ASTM F136
ASTM F136
Unalloyed Titanium, for Surgical Implant Applications
ASTM F136 is a standard specification established by ASTM International for a titanium alloy specifically intended for surgical implants. This alloy is commonly known as Ti-6Al-4V ELI (Extra Low Interstitial), which is a variation of the widely used Ti-6Al-4V titanium alloy but with tighter control on the levels of certain impurities to improve its biocompatibility and mechanical properties. Here’s an overview:
Composition:
- Titanium (Ti): Balance
- Aluminum (Al): 6%
- Vanadium (V): 4%
- Oxygen (O): ≤ 0.13%
- Carbon (C): ≤ 0.08%
- Nitrogen (N): ≤ 0.05%
- Hydrogen (H): ≤ 0.0125%
- Iron (Fe): ≤ 0.25%
- Other elements: Trace amounts
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Chemical and Mechanical Properties
Biocompatibility:
- Medical Grade: ASTM F136 is specifically designed for medical implants, ensuring that the material is biocompatible and does not cause adverse reactions in the human body.
- Corrosion Resistance: The alloy offers excellent resistance to corrosion in the human body, particularly against bodily fluids and tissues, which is critical for long-term implant performance.
Mechanical Properties:
- High Strength: Ti-6Al-4V ELI offers high tensile strength, making it suitable for load-bearing implants like bone screws, plates, and joint replacements.
- Ductility and Toughness: The material retains good ductility and toughness, even at the low impurity levels specified by ASTM F136, which allows it to withstand the mechanical stresses encountered in the body.
- Fatigue Resistance: Excellent fatigue resistance, which is crucial for implants that endure repetitive motion and stress over long periods.
Microstructure:
- Alpha-Beta Alloy: Ti-6Al-4V ELI is an alpha-beta alloy, meaning it has a mixture of both alpha (hexagonal close-packed) and beta (body-centered cubic) phases. This combination provides a balance of strength, toughness, and ductility.
Heat Treatment:
- Solution Treated and Aged (STA): The alloy can be heat-treated to improve its mechanical properties further. The STA process enhances strength while maintaining adequate ductility and toughness.
Manufacturability:
- Machinability: While titanium alloys are generally more challenging to machine than other metals, Ti-6Al-4V ELI can be successfully machined into complex shapes with the right techniques and tools.
- Weldability: The alloy can be welded, which is important for the production of certain types of implants, though care must be taken to avoid contamination and defects.
Orthopedic Implants: Widely used in the production of hip and knee replacements, bone plates, screws, and spinal fixation devices due to its strength and compatibility with human tissue.
Dental Implants: Employed in dental implant components such as abutments and frameworks, where both strength and biocompatibility are essential.
Craniofacial Implants: Used in reconstructive surgery for facial bones, offering both structural support and integration with existing bone.
Cardiovascular Devices: Utilized in devices like heart valves and pacemaker enclosures where biocompatibility and resistance to fatigue are critical.
Versus ASTM F67 (Commercially Pure Titanium): While ASTM F67 titanium offers excellent biocompatibility, ASTM F136 (Ti-6Al-4V ELI) provides superior strength and is preferred for load-bearing implants.
Versus Ti-6Al-4V (Standard Grade): The ELI version (ASTM F136) has lower levels of oxygen and other interstitial elements, improving ductility and toughness, which are particularly important in medical applications.
Versus ASTM F1295 (Ti-6Al-7Nb): Both are used in medical implants, but Ti-6Al-4V ELI (ASTM F136) is more widely used due to its well-established track record. Ti-6Al-7Nb, on the other hand, offers a vanadium-free alternative, which some practitioners prefer for biocompatibility reasons.
Cost: Titanium alloys, especially those made to medical-grade specifications like ASTM F136, are more expensive than other materials.
Manufacturing Challenges: The high strength and specific properties of the alloy can make it difficult to machine and process, requiring specialized equipment and expertise.