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Ti Grade 5 ELI / UNS R56401

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Ti Grade 5 ELI — Titanium – 6% Al – 4% V, extra low interstitial (ELI) — is a variant of Ti Grade 5 used in medical applications for its good biocompatibility, excellent fracture toughness and crack propagation behaviour. It retains its toughness even at temperatures as low as -253 °C. Alongside these properties, its low modulus of elasticity and good fatigue strength arise from the relatively low content of elements such as oxygen and iron.

Application areas: medical technologies.​‌​​‌​

Machinability: It machines in much the same way as Ti-6Al-4V, but its low oxygen content makes it easier to machine. It can be processed on CNC machines by operations such as milling, turning and drilling. Better machinability also makes the production of high precision parts possible.

Machining notes: The low oxygen content means less abrasion of cutting tools, which gives longer tool life during machining. For welding, the alloy is suitable for TIG (Tungsten Inert Gas) welding, and shielding with inert gases such as argon should be provided during welding.​‌​​‌​

Advantages. Its low interstitial element content gives superior properties in many areas:

Biocompatibility: particularly important for biomedical applications such as medical implants. The low oxygen level gives less oxidation and smoother surfaces, which improves compatibility with tissue and minimises biological reactions.​‌​​‌​

Better machinability: the low oxygen content makes the alloy easier to machine. In the production of high precision parts in particular, Ti Grade 5 ELI has an advantage in machinability over standard Ti-6Al-4V.

High strength-to-weight ratio: it offers high strength while remaining light, which makes it ideal for applications such as the aerospace industry.​‌​​‌​

High corrosion resistance: it shows high corrosion resistance to chemical and marine environments, and for that reason it is also chosen in the petrochemical, marine and energy sectors.

Better toughness: the low oxygen content allows the alloy to show higher toughness, which is an advantage in applications involving high impact.​‌​​‌​

What Ti-6Al-4V ELI Is — and Why Grade 5 and Grade 23 Are NOT THE SAME MATERIAL​‌​​‌​

Titanium Grade 23, known commercially as Ti-6Al-4V ELI (“Extra Low Interstitial”), is the version of the world’s most widely used titanium alloy in which the interstitial elements have been deliberately tightened. The nominal composition is unchanged: 6 % aluminium, 4 % vanadium, balance titanium, and the microstructure is still an α+β alloy. The only thing that changes is the ceiling on oxygen, nitrogen, iron and hydrogen. And that “only thing” is the entire reason the grade exists.

First, the critical warning: “Grade 5” and “Grade 5 ELI / Grade 23” are not the same material and cannot be ordered interchangeably. The phrase “Ti Grade 5 ELI” is common in the market and in practice means Grade 23 — but it is not an official ASTM grade name. ASTM recognises two separate grades: Grade 5 (R56400) and Grade 23 (R56407). If a certificate says “Grade 5”, what you hold is not ELI, whatever its elongation and oxygen figures happen to be.​‌​​‌​

And now the price: on specification, ELI is WEAKER​‌​​‌​

This is the most frequently hidden fact about ELI and it should be stated plainly. Oxygen is a strengthener; take it out and strength goes with it. The ASTM minimums:

The commercial consequence in one sentence: ELI buys damage tolerance with part of its strength. That trade is right when you fear the growth of a crack rather than the part yielding under load: load-bearing implants, cryogenic pressure vessels, deep-sea hardware, fracture-critical aerospace parts. Moving to ELI on a part sized by static strength is paying a premium to lose performance.

Standards by Product Form

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Grade 23 shares nearly all of the Grade 5 standards map — but there are two important gaps and one large divergence. The gaps are tube and castings; the divergence is that the industrial (B-series) and implant (F-series) routes define the same alloy with different numbers and different chemical limits.

