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The deformation systems in hexagonal close-packed (hcp) metals are not as symmetrically distributed as in cubic ones. Furthermore, because the primary slip systems are not as numerous and are limited to deformations in the a direction, twinning competes with slip in plastic deformation and can, depending on the deformation conditions, play an essential role. In order to explain the conditions in Zirconium and Zircaloy, the well-established relationships of hcp metals are discussed and so are their dependencies on the metal-specific parameters of the hexagonal structure. The interactions between deformation mechanisms and texture formation on the one side and deformation mechanisms and mechanical anisotropy on the other can be likewise transferred to other hcp metals, if one takes into account the differences in dependence of the metal-specific parameters.
This paper briefly reviews work by the author identifying and describing in-reactor deformation mechanisms of materials and structures used in nuclear reactors, in particular, Zircaloy-2, Zircaloy-4, and Zr-2.5Nb, and the CANDU fuel channel (comprising Zr alloy pressure tubes, calandria tubes, and spacers). The discussion is set in the context of contemporary findings of other workers in the international community. The following themes are highlighted: The contributions of creep and growth to deformation; c-component dislocations and the fluence dependence of irradiation growth; anisotropy of irradiation growth; deformation equations and pressure tube-to-calandria tube contact in CANDU reactors; low temperature flux (damage rate) dependence of deformation rates. The first developments were reported in 1976 at the third conference in this series and there are ongoing developments in all areas. The linear low temperature flux dependence of creep and growth rates is yet to be satisfactorily explained.
We report the development of intergranular and interphase constraints in textured Zircaloy-2, Zr-2.5Nb, and Excel alloy during room temperature tension and compression loading in two or three directions relative to the parent texture. Neutron diffraction was used to track the lattice strain development in the ?-phase (all alloys) and ?-phase (Zr-2.5Nb and Excel) in three principal directions relative to the parent texture. Zircaloy-2 at room temperature is essentially single phase hcp ?Zr. The active deformation mechanisms appear to be, in order of increasing critical resolved shear stress, prism (a) slip, basal (a) slip, tensile twinning and pyramidal (c+a) slip. No compressive twinning was observed. Combined with intergranular constraints due to prior thermal treatment, these mechanisms result in substantial asymmetry in the yield stress and lattice strain development (compression versus tension). In Zr-2.5 Nb and Excel, the ?-phase appears to deform by the same slip mechanisms as Zircaloy-2, and similar assymmetry of the yield stress and lattice strain development is observed. However, the existence of tensile twinning is not clearly evidenced. The ?-phase also deforms by slip, but the critical resolved shear stress is much higher than that for the slip mechanisms in the ?-phase, leading to the development of very large interphase constraints in the plastic deformation regime. This is attributed to a combination of solution strengthening of the ?-phase (by Nb and, in Excel, Mo) and by grain size.