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Titanium for Medical Implants - 1

Ti6Al7Nb vs Ti6Al4V ELI: A Comprehensive Comparison

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Introduction

When it comes to selecting materials for medical devices and implants, titanium alloys are often the go-to choice due to their exceptional biocompatibility, strength, and corrosion resistance. Among the most widely used titanium alloys in the medical field are Ti6Al7Nb and Ti6Al4V ELI (Extra Low Interstitial). Both alloys have unique properties that make them suitable for various applications, but understanding their differences is crucial for making informed decisions in medical equipment design and manufacturing.

In this blog post, we’ll explore the properties, advantages, and applications of Ti6Al7Nb vs Ti6Al4V ELI, helping you determine which alloy best suits your specific medical needs.

Ti6Al7Nb vs Ti6Al4V ELI: Chemical Composition

What is Ti6Al4V ELI (Grade 23 Titanium/Grade 5 ELI)?

Ti6Al4V ELI is a high-purity variant of the Ti6Al4V alloy, designed specifically for medical and aerospace applications. Its composition includes:
6% Aluminum (Al)
4% Vanadium (V)
Extra Low Interstitial (ELI) elements (oxygen, nitrogen, carbon, and iron)
Balance Titanium (Ti)

The “ELI” designation indicates that this alloy has reduced levels of interstitial elements, which enhances its ductility, fracture toughness, and biocompatibility. Ti6Al4V ELI is widely used in critical medical applications, such as orthopedic and spinal implants.

What is Ti6Al7Nb?

Ti6Al7Nb is a titanium alloy developed as an alternative to Ti6Al4V, particularly for medical applications. Its composition includes:
6% Aluminum (Al)
7% Niobium (Nb)
Balance Titanium (Ti)

Adding niobium instead of vanadium addressed concerns about potential vanadium toxicity and improved long-term biocompatibility. Ti6Al7Nb is increasingly being adopted in Europe for medical implants and devices.

Titanium for Medical Implants

Titanium for Medical Implants

Ti6Al7Nb vs Ti6Al4V ELI: Properties

Property Ti6Al4V ELI (Grade 23) Ti6Al7Nb (Grade 7)
Biocompatibility Excellent Superior (no vanadium)
Strength High Comparable to Ti6Al4V ELI
Corrosion Resistance Excellent Excellent
Density 4.43 g/cm³ 4.52 g/cm³
Elastic Modulus 110 GPa 105 GPa
Fatigue Resistance High Slightly higher
Fracture Toughness Excellent (due to ELI) Good
Wear Resistance Good Good

Advantages of Ti6Al7Nb Over Ti6Al4V ELI

  1. Improved Biocompatibility
    The absence of vanadium in Ti6Al7Nb makes it a safer choice for long-term implants. Vanadium, though present in small amounts in Ti6Al4V ELI, has raised concerns about potential toxicity over time. Conversely, Niobium is highly biocompatible and inert, making Ti6Al7Nb a preferred option for implants that remain in the body for extended periods.
  2. Enhanced Fatigue Resistance
    Ti6Al7Nb exhibits slightly better fatigue resistance than Ti6Al4V ELI, which is critical for implants subjected to cyclic loading, such as hip and knee replacements.
  3. Better Long-Term Performance
    Studies have shown that Ti6Al7Nb has superior long-term stability in physiological environments, reducing the risk of implant failure or adverse reactions.

Advantages of Ti6Al4V ELI Over Ti6Al7Nb

  1. Proven Track Record
    Ti6Al4V ELI has been used in medical applications for decades, and its success is well documented. Its properties and performance are well understood, making it a reliable choice for many applications.
  2. Superior Fracture Toughness
    The extra-low interstitial (ELI) composition of Ti6Al4V ELI enhances its ductility and fracture toughness, making it ideal for applications where mechanical integrity is critical, such as spinal implants.
  3. Wider Availability
    Due to its long-standing use, Ti6Al4V ELI is more readily available and often more cost-effective than Ti6Al7Nb.
  4. Established Manufacturing Processes
    The manufacturing and machining processes for Ti6Al4V ELI are well-established, making it easier to produce complex components with consistent quality.

Applications in Medical Devices

Ti6Al4V ELI Applications

  1. Orthopedic Implants: Hip stems, knee replacements, and bone plates.
  2. Spinal Implants: Rods, screws, and cages due to their high fracture toughness.
  3. Dental Implants: Crowns, bridges, and dental screws.
  4. Surgical Instruments: Scalpels, forceps, and retractors.