The warning that must be published about ISO 5832-3

Contrary to widespread belief, ISO 5832-3 is NOT an ELI standard. Its own composition table gives oxygen ≤0.20 %, iron ≤0.30 %, aluminium 5.5–6.75 % — that is the standard Grade 5 composition. The standard merely carries a note in the text that “a grade with more restrictive limits of oxygen and iron is known under the term ELI”; it does not define ELI in its own table.
The mechanical minimums of ISO 5832-3 also differ from ASTM: for bars and sheets up to 75 mm maximum dimension, Rm ≥860 MPa, Rp0.2 ≥780 MPa, elongation ≥8 % (sheet) or ≥10 % (bar).
Practical consequence: writing “ISO 5832-3” on an implant order does not mean you have asked for ELI. If ELI is wanted, write ASTM F136 (or explicitly “ISO 5832-3, ELI grade, O ≤0.13 %”). This is the single most expensive error we see in the implant supply chain.​‌​​‌​

ASME Code Acceptance and Maximum Code Temperatures

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Two separate questions must not be conflated. First: “has ASME adopted the material specification?” Second: “has ASME published a design stress for this grade?” In code work it is the second that decides.

ELI’s own regulatory world, by contrast, is very strong: ASTM F136 in medical devices, AMS 4907 / 4930 / 4931 / 4956 in aerospace and ASTM F3001 in additive manufacturing are fully established. Grade 23 is not a weak material because it is “not in the code”; the ASME pressure code simply is not its market.

Product Forms With NO Standard — the Commercially Valuable Section​‌​​‌​

This is the section your sales engineers should memorise.

Chemical Composition

The table below shows how the same material is defined differently in three documents. These differences are not small and they have direct consequences at certificate review.​‌​​‌​

Why so much care? Because the difference is only 700 ppm of oxygen — and in a melting furnace, a scrap charge or an uncontrolled heat treatment, 700 ppm of oxygen is easily gained. ELI is not a quality category but a process discipline that must be protected end to end: sponge selection, scrap mix, number of vacuum arc remelting cycles, hot-working atmosphere and final annealing environment. The ELI premium is the cost of that discipline.​‌​​‌​

Mechanical Properties

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Fracture toughness — the reason ELI exists​‌​​‌​

The numbers in this section are the only thing that justifies the ELI premium, so the quality of the source matters.
The soundest data comes from a national standards laboratory study of low-temperature fracture in Ti-6Al-4V ELI. Measured K_Ic values in the base metal are approximately 100–110 MPa√m at room temperature and about 60.8 MPa√m at 76 K (−197 °C), with an abrupt transition between 76 and 125 K. In the same study, electron beam welding reduced toughness at the HAZ boundary by 16 % relative to the base metal. Fatigue crack growth rates were found to be insensitive to temperature and to weld-induced microstructural change over the stress intensity ranges tested.
Direct comparison with Grade 5: a secondary source gives ~55 MPa√m for Grade 5 and ~75 MPa√m for Grade 23 (about 36 % higher). [Single source — treat as indicative, do not use in design.] Producer datasheets give no numerical comparison and say only that “the ELI grade should be specified whenever toughness is a priority”. The order of magnitude is not in dispute; the exact figure depends on product form, microstructure and orientation and must be requested from the mill per project.

Fatigue​‌​​‌​

The producer’s published axial fatigue data for ELI (R = 0.06–0.1): smooth 400–700 MPa (60–100 ksi), notched (K_t = 3) 140–270 MPa (20–40 ksi). Note the magnitude of the notch sensitivity: a notch cuts fatigue strength to roughly one third — a general property of titanium and not a problem ELI solves. ELI improves how fast the crack leaving the notch root propagates; it does not forgive the notch. Design rule: surface finish, radii and shot peening matter more than the choice of grade.

