Dynamic Recrystallization of Zircaloy-4 during Working within the Upper α-Range

Abstract

Compression tests were used to simulate simple deformation paths within the upper a-range of Zircaloy-4 (i.e. 500°C-750°C). The mechanical behaviour reveals two different domains : at low temperatures and large strain rates, strain hardening takes place before flow softening, whereas this first stage disappears at lower flow stress levels. Strain rate sensitivity and activation energy were determined for both domains. Dynamic recrystallization was investigated using the Electron BackScattering Diffraction (EBSD) technique. It appears that the mechanism involved here is continuous dynamic recrystallization (CDRX), based on the increasing misorientation of subgrain boundaries and their progressive transformation into large angle boundaries. At low strains (e £ 0.3), CDRX kinetics are similar whatever the deformation conditions, while higher temperatures and lower strain rates promote recrystallization at large strains.

In Situ Observation of Dynamic Recrystallization in the Bulk of Zirconium Alloy

“…It should be noted that upon recovery, highly perfect, virtually dislocated free sub-grains are created for a short time showing low mosaic spread and, thus, are found less probable to fulfill the Ewald condition, which describes the observed decrease in intensity in the timeline of the grain. The regime of dynamic recrystallization is entered upon further increase in temperature, strain, and strain rate (as necking occurs) and it is characterized by the sudden appearance and fluctuation of new orientations [16] starting slowly after t 4 ¼ 35 s and escalating until rupture of the specimen at t 5 ¼ 49 s, after which the load drops to zero and static recovery and grain growth takes place, indicated by straight, coarsening timelines. Some of the new grains are temporal islands in orientation space, particularly well seen in the b-Zr 110 and b-Zr 211 maps of Figure 3, extending azimuthally up to 108 and with a well-expressed sub-grain orientation structure.…”