Revealing the As-Cast and Homogenized Microstructures of Niobium-Bearing Nickel-Based Superalloy

The as-cast structure and its evolution during homogenization heat treatment for a typical nickel-based superalloy (Inconel 718) were studied based on different etching methods. Unambiguous revealing of the dendritic microstructure and the austenite/Laves eutectic constituent in the interdendritic areas was considered as the basis for the judgment for appropriateness of the etching techniques. Both chemical etching and electrolytic etching techniques were taken into account. The waterless Kalling's reagent number 2 was found to be a suitable etchant for revealing the macrostructure of the alloy. It was also concluded that electroetching by a H₂SO₄ solution and by a solution containing HCl, HNO₃ and glycerol are appropriate for studying the as-cast and homogenized microstructures based on optical microscopy and scanning electron microscopy (SEM), respectively. The dissolution of the Laves phase, the coarsening of the grains, and the development of annealing twins during exposure at elevated temperatures were also briefly discussed.