Material of the month – Nickel

Materials World magazine
,
3 Sep 2015

From the loose coins in our pockets to turbine blades and rechargeable batteries, they all have one thing in common – nickel. This month, Anna Ploszajski explores its history. 

This column has featured a great many lustrous transition metals in its time. But how many of those have caused a global extinction, enabled globalisation by air travel, can remember what you do to it, or are named after the devil?

The benefits of nickel as an alloying element have been used for millennia – the earliest discovered example dates back 5,000 years, found in a nickel-iron alloy in Syrian bronzes, containing just 2% nickel. Fast-forward to the 17th Century, and nickel’s red, devilish ore was driving German miners crazy – they believed it contained copper but couldn’t for the life of them extract it. For this reason, the miners called it Kupfernickel, or Old Nick’s Copper – Old Nick being the devil. It wasn’t until 1751 that Swedish scientist Baron Axel Fredrik Cronstedt isolated the metal and named it nickel, after Old Nick. 

Nickel reacts with oxygen in the air to form a passive layer of surface oxide, protecting the metal underneath and healing it when scratched. This surface layer makes nickel unreactive – a feature that is exploited by chemists every time they use a spatula to measure out a chemical. However, nickel’s reactivity means that it is rarely found in its metallic state on Earth. Rather, it is tied up with other elements, such as iron, sulphur and arsenic in the Earth’s crust, and, along with iron, makes up a large portion of our planet’s inner core. Most of the nickel extracted comes from the Philippines, Indonesia, Canada, Russia and Australia in the form of laterite and sulphide deposits. There are also deep-sea resources in the Pacific Ocean. 

Conventional roasting and reduction procedures are used to extract nickel from these ores. This achieves a 75% pure product, which is then refined by the Mond process, reaching 99.99% pure nickel. The method was invented and patented by German-born chemist and industrialist Ludwig Mond in 1899. It involves reacting impure nickel oxide with syngas at 50°C, reducing it to impure nickel metal, which is further heated with carbon monoxide at 50–60°C with a sulphur catalyst to form nickel carbonyl. This then decomposes at 230°C to produce pure nickel and carbon monoxide as a by-product.   

The coins market

Nickel has been used in coins since the mid-19th Century across the world. During the Second World War, many of the nickel-based coins in the USA and Canada were taken out of circulation, due to its value as a military material. Canada saw pure nickel coins returned after the war, though the rising price of the metal by 2000 led to its replacement with cheaper materials. In 2007, rising prices caused problems in the USA – nickel inflated so much that the value of the metal in a five-cent piece (‘a nickel’) was worth nine cents. The USA mint hurriedly criminalised the melting and export of coinage, with violators threatened with imprisonment and fines. By 2013, the metallic value of a nickel was down to 90% of the face value of the coin. Today, €1 and €2 coins, 5¢, 10¢, 25¢, 50¢ USA coins and 20p, 50p, £1 and £2 UK coins all contain nickel. 

Nickel is a good choice for coins, due to its inertness – get caught in the rain or fall in a lake, and your cash will still be safe, and pure nickel coins are also magnetic. In fact, nickel is one of only four elements (the others being iron, cobalt, and the often forgotten gadolinium), which are ferromagnetic at room temperature. Alloying iron with nickel, aluminium and cobalt produces alnico – a strongly ferromagnetic material, used for making permanent magnets in electric motors, microphones, sensors, loudspeakers, magnetron tubes and guitar pickups to name a few. Until the 1970s, alnico magnets were the strongest known, until the discovery of rare earth magnets. Since then, rare earth magnets have begun to replace their alnico counterparts due to their stronger magnetic fields, which allows for smaller and lighter systems.

Mu-metal is an alloy comprising nickel (80%), iron (15%) and molybdenum (5%). Its high permeability makes it an excellent magnetic shield of static or low frequency magnetic fields. The high permeability provides a path for the lines of the incoming magnetic field, guiding it around the protected space. The best shape for mu-metal shields is a closed container surrounding the shielded area. These shields are less effective for low or high magnetic field strengths, so mu-metal shields are often made of several layers which successively reduce the magnetic field.     

Aerospace applications

One application of nickel is an undisputed triumph of materials engineering. Turbine blades made from nickel superalloys boast excellent mechanical strength, corrosion resistance and, most importantly for their role in the heart of a turbine engine, resistance to thermal creep. The secret to these properties is in part, due to the elimination of grain boundaries, by casting the blade as a single crystal. This is achieved by gradually solidifying the molten cast upwards through a curly pigtail section, which acts as a single crystal selector and only a single crystal grows in the main blade section of the casting.   

