ASTM F136 Ti-6Al4V ELI Grade 23 Round Bar for Medical and Dental Implants
Oct 23, 2025 ASTM-F136.pdf
Medical-Grade Titanium: Standards and Designations
Ti-6Al4V ELI Grade 23 is the dominant titanium alloy for load-bearing medical implants. It is specified under ASTM F136 for wrought surgical implant material and ISO 5832-3 for implant alloys, with UNS designation R56401. In bar form it also falls under ASTM B348. The ELI designation means Extra Low Interstitials: oxygen, iron, carbon and nitrogen are held to tighter maximums than in Grade 5, which directly improves the fracture toughness and fatigue resistance needed in implants that must survive decades of cyclic loading inside the body.
Chemical Composition and Purity
| Element | Specified Maximum or Range |
|---|---|
| Titanium | Balance |
| Aluminum | 5.5-6.5% |
| Vanadium | 3.5-4.5% |
| Iron | 0.25% max |
| Oxygen | 0.13% max |
| Carbon | 0.08% max |
| Nitrogen | 0.05% max |
| Hydrogen | 0.012% max |
The tight oxygen and iron limits distinguish implant-grade material from standard Grade 5. Lower interstitial content raises ductility and fracture toughness, so cracks initiated at notches or surface damage are less likely to propagate to failure.
Mechanical Properties
Annealed Grade 23 bar typically delivers a tensile strength of 895-1000 MPa with a yield strength of 825-900 MPa and elongation of 10-14% per ASTM B348. For ASTM F136 implant material, the minimum tensile strength is 860 MPa and the minimum yield strength is 795 MPa with at least 10% elongation and 25% reduction of area. The modulus of elasticity is about 114 GPa, roughly half that of stainless steel and cobalt-chrome alloys, which helps implants share load with surrounding bone and reduces stress shielding.
| Property | Typical Annealed Value |
|---|---|
| Tensile strength | 895-1000 MPa |
| Yield strength, 0.2% offset | 825-900 MPa |
| Elongation | 10-14% |
| Reduction of area | 25-35% |
| Fatigue strength (10^7 cycles, smooth bar) | About 500-600 MPa |
Biocompatibility and Osseointegration
Titanium and its alloys are classified as bioinert metals. A dense, stable titanium dioxide film forms instantly on the surface, so the material does not corrode in body fluids, does not release significant metal ions, and does not trigger allergic reactions in most patients. Dental and orthopedic implants rely on osseointegration: living bone grows into direct contact with the titanium surface without an intervening fibrous layer. Surface roughness from machining, grit blasting or acid etching increases the contact area and accelerates osseointegration, which is why implant manufacturers control surface preparation carefully.
Applications in Medical and Dental Devices
Dental implants, abutments and prosthetic frameworks
Orthopedic trauma: bone plates, screws, intramedullary nails
Joint replacement: hip stems, femoral heads, knee components
Spinal fixation: pedicle screws, rods, cages
Cardiovascular: pacemaker housings, heart-valve frames, vascular clips
Surgical instruments that require high strength with light weight
Machining and Surface Finishing
Ti-6Al4V ELI is machinable but demanding. Its low thermal conductivity concentrates heat at the cutting edge, and the alloy work hardens quickly. Successful machining uses sharp inserts with positive rake geometry, moderate cutting speeds, constant feed rates and high-pressure, high-volume coolant. After machining, implant components are typically cleaned, deburred and passivated per ASTM F86 to restore the protective oxide film. Anodizing, bead blasting and electropolishing are common finishing routes that also create controlled surface texture for bone contact.
Sourcing and Certification
Because implants are regulated medical devices, raw material traceability is essential. Buyers should require an EN 10204 3.1 mill certificate listing the heat number, chemical composition, mechanical test results and condition for every lot of bar. Common stock diameters for dental and orthopedic work range from about 3 mm to 80 mm, with longer bars supplied in 3 m or 4 m lengths or cut to size. Confirming interstitial limits, ultrasonic soundness and surface quality before machining avoids expensive rejections later.
Frequently Asked Questions
1. Why is Grade 23 preferred over Grade 5 for implants?
The extra-low-interstitial limits give Grade 23 better fracture toughness, ductility and fatigue crack growth resistance, which are the properties that matter most for implants subjected to long-term cyclic loading.
2. Does ASTM F136 bar also meet ISO 5832-3?
The two standards define very similar chemistry and property requirements for Ti-6Al4V ELI implant alloy. Many mills certify material to both so that it can be used in markets accepting either standard.
3. Can dental implant bars be supplied in small diameters?
Yes. Round bar for dental implants is commonly available from about 3 mm to 25 mm diameter, and many suppliers also offer wire and rod forms in the same grade.
4. What surface finish should implant bar have?
Ground, centerless-ground or polished surfaces are preferred because surface defects reduce fatigue strength. The final implant surface texture is then created during component finishing, not on the raw bar.
5. Is Ti-6Al4V ELI magnetic?
No. Titanium alloys are non-magnetic, which is another reason they are used in devices that must be compatible with MRI and other imaging systems.
6. How is the oxide film restored after machining?
By passivation: cleaning in acid solutions followed by rinsing, per ASTM F86. This removes embedded iron and re-forms a uniform titanium dioxide layer that protects the surface.
