**6. Challenges for application of nanocrystalline alloys at high temperature**

For high temperature application, a major problem is thermal stability of the nanocrystalline structure. Thermodynamically, such fine structures could not survive for long times at high temperatures because of the large specific surface energy (driving force) for grain growth. An effective way to prevent this grain growth is by introducing stable second-phase particles in the microstructure, where they play a particle pinning effect (Zener effect) on grain growth [120]. Surprisingly, the coarsening kinetics for nanocrystalline alloys prepared by sputtering is rather slow even without apparent second-phase particles in the microstructure. Lou et al. [19] found that after 100 h oxidation in air at 1000°C, the grains of a sputtered K38G nanocrystalline coating were somewhat coarsened, but still rather finegrained, growing from 20-100 nm to 200-1000 nm. It is not clear if this high stability against grain growth comes from grain boundary segregation of alloy elements (e.g. K38G alloy contains 3.6% Ti, 1.7% Mo, 2.6% W, 1.7% Ta in the Ni-Cr-Al base), which anchor or retard the grain boundary movement (similar to Zener effect) although no particle formation was apparent, or from the sputtered coating structure itself (columnar structure). Unfortunately, no systematic investigation of temperature and alloy composition effects on the thermal stability of these nanocrystalline alloys is available to date, future research should be carried out for high temperature application of nanocrystallline alloys.

Another problem limiting the application of nanocrystalline materials is preparation of nanocrystalline alloys. Currently, the bulk metallic nanomaterials can only be prepared at the laboratory scale, usually by compacting prepared nanocrystalline powders. However, consolidation of the nanopowders into bulk materials needs high temperature and pressure which may considerably coarsen the structure. Because of this difficulty, surface nanocoating has been considered a potential industry application. Nanocrystalline costing are often prepared by chemical vapour deposition (CVD), physical vapour deposition (PVD), electrochemical deposition, electro-spark deposition, and laser and electron beam surface treatment.
