PROTON IRRADIATION

The sub-surface of 1 dpa H samples consisted of a zone of nanograins as these samples were mechanically polished. However, to observe the irradiation induced defects, few microns from the surface was removed during TEM sample preparation. No such issues persisted for 2 dpa – H sample.

Frank loops were present in the microstructure post to proton irradiation. TEM DF images of the four different families of the loops with corresponding diffraction pattern are shown in Figure 2-24. These images were obtained by selecting either of the streaks (indicated by red arrows) presents in the diffraction pattern.

Though the Frank loops were observed in all the samples (at surface doses as well as at irradiation peak doses), cavities were observed only in 1 dpa H sample at irradiation peak dose. The cavities observed were facetted in nature and their spatial distribution was homogeneous. Figure 2-25 shows the DF TEM image of the Frank loops and BF TEM image of the cavities observed at the irradiation peak damage (18 dpa KP) in the 1 dpa H sample.

The quantitative assessment of these defects included the estimation of density and size distribution of the defects. This was done using Visilog software. In the case of Frank loops, the quantification was performed on 3 different images for each dose. To estimate the density of the loops, the mean thickness of the TEM foils used was assumed to be 100 nm. As only the TEM foils of thickness ranging between 70 nm to 150 nm are transparent under TEM, the choice of sample thickness 100 nm is justified. As no actual thickness measurements were performed, the thickness chosen could be a source of error in estimating density. Hence, the error in the density value was estimated by assessing the density using foil thicknesses of 70 nm and 150 nm.

Figure 2-24: Four families of Frank loops observed at a depth of 30 µm (~18 dpa K-P) on 1.0 dpa-H proton irradiated TENUPOL prepared sample along with diffraction pattern.

Figure 2-25: a) Dark Field image of the Frank loops observed at the irradiation surface (~ 1 dpa K-P) b) cavities observed at the irradiation peak (~18 dpa K-P) in the 1 dpa-H proton irradiated sample.

Material Investigation

The results of the quantitative assessment of the Frank loops are tabulated in Table 2-6.

Sample Dose (dpa K-P)

Location Frank loops density (x 1022 loops/m3)

Mean Frank loops size (nm)

1 dpa – H 1 Irradiated surface 1.5 ± 0.6 13.6 ± 4.4 2 dpa - H 2 Irradiated surface 3.6 ± 1.5 13.8 ± 4.8

0.25 dpa – H 3 Irradiation peak 1.6 ± 1.0 18 ± 4

1 dpa – H 18 Irradiation peak 21.0 ± 8.0 4.6 ± 1.6

Table 2-6: Comparison of the irradiated microstructure observed for different doses in proton irradiated samples.

Trend in the loops number density and size may not be clear from the Table 2-6 but when plotted as a function of dpa on a lognormal graph, saturation in density and size was observed which was in accordance with literature (see § 2.3.4). The only exception was the size corresponding to damage of 18 dpa K-P.

The quantitative assessment of cavities yielded a mean density of 3.6 ± 1.45 x 1021 cavities/m3 and a mean size of 2.7 ± 0.1 nm. These quantifications were done on 2 different images and the TEM foil thickness of 100 nm was chosen.

Comparison of these results with the literature is provided in the section § 2.3.4.

2.3.2.2. SELF ION IRRADIATION

All the TEM characterizations, in this case, were done at the irradiated surface as the irradiated zone was just 2.5 µm deep in the material. Like proton irradiated samples, Frank loops were observed in all the samples. However, due to small penetration depth, a strong impact of surface preparation was observed in this case.

Starting with mechanically polished samples (10 dpa Fe (mech.)), post irradiation the subsurface microstructure consisted of nanograins. The size of these nanograins ranged between 100 – 300 nm. The nanograins were present prior to the irradiation as well. However, in irradiated samples, some of the nanograins had a low content of Cr and Ni (EDX chemical composition: 89 at % Fe, 10 at % Cr and 1 at % Ni) compared to the nano austenite grains. The diffraction pattern indexed these grains to be Body Centered Cubic (Im3m space group and a = 2.86 Å). This lead to the conclusion that these grains were martensite (marked as M in Figure 2-26a) depleted in Cr and Ni.

