Rod-Textured Zircaloy-2: A Case Study of an Anisotropic Metal with Multiple Deformation Modes, Strong Texture and Thermal Strains

Hexagonal close-packed and lower symmetry metals often exhibit anisotropic mechanical properties because the dominant slip system forbids slip in certain lattice directions. Rod-textured Zircaloy-2 is a model system which can act as a road map for understanding more complex cases. In this case prism slip is dominant and pyramidal slip is only initiated at higher applied stresses. Tensile twinning does not always play a role since its initiation depends on the starting texture. Along the rod axis, Zircaloy-2 exhibits a very strong weighting of poles lying within the basal plane of the structure such as <10 1 0>, <11 2 0> etc. However, perpendicular to the rod axis, all poles are present. The coefficients of thermal expansion are unequal along the a- and c-axes of the crystal structure, so there are always large intrinsic thermal strains. Likewise, the mechanical properties perpendicular to the rod axis are dominated by the interaction of grains with hard and soft plastic response. Over two decades, the residual strains and the in-situ strain response parallel and perpendicular to the rod axis have been measured by neutron diffraction for both tensile and compressive applied stress. The paper reviews our understanding of the strain development for tensile and compressive applied stress in Zircaloy-2 in terms of slip and tensile twinning, the crucial part played by the thermal strains and the simplifying role of the strong texture.