**5.1.2 SIMS depth profile of oxidized Fe-10Cr alloys**

In order to understand the mechanism of improved oxidation resistance of nanocrystalline Fe-10Cr alloy, the composition, including Cr content of the thin oxide films developed on the nanocrystalline and microcrystalline alloys was characterized. The thin oxide films formed over nanocrystalline Fe-10Cr and microcrystalline Fe-10wt%Cr alloys at 300, 350 and 400oC in air were characterised by SIMS depth profiling [12,39].

Oxidation resistance of Fe-Cr alloys was associated the development of a protective layer of Cr2O3. Depth profiles for Cr, O and Fe for the nanocrystalline and microcrystalline Fe-10Cr alloys oxidized at the three temperatures for 30, 120 and 3120 min were obtained [12,32,33]. It was found that the oxide film developed on microcrystalline Fe-10Cr alloy is considerably thicker than that on nanocrystalline Fe-10Cr alloy at the three test temperatures [12,39].

The most relevant findings of the SIMS analyses as reported in the literature [12,38,39] are the depth profiles of chromium and their consistency with the trends of oxidation kinetics. The Cr depth profiles obtained after 52 hours of oxidation are presented in the Figures 8-10. At each of the oxidation temperatures, Cr content of the inner layer of nanocrystalline Fe-10Cr alloy was invariably found to be considerably higher than the highest Cr content in the inner layer of microcrystalline Fe-10Cr alloy. This provides an explanation for the greater oxidation resistance of the nanocrystalline Fe-10Cr alloy (as shown in Figures 8-10), since oxidation resistance of Fe-Cr alloys is governed primarily by the Cr content of the thin oxide scale.

Fig. 7. Oxidation kinetics (weight-gain vs time plot) of nanocrystalline (nc) and

**5.1.2 SIMS depth profile of oxidized Fe-10Cr alloys** 

0

1

2

3

4

Weight gain per unit area

(mg/cm2

)

5

6

7

 nc Fe10Cr mc Fe10Cr

400oC in air were characterised by SIMS depth profiling [12,39].

oxidation).

scale.

microcrystalline (mc) Fe-10Cr alloys, during oxidation at 400°C for 3120 min in air [12,39]. Inset shows a zoom of the region of initial periods of oxidation (up to 240 minutes of

0 500 1000 1500 2000 2500 3000 3500

Time (min)

0.0 0.5 1.0 1.5 2.0 2.5

0 50 100 150 200 250

In order to understand the mechanism of improved oxidation resistance of nanocrystalline Fe-10Cr alloy, the composition, including Cr content of the thin oxide films developed on the nanocrystalline and microcrystalline alloys was characterized. The thin oxide films formed over nanocrystalline Fe-10Cr and microcrystalline Fe-10wt%Cr alloys at 300, 350 and

Oxidation resistance of Fe-Cr alloys was associated the development of a protective layer of Cr2O3. Depth profiles for Cr, O and Fe for the nanocrystalline and microcrystalline Fe-10Cr alloys oxidized at the three temperatures for 30, 120 and 3120 min were obtained [12,32,33]. It was found that the oxide film developed on microcrystalline Fe-10Cr alloy is considerably thicker than that on nanocrystalline Fe-10Cr alloy at the three test temperatures [12,39].

The most relevant findings of the SIMS analyses as reported in the literature [12,38,39] are the depth profiles of chromium and their consistency with the trends of oxidation kinetics. The Cr depth profiles obtained after 52 hours of oxidation are presented in the Figures 8-10. At each of the oxidation temperatures, Cr content of the inner layer of nanocrystalline Fe-10Cr alloy was invariably found to be considerably higher than the highest Cr content in the inner layer of microcrystalline Fe-10Cr alloy. This provides an explanation for the greater oxidation resistance of the nanocrystalline Fe-10Cr alloy (as shown in Figures 8-10), since oxidation resistance of Fe-Cr alloys is governed primarily by the Cr content of the thin oxide

Fig. 8. SIMS depth profiles for Cr the in the oxide scale developed during oxidation of nanocrystalline (nc) and microcrystalline (mc) Fe-10Cr alloys at 300°C for in air for 3120 minutes, using a Cameca ims (5f) dynamic SIMS instrument. SIMS parameters were: Cs+ ion primary beam (10 nA), depth profiling of craters of 250 µm × 250 µm area [12,39].

Fig. 9. SIMS depth profiles for Cr the in the oxide scale developed during oxidation of nanocrystalline (nc) and microcrystalline (mc) Fe-10Cr alloys at 350°C for in air for 3120 min, using a Cameca ims (5f) dynamic SIMS instrument. SIMS parameters were: Cs+ ion primary beam (10 nA), depth profiling of craters of 250 µm × 250 µm area [12,39].

