Texture Evolution and Variant Selection in Zr-2.5Nb During the α-β Phase Transformation
Mosbrucker, Paula L.
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Zr-2.5Nb is used as the pressure tube material for 2nd and 3rd generation CANDU reactors. The physical properties of pressure tubes in service, including strength, dimensional stability, and delayed hydride cracking resistance, are largely dependent upon the crystallographic texture of the hcp α-phase, whose texture is predominantly developed during the extrusion stage of manufacturing. During extrusion and subsequent cooling, the formation of α may occur by transformation of the bcc β-phase to α according to the Burgers relationship and influenced by variant selection – that is, a preference for one or more of the twelve possible orientations of the hcp lattice relative to the bcc lattice. Variant selection has been observed in other Zr alloys, including the heat-treated zone in pressure tube welds and the bulk texture of heat-treated pressure tubes. Further, it has been proposed as a possible explanation for texture characteristics in pressure tubes that are not explained by the deformation mechanics of extrusion. However, the criteria for variant selection are unclear. In this work, an understanding of the criteria for variant selection is developed through observations of the differing mechanisms at play during both directions of transformation, from α-β and β-α. Transformation via the Burgers relationship was confirmed; the existence of variant selection is also established. In thermal cycles to the β-regime, this selection manifests as the selection of a new (0002) variant, as driven by anisotropic thermal stresses generated during heating. Upon cooling, the high-temperature β texture is inherited by the α grains via the Burgers relationship; the magnitude of the texture maxima is driven by elastic transformation strains. Further thermal cycles to the β regime demonstrate texture memory, with some development of cubic symmetry due to grain growth during the hold in the β-phase. No texture changes are observed if samples are not heated fully into the beta regime. Finally, a study of the biasing effects of both residual and external stresses is discussed. While the external stress did not appear to be capable of biasing variant selection during either heating or cooling, some texture changes were observed, likely due to deformation at high temperature.