Dislocation structures in zirconium and zircaloy-4 fatigued at different temperatures
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INTRODUCTION
STUDY of the substructure developed during cyclic deformation is one of the principal subjects of investigations in fatigue. It is helpful to give microscopic support for the fatigue deformation mechanism, establish the correlation of mechanical behavior with dislocation arrangements, and determine an effective method to improve the fatigue properties for materials. However, the majority of these investigations have been conducted on cubic metals and alloys. Scarce information is available about the fatigue dislocation structure for hexagonal-close-packed (hcp) metals such as zirconium and its alloys, which are important for nuclear technology. The great variety of available deformation modes stems from the transmission electron microscopy (TEM) examinations of the development of the deformation texture during working due to the high plastic anisotropy for zirconium and zircaloy*,[1–5] and of the dislocation *ZIRCALOY is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
substructures produced during monotonic deformation.[6–10] It is well established that slip on {1010} prismatic planes is the most active deformation mode at all testing temperatures in zirconium and zircaloy.[1–4,8,9] Some authors have reported the presence of pyramidal slip on {1101} planes.[2–5] Pochettino et al.[1] and Holt et al.[7] have shown evidence for ^c 1 a& pyramidal slip activity. It is believed that the critically resolved shear stress (CRSS) value for ^c 1 a& LIN XIAO, Associate Professor, and HAICHENG GU, Professor, are with the Research Institute for Strength of Materials, Xi’an Jiaotong University, Xi’an, 710049, China. Manuscript submitted July 9, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
slip is higher than that corresponding to prismatic slip,[4] hence, under a constraint condition and with an increasing test temperature, slip on pyramidal planes {1011} or {1121} in ^c 1 a& directions was possible to obtain.[4] Akhtar[9] has analyzed the possible slip planes in zirconium single crystals of varying orientations during tension. He has suggested that the prismatic slip needs much less CRSS than it does on basal or pyramidal planes, that the basal slip occurs only above 873 K, and that the angle between the tensile axis and the basal plane is larger than 607. To the authors’ knowledge, little systematic work has been done on the development of fatigued dislocation substructure in zirconium and zircaloy-4; the only example of an investigation of this type is that of Alvarez-Armes and coworkers,[11,12] who studied the development of characteristic dislocation structure at a constant strain range of 0.5 pct between room temperature (RT) and 973 K, and concluded that the operating Burgers vector is 1/3[2110] in a zircaloy-4 specimen fatigued at 873 K.[12] However, the slip planes remained uncertain. The present investigation was designed to study in great detail the possible slip or twinning modes and the corresponding dislocation configurations produced in zirconium and zircaloy-4 during cycling.
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