Quasi-In Situ EBSD Study of Anisotropic Mechanical Behavior and Associated Microstructure Evolution in Zircaloy-4

The anisotropic mechanical behavior and associated microstructure evolution in annealed Zircaloy-4 were investigated at room temperature, using quasi-in situ tensile tests along the typical direction, rolling direction (RD), and transverse direction (TD). Herein, the in-grain misorientation axes (IGMA) and the nominal Schmid factors were evaluated to analyze the slip mode based on the electron backscatter diffraction. The IGMA result shows that there were anisotropic slip modes within grains, whose basal poles were parallel with the TD (TB) and placed within 40 to 50 degrees from the normal direction (ND) to the transverse direction (N (40°–50°) TB)), under different loading directions. When loading along the RD, the basal <a> slips were activated in the N (40°–50°) TB and TB orientation grains, while the second-order pyramidal slips were activated in the grains when loading along the TD. The relatively higher ultimate tensile strength and elongation in Zircaloy-4 when tensile along RD occurs due to its much higher frequency of soft grains (88.54%) than the TD sample (64.29%), and the synergy deformation among local grains. The present study demonstrated that the anisotropic mechanical behavior of Zircaloy-4 was attributed to the combined effects that exist between the anisotropic slip behavior and the different compatible deformation capabilities. Many shallow dimples and cleavage regions were observed on the fracture surface in the TD sample. Such fracture features are consistent with the lower ultimate tensile strength ~470 MPa and elongation ~14.5% in the deformed tensile Zircaloy-4 along the TD.