Oxidation Resistance of Nanocrystalline Alloys 233

Fig. 11. Comparison of oxidation resistance of nanocrystalline Fe10Cr alloy with that of

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

0 500 1000 1500 2000 2500 3000 3500

Time (min)

 nc Fe10Cr mc Fe20Cr

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

microcrystalline Fe20Cr alloy at 350°C in air [12,39].

0.00

0.05

0.10

0.15

Weight gain per unit area

(mg/cm2

)

0.20

0.25

0.30

0.35

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

of Al in the formed oxide scale [45,118].

SIMS analysis as carried out in our previous work [12,39] provides a qualitative analysis of Cr enrichment of the surface. Based on such qualitative analysis of Cr content, a Cr2O3 oxide layer was proposed to develop in nanocrystalline alloy, whereas, it was proposed that a mixed Fe-Cr oxide layer forms in case of microcrystalline alloy. A Future study quantifying the Cr, Fe and O contents of oxide layer and their oxidation states using techniques such as X-ray photoelectron spectroscopy (XPS) must provide a better understanding of the effect of nanocrystalline structure on the chemical composition of oxide layer.

Fig. 10. SIMS depth profiles for Cr the in the oxide scale developed during oxidation of nanocrystalline (nc) and microcrystalline (mc) Fe-10Cr alloys at 400°C for in air for 3120 min, using a Cameca ims (5f) dynamic SIMS instrument. SIMS parameters were: Cs+ ion primary beam (10 nA), depth profiling of craters of 250 µm × 250 µm area [12,39].

#### **5.1.3 Oxidation resistance of nanocrystalline Fe10Cr versus oxidation of microcrystalline Fe20Cr alloy**

For developing an understanding of how the considerably greater oxidation resistance of nanocrystalline Fe-10Cr alloy (in comparison with microcrystalline Fe-10Cr alloy) compares with the resistance of an alloy with much higher Cr content, samples of microcrystalline Fe-20Cr alloys were also oxidized at 350 °C for durations up to 3120 minutes [12,39]. However, what is most relevant to note is that the weight gain at the end of 3120 minutes of oxidation of microcrystalline Fe-20Cr alloy is similar to that of the nanocrystalline Fe-10Cr alloy at 350 °C for same period of time (shown in Figure 11), suggesting the degree of oxidation resistance conferred due to nanocrystalline structure at only 10% chromium to be similar to that of the alloy with 20% chromium but microcrystalline structure. This finding may have wide industrial applications in developing steel with low Cr but very high oxidation resistance as exhibited by Fe20Cr alloy.

SIMS analysis as carried out in our previous work [12,39] provides a qualitative analysis of Cr enrichment of the surface. Based on such qualitative analysis of Cr content, a Cr2O3 oxide layer was proposed to develop in nanocrystalline alloy, whereas, it was proposed that a mixed Fe-Cr oxide layer forms in case of microcrystalline alloy. A Future study quantifying the Cr, Fe and O contents of oxide layer and their oxidation states using techniques such as X-ray photoelectron spectroscopy (XPS) must provide a better understanding of the effect of nanocrystalline structure on the chemical composition of

Fig. 10. SIMS depth profiles for Cr the in the oxide scale developed during oxidation of nanocrystalline (nc) and microcrystalline (mc) Fe-10Cr alloys at 400°C for in air for 3120 min, using a Cameca ims (5f) dynamic SIMS instrument. SIMS parameters were: Cs+ ion primary

Sputtering time (s)

100 1000 10000

 nc Fe10Cr mc Fe20Cr

For developing an understanding of how the considerably greater oxidation resistance of nanocrystalline Fe-10Cr alloy (in comparison with microcrystalline Fe-10Cr alloy) compares with the resistance of an alloy with much higher Cr content, samples of microcrystalline Fe-20Cr alloys were also oxidized at 350 °C for durations up to 3120 minutes [12,39]. However, what is most relevant to note is that the weight gain at the end of 3120 minutes of oxidation of microcrystalline Fe-20Cr alloy is similar to that of the nanocrystalline Fe-10Cr alloy at 350 °C for same period of time (shown in Figure 11), suggesting the degree of oxidation resistance conferred due to nanocrystalline structure at only 10% chromium to be similar to that of the alloy with 20% chromium but microcrystalline structure. This finding may have wide industrial applications in developing steel with low Cr but very high oxidation

beam (10 nA), depth profiling of craters of 250 µm × 250 µm area [12,39].

**microcrystalline Fe20Cr alloy** 

0.0

2.0x106

4.0x106

Counts per s

6.0x106

8.0x106

resistance as exhibited by Fe20Cr alloy.

**5.1.3 Oxidation resistance of nanocrystalline Fe10Cr versus oxidation of** 

oxide layer.

Fig. 11. Comparison of oxidation resistance of nanocrystalline Fe10Cr alloy with that of microcrystalline Fe20Cr alloy at 350°C in air [12,39].
