**5.2 Ni-Cr-Al based alloys**

Oxidation resistance of Ni-Cr-Al based alloys largely depend upon the chemical and physical properties of alumina scale formed during high temperature oxidation. Most of the Ni-Cr-Al alloy contains enough Cr to form external Cr2O3 scale but for the application over 1000°C, the chromia scale does not provide any beneficial effect due to the volatilisation problems. Formation of a compact protective Al2O3 scale is most efficient in protecting material from the high temperature oxidation. In general, a conventional Ni-20Cr-Al system requires more than 6 wt% of Al to form protective oxide scale which largely depends upon the a) diffusion coefficient of the Al from the bulk to alloy/oxide interface and b) diffusion of Al in the formed oxide scale [45,118].

Since nanocrystalline materials possess significantly higher diffusion coefficient caused by higher fraction of grain boundaries therefore Al required for formation an exclusive Al oxide film can be reduced significantly and nanocrystalline materials should show improved oxidation resistance [88-93,119]. Wang et al [119] were among first researchers to investigate the oxidation resistance of nanocrystalline Ni-Al-Cr alloys and reported a significant improvement in the oxidation resistance of NiCrAl alloys due to nanocrystalline structure. Later, various authors have investigated the oxidation resistance of nanocrystalline NiCrAl alloys with various Al and Cr contents and produced by different methods but in all the cases nanocrystalline structures was reported to enhance the oxidation resistance. Most comprehensive work showing effect of the nanocrystalline structure on NiCrAl alloys was that of Gao et al [88] who reported excellent oxidation behaviour of a nanocrystalline coating of Ni20CrAl alloy over its microcrystalline

Oxidation Resistance of Nanocrystalline Alloys 235

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

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

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

Nanocrystalline materials are being investigated due to their unique properties. More generally, development of materials resistant to environmental degradation is not the main focus of nanocrystalline metals research to date, but it seems possible that nanocrystalline metallic materials may lead to a substantial increment in oxidation resistance; caused by promoted oxide scale formation, improved adherence and reduced spallation tendency of the oxide scale. Nanocrystalline Fe-Cr and M-Cr-Al alloys have demonstrated improved oxidation resistance and present potential to be used for high temperature applications in future. More fundamental investigations are required to fully characterise the oxidation

[3] C. C. Koch, K.M. Yousef, R. O. Scattergood, K. L. Murty, Advanced Eng. Mater. 7 (2005)

phenomenon and underlying principles for nanocrystalline materials.

[2] G. Palumbo, S. J. Thorpe, K. T. Aust, Scripta Metall. Mater. 24 (1990) 2347.

[5] R. Birringer, H. Gleiter, H. P. Klein, P. Marquardt, Phys. Lett. A 102 (1984) 365. [6] M. A. Meyers, A. Mishra, D. J. Benson, Progress in Materials Science 51 (2006) 427.

[1] H. Gleiter, Progress in Materials Science, 33 (1989) 223.

[4] R. W. Siegel, NanoStructured Materials 4 (1994) 121

the temperature range of 600-800°C.

oxidation resistance but less Cr content.

structure on the oxidation resistance of an alloy.

**8. Concluding remarks** 

**9. References** 

787

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 content.