Cryogenic behaviour​‌​​‌​

This is ELI’s second large market, and the reason is directly the oxygen ceiling. Titanium gains strength at low temperature but loses toughness, and the size of that loss scales directly with interstitial content. The laboratory data above quantifies it: ELI’s K_Ic falls from ~100–110 MPa√m at room temperature to ~60.8 MPa√m at 76 K — more than half is retained. Standard Grade 5 follows the same decline from a far lower starting point and is generally not considered suitable for cryogenic use.
Commercial consequence: for pressure vessels, valves and structural parts in liquid nitrogen (77 K) and liquid hydrogen (20 K) service, ELI is specified and standard Grade 5 is not. A cryogenic specification that says “Ti-6Al-4V” without stating ELI is incomplete.

Fall-off at elevated temperature​‌​​‌​

Ti-6Al-4V is not a high-temperature alloy. The producer recommends about 350 °C (660 °F) as the service temperature. Published elevated-temperature values (for Grade 5, from a European mill sheet): at 315 °C, Rp0.2 620 / Rm 689 MPa; at 425 °C, 516 / 620 MPa; at 540 °C, 413 / 482 MPa. No separate elevated-temperature table for Grade 23 was found; ELI values are expected to be somewhat lower — ask the mill if you need numbers.
And the real limitation is not strength. Above roughly 400 °C, heating makes the softening permanent; above 600 °C in air an alpha case forms at the surface; and prolonged high temperature also brings creep and Ti₃Al (α₂) formation risk. For hot forming this is an advantage: heating to only 427 °C gives roughly a 40 % reduction in yield strength.

Physical Properties​‌​​‌​

Heat Treatment and Thermal Stability​‌​​‌​

Ti-6Al-4V is one of the few titanium alloys that genuinely hardens by heat treatment — unlike the unalloyed grades. But doing so to ELI is usually wrong: solution treating and ageing raises strength and takes back exactly what you paid for, the fracture toughness. That is why the ELI specifications (AMS 4907 / 4930 / 4931) cover the annealed condition, while the STA specification (AMS 4965) is a Grade 5 document.

The good news on thermal stability: Ti-6Al-4V has no sigma phase, no ordering embrittlement and no carbide sensitisation. The real risks are three: (1) interstitial contamination at every hot operation; (2) accidentally crossing the β transus — in ELI that threshold is about 20 °C lower than in Grade 5; (3) Ti₃Al (α₂) precipitation after long exposure at high temperature.

Welding​‌​​‌​

One of ELI’s least discussed but most real advantages is welding. Weld embrittlement in titanium comes from interstitial pickup; if the base metal already starts low, there is more margin in the total interstitial budget. Under the same shielding regime an ELI weld is more ductile and tougher than a standard Grade 5 weld. This is why ERTi-23 filler is used deliberately even on Grade 5 base metal.

Machining

Ti-6Al-4V is one of the most difficult materials routinely machined in industry; its machinability rating is given as 22 % of AISI B1112 steel. The cause is not hardness. Thermal conductivity is 6.6–7.5 W/m·K — the heat generated in the cut does not leave with the chip, it stays at the tool tip. Hot titanium reacts with carbide and wears the tool chemically; and on top of that comes the low modulus: the part springs away from the tool.​‌​​‌​

A reassuring note: there is no practical difference in machining parameters between ELI and Grade 5. ELI is slightly softer and more ductile, so it is marginally gummier. The ELI premium comes from melting discipline and documentation, not from manufacturing difficulty.

Corrosion — Where It Excels, and WHERE IT FAILS

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An honest frame first: the corrosion resistance of Ti-6Al-4V is close to that of unalloyed titanium but not identical to it. Protection again comes from the surface TiO₂ passive film, which repairs itself within seconds given ppm levels of moisture or oxygen. Aluminium and vanadium do not disrupt that film; but crack sensitivity, crevice corrosion thresholds and hydrogen behaviour are somewhat more complex in the alloyed grade.​‌​​‌​

