Titanium Sample Preparation

Introduction

Titanium and its alloys are important materials in aerospace, medical, and industrial applications. Proper preparation is essential to reveal the true microstructure without introducing artifacts such as deformation, scratches, or contamination. Titanium is particularly challenging due to its reactivity and tendency to form surface oxides. This guide will walk you through the complete preparation process.

Common titanium alloys include Ti-6Al-4V (Grade 5), the most widely used titanium alloy, as well as commercially pure titanium (Grade 2)and various alpha-beta alloys like Ti-3Al-2.5V (Grade 9). Titanium can be challenging due to its reactivity with oxygen and tendency to form surface contamination. The key is to use appropriate abrasives, maintain consistent pressure, and avoid contamination throughout the process. Titanium alloys vary in hardness, with some being relatively soft while others can be quite hard.

Sectioning

When sectioning titanium samples, use a slow cutting speed to minimize heat generation and deformation. A cutting speed of 100-200 RPM is typically appropriate for most titanium alloys, though harder alloys like Ti-6Al-4Vmay require slightly slower speeds. Titanium's reactivity requires careful handling to prevent contamination.

MAX-D or MAX-VHS abrasive cut-off blades suitable for titanium. For harder titanium alloys, MAX-VHS series blades may be more appropriate. Thin blades (0.5-1.0 mm) minimize heat generation and deformation.

• Use MAX-D series blades for general titanium alloys, or MAX-VHS series for harder titanium alloys
• Use a thin abrasive cut-off wheel (0.5-1.0 mm thickness)
• Apply steady, moderate pressure
• Use adequate coolant to prevent overheating and contamination
• Allow the wheel to do the cutting - avoid forcing
• Clean sample immediately after sectioning to remove cutting fluid and prevent contamination

Mounting

Mounting provides edge retention and easier handling. For titanium, compression mounting with phenolic or epoxy resins works well. Cold mounting with epoxy is also suitable and avoids potential heat-related issues. Ensure the sample is thoroughly cleaned before mounting to prevent contamination.

Compression Mounting

1. Clean the sample thoroughly to remove cutting fluid and debris
2. Place sample in mounting press with appropriate resin
3. Apply pressure: 3000-4000 psi for phenolic, 2000-3000 psi for epoxy
4. Heat to 150-180°C and hold for 5-8 minutes
5. Cool under pressure to room temperature

Cold Mounting

1. Clean and dry the sample thoroughly
2. Place in mounting cup with epoxy resin
3. Allow to cure at room temperature (typically 4-8 hours)
4. Cold mounting avoids heat that could affect titanium microstructure

Grinding

Grinding removes sectioning damage and prepares the surface for polishing. Start with coarse grits and progressively move to finer grits. For titanium, we recommend the following sequence. Titanium can work-harden, so maintain consistent pressure and avoid excessive grinding time.

Silicon carbide (SiC) grinding papers in various grit sizes (120, 240, 400, 600) for progressive grinding. Rotate sample 90° between each grit to ensure complete scratch removal.

Grinding Sequence

1. 120 grit: Remove sectioning damage (30-60 seconds per step)
2. 240 grit: Remove previous scratches (30-60 seconds)
3. 400 grit: Further refinement (30-60 seconds)
4. 600 grit: Final grinding step (30-60 seconds)

Important: Rotate the sample 90° between each grit to ensure complete removal of previous scratches. Use water as a lubricant and maintain consistent pressure. Clean the sample thoroughly between steps to prevent contamination. Titanium can work-harden, so avoid excessive grinding time.

Polishing

Polishing removes grinding scratches and prepares a mirror-like surface. For titanium, diamond polishing followed by oxide polishing typically yields excellent results. Use appropriate cloths and maintain consistent pressure to avoid contamination and relief.

Polycrystalline diamond compound provides consistent cutting action for titanium alloys.

Various polishing pads and cloths for different polishing stages. Select pad hardness based on titanium alloy and polishing stage.

