Abstract
On slow cooling high purity zirconium containing 40 ppm hydrogen, y-hydride (ZrH) precipitates as acicular plates. This paper considers the possibility that such plates form by a displacive or shear transformation. The habit planes and orientation relationships of hydrides have been determined and related to possible dislocation mechanisms for the transformation. It is shown that habit planes close to {1017}Zr are predicted for hydrides having a [110]γ||[1210]zr, (111)γ ∼ ||(0001)Zr orientation relationship. Hydrides with {1010} habit planes have a [110]γ or [101]γ||[1210]Zr, and [001]γ, or [010]γ||[0001]Zr orientation relationship. In this case, the observed habit plane can only be invariant if the pure lattice strain is succeeded by a pure shear shape change. Experimentally, internal twinning in the plates and dislocation arrays at the interface are observed, and it is suggested that these accomplish the required shape change. In the final section of the paper, these results are related to the problem of hydride reorientation and hydride cracking in zirconium alloys.
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