ELI’s real advantage on the corrosion side

This is ELI’s least known but best documented superiority. A titanium producer states in its own corrosion manual that “6-4 ELI (low oxygen content) is considered one of the best of the high strength titanium-base alloys for seawater service”. The reason is not the general corrosion rate — both grades are excellent there — but resistance to stress corrosion cracking in pre-cracked specimens. High-oxygen Ti-6Al-4V can show SCC susceptibility in seawater when pre-cracked, whereas ELI is markedly more resistant under the same conditions. This is why ELI is preferred in deep-sea, submarine and offshore hardware, and it is an argument separate from the toughness one.​‌​​‌​

Where it excels

Seawater: negligible corrosion to 260 °C (500 °F); no pitting, no attack even under biofouling. Fatigue: titanium suffers no significant loss of fatigue strength in seawater. Erosion-corrosion: withstands flow velocities of 30 m/s.
Oxidising environments: nitric acid, oxidising chlorides, chlorinated water, hypochlorite, chlorine dioxide, chlorate, perchlorate — full resistance.
Body fluid: biocompatibility and passive film stability underpin ELI’s position in the implant market; being non-magnetic gives MR compatibility.​‌​​‌​

WHERE IT FAILS — this section is mandatory

Frequently Asked Questions

We use Grade 5. How much is it worth paying to move to Grade 23?​‌​​‌​

There is one honest criterion: what is the failure mode of the part?
If the part is sized by static strength, the switch is harmful. ELI’s specification minimums are 67 MPa lower in tensile and 69 MPa lower in yield (7.5 % and 8.3 %). To carry the same load you must increase section, weight goes up, and on top of that you pay 1.2–1.5× more per kilogram — a loss on both sides.
If the part is limited by crack growth, cyclic load or low temperature, the switch is a real and measurable gain. Fracture toughness rises markedly (measured ELI values are of the order of ~100–110 MPa√m at room temperature), damage tolerance improves, cryogenic behaviour improves, the weld is more ductile, and SCC resistance in pre-cracked seawater specimens rises.
And there is a third category: regulatory necessity. For load-bearing implants the choice is no longer engineering but compliance — the ASTM F136 route is required; cryogenic and fracture-critical aerospace specifications likewise mandate ELI outright.
In short: buy it not as “the better material” but as “the right material for my failure mode”. The price difference comes from melting discipline, traceability and documentation, not from manufacturing difficulty.

Are ASTM B348 Grade 23 and ASTM F136 the same material? Can I substitute one for the other?​‌​​‌​

Same alloy, different document — and no, they are not interchangeable.
The differences are concrete. First, the UNS number: the B-series says R56407 for Grade 23, F136 says R56401. Second, nitrogen: ASTM Grade 23 says 0.03 %, while two independent producer sheets give 0.05 % for F136 — the implant standard is looser on this one item. Third, and most important: F136 goes beyond chemistry. It imposes microstructure, grain size, α-phase morphology, surface condition and a full traceability chain. None of that is in B348.
The practical consequence runs both ways. A bar certified to B348 Grade 23 cannot be used in an implant that requires F136 — even if the chemistry conforms, the microstructure and traceability evidence are missing. In the other direction, F136 material is more than adequate for industrial work but needlessly expensive; and because its nitrogen ceiling is looser, it may not automatically satisfy an aerospace specification that demands tight nitrogen.
Ordering rule: write the specification of the target application; do not settle for the words “Grade 23” or “ELI”. For medical work, “ASTM F136”; for industrial or aerospace work, “ASTM B348 Grade 23, UNS R56407” plus the numerical interstitial ceilings required.

