Investigations of deformation-induced δ → ζ phase transformation in zirconium hydride by in situ high-energy X-ray diffraction

In situ high-energy X-ray diffraction (HE-XRD) was used to study the micromechanical behavior of Zircaloy-4 during tensile loading, revealing for the first time a deformation-induced phase transformation from δ-hydride to ζ-Zr2H in the hydride at a critical stress of approximately 370 MPa. Detailed investigations using transmission electron microscopy confirmed that the ζ-Zr2H phase had a “hexagon-on-hexagon” orientation relationship with the α-Zr matrix, i.e., (0002)ζ//(0002)α and [21¯1¯0]ζ//[21¯1¯0]α. The ζ phase has almost the same lattice parameters for a and b and twice the c parameter as those of α-Zr. The affinity in crystal structure and lattice parameters between ζ-hydride and α-Zr suggests that ζ-Zr2H as an intermediate phase could be important to the dissolution and reprecipitation of δ-hydride during plastic deformation. The HE-XRD experiments provide evidence that the fracture of ζ-hydride plates aligned perpendicular to the loading direction (LD) causes the premature failure of the grains oriented with [0001]//LD in the matrix. The new finding regarding the deformation-induced phase transformation in the hydride is important to deepen the understanding of delayed hydride cracking in Zr alloys.

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