In situ observation of dynamic recrystallization in the bulk of zirconium alloy

In-situ observations of dynamic recrystallization in individual bulk crystallites of a macroscopic Zircaloy-4 specimen under thermo-mechanical load, was reported. The nuclear reactor structural material Zircaloy-4 grade R60804 of nominal composition Zr, 1.5% Sn, 0.2% Fe, 0.1% Cr, 0.1% O was used for the study. It crystallizes in a hexagonal closed packed (hcp) α-Zr structure below 1083 K and a body centered cubic (bcc) β-Zr structure above 1253K. The 5-mm-thick specimens were put into a servo-hydraulic load frame with the load axis perpendicular to the incoming X-ray beam of 0.2 × 0.2 mm ² size. The sample was heat treated in air with zero load in order to allow recovery and growth of distinguishable grain sizes by ramping the temperature up to 1277K and subsequent cooling. Diffraction patterns were then recorded upon application of a constant load of 225 N. It was observed that upon heating little growth occurs in the low-temperature α-Zr phase which then transforms gradually into β-Zr.