Inconel 718

Use slow-speed diamond saw or abrasive cut-off wheel designed for non-ferrous materials. Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating and minimize deformation. Cutting speed: 150-250 RPM for most cut-off saws. Apply steady, moderate pressure - the material allows for reasonable feed rates. Avoid forcing the cut which can cause wheel wear and sample damage. Leave adequate allowance (~2-3 mm) for grinding away the heat-affected zone from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the microstructure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. Ensure proper cooling under pressure to minimize shrinkage.

The hardness (352 HB) of Inconel 718 requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (30-40 N per 30 mm sample) - the hard material may require longer grinding times. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 120 grit: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove heat-affected zone.
• 240 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 320 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Adequate water lubrication is critical - avoid drying during grinding which can cause smearing.

The hardness requires careful polishing. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent deformation.

Diamond polishing sequence:

• 9μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (30-40 N per 30 mm sample). Start with 9μm to minimize damage. The hard material may require longer polishing times.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (25-35 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a medium-hard synthetic pad with lighter pressure (25-35 N). These pads provide gentle material removal.
• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with very light pressure. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing or excessive relief.

Inconel 718 is a precipitation-hardened Ni-Cr-Fe superalloy strengthened by γ' (Ni₃Al/Ti) and γ" (Ni₃Nb) precipitates. Proper etching is critical for revealing these fine precipitate phases, δ (Ni₃Nb) phase, grain boundaries, and carbides. Two primary etchants are recommended:

Glyceregia (Chemical Etching) - Primary choice for general microstructure:

• Composition: 15ml HCl + 5ml HNO₃ + 10ml glycerol. Add acids to glycerol slowly with stirring.
• Application: Immerse or swab for 10-30 seconds. Prepare fresh — decomposes within hours. The glycerol moderates the reaction rate for controlled etching.
• Reveals: Grain boundaries, twin boundaries, δ phase (needle-like), and general microstructure. Good for overall assessment but does not reliably reveal fine γ'/γ" precipitates.
• Rinse: Immediately with water, then ethanol. Dry with compressed air or warm air to avoid staining.

Electrolytic 10% Oxalic Acid (Electrolytic Etching) - Essential for precipitate characterization:

• Composition: 10g oxalic acid dihydrate in 100ml H₂O
• Application: Electrolytic — sample as anode, stainless steel cathode. Apply 3-6V DC for 5-15 seconds. Lower voltage (3V) gives gentler etching for γ'/γ" revelation; higher voltage (6V) for grain boundaries and carbides.
• Reveals: γ' and γ" precipitates, δ phase, grain boundaries, carbides (NbC, TiC), and sensitization. This is the preferred etchant for detailed precipitate characterization in Inconel 718.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Etching Strategy:

• Start with Glyceregia for general microstructure, grain boundaries, and δ phase identification
• Use electrolytic 10% oxalic for γ'/γ" precipitate characterization — this is the preferred method for detailed phase analysis
• For electrolytic etching, start at 3V for 5 seconds and increase time/voltage gradually
• Always clean and degrease before etching — residual polishing compound will interfere
• Use short initial etch times, check under the microscope, and repeat if needed

Safety: Glyceregia produces toxic fumes — use in fume hood. Electrolytic etching requires proper electrical setup. Wear gloves, safety glasses, and lab coat for all etchants.