Metallic materials that have high-temperature applications in gas turbines require a combination of excellent mechanical strength and resistance to creep — deformation resulting from long-term applied stress. Nowadays, nickel-based 'superalloys' are key materials in the hottest sections of gas turbines, largely on account of their exceptional creep resistance at temperatures of up to 1,100 °C (almost 90% of their melting temperature)1. The use of these superalloys has led to a spectacular decrease in the fuel consumption of gas turbines over the past 40 years, reducing their environmental impact2. Searching for new lightweight metallic materials of even greater high-temperature capability than the superalloys is one of the biggest challenges the aerospace industry faces in reducing its contribution to global warming3. On page 378, Darling et al.4 report the development of a nanocrystalline alloy that combines impressive mechanical strength with high-temperature creep resistance.

Nanocrystalline metals and alloys are made of minuscule grains with diameters typically smaller than 100 nanometres5. Because of this structure, nanocrystalline materials have excellent mechanical strength at low temperatures (up to a few hundred degrees Celsius). However, the poor creep resistance of such materials has always prevented their use for high-temperature applications6.

Darling and colleagues' nanocrystalline alloy is based on a system consisting of copper grains with average diameters of about 50 nm (Fig. 1). The authors introduce particles of the metal tantalum with diameters of between 3 and 32 nm to the boundaries between the grains. To process the alloy, the copper and tantalum particles are first mechanically milled for 4 hours at a very low temperature (−196 °C) to produce a powder with the desired particle sizes. The powder is then pushed through a channel at 700 °C to form a bar of the alloy called a billet. The authors repeat this last process four times, which results in severe plastic deformation of the alloy — the billet is 460% longer by the end of the process, ensuring a fine grain size.

Figure 1: A creep-resistant nanocrystalline alloy.
figure 1

Darling et al.4 construct a nanocrystalline alloy that has high-temperature resistance to creep — deformation under continuous stress. a, The alloy is based on a system of grains that are separated by boundaries made of copper atoms (a single grain is illustrated here). b, When long-term stress is applied to a typical system, the copper atoms diffuse to new positions, increasing the size of the grain. c, The authors add tantalum particles to a grain boundary. When long-term stress is applied, the size of the grain does not increase as significantly as in the case of pure copper — the alloy has greater creep resistance.

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The alloy's exceptional creep properties result mainly from the stability of its microstructure — the tantalum particles pin down the grain boundaries, preventing them from moving to new positions in response to stress at high temperatures. The authors find that the rate at which creep occurs in the alloy is six to eight orders of magnitude lower than in most other nanocrystalline metals, implying a spectacular improvement in durability. However, the alloy is not a direct candidate material for the hottest sections of gas turbines because it has high strength and creep resistance only at rather low temperatures (up to 600 °C), compared with the current nickel- and cobalt-based superalloys. But its development opens the door for new types of nanocrystalline alloy, provided that several key issues are addressed.

The first main concern for the industrial use of these nanocrystalline alloys is the processing route, especially in the aerospace industry, which requires reliable and stable processes. For alloys such as that developed by Darling et al., a uniform dispersion of grain-boundary pinning particles, as well as a controlled grain size over a large volume, would be necessary for high-temperature components in gas turbines such as blades, vanes or disks. However, it would probably be difficult to achieve these two properties using the authors' processing route (particularly during the milling and severe plastic-deformation stages).

A second concern is resistance to oxidation, another design criterion for high-temperature industrial applications. Increasing the density of grain boundaries in metallic systems generally enhances oxidation because it raises the rate of grain-boundary diffusion7. However, a nanocrystalline alloy could be made more resistant to oxidation if a dense and protective outer oxide layer were to be rapidly grown. This could be achieved by adding to the alloy elements such as aluminium or chromium, which are widely known to improve the environmental resistance of metallic materials7. Alternatively, a specific coating might be developed for the nanocrystalline alloy. Such a coating would need to be chemically compatible with the alloy (having similar chemical composition and thermal expansion) and contain high levels of aluminium and chromium to ensure excellent oxidation and corrosion resistance in harsh environments.

Finally, the authors' alloy would need to retain outstanding strength and creep resistance at temperatures considerably higher than 600 °C to be used for the above-mentioned gas-turbine components. Instead of copper, the grains could be made of elements such as nickel or cobalt, which have a higher melting point and a stable crystallographic structure over the entire temperature range required. With the addition of a second source of strengthening (such as intragranular precipitation8, a process that provides efficient obstacles to the irreversible elongation of the grains), the creep properties of the alloy might reach the level of the nickel-based superalloys that are used today, but with greater mechanical strength. This increased strength could allow load-bearing sections of the components to be made smaller, and hence vastly lower in weight — improving gas-turbine efficiency and reducing fuel consumption.Footnote 1