Grade 23 Titanium Explained: Properties, Applications, Machining & Design Guidelines

Grade 23 titanium, also known as Ti-6Al-4V ELI (Extra Low Interstitial), has a unique and irreplaceable place in the hierarchy of titanium alloys. From an engineering point of view, it is not meant to maximize raw tensile strength. Instead, it is meant to maximize reliability, fracture toughness, and fatigue safety in situations where failure is not an option.

Grade 23 is based on the industry-standard Grade 5 (Ti-6Al-4V) and has been refined to greatly lower the amount of interstitial elements, mainly oxygen, nitrogen, and iron. This level of chemical purity makes a cleaner α−β microstructure that is more ductile, better at resisting crack growth, and very tough at low temperatures. In real life, Grade 23 gives up a little bit of static strength for a lot more safety against brittle fracture. This is why it is now the most common material for medical implants that have to support weight and important parts of airplanes, where following the rules and being able to handle damage are more important than the cost of the material. This article gives a full technical look at Grade 23 titanium, looking at the engineering reasons for choosing it and the specific manufacturing steps needed to keep its life-critical properties.

What Is Grade 23 Titanium? Definition and Requirements

The “ELI” Difference: Extra Low Interstitials

Grade 23 Titanium is basically a purer form of Grade 5. The “ELI” label means “Extra Low Interstitials.”

• What are interstitials? They are things like oxygen (O), nitrogen (N), and carbon (C) that fit between the titanium atoms in the crystal lattice.

• The Engineering Impact: In standard Grade 5, these elements make things stronger but also less flexible. The material becomes more ductile and less likely to break when it is lowered to Grade 23. It changes the alloy from a strong structural material to one that can withstand damage and is safe for living things.

Standards and Designations Around the World

Because regulated industries like medicine and aerospace use Grade 23, it is very important to specify the right standard.

Tip for buying: Don’t just say “Ti-6Al-4V ELI.” You need to specify ASTM F136 for medical devices. This standard says that you have to check both the chemistry and the microstructure to make sure there are no alpha-case defects or inclusions that could cause fatigue failure.

The chemical makeup and the ways that microstructures work

The strict chemical control of Grade 23 is what makes it work so well. It has the same amount of Aluminum and Vanadium as Grade 5, but the limits on impurities are much stricter.

Limits on Chemical Composition (wt.%)

The next table shows the difference between standard Grade 5 and Grade 23 ELI.

≤0.13%

≤0.20%

≤0.25%

≤0.40%

≤0.05%

≤0.05%

≤0.08%

≤0.08%

Microstructure: Making it Tougher

Grade 23 keeps the two-phase α+β structure. But the lower oxygen level lowers the “frictional” stress on dislocations that are moving through the α lattice.

• Plastic Zone Size: This makes the plastic zone at the tip of a crack bigger, which slows down the growth of the crack and absorbs energy.

• Result: The material is less likely to break suddenly and become brittle, even at very low temperatures or when it is under the complex loads that the human body puts on it.

Performance Envelope and Mechanical Properties

When the design calls for a material that will bend before it breaks and stop cracks from spreading, grade 23 is chosen.

Common Mechanical Properties (at Room Temperature)

Analysis of Engineering:

• Strength vs. Toughness: Grade 23 gives up about 5–10% of the yield strength of Grade 5. In return, it gets a big boost in Fracture Toughness(KIc). This trade-off is almost always necessary for parts that are important for safety and need to be able to “leak before break” or be tolerant of damage.

• Modulus: The modulus stays at about 110 GPa, which is half of steel. This is good for bone implants because it lowers “stress shielding,” which happens when an implant is too stiff and causes the bone around it to atrophy.

How Fatigue Works and How Failures Happen

Fatigue life is the most important factor in medical implants (like hip stems) and aerospace fittings.

Resistance to Growth of Fatigue Cracks

Grade 23 is very good at putting off the start of fatigue failure.

• Crack Initiation: A cleaner microstructure with fewer interstitial defects means that there are fewer places where micro-cracks can start.

• Crack Propagation: The ELI chemistry makes cracks grow more slowly (da/dN) than standard Grade 5 once they form. This gives a bigger safety margin between finding a crack and having a catastrophic failure.

Common Ways Things Go Wrong

• Surface Driven: Most of the time, fatigue failure in Grade 23 is caused by the surface. Marks from machining, scratches, or a bad surface finish can greatly shorten the life of a part.

• Alpha Case: If the surface layer is not treated properly, like if it is exposed to air at a high temperature during heat treatment, it becomes brittle and rich in oxygen. When this layer is put under stress, it cracks right away, which quickly leads to fatigue failure. Alpha case on finished parts is strictly forbidden by ASTM F136.

Resistance to corrosion and compatibility with living things

In Grade 23, corrosion resistance means more than just not rusting. It also means not reacting with chemicals in the body.

Stability of Passive Oxide Film

The Titanium Dioxide (TiO2) Layer on Grade 23 is chemically the same as Grade 5, but it is more structurally stable because it has fewer impurities.

• Biocompatibility: The stable oxide layer keeps Vanadium or Aluminum ions from getting into nearby tissue, which stops allergic or toxic reactions. The fact that Grade 23 can osseointegrate is what makes it the best choice for bone screws and plates.

• It is not affected by body fluids, seawater, or most sterilization environments (like steam and autoclaving).

Corrosion by galvanism

Grade 23 is a noble metal, but when it is combined with stainless steel screws in a saline environment (like the body), it can cause galvanic corrosion of the stainless steel. This is usually not a problem because medical assemblies are made of all titanium or are made of compatible Cobalt-Chrome alloys.

