**7. Future research**

All studies reporting excellent oxidation resistance of nanocrystalline NiCrAl alloys were conducted at high temperatures. However, the grain growth behaviour of the alloys at these elevated temperatures was not determined and therefore many questions remain unanswered. Detailed studies investigating the effect of such fine grain size on the oxide scale formation along with grain growth of material will be helpful in understanding the underlying mechanism of the improved oxidation resistance.

counterpart. They reported a coating with grain size less than 70 nm may increase the oxidation rate by 4 times. Increment in oxidation resistance due to grain refinement was more pronounced when grain size was below 100 nm. It has been shown than Al content required to prevent external oxidation can be reduced from 6% to only 2% by reducing the grain size to ~ 60 nm [88-93]. These findings may have large industrial implications as it would provide an opportunity to achieve the desired oxidation resistance with lower Al

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

out for high temperature application of nanocrystallline alloys.

underlying mechanism of the improved oxidation resistance.

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

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

All studies reporting excellent oxidation resistance of nanocrystalline NiCrAl alloys were conducted at high temperatures. However, the grain growth behaviour of the alloys at these elevated temperatures was not determined and therefore many questions remain unanswered. Detailed studies investigating the effect of such fine grain size on the oxide scale formation along with grain growth of material will be helpful in understanding the

content.

surface treatment.

**7. Future research** 

Oxidation behaviour of Fe-Cr alloys as described in the section 5.1 was investigated at moderate temperatures [12,37-39]. Choice of moderate temperature was motivated by: 1) a very high difference in the grain boundary and lattice diffusion coefficient values at moderate temperatures and 2) higher grain growth at elevated temperatures. Grain growth of the nanocrystalline materials at high temperatures limits their use for high temperature applications. However, it was shown recently that addition of Zr to Fe-Cr based alloys prevents grain growth of these materials [120,121] and therefore such alloys with small amount of Zr (i.e., Fe-Cr-Zr alloys) will be ideal for investigation of oxidation resistance in the temperature range of 600-800°C.

Grain size of Fe-Cr alloys (used for the investigation of the effect of nanocrystalline structure on oxidation resistance) was limited to 54 (±4) nm which could be further decreased with the recent advancements in the sample preparation techniques such as one recently developed by Gupta et al [122] where an artefact free FeCrNi alloy with a grain size less than 10 nm was produced by in-situ consolidation technique. Further investigations on such alloys with grain size below 10 nm will demonstrate pronounced effect of triple points and grain boundaries and it may be possible to develop stainless steels with further improved oxidation resistance but less Cr content.

Improved oxidation resistance of nanocrystalline Fe-Cr or Ni-Cr-Al alloys have been attributed to the greater Cr and/or Al enrichment of the oxide scale (i.e., change in chemical composition of the oxide scale) due to faster diffusion of Cr and/or Al. However, physical properties of oxide scale, which are very important in determining the oxidation resistance of an alloy, have attracted only a little research attention. Investigation of the physical properties (grain size, morphology, crystallographic details etc.) of the oxide scale formed on the nanocrystalline alloys will further help in understanding the effect of nanocrystalline structure on the oxidation resistance of an alloy.
