Ti-6Al-2Sn-4Zr-2Mo (Grade 5)

Use slow-speed diamond saw with continuous coolant flow. Diamond saws are preferred for titanium alloys to minimize deformation and work hardening. Standard diamond blade (0.3-0.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating and minimize work hardening. The reactive nature of titanium requires careful handling. Cutting speed: 150-250 RPM for most diamond saws. Apply light to moderate pressure - the material can work-harden if excessive pressure is applied. Avoid forcing the cut which can cause sample damage and work hardening. Leave adequate allowance (~1-2 mm) for grinding away the heat-affected zone and any deformation 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 material requires careful handling during mounting to avoid deformation. For critical applications, ensure the mounting material is compatible with the intended use environment.

The hardness (365 HB) of Ti-6Al-2Sn-4Zr-2Mo (Grade 5) 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.

Ti-6Al-2Sn-4Zr-2Mo is a near-alpha high-temperature alloy where the microstructure is predominantly alpha with a small volume fraction of beta phase. Etching reveals the alpha platelet/lath morphology and prior beta grain boundaries critical for creep performance evaluation. Kroll's Reagent (Chemical Etching) - Standard etchant:

• Composition: 2-3ml HF + 5ml HNO₃ + 100ml H₂O
• Preparation: Add acids to water slowly with stirring in a plastic container (HF attacks glass). Prepare fresh for best results.
• Application: Swab for 5-15 seconds. The predominantly alpha microstructure may etch more slowly than Ti-6Al-4V due to lower beta content.
• Reveals: Alpha platelets and Widmanstätten colonies within prior beta grains. Thin beta phase at alpha lath boundaries appears dark. Prior beta grain boundaries visible with moderate etch times.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Modified Kroll's Reagent (Chemical Etching) - For controlled etching:

• Composition: 1ml HF + 3ml HNO₃ + 100ml H₂O
• Application: Swab for 10-30 seconds. Better for resolving aligned alpha platelet colonies typical of this near-alpha alloy.
• Reveals: Fine alpha colony structure and alpha lath width relevant to creep and fatigue performance.

Etching Strategy:

• Start with Kroll's at 5-10 seconds for general microstructure
• Near-alpha alloys have less beta to darken, so contrast may be more subtle than Ti-6Al-4V
• For high-temperature service evaluation, assess alpha colony size and prior beta grain size
• Polarized light can reveal alpha platelet crystallographic orientation within colonies

Safety: HF is extremely hazardous — always use in a fume hood with HF-rated gloves, face shield, and lab coat. Have calcium gluconate gel available. Store all HF solutions in plastic containers.