Ti6Al7Nb Applications

  1. Long-Term Implants: Hip and knee replacements, particularly in Europe.
  2. Cardiovascular Devices: Stents and pacemaker casings.
  3. Dental Implants: Increasingly used for their superior biocompatibility.
  4. Trauma Fixation Devices: Plates and screws for bone fracture repair.

Ti6Al7Nb vs Ti6Al4V ELI: Which Alloy Should You Choose?

The choice between Ti6Al7Nb and Ti6Al4V ELI depends on the specific requirements of your medical application:
Choose Ti6Al7Nb if:

  1. Long-term biocompatibility is a top priority.
  2. You are designing implants for regions with stricter regulatory standards (e.g., Europe).
  3. Fatigue resistance and long-term stability are critical.

Choose Ti6Al4V ELI if:

  1. Fracture toughness and mechanical integrity are paramount (e.g., spinal implants).
  2. You need a material with a proven track record in medical applications.
  3. Cost and availability are significant factors.

Conclusion

Both Ti6Al7Nb and Ti6Al4V ELI are exceptional titanium alloys with unique advantages for medical applications. While Ti6Al4V ELI remains a reliable and widely used option, particularly for its superior fracture toughness and proven performance, Ti6Al7Nb offers enhanced biocompatibility and long-term stability, making it an excellent choice for next-generation medical devices and implants.

When selecting an alloy, consider factors such as biocompatibility, fatigue resistance, fracture toughness, regulatory requirements, and cost. Understanding each material’s strengths and limitations allows you to make informed decisions that ensure your medical devices’ safety, efficacy, and longevity.

If you have further questions or need assistance selecting the right material for your medical application, please contact our team of experts at [email protected]. We’re here to help you navigate the complexities of material science and deliver solutions that meet the highest quality and performance standards.

Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450: A Technical Guide

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Introduction

Initially developed for application as an aerospace alloy because of its relative ease of formability, high strength, and ductility at temperatures up to 427°C (801°F), Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 is an ideal candidate for rivets that secure aluminum aircraft panels, particularly in areas exposed to high engine exhaust temperatures.

Low Elastic Modulus

This alloy has been used as welded flange components that secure high-energy physics RF linear accelerator cavities (made from high-purity niobium), and has been drawn into superconductivity wire. It is used as a medical device material for cochlear hearing implants, and its low elastic modulus makes it a consideration for various medical and dental devices.

Machinability

Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 is easily fabricated into structures and components. Although it is considered a refractory metal, processing techniques are the same as those for other titanium alloys. Parts are typically welded using the GTAW process but require the addition of trailing and backing shields. Due to its simple metallurgical structure, there is no requirement for post-weld heat treatment. Machining requires the use of solid fixture setups and high torque at low RPM ridge machine tools with sharp high rake cutters.

High Formability

With a minimal bend radius capability (down to a bend radius of 1t where t=sheet thickness) and low modulus of elasticity, Ti-45Nb Alloy is highly formable yet retains excellent tensile and elongation properties. It can also be cast into shapes with the same processes used to cast CP titanium.

Surface Oxidation Treatment

Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 readily accepts surface oxidation treatment to enhance its hardness and wear resistance. “Nobelizing” is a common commercially available process to apply this type of surface treatment.

Corrosion Resistance Higher Ignition Resistance

Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 provides improved corrosion resistance along with higher strength compared to that of Ti-7, 12, 16, and 26 in many chemical environments. Ti-45Nb Alloy has also proven to be a great success in the autoclave processing of gold ore. For many years gas vents, oxygen lances, steam spargers, valves and other pressure oxidation reactor components have been produced, put into service, and survived hot, concentrated sulfuric or hydrochloric acid. In addition, the metal has been shown to have higher ignition resistance when compared to CP Ti, thereby allowing further process optimization.

Another chemical processing application for Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 is the oxygen-based, low-pressure, wet oxidation process for the treatment of wastewater and sludge. Because of its corrosion and higher ignition resistance, this material is an ideal candidate for the 200°C (392°F), 20 bar pressure with oxygen environment. The superior corrosion resistance of Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 makes it well suited for service in various environments, including most organic and mineral acids below 150°C (302°F). Because of its stable oxide film, Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 has excellent resistance to oxidizing acids, including mixtures of HNO3 and HCL; however, it is susceptible to attack from HF and other fluoride ions.

Acids and corrosive solutions that Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 has shown good chemical resistance to include:
Hydrochloric (HCL)
Sulfuric (H2SO4)
Hydrobromic (HBr)
Hydroiodic (HI)
Saltwater / Brine / Salt Solutions

Potential Applications

Aerospace rivet material
High pressure oxygenated gas vents
Oxygen lances for pressure oxidation reactors
Valves for corrosive oxygenated processes
Chemical corrosion resistance
Medical implant devices
High energy physics and superconducting wire

Figure 1. Ferrallum 255 super duplex stainless (top coupon) showed extensive corrosion (50% of wall thickness) after eight months in a mining autoclave. Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450 (bottom coupon) showed no measurable corrosion after four months and has tested well for nearly two years.

Ferrellum 255 SDSS vs. Ti-45Nb

Specifications & Certificates

Plate, sheet, tube, pipe, bar, billet, extrusions, wire, castings.

Product Forms

ASTM
B265 – Titanium and Titanium Alloy Strip, Sheet and Plate
B338 – Seamless and Welded Titanium and Titanium Alloy Tubes
B348 – Titanium and Titanium Alloy Bars and Billets
B381 – Titanium and Titanium Alloy Forgings
B363 – Seamless and Welded Unalloyed Titanium and Titanium Alloy Welding Fittings
B861 – Titanium and Titanium Alloy Seamless pipe
B862 – Titanium and Titanium Alloy Welded Pipe
B863 – Titanium and Titanium Alloy Wire

AWS
AWS A5.16/A5.16M:2007 – Specification for Titanium and Titanium-Alloy Welding Electrodes and Rods. Ti Grade 36 weld wire is designated in AWS A5.16/A5.16M as ERTi-36.
AWS G2.4/G2.4M:2007 – Guide for the Fusion Welding of Titanium and Titanium Alloys

Chemical Composition

Nominal 55 wt% Titanium, 45 wt% Niobium

Ingot Chemistry Nb O C N H Fe Ti
Typical +/- 1.5% 850 175 75 5 250 +/- 1.5%
Specification 42.0-47.0 < 1600 < 400 < 200 < 35 < 300 Balance

Note: Interstitials are in units of ppm. To obtain units of wt%, divide by 10,000.

Physical Properties

Property Value
Density 5.7 g/cm³ (0.206 lb/cu.in)
Melting Point 1900°C (3452°F)
Phase Transition or Beta Transus No beta transus. Alloy is beta phase to room temperature.
Coefficient of Thermal Expansion (CTE) 9.03 x 10^-6/°C (5.02 x 10^-6/°F)
Thermal Conductivity 10 W/m·K (5.78 BTU/hr·ft·°F)
Specific Heat 0.427 J/g·°C (0.102 BTU/lb·°F)
Thermal Diffusivity 4.3 x 10^-6 m²/s (46.3 ft²/sec)

Mechanical Properties

Property Value
Ultimate Tensile Strength (at RT) 450 MPa (65,000 psi)
Yield Strength (at RT) Minimum 410 MPa (60,000 psi), Maximum 655 MPa (95,000 psi)
Elongation (at RT) 10% (in 2 inches)
High Temperature Strength Graphs See Figure 2 and Table 4
Bend Radius Under 0.070 inch (1.8 mm) thickness = 4.5T<br>0.070 to 0.187 inch (1.8-4.75 mm) thickness = 5T
Modulus of Elasticity 62.05 GPa (9 x10^6 psi)

Tensile StrengthTable 4. Mechanical Properties for Titanium Grade 2 and Niobium vs. Ti-45 Niobium

Material Elastic Modulus (psi) Yield Strength (psi) Ultimate Strength (psi) Integrated Thermal Contraction 293K to 1.88K (in/in)
Niobium 1.52E+07 55000 46000 0.0014
Ti-45 Niobium 9.00E+06 69000 79000 0.0019
Titanium Grade 2 1.55E+07 40000 50000 0.0015

Other Properties and Pertinent Information

Ti Ignition Curve

Ti-45Nb, Grade 36, 55Ti-45Nb, UNS R58450, provides a unique solution for demanding applications requiring both high performance and biocompatibility. Its advantages are clear, from medical implants to high-stress aerospace parts. By understanding its properties and choosing it wisely for the right applications, you can harness the potential of Ti-45Nb to improve outcomes, extend product life, and contribute to advancements in multiple industries.