as perimeter, area and Feret diameters of the grain and primary and secondary gamma' binary images were measured. Incomplete objects at the edge of the binary image were removed using the image analysis software. The number of objects analysed for grain size, primary gamma' and secondary gamma' particle size were at least 100, 500 and 200 respectively.
Reference: C. Schoettle, P.A.S. Reed, M.J. Starink, I. Sinclair, D.J. Child, G.D. West, R.C. Thomson, SUSTAINED MACROSCOPIC DEFLECTED FATIGUE CRACK GROWTH IN NICKEL BASED SUPERALLOY 720LI, In book: Superalloys 2012, pp. 395-402.