Careful inspection revealed the presence of irradiation induced defects in few nano austenite grains. BF TEM image (on zone axis [011]) of one of the nano austenite grain

containing Frank loops is shown in Figure 2-26b. No irradiation induced defects were observed in martensite grains and hence, quantification of these loops was done by just considering the austenite grains volume. The average density estimated using the TEM foil thickness of 100 nm was 6 ± 2 x 1020 m-3 and the average size was 20.3 ± 2.7 nm.

Figure 2-26: Bright Field TEM images showing a) the nano martensite grains (marked as M and Cr23C6 carbides marked as X) along with associated diffraction pattern b) Irradiation induced defects

observed in few nano austenite grains in the 10 dpa Fe (mech) irradiated samples.

Frank loops were observed in both 5 dpa Fe (Figure 2-28a) and 10 dpa Fe samples as well. To recall, these samples were vibro-polished and had no nano grains in the subsurface. The DF TEM image (g = ½(3-11) on zone axis [011]) and high resolution BF TEM image of the Frank loops observed in 5 dpa sample is shown in Figure 2-27. In 5 dpa Fe sample, the size of the majority of the loops observed ranged between 6 and 14 nm. The largest loop size observed was 30 nm while no loop smaller than 2 nm was accounted. The size distribution of Frank loops (Figure 2-28b) appeared to be an asymmetric distribution that extended up to 30 nm similar to what has been reported in literature for neutron irradiated SS 304L [18]. The average number density and diameter of dislocation loops observed were 5 ± 3.1 x 1021 m-3 and 13.4 ± 1.9 nm respectively. The results of the quantitative assessments of the irradiation induced Frank loops are summarized in Table 2-7. As evident, the density of the Frank loops was smaller by a factor of ~ 40 in 10 dpa – Fe (mech.) compared to 10 dpa – Fe despite the same dose. This suggests that the nanograins highly suppressed the density of irradiation induced defects. Considering the error in density measurements due to the choice of TEM foil thickness, a

Material Investigation

slightly lower density of loops in 5 dpa – Fe sample was observed compared to 10 dpa – Fe sample. Detailed comparison of these results with literature is done in § 2.3.4.

Figure 2-27 : a) Rel-rod DF TEM image b) High resolution BF TEM image along with Fourier transform (inset) of Frank loops observed in 5 dpa Fe sample.

Figure 2-28: a) Bright Field TEM image indicating the size of few Frank loops observed b) size distribution of the Frank loops observed in 5 dpa Fe sample.

Sample Dose (dpa K-P)

Location Frank loops density (x 1022 loops/m3)

Mean Frank loops size (nm)

10 dpa – Fe (mech)

10 Irradiated surface 0.06 ± 0.2 20.3 ± 2.7 5 dpa – Fe 5 Irradiated surface 0.5 ± 3.1 13.4 ± 1.9 10 dpa – Fe 10 Irradiated surface 2.55 ± 10.5 14.9 ± 3.6 Table 2-7: Comparison of the irradiated microstructure observed in Fe irradiated samples.

Beside Frank loops, few cavities and irradiation – enhanced carbides (indexed as Cr23C6 in

FCC cell with Fm3M space group and cell parameter a = 10.65 Å and marked as X in Figure 2-26) were observed in 10 dpa Fe (mech) sample (Figure 2-29). Cavities were facetted and their size ranged between 8.2 nm to 28.47 nm giving a mean size of 18 nm. Their distribution was highly inhomogeneous. As they were observed only in one or two nano – austenite grains, density estimation was not performed. But no such defects were observed in any other sample. This suggests that the behavior of mechanically polished material to Fe irradiation was different compared to vibratory polished material due to the presence of nanograins in the former.

Figure 2-29: BF TEM image of a) cavities observed in a nano austenite grain b) carbides (marked as C) with associated diffraction pattern which indexed it as Cr23C6.