Microstructure and Formation Mechanisms of δ-Hydrides in Variable Grain Size Zircaloy-4 Studied by Electron Backscatter Diffraction

Abstract

Microstructure and crystallography of δ phase hydrides in as-received fine grain and 'blocky alpha' large grain Zircaloy-4 (average grain size ~11 µm and >200 µm, respectively) were examined using electron backscatter diffraction. Results suggest that the matrix-hydride orientation relationship is {0001} ||{111} ; < 112 ̅ 0 > || < 110 > for all the cases studied. The habit plane of intragranular hydrides and some intergranular hydrides has been found to be {101 ̅ 7} of the surrounding matrix. The morphology of intergranular hydrides can vary depending upon the angle between the grain boundary and the hydride habit plane. The misfit strain between α-Zr and δ-hydride is accommodated by high density of dislocations and twin structures in the hydrides, and a mechanism of twin formation in the hydrides has been proposed. The growth of hydrides across grain boundaries is achieved through an autocatalytic manner similar to the growth pattern of intragranular hydrides. Easy collective shear along < 11 ̅ 00 > makes it possible for hydride nucleation at any grain boundaries, while the process seems to favour grain boundaries with low (<40°) and high (>80°) c-axis misorientation angles. Moreover, the angle between the grain boundary and the adjacent basal planes does not influence the propensity for hydride nucleation.

Effect of Sn on Generalized Stacking Fault Energy Surfaces in Zirconium and its Hydrides

“…The presence of stacking faults could be explained by a strain accumulation, originating from the lattice mismatch between the forming hydride and α-Zr matrix. Wang et al [36] previously showed that α-Zr grains within such hydride pockets exhibit a difference in orientation compared to the parent grain. Therefore, further growth of the hydride can be attributed to a large strain induced deformation, which causes stacking faults to propagate from the interface into the matrix.…”

Hydride Growth Mechanism in Zircaloy-4: Investigation of the Partitioning of Alloying Elements

“…As discussed above and by Wang et al, there are two types of grain boundary hydrides [18], classified based on their growth parallel or normal to the boundary and referred to respectively as a-type and btype. Note that the hydride studied in our preliminary work, ref.…”

“…It has been recently reported that H/D charged Zircaloy-4 sample shows, two types of (>μm sized) intergranular hydrides, described in two distinct type if the hydride is along (a-type) or perpendicular (btype) to the grain boundary, that appear to be different to those growing at twin boundaries (for more details see ref [18]). At the length scale seen in the SEM or during FIB preparation, please note that what we tend to refer to as a hydride is often actually an assembly of smaller hydrides, i.e.…”

“…In part this was motivated by trying to limit the grain boundary area and hence assist in understanding the nucleation of hydrides. Two types of intergranular hydrides were reported, with a growth either parallel (a-type) or perpendicular (b-type) to the grain boundary [18], in addition to intragranular hydrides. Hydrides were also reported at the boundary of deformation twin boundaries formed during processing at room temperature [18].…”

The mechanical property evolution and grain boundary accommodation during hydride transformation in commercial pure titanium

“…Grain size effect on hydride precipitation in Zircaloy-4 plate has been studied by Wang et al. [9], more intergranular hydrides were found in the fine grain sample with higher density of preferential nucleation sites, that is, grain boundary (GB). The hydride precipitation at grain boundary can produce orientation gradients in the adjacent grain [9] resulting in the nucleation of cross-boundary hydride via an auto-catalytic mechanism [10,11] which related to the collective shear on the slip plane in both grains and the grain boundary energy.…”