Material of the month - Titanium

Materials World magazine
,
13 Oct 2014

This month, Anna Ploszajski reveals the history of titanium.

It gives rubies their starry shine and forms alloys as hard as sapphire. It supports and protects the human body on both the inside and outside, and has the potential to solve the global energy crisis. But how can a single element be so multifaceted?

Compared to some ancient metals, titanium is a relatively young element. In 1791, William Gregor discovered a strange white metallic oxide in Cornwall, which was later named for the Titans of Greek Mythology after being rediscovered by Martin Heinrich Klaproth. But it wasn’t until 1910 that pure metallic titanium was first isolated from its oxides by Matthew A Hunter, who heated titanium tetrachloride (TiCl4) with sodium at 800 degrees C. Even then, it still took 30 years before William Justin Kroll refined this process and industrial-scale production took off.

The Kroll process is still used for commercial titanium production today. Titanium reacts with oxygen at high temperatures, so it is not possible to produce it by straightforward reduction. Instead, the Kroll process involves firstly converting rutile, a mineral primarily composed of titanium dioxide, to TiCl4 using chlorine gas in the presence of carbon at 1,000degrees C. Other volatile chlorides are formed too, and must be removed by continuous fractional distillation during this step. The TiCl4 is then reheated to 850 degrees C and reduced by liquid magnesium or sodium in a stainless steel vessel to form a porous titanium metal sponge. This sponge is purified by leaching or heated vacuum distillation, then crushed and pressed. It is melted in a consumable electrode vacuum arc furnace, and then allowed to solidify under vacuum into a solid ingot. Further re-melts improve the purity of the product.

This is a complex and highly expensive batch process, due to the multiple stages of heating to high temperatures and the sacrificial use of expensive magnesium. It renders titanium around six times more expensive than stainless steel, even though its most common ores, rutile and ilmenite, are globally abundant. However, a relatively new production method called the FFC Cambridge process may be able to drive down the price of titanium by using molten salt electrochemistry to turn mixed oxide powders directly into titanium metal, eliminating the need for costly multistep melting (see A salt solution, October 2012 Materials World).

Uses and applications
In the early years, titanium was viewed as a highly strategic commodity after being used in Soviet military applications in the 1950s and 1960s during the Cold War. Furthermore, the US Department of Defence encouraged efforts for commercialisation of the new metal and also kept it at the Defense National Stockpile Center. Today, titanium alloys still find extensive use in military, aerospace and marine applications, due to its high tensile strength-to- density ratio, corrosion resistance, fatigue resistance and low propensity to creep.

Titanium has proved an important material for the aerospace industry. The world record for the fastest manned air-breathing jet aircraft remains unbroken since 1976 at 3,531kph, achieved by the Lockheed SR-71 Blackbird. It managed this amazing speed partly because the aircraft was made from 85% titanium and 15% polymer composite materials. Titanium has the highest strength to density ratio of any metallic element and is 45% less dense than some steels. It also has a high melting point (1,650 degrees C), making it able to withstand Blackbird’s high surface and internal operating temperatures.

Titanium-based military aerospace technology has filtered down into pedestrian aircraft, used for rotors, compressor blades and components of hydraulic systems in the jet engines that transport us. Most of the titanium present in aircraft is an alloy containing 6wt% aluminium and 4wt% vanadium (Ti-6Al-4V). The solubility of these inclusions varies dramatically with temperature, which allows post-production heat treatments to bring precipitation strengthening and fine-tuned performance.

But this alloy isn’t just useful for taking us on holiday. Ti-6Al-4V is also used extensively for load-bearing orthopaedic implants on account of its inherent biocompatibility. Titanium has a well-matched Young’s modulus with bone, and so skeletal loads are distributed equally between the host bone and the implant. This leads to a lower risk of bone degradation from stress shielding and subsequent fracture. A titanium hip replacement can remain fully functional for 20 years, and titanium dental implants have a lifetime of more than 30 years. Furthermore, titanium is non-ferromagnetic, so patients with implants can be safely examined using MRI scanners.

However, despite their matching elastic properties, titanium is more than twice as stiff as bone, so the bone bears a greatly reduced load, which can lead to deterioration. This can cause loosening of the prosthesis and further complications over time. Furthermore, Ti-6Al-4V has poor shear strength under certain loading conditions, making it unsuitable for use in screws or plates, and its undesirable surface wear makes can often make it seize if used in sliding contact with itself or other metals. What’s more, vanadium has demonstrated cytotoxic outcomes when isolated. For these reasons, an alternative alloy with matching strength and biocompatibility to Ti-6Al-4V but containing 7% niobium instead of vanadium (Ti-6Al-7Nb) is preferred.

Many familiar everyday objects are made from titanium, including sporting goods, backpacking equipment, horseshoes, mobile phones and even the casing of a laptop. At a larger scale, the Guggenheim Museum in Bilbao, Spain, (pictured) is famously sheathed in curvaceous titanium panels, and has been repeatedly hailed as one of the greatest architectural works of our time.

Upon exposure to the air, titanium immediately forms a passive oxide coating, which protects the bulk metal from further oxidation. This protective layer renders titanium immune from attack by most acids and chlorine solutions. Exceptional corrosion resistance combined with excellent strength properties make titanium the perfect candidate for drip shields in nuclear waste storage to provide greater corrosion resistance. Other applications for titanium in highly corrosive environments include desalination plants, fishing equipment, and ocean surveillance and monitoring devices used by scientists and the military.

All of the applications discussed so far have dealt with titanium in its metallic state. However, this accounts for just 5% of the titanium extracted from the ground. The remaining 95% is destined for application as titanium dioxide. TiO2 is bright white and highly opaque, so it is used as white pigment in paints, paper, toothpaste and plastics. It is also used in cements, and as a strengthening agent in graphite composite fishing rods and golf clubs.

TiO2 has generated great excitement in recent years at the nanoscale. Nanoparticles less than 50nm in diameter absorb UV light and scatter visible light, making them excellent additives for sunscreen, which is invisible on the skin. Although there has been some public concern over the health risks surrounding nanomaterials used in cosmetics, researchers have found that TiO2 nanoparticles do not penetrate undamaged skin.

Titanium dioxide even promises to pave the way towards a greener future. In 1972, a phenomenon called the Honda-Fujishima effect was published, which describes the photocatalytic properties of nanoscale TIO2. When a voltage is supplied, TiO2 is able to carry out hydrolysis, the splitting of water into its constituent elements, hydrogen and oxygen. If this process is completly powered by renewable energy sources such as solar panels, carbon-neutral hydrogen can be produced from water. This hydrogen could be used in sustainable power technologies such as fuel cells, alleviating our reliance on fossil fuels forever.

This photocatalytic response can also be put to use in a very different setting. Thin films of TiO2 coated onto glass exhibit two separate phenomena, which make it self-cleaning. The first is the photocatalytic breaking down of adsorbed organic matter when TiO2 interacts with UV radiation in sunlight. The second is superhydrophilicity, whereby UV radiation creates surface oxygen vacancies, converting Ti4+ to Ti3+ sites, which are favourable for dissociative water adsorption and produce hydrophilic domains. So when it rains, the surface is easily wet by water droplets, which carry away the dirt as they roll down the glass.

From bulletproofing to aerospace, and photocatalysis to prosthese, titanium boasts a magnificent array of diverse applications. In its brief lifetime it has infiltrated our everyday lives at an astounding rate. With economical processing routes just around the corner, it seems likely that we’ll be seeing a lot more from titanium in the future.