Published: Jan 2010
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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.
The low offer of slip systems, their asymmetrical distribution (prism slip in <a> directions and — under constraint — pyramidal slip in <c+a> directions) as well as the strict crystallographic orientation relationships of first and second order pyramidal twinning result in the formation of a strong deformation texture. By virtue of twinning, even small deformation rates lead to large lattice rotations, which change the orientation of the crystallites where all basal poles align in the direction of the compressive force. The fact that in Zirconium and Zirconium base alloys the preferred crystallographic orientation, which is spread in the transverse direction, is also retained as the final stable position is explained by <c + a> pyramidal slip.
The decisive factor in texture development is the material flow, the degree of freedom of which is lowest for tube reducing as compared to sheet rolling and wire drawing processes. Therefore tube reducing (characterized by reductions in cross-section
For textured materials, on the other hand, the deformation mechanisms are also responsible for the strong anisotropy of the mechanical properties. This is discussed on the example of specially prepared Zircaloy tubes, which were machined out of a sufficiently thick Zircaloy plate with pronounced sheet texture. By this procedure, one obtains around the circumference of the tubing continuously changing preferred orientations of the basal poles with the extreme orientations possible in Zircaloy tubing. For the different uni- and multiaxial loading conditions applied, the theoretical predictions of the mechanical behavior agrees in any case with the experimental results.
In nuclear application, the anisotropic behavior of biaxial loading conditions is represented by yield loci, creep loci, or burst loci according to the respective criteria yield stress, creep rate, or fracture stress. Depending on the texture and the loading conditions, an attempt is made to correlate the shape of the loci to the operative deformation mechanism. In this way, it is possible to find selection criteria for the desirable texture in Zircaloy cladding tubes. The original paper was published by ASTM International in the Journal of ASTM International, April 2005.
Zirconium, Zirconium base alloys, deformation mechanisms, texture, mechanical anisotropy
Energy Technology, Erlangen,