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J.L. Johnson, L.K. Tan, P. Suri, and R.M. German, “Mechanical Properties and Corrosion Resistance of MIM Ni-Based Superalloys,” Advances in Powder Metallurgy and Particulate Materials, R.A. Chernenkoff and W.B. James (eds.), MPIF, Princeton, NJ, 2004, pp. 4.89-4.101.

 

Abstract

    Superalloys based on nickel exhibit a combination of strength and resistance to surface degradation that make them useful for many applications. Metal injection molding (MIM) of HX, 718, and 625 is evaluated. Sintering conditions for these alloys are optimized to achieve maximum density and their microstructures are characterized. 718 and 625 achieve high sintered densities via supersolidus liquid phase sintering, but pore-grain boundary break-away occurs for HX before liquid forms so lower sintered densities are achieved. The mechanical properties of MIM 718 and 625 are compared to cast, wrought, and previously reported MIM properties. The corrosion resistance of these materials is tested in various media and compared to 316L stainless steel and commercially pure Ni. While these two materials perform well under oxidizing and reducing conditions respectively, the superalloys, especially 625, perform well in both types of corrosive environments.

 

Conclusions

    Achieving high densities with 718 and 625 requires sintering at or just above the solidus temperature. For the HX powders investigated in this study, sintered densities are limited by the lack of a liquid phase up to a temperature at which pore-grain boundary separation occurs. Powder chemistry is a key factor affecting liquid phase formation and the processing window for sintering to high density and optimal mechanical properties. MIM can produce alloys with a range of Ni contents to tailor corrosion properties for specific applications. MIM 316L has good corrosion resistance to oxidizing environments such as nitric acid and bleach, but MIM 270 is better suited to reducing environments such as sodium hydroxide and dilute solutions of sulfuric acid. MIM 718 is suitable for both oxidizing media such as nitric acid and reducing media such as sodium hydroxide, but performs poorly in hydrochloric acid and sulfuric acid where MIM HX performs better. MIM 625 has excellent resistance to all of these corrosive media.

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