Transmission Electron Microscopy Sample Preparation of INCONEL 738 Nickel-Base Superalloy

Abstract

Size, shape, volume fraction and distribution of embedded g^/ phase in g phase has direct effect on strength of INCONEL alloy. Microstructure parameters of INCONEL phases are quantified from microstructure images using transmission electron microscopy (TEM). Different TEM sample preparation techniques were used to study INCONEL 738 alloy microstructure for transmission electron microscopy (TEM). The INCONEL 738 was first cut into a 1 × 1 cm slice with 600 µm thickness with diamond wire cut. INCONEL sample was mounted by wax (M135), after initial grinding and polishing. The molded INCONEL sample was further polished by different grit size SiC paper to reduce the thickness below 80 micron. At this stage, 3 mm discs were cut from the thin slice of INCONEL alloy by mechanical punch machine. The 3 mm discs of INCONEL alloy were used for TEM sample preparation. Three methods of electro-jet polishing, ion milling and micro control dimpling were employed to prepare transparent TEM sample to observe the surface microstructure details of INCONEL 738 alloy. Electro-jet polishing TEM sample preparation technique could reveal microstructure details of INCONEL alloy g and g^/ phases using 42% H₃PO₄, 34% H₂SO₄, 24% H₂O electrolyte at about – 40⁰C bath temperature and applied voltage of 30 V.