Extraction of Non-metallic Inclusions in Nickel-Based Superalloy Powders by Stepwise Acid Dissolution Method

A stepwise acid dissolution method to extract non-metallic inclusions accurately in nickel-based superalloy powders for quantitative characterization was proposed in this work. The superalloy powder was firstly dissolved by the mixed concentrated acid with the optimal ratio of HCl: HNO3=6:1, and the acid soluble products and insoluble metal elements were then completely dissolved using the H2O2-oxalic acid system and dilute acid. The quantity, morphology, composition, and size distribution of the acquired non-metallic inclusions were analyzed by SEM, EDS and other characterization methods. The results indicate that this extraction method has the advantages of simplicity, short time, high separation efficiency, high recovery rate, and less influence on the inclusions. It is suitable for extracting a wide range of inclusion particle sizes. In addition, the extraction effects of the stepwise acid dissolution method and the water elutriation method were compared. It is found that both methods have similar extraction effects for the inclusions with a particle size greater than 50 mu m. However, the stepwise acid dissolution method exhibits significantly improved extraction effects for the inclusions less than 50 mu m.