We are building a cryogenic pressure vessel. Is ELI mandatory?​‌​​‌​

Yes — and this is the least disputable justification for ELI.
Titanium gains strength at low temperature but loses toughness, and the size of that loss scales directly with interstitial content. This is not a matter of preference but measured material behaviour: ELI’s fracture toughness falls from ~100–110 MPa√m at room temperature to ~60.8 MPa√m at 76 K (−197 °C) — more than half is retained — with an abrupt transition between 76 and 125 K. Standard Grade 5 follows the same curve from a far lower starting point.
Three practical warnings. First, welding: in the same study electron beam welding cut HAZ-boundary toughness by 16 %; a cryogenic vessel must be designed from that reduced value, not from the base metal. Second, hydrogen: ELI’s hydrogen ceiling is lower than Grade 5’s and that margin is valuable in cryogenic hydrogen service — ask for the actual measured value on the certificate, not just the ceiling. Third, ASME: if this is to be a code vessel, a II-D listing for Grade 23 could not be verified; the code route must be addressed separately.
And a specification-writing rule: writing “Ti-6Al-4V” without stating ELI gives the supplier the right to ship standard Grade 5. A cryogenic specification must state the grade number and the numerical oxygen ceiling explicitly.

Can I solution treat and age ELI to harden it?​‌​​‌​

Technically yes; commercially usually wrong.
Ti-6Al-4V is an α+β alloy that hardens by heat treatment. The typical Grade 5 route is solution treatment at 913–954 °C for 1 hour with a water quench, then ageing at 524–552 °C for 4–8 hours; hardness rises from 30–34 HRC to 35–39 HRC.
But doing this to ELI takes back the reason you bought it. STA raises strength and lowers fracture toughness — you spend on heat treatment the premium you paid to remove 700 ppm of oxygen. That is why the ELI specifications (AMS 4907 / 4930 / 4931) cover the annealed condition while the STA specification AMS 4965 is a Grade 5 document.
Two further warnings. Section thickness: the AMS 4965 minimums fall with section — 1138 MPa at ≤12.7 mm but 896 MPa at 76–102 mm, i.e. down to annealed Grade 5 level; in heavy section the money spent on STA is wasted. β transus: ELI’s transus is about 20 °C lower than Grade 5’s, so a solution-treatment recipe written for Grade 5 can accidentally take ELI into the β field. Furnace recipes must be written per grade.
If high strength is genuinely required, the right answer is usually not to harden ELI but to buy STA Grade 5. Where a fracture-critical part needs higher performance, the correct ELI route is β annealing or recrystallisation annealing — those raise toughness rather than lowering it.