Diamond Polishing

1. 9 μm diamond: 3-5 minutes on a hard cloth (e.g., Texmet)
2. 3 μm diamond: 3-5 minutes on a medium-hard cloth
3. 1 μm diamond: 2-3 minutes on a soft cloth

Final Polishing

1. 0.05 μm colloidal silica: 1-2 minutes on a soft cloth
2. Rinse thoroughly with water and dry with compressed air

Use appropriate polishing lubricants and maintain consistent pressure. Over-polishing can introduce relief, especially around inclusions or second phases. Clean the sample thoroughly between steps to prevent contamination. Titanium is reactive, so avoid prolonged exposure to polishing solutions.

Etching

Etching reveals the microstructure by selectively attacking grain boundaries and phases. The choice of etchant depends on the titanium alloy and what features you want to reveal. Common etchants include Kroll's reagent, hydrofluoric acid solutions, and various electrolytic solutions. Titanium requires careful etching due to its reactivity.

Common Etchants for Titanium

• Kroll's Reagent: General purpose, most commonly used for titanium. Reveals grain boundaries and alpha/beta structure (2-3ml HF, 5-6ml HNO₃, 100ml H₂O). Works well for Ti-6Al-4V and most alpha-beta alloys.
• Modified Kroll's: For alpha-beta titanium alloys. More aggressive (1-2ml HF, 2-3ml HNO₃, 100ml H₂O). Useful for harder alloys that require more aggressive etching.
• Weck's Reagent: For revealing grain boundaries in alpha titanium (100ml H₂O, 4g NaOH, 4g KMnO₄). Particularly effective for commercially pure titanium (Grade 2) and other alpha-phase materials.
• Electrolytic Etching: For sensitive microstructures. Use appropriate voltage and time settings. Can be useful for Ti-3Al-2.5V and other alloys where chemical etching may be too aggressive.
• Hydrofluoric Acid (HF): Dilute solutions (0.5-2%) for general etching

Etching solutions and reagents for titanium. Common etchants include Kroll's Reagent, Modified Kroll's, Weck's Reagent, and electrolytic solutions. Etching time typically ranges from 5-30 seconds depending on the etchant and alloy. Warning: Hydrofluoric acid is extremely hazardous and requires proper safety equipment.

Etching Procedure

1. Ensure sample is clean and dry
2. Apply etchant with cotton swab or immerse sample
3. Etch for 5-30 seconds (time varies by etchant and alloy)
4. Immediately rinse with water, then alcohol
5. Dry with compressed air

Important Safety Note: Many titanium etchants contain hydrofluoric acid (HF), which is extremely hazardous. Always use appropriate personal protective equipment (PPE) including gloves, safety glasses, and proper ventilation. Have calcium gluconate gel available as first aid for HF exposure.

Tip: Start with shorter etching times (5-10 seconds) and increase if needed. Over-etching can obscure fine details and create pitting. For electrolytic etching, use appropriate voltage and time settings. Kroll's reagent is the most commonly used etchant for titanium and typically provides good results for most alloys.

Troubleshooting

Common Issues and Solutions

• Scratches remaining: Insufficient grinding/polishing time or skipped grits. Ensure complete scratch removal at each step.
• Contamination: Titanium is highly reactive. Clean between steps thoroughly, use fresh abrasives, and avoid cross-contamination from other materials.
• Surface oxidation: Titanium forms oxides easily. Minimize exposure to air, clean immediately after polishing, and etch promptly after final polish.
• Relief around second phases: Over-polishing or too soft a cloth. Reduce polishing time or use slightly harder cloth.
• Work-hardening: Excessive grinding or polishing can work-harden titanium. Use appropriate pressure and avoid excessive time.
• Over-etching: Reduce etching time or dilute etchant. Start with shorter times (5-10 seconds).
• Pitting after etching: Etchant too strong or etching time too long. Dilute etchant or reduce time. HF-based etchants are particularly aggressive.
• Poor edge retention: Consider using phenolic mounting material or different mounting technique.
• Inconsistent etching: Ensure sample is clean and dry before etching. Surface contamination can cause uneven etching.