You specify Ti-6Al-4V on an implant drawing, the supplier asks “F136 or F136 ELI?”, and the difference decides fatigue life under 10 million load cycles. This guide separates what the names actually mean, why interstitials matter, and how to write the callout so you get the alloy you paid for.
For any implant that carries load over a long period — hip and knee femoral components, spinal rods and cages, dental implants, trauma plates and screws — specify Ti-6Al-4V ELI (Grade 23, ASTM F136). The “ELI” suffix means Extra Low Interstitial: tighter maximums on oxygen, iron, nitrogen and carbon, which buys higher fracture toughness and better fatigue resistance where a device sees millions of cycles.
Where Grade 5 (Ti-6Al-4V, ASTM B348 / ASTM F1472) remains acceptable: trial implants removed during surgery, surgical instruments, non-load-bearing spacers and washers, and external fixation hardware — applications where a fatigue-driven fracture does not put a permanent implant at risk. Many regulatory schemes (FDA 21 CFR 888, EU MDR 2017/745) treat Grade 5 as not acceptable for permanent load-bearing implantation, so when in doubt, choose ELI.
ELI is not a different alloy; it is the same Ti-6Al-4V system with tighter maximums on the interstitial elements (oxygen, nitrogen, carbon, hydrogen) and iron. Interstitials sit in the metal lattice and act as crack-initiation sites under cyclic load. The two limits that matter most:
| Element | Grade 23 ELI (ASTM F136) | Grade 5 (ASTM B348) | Why it matters |
|---|---|---|---|
| Oxygen, max | 0.13% | 0.20% | Primary driver of fracture toughness and fatigue |
| Iron, max | 0.25% | 0.40% | Impurity; beta stabilizer, affects microstructure |
| Nitrogen, max | 0.05% | 0.05% | Embrittling interstitial |
| Carbon, max | 0.08% | 0.08% | Interstitial; kept low for weldability and toughness |
The oxygen limit is the single biggest difference: 0.13% vs 0.20%. Lower oxygen means the alloy can tolerate more plastic strain before a crack grows — exactly what a press-fit femoral stem or a cyclically loaded bone screw needs.
Implants are designed around fatigue, not static strength. A hip stem is cyclically loaded roughly 1 million times per year of use; ASTM F136-based implant testing commonly targets 10⁷ (10 million) cycles. Higher interstitial content lowers the stress intensity the material can survive at the crack tip.
| Property (annealed) | Grade 5 (Ti-6Al-4V) | Grade 23 ELI (ASTM F136) |
|---|---|---|
| Tensile strength, min | 895 MPa (130 ksi) | 860 MPa (125 ksi) |
| Yield strength 0.2%, min | 828 MPa (120 ksi) | 795 MPa (115 ksi) |
| Elongation, min | 10% | 10% |
| Fracture toughness KIc (typical) | ~55 MPa√m | ~75–90 MPa√m (single-source: makerstage) |
| Fatigue endurance limit R=-1 (typical) | ~517 MPa | ~621–655 MPa (single-source: makerstage) |
| Elastic modulus | ~110 GPa | ~110 GPa |
| Density | 4.43 g/cm³ | 4.43 g/cm³ |
ELI gives up a little static strength (860 vs 895 MPa minimum) to gain roughly 20% higher fatigue endurance and 35–60% higher fracture toughness. For a permanent implant, that trade is almost always the right one.
Grade 5 (ASTM B348 / F1472 / AMS 4928) is not “bad” titanium — it is the workhorse aerospace/industrial alloy with slightly higher strength and a lower price. The acceptability question is regulatory and risk-based:
ELI costs more per kilogram and is often available from fewer certified mills with fewer heat lots per year — which can stretch lead time when a specific heat number is required for traceability. But the premium is small relative to the whole device.
Industry sourcing experience puts the Grade 23 material premium at roughly 2–5% of the total finished implant cost (machining, surface treatment, sterilization and regulatory compliance dominate the rest) (single-source: hontitan). The expensive outcome is not the ELI bar — it is a material substitution discovered after a device is in the field, or a certified lot scrapped because the bar was not actually F136.
An unambiguous callout prevents the supplier from shipping the wrong grade:
No — ELI is slightly weaker in static strength but tougher and more fatigue-resistant. Grade 23 ELI minimum tensile is 860 MPa vs 895 MPa for Grade 5, but ELI has roughly 20% higher fatigue endurance and 35–60% higher fracture toughness because of the lower interstitial (especially oxygen) limits. For cyclically loaded implants, toughness and fatigue matter far more than the static-strength difference.
Only for non-load-bearing or temporary applications. Trial implants, surgical instruments, spacers and washers can use Grade 5. For permanent load-bearing implants (hip/knee components, spinal rods, dental implants, trauma plates), regulatory practice treats Grade 5 chemistry per ASTM B348/F1472 as not acceptable — and if your drawing cites ASTM F136, Grade 5 cannot be certified against it at all.
No — cutting parameters are essentially identical. Both are α-β titanium: carbide tools at roughly 80–155 SFM (24–47 m/min) with PVD TiAlN inserts and high-pressure coolant. The real differences are documentation and inspection (F136 material cert, full traceability, first-article per the device QMS), not feeds and speeds.
Typically 2–5% of the total finished implant cost. The per-kilogram bar premium is real and ELI heat lots are fewer, but machining, surface treatment, sterilization and regulatory compliance dominate device cost. The expensive failure mode is a material-substitution discovery after field use — far more costly than the ELI bar itself (single-source: hontitan).
A mill test certificate (MTC, EN 10204 3.1) that names ASTM F136 and the heat number. It must list the actual oxygen and iron values (not just “within spec”), the mechanical test results for that heat, and a chemistry trace to the certified melt. Keep it in the Device History Record with the machining and passivation records.
Not for a load-bearing implant, and not if your drawing cites ASTM F136. “Medical grade” is marketing, not a standard. Verify the actual specification: F136 = ELI (Grade 23), B348/F1472 = Grade 5. Demand the standard designation and the F136 material certificate, or treat the offer as a substitution.
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