Common Datasheet Errors and Purchasing Traps​‌​​‌​

1. Grade 5 mechanical data printed under a Grade 23 heading — THE MOST COMMON ERROR. If a page shows tensile ≥895 MPa, yield ≥828 MPa under a “Grade 23 / ELI” heading, that is Grade 5 data. The ASTM minimums for Grade 23 are ≥828 / ≥759 MPa. A buyer who sees 828 MPa yield never learns that ELI is weaker and builds the design on the wrong basis.
2. “ISO 5832-3 = ELI” — WRONG, and expensive. The composition table of ISO 5832-3 gives O ≤0.20 %, Fe ≤0.30 %, Al 5.5–6.75 %, i.e. standard Grade 5. The standard only carries a note that “a grade with more restrictive limits of oxygen and iron is known under the term ELI”. Writing “ISO 5832-3” on an implant order does not mean asking for ELI.
3. “W.Nr. 3.7235” shown as ELI — WRONG. 3.7235 is Titanium Grade 7 (Ti-0.2Pd, UNS R52400), palladium-bearing unalloyed titanium. Five independent European suppliers list it that way. It has nothing to do with Ti-6Al-4V.
4. Attributing a separate Werkstoffnummer to ELI. In Europe Grade 23 also uses 3.7165 — the same number as Grade 5. 3.7164 is also Ti-6Al-4V (the aerospace number). ELI cannot be ordered by W.Nr.; the grade number or specification name is required.
5. Treating R56401 and R56407 as identical, or one of them as “wrong”. Both are real and belong to different documents: the current ASTM B-series scope texts number Grade 23 as R56407, while ASTM F136 numbers it R56401. Do not use a UNS number alone on an order.
6. Assuming F136 and ASTM Grade 23 are identical. The nitrogen ceiling differs (0.05 % for F136 · 0.03 % for ASTM Grade 23), the hydrogen ceiling varies by publisher, and F136 imposes microstructure, grain size and traceability requirements beyond chemistry. Neither substitutes for the other.
7. “ASTM B338 Grade 23 tube” — WRONG. B338 contains no Ti-6Al-4V at all — neither Grade 5 nor Grade 23. Its 28 grades are the unalloyed, Pd/Ru-bearing, Gr 9 and Gr 12 grades.
8. Offering an “ELI casting”. The only Ti-6Al-4V casting grade in ASTM B367 is C-5 (R56409), and it is the standard Grade 5 composition. No standardised ELI casting grade exists.
9. Confusion matching AMS numbers to grades. ELI: AMS 4907 (sheet/plate), 4930 (bar, wire, forgings, rings), 4931, 4996 (billet), 6932, 4956 (wire). Grade 5: AMS 4911 (sheet/plate), 4928 (bar/forging), 4967, 4965 (STA), 4954 (wire), 4985 (castings). Citing AMS 4911 or 4928 as an ELI document is a common error.
10. Citing AMS 4965 for ELI. AMS 4965 is a Grade 5 STA document and contradicts ELI’s annealed philosophy. Its minimums also fall markedly with section: 1138 MPa at ≤12.7 mm, 896 MPa at 76–102 mm.
11. The β transus habit. “The β transus of Ti-6Al-4V is 995 °C” is approximately right for Grade 5 (~982–1010 °C); for ELI the transus is LOWER (~963–991 °C). The reason is metallurgical: oxygen is an α stabiliser. A furnace recipe written for Grade 5 can take ELI into the β field.
12. Dropping the iron and aluminium limits. Iron: ASTM 0.40 % for Grade 5, AMS and European practice 0.30 %, Grade 23 0.25 % on both routes — for an aerospace buyer that is the accept/reject line. Aluminium: Grade 5 5.50–6.75 % · Grade 23 5.50–6.50 %, narrowed at the top because high aluminium promotes Ti₃Al (α₂).
13. “ELI is better in every respect” — WRONG. ELI is weaker on specification (67 MPa in tensile, 69 MPa in yield), more expensive and harder to source. Buy it only when fracture toughness, damage tolerance, cryogenic behaviour or regulatory compliance require it.
14. “ELI is only a medical grade” — WRONG, and it loses business. Its strongest technical justifications are cryogenic service, deep-sea hardware and fracture-critical aerospace parts; a producer calls ELI “one of the best of the high strength titanium-base alloys for seawater service”.
15. Mistaking density and conductivity spreads for a grade difference. Density is published as 4.42 / 4.43 / 4.47 g/cm³ and thermal conductivity as 5.8 / 6.6 / 6.7 / 7.5 W/m·K. These are measurement and rounding differences, not the Grade 5 / Grade 23 difference.
16. “We used ELI, so we have no toughness problem” — invalid at the weld line. Measured data: in an electron beam weld, fracture toughness at the HAZ boundary fell 16 %. In a welded structure the design follows the value at the joint.
17. Confusing ELI and standard grade in additive manufacturing. Powder bed fusion has separate standards: ASTM F3001 = ELI, ASTM F2924 = standard Ti-6Al-4V.
18. “Grade 23 is ASME approved” — incomplete. SB-265 and SB-348 do include Grade 23 as a material specification; but the II Part D allowable stress listing required for code design could not be verified. The Titanium Association’s presentation speaks of 17 approved grades for VIII-1 and shows Gr 1, 2, 2H, 12, 28; Ti-6Al-4V is not mentioned.
19. Dropping the wear and galling warning. The producer states plainly that the alloy is “not recommended for wear applications”. Sliding titanium-on-titanium contact must be avoided.

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