Recrystallization mechanism and texture evolution of pre-twinned Zr702 during annealing

With the aim of tailoring the texture of Zr alloys by twin recrystallization, the recrystallization mechanism and texture evolution of a tensile-twin-containing Zr702 were investigated in this work. Two types of rolling modes, simple rolling along former normal direction (ND) and cross rolling along former ND and rolling direction (RD) at cryogenic temperature, were conducted in a Zr702 sheet with a basal texture to activate high density tensile twins, and subsequent annealing was applied to induce twin recrystallization. During cryogenic rolling, {1012} and {1121} tensile twins were activated, which resulted in a texture change of c-axis keeping away from ND and tilting close to transverse direction (TD) along the ND-TD plane. And a {1121} twin band- {1012} twin band{1121} twin band lamellar structure formed inside the twinned grains at high thickness reductions due to the growth of {1012} twins between parallel {1121} twins. When rolling direction was RD for cross rolling, the grain orientations were more beneficial to the activation of prismatic slip than tensile twinning, leading to lower area fractions of tensile twins and sluggish texture change in cross-rolled samples. However, the final textures of the two types of rolled samples at 30% thickness reduction were similar. During annealing, twin recrystallization in tensile twins featured by bulging of twin boundaries and conventional discontinuous recrystallization at original grain boundaries occurred in twinned grains. Recrystallized grains formed in tensile twins had orientations closed to the un-recrystallized twins and contributed to the generation of a uniform texture of c-axis tilted close to TD along ND-TD plane in rolled and annealed samples, regardless of the different area fractions of twins and the various rolled textures at different thickness reductions. Few recrystallized grains nucleating in {1012} twins gave rise to the disappearance of c-axis parallel to TD in the two kinds of rolled samples after annealing. The peaks of c-axis stayed closer to TD in the simple rolled samples after annealing as a result of weak discontinuous recrystallization at the original grain boundaries, which induced a non-twin orientation in the recrystallized grains. In addition, the single nucleation in {1121} twins and higher stored energy lead to larger grain size in simple rolled and annealed samples.

Automated summary

This study investigated the recrystallization mechanism and texture evolution of Zr702 under different rolling modes. The results showed that cryogenic rolling activated {1012} and {1121} tensile twins, leading to a change in texture. Cross rolling activated prismatic slip more than tensile twinning, resulting in lower area fractions of tensile twins and slower texture change. