Titanium Alloys and Their Use in Bike Frames

Back to top

Introduction

     Titanium (Ti) is often seen as the perfect frame material for the aspiring cyclist, whether for off-road racing or the daily commute, but what exactly is it that makes this metal such a sought after material, and why is it that it is not actually that popular?

Titanium Crystal Bar

     In this investigation, I intend to use Physics to explain the properties of this wonder metal, find out why these properties are so useful to cyclists, and why it is that Titanium is so good at what it does.

    

Structure of Titanium

     Chemically, Titanium’s structure is a giant metallic bond structure. Physically, these bonds hold the nuclei in a hexagonal form, at room temperature. However, Titanium is a Dimorphic Allotrope, and so at 882 ͦc Titanium’s structure changes into a cubic arrangement. Allotropy is the ability of a chemical element to form different structures while remaining pure. An example of another allotropic element is Carbon, which forms a tetrahedral lattice (Diamond), sheets of hexagonal lattice (Graphite) and many other structural shapes.

Titaniums Structure - Self Generated in Jmol

Physical Properties

     Pure Titanium is best known for its excellent strength to weight ratio; this means, that for a solid material of relatively low density, it requires a large amount of stress to break. This is because the mass of a Titanium atom is relatively low, but Titanium’s crystal structure is incredibly strong, as each atom is metallically bonded to four other atoms. This combination is means that you have a very light structure that doesn’t fall apart. It is also reasonably ductile, or able to be drawn out into thin sheets, and is a poor conductor of both heat and electricity. It is not magnetic, and also has quite a high melting point, which can cause issues when forging, as Titanium burns in oxygen at a lower temperature than it melts at. Most commercial grades of Titanium have a similar tensile strength to low grade steel, but can be 45% lighter.
     Aluminium is lighter still, but Titanium is twice as strong as the most common of aluminium alloys. Some Titanium alloys have been known to have tensile strengths over 1400MPa, but they all lose strength above 430 ͦc. In most cases Titanium isn’t very stiff (it does not require much force to bend or twist). For this reason, it is rarely used for structural applications which require high rigidity. Titanium has a reasonably high tensile elasticity for its strength, its Young’s Modulus being similar to that of weaker metals like brass and bronze, and far lower than that of Steel.

Physical Properties - Source 2

Chemical Properties

     Titanium itself is a very reactive metal, but it is because of this reactivity that it is also considered extremely resistant to corrosion, almost as resistant as Platinum. Only the most concentrated of acids can successfully attack this metal, even a reactive gas like Chlorine cannot change it. This is because Titanium forms an oxide layer almost instantly in air, and TiO is very unreactive. Neither does Titanium readily react with water or oxygen at room temperature, however, once heated, this is a different matter.
     At 1200 ͦc Titanium reacts with Oxygen in air, combusting to produce Titanium Dioxide. In pure Oxygen this is cut to 610 ͦc. Thus, Titanium has to be melted down in a vacuum, else it burns before it is hot enough. It even burns in Nitrogen, one of the few elements that does.

Miscellaneous Properties - Source 2

Uses of Titanium

      Pure Titanium, as with most elemental metals, is rarely used commercially, but alloys of Ti are frequently used for a wide variety of purposes. Titanium’s excellent strength to weight ratio and its resistance to corrosion, plus its crack resistance and resistance to fatigue, makes it perfect for use in aerospace (Aircraft, Missiles, Rockets, etc...), military/marine (firearms, naval ship propellers, etc...) and other more commercial markets, such as Mobile Phone, laptop and bike frames.
      Titanium is also considered to be the most biocompatible metal that exists; it is non-toxic and is not rejected by the body, and so it is frequently used for things like hip replacements and other medical and dental implants. This, combined with Titanium’s stunning white-metal complexion means that it is also used more and more frequently for jewellery; Wedding rings, necklaces and even watches are all coming Ti plated or even made of solid Ti.
      Surprisingly the most common use of Titanium is actually as whitener. About 95% of Titanium ore is refined into TiO2, or Titanium Dioxide. This is a bright white compound that is used in paints, plastics, paper and toothpastes. It doesn’t fade, is opaque and is chemically inert, making it perfect for pigment use.

Titanium Ring - Source 3

What Makes Titanium the Cyclists 'Dream Material'?

      Usually, Grade 9 Titanium is used for bike frames. This alloy contains 94.5% Ti, 3% Aluminium and 2.5% Vanadium, and is used because it combines the variety of benefits of Grade 5 with an ease of machining you don’t get in other similar grades. This means that it is significantly stronger than the ‘pure’ grades of Titanium (1-4), but is easier to weld and make into something useful, essential for keeping prices in commercial range.
      Titanium’s excellent strength combined with its corrosion resistance, fatigue resistance and crack resistance means that a frame made of Titanium can last for a lifetime, unlike other bikes that can, without maintenance, swiftly fall into disrepair. Titanium is also well renowned for its excellent ride comfort, due to the fact that it is not stiff. This also means that in an accident, Titanium will not ‘snap’ like other, stiffer metals, it will in fact give a little, and thus survive in situations that other bikes would not. And, of course, weight being all important for cyclists, Titanium’s relatively low density is a dream come true. The only material to match Titanium for all of its other features, Steel, is far surpassed in this field.

Titanum Bike Frame - Source 4

Whats Not So Good?

      Titanium’s high reactivity means that it is difficult and expensive to mould or weld, and when it is welded, the joints are all weakened because they’ve been heated, and all of the stress focuses on these points, increasing the chances of cracks propagating from here. This means that joint have to be ‘butted’, a time consuming process requiring very skilled workmanship. All of these add to the cost of producing the frame, and this is the main problem with Titanium; cost.
     For the ninth most common element in the lithosphere, Titanium’s difficulties in production make it much more expensive to produce than you would expect. It also has to be refined before it’s machined, as it’s almost never found pure, which adds to the cost again. Also, Titanium may be light, but its lack of stiffness means that there is more power wasted with every pedal push than say, a steel frame, due to the slight flex you can achieve.

Titanium is Expensive! - Source 5

Comparison

There are five key Physical Properties that affect materials usefulness in bike frames:

  1. Material Density
  2. Stiffness of material
  3. Yield strength
  4. Elongation
  5. Fatigue and Endurance limits

Points 1 and 2 are all about weight and ride quality; points 3 and 4 are about how well a bike will respond in a crash, and point 5 is how long a frame will last under the stresses of use. This source details the differences between each. Evidently, this is just a rating for each property, and does not include any evidence or data, but it gives a good representation of what makes Titanium so good:

Why Ti Chart - Source 6