Characterisation of Hydrides in a Zirconium Alloy, by EBSD

Introduction

Zirconium alloys are used in nuclear reactors owing to their low capture cross-section for thermal neutrons and good mechanical and corrosion properties. However, they suffer from delayed hydrogen cracking (DHC) due to formation of hydride particles [1,2]. This study shows how the electron backscattered technique (EBSD) [3] can be used to characterise hydrides in terms of their orientation relationship with the matrix and internal structure and local misorientation.

Methodology

Hydrided electron beam welded zirconium alloy specimens were prepared by electro-polishing, using a 10% perchloric acid and 90% methanol electrolyte at -15°C and 20V. [4]

The specimens were characterised in a FEG SEM and the EBSD technique using both the OI NordlysNano and NordlysMax² detectors and Oxford Instruments' AZtecHKL Software. The SEM was operated at 20kV and EBSD data was indexed using the zirconium – Zr (space group 194, a=0.32, b=c=0.51nm) and zirconium hydride – ZrH₂ (space group 225, a=b=c=0.48nm) phases.

Results and Discussion

The electro-polishing conditions used were successful in not only retaining the hydrides, which appear as long dark plates with a large aspect ratio as shown in the FSD image in Figure 1, but also polishing them such that high quality EBSPs were produced from these particles, as illustrated in a series of EBSPs in Figure 2. Successful EBSD mapping was hence possible as illustrated in the series of Figures 3–5. Figure 3a–c shows a low magnification raw EBSD pattern quality, phase, and IPF Z maps, collected using the NordlysMax² detector. As a step size of 1µm was used for this map only a few pixels in the fine hydride particles were indexed; however, these maps clearly show the grain structure of the zirconium that forms during welding.

Figure 1. FSD image showing the microstructure of the hydrided specimen. Hydrides are appear as dark plate like particles with a large aspect ration, some particles are highlighted with arrows.

Figure 2. raw and indexed patterns from a) the matrix and b) from the hydrides.

Figures 4–6 show high resolution, high magnification maps from the zirconium hydride particles.

Data shown in Figures 4–5 were collected using the NordlysMax² detector while data in Figure 6 was collected using the NordlysNano detector. It is clear from these figures that the hydride particles exist both intra- and trans-granularly and at grain boundaries. It is also evident from the IPF X and Z maps that a strong orientation relationship exists between the hydrides and the zirconium matrix.

An inspection of the pole figures from these phases shown in Figure 7 shows that Blackburn's orientation relationship (0002)Zr//(111)hydride and [11-20]Zr//[1-10]hydride exists between these two phases.

It is also evident from the analysis in Figure 7 that this orientation relationship is maintained across boundaries. This is possible because these boundaries have 60° rotation about the [11-20] axis, which provides the requisite (0002) planes on which the same (111) planes in the hydride can continue to form.

Figure 7. EBSD IPF X map as in Figure 4 and 3D crystal views with respective pole figures showing rotation of matrix grains A and B and hydride and the Blackburn’s orientation relationship between the hydrides and the matrix

Figure 8. EBSD IPF X map as in Figure 5 and 3D crystal views with respective pole figures showing the sigma 3 twin relationship in the hydride and the Blackburn’s orientation relationship between the matrix and hydride respectively

Analysis of the boundaries within the hydride particles (Figure 8) shows that these are Sigma 3 twin boundaries with an angle axis pair 60° about [111] axis. Despite the twinned boundaries, the Blackburn's orientation relationship with the matrix is conserved as rotation about the same [111] variant in the hydride is maintained about the [0002] axis in the matrix grain.

Local mis-orientation maps for the three data sets in Figures 4–6 are shown in Figure 9, from which it is clear that hydrides are associated with higher mis-orientations than the matrix. Local misorientations could have arisen in the particles during their nucleation and growth phase. Additionally, twin boundaries observed in hydrides may have formed to accommodate strains associated with this process.

Conclusions

Careful preparation of hydrided zirconium alloy can yield specimens suitable for EBSD characterisation to determine the orientation relation between hydrides and matrix and also to get detailed insight into the internal structure of the hydrides in terms of twins and local mis-orientations.

References

1. Delayed hydride cracking in zirconium alloys in pressure tube nuclear reactors, IAEA-TECDOC-1410, VIENNA, 2004.
2. Delayed Hydride Cracking of zirconium Alloy Fuel Cladding, IAEA-TECDOC-1649, VIENNA, 2010.
3. Microtexture Determination and Its Applications: V. Randle, Maney Publishing (2008).
4. H S Ubhi. Unpublished work.