The alloy itself is a concoction of elements from across the periodic table, each bestowing essential properties to the final material. Aluminium and chromium are added to provide oxidation and corrosion resistance by forming passive surface oxide layers, boron and yttrium aid the adhesion of this layer to the bulk metal. The strength and creep resistance of the material comes from a phase within the alloy called the y’-phase (gamma-prime), formed by alloying nickel with aluminium, tantalum, niobium, titanium and vanadium. These elements preferentially partition to the gamma-prime precipitates, whereas chromium, iron, cobalt, molybdenum and rhenium partition to the matrix providing solid solution strengthening throughout the microstructure. 

The gamma-prime phase appears as cube-shaped precipitates about 0.5 microns across, which typically make up about 70% of the volume of the material. This phase is highly coherent with the matrix and acts as a barrier to dislocation motion, but its most remarkable property is its strength with response to temperature. When dislocations meet the gamma-prime phase, they dissociate, forming an anti-phase boundary. At high temperatures, the free energy of this anti-phase boundary is significantly reduced if its bordering partial dislocations lie along certain planes, which are coincidentally non-slip planes. It is more energetically favourable for the dislocations to cross-slip onto these low energy non-slip planes, in which they are then effectively stuck. This phenomenon causes the yield strength of the gamma-prime phase to actually increase with temperature up to about 1,000°C. 

But these blades must withstand temperatures well above this temperature inside the engine. They are protected from this harsh environment by thermal barrier coatings. These ceramic layers are made from yttria-stabilised zirconia (YSZ) – a material that has extremely low thermal conductivity due to the high concentration of point defects, which scatter lattice vibrations. Furthermore, its feather-like sub-columnar structure provides high strain tolerance against thermal cycling, allowing the surfaces to withstand extreme thermal gradients and, along with a complex network of cooling holes in the metal itself, ultimately allows the blades to operate in temperatures 200°C above its melting point. 

The combination of ingenious phase control and coating means that in a modern jet engine, each blade can extract the same power as a Formula 1 engine, and there are 68 per jet engine. 

Monel alloys 

As well as nickel-based superalloys, nickel is a crucial ingredient in a vast number of key engineering alloys, and is commonly combined with copper, chromium, iron, aluminium, lead, cobalt, silver and gold. In July’s Material of the Month, we saw that nickel was the critical element to the commercialisation of stainless steels – the nickel stabilises the austenitic phase, making the material much less brittle at low temperatures.  

A popular group of nickel-copper alloys are called monel, comprising at least 63% nickel, and the rest copper, and a small percentage of manganese and iron. These alloys are used in settings such as oil refining and marine environments when a higher strength than pure nickel is required, with first class corrosion resistance to ensure a long lifetime. Monel is also used for safety wiring in aircraft to ensure that fasteners do not undo due to corrosion, particularly in high-temperature places, and also as the piston material in high-end brass instruments.

Combining nickel with titanium in roughly equal quantities forms nitinol – a remarkable shape-memory and superelastic intermetallic compound. Discovered in 1959, it can undergo deformation at one temperature, and then spontaneously recover to its original shape by heating above its transformation temperature. This bewildering property is due to a reversible solid-state phase transformation called the martensitic transformation – a shift between a deformed low-temperature martensite phase and a high-temperature austenite phase. The effect is exploited to make self-expanding nitinol stents. They are inserted into a patient’s blood vessel by the surgeon in a collapsed form, and the body heat is sufficient to trigger the expansion of the stent inside the vessel to hold it permanently open.

Recharge the batteries

Nickel has been a crucial element in many rechargeable batteries, such as the early nickel-cadmium battery and its successor, the nickel-metal hydride battery. Nickel-metal hydride batteries have mostly replaced nickel-cadmium units thanks to their higher capacity, non-toxic components and high-energy density. These favourable properties have made nickel-metal hydride batteries the device of choice for electric and hybrid vehicles.

In June 2015, the dawning of the next great extinction event was announced by scientists Pimm and coworkers in the journal Science, forecasting that the 1000-fold increase in the rate of extinction of species may be indicative of the demise of life on Earth as we know it. The finger of blame is, of course, pointed towards global warming although, surprisingly, nickel may have played a major role in a previous mass extinction event 252 million years ago, called the Permian-Triassic extinction. During this time, microbes known as methanosarcina acquired a new metabolic pathway via gene transfer, enabling them to metabolise acetate into methane – a process for which nickel acts as a cofactor in one of the enzymes involved. Volcanic eruptions happened to release vast quantities of nickel and promoted a dramatic increase in methane and carbon dioxide in the Earth’s oceans and atmosphere, rendering it uninhabitable for life on the planet.