Making things: welding, machining, and heat treatment

To process Grade 23, you need to think like a doctor: cleanliness and temperature control are very important.

Instructions for CNC Machining

Grade 23 machining is like Grade 5 machining, but the stakes are higher when it comes to surface integrity.

• Tools: Use carbide tools with a sharp, positive rake. When tools are dull, they make work harder and get too hot, which can make a microscopic alpha-case layer on the machined surface.

• Coolant: You must use high-pressure water-soluble coolant. People often stay away from oil-based coolants in medical parts to make cleaning validation easier.

• Surface Finish: Machined surfaces are often polished or shot-peened to get rid of micro-machining marks that can cause stress risers in parts that are critical for fatigue.

Welding and Joint Strength

You can weld Grade 23, but you need to protect it to keep its “ELI” status.

• Shielding: If the weld takes in oxygen from the air, the weld zone becomes “Standard Grade 5” (or worse) and loses its ductility. You need 99.999% argon shielding.

• Laser welding is often used to put together the housing of medical devices because it doesn’t use a lot of heat.

Treatment with heat

• Annealing: Most Grade 23 is used when it is annealed (700°C–785°C) to make it as tough as possible.

• No Hardening: Grade 23 is not often Solution Treated and Aged (STA) for high strength, unlike industrial Grade 5. This is because it makes the material less tough, which is the opposite of what ELI was chosen for in the first place.

How to Choose Between Grade 23, Grade 5, and Grade 9

Logic for Choosing:

• Select Grade 23 for medical implants (ASTM F136 is required) or cryogenic parts where standard titanium becomes brittle.

• Select Grade 5 for general aerospace brackets or industrial shafts where the highest strength is needed and the risk of breaking is controlled.

• If the part needs a lot of cold forming (bending or tubing), choose Grade 9.

Common Uses of Grade 23 Titanium in Industry

1. Implants for the bones

   • Parts: hip stems, knee femoral parts, bone plates, and spinal cages.

   • Why Grade 23? High cycle fatigue strength (walking creates millions of cycles), biocompatibility, and a modulus that is closer to bone than steel.

2. Implants for teeth

   • Parts: screws and abutments for the implant.

   • Why Grade 23? Able to handle a lot of chewing force without breaking, and it fits perfectly with jawbone tissue.

3. Aerospace Parts for Cryogenics

   • Parts: tanks for liquid oxygen and hydrogen fuel and high-pressure vessels.

   • Why the 23rd grade? At -250°C, Standard Grade 5 loses its ability to bend. Grade 23 stays very tough even at cryogenic temperatures.

Common Design Mistakes with Grade 23 Titanium

1. Saying “Grade 5” for medical parts

Designers frequently consider Grade 5 to be “satisfactory.”

• Risk: Standard Grade 5 has more iron and oxygen, which can cause bad biological reactions or early fatigue failure in vivo. Regulatory bodies (FDA/EMA) usually don’t allow non-ELI titanium for long-term implants.

2. Not paying attention to the surface finish

Making an implant that is critical for fatigue but leaving the surface “As Machined.”

• Risk: Tool marks cut down on fatigue life by a lot. It is important to specify in the print what the standard finishing steps are, such as polishing, passivation, or anodizing.

3. Putting too much faith in strength

Choosing Grade 23 just for its strength.

• Correction: If you only need strength and don’t care about fracture toughness, standard Grade 5 is stronger and cheaper. Grade 23 is a specific choice for toughness.

Summary — Important Points for Engineers

The titanium world has a high-reliability version called Grade 23 Titanium (Ti-6Al-4V ELI). It turns the workhorse Grade 5 alloy into a material that can hold life-threatening loads in the human body and the harsh vacuum of space by strictly limiting interstitial elements.

• Safety First: It gives up a little bit of static strength in exchange for a huge increase in Fracture Toughness and Fatigue Crack Growth Resistance.

• Medical Standard: Following ASTM F136 is the first step to getting into the medical device market.

• Process Sensitivity: The quality of its properties depends on how it was made. To keep its ELI status, machining heat, surface finishing, and contamination must be tightly controlled.

Choosing Grade 23 means that the engineer won’t let anything go wrong. Grade 23 gives critical applications the performance margin they need, whether it’s to support a broken bone or hold cryogenic fuel.

FAQ

Q1: Is Grade 23 titanium stronger than Grade 5?
No. Grade 23 (Yield ~795 MPa) is usually 5–10% weaker than Grade 5 (Yield ~880 MPa). The decrease in oxygen (a strengthener) makes static strength weaker but makes toughness and ductility much stronger.

Q2: Is it okay to use Grade 23 in factories?
Yes, but it’s usually too much. Standard Grade 5 is cheaper and easier to find unless the application needs cryogenic temperatures or very high fracture toughness.

Q3: Does Grade 23 titanium have a magnetic field?
No, it does not have a magnetic field. This is very important for patients with implants who might need MRI scans (Magnetic Resonance Imaging) because magnetic materials would heat up or move.

Q4: How do ASTM F136 and ASTM B348 differ from each other?
For Grade 23, ASTM F136 is the medical implant standard. It calls for strict testing of microstructure and biocompatibility. ASTM B348 is the standard for industrial bars. There is a B348 Grade 23, but it does not require the same level of quality control as implants.

Q5: Is it hard to polish Grade 23?
It shines well, but you have to be patient. It can “smear” if you polish it too hard because it is softer and more ductile than Grade 5. To get a mirror finish, medical polishing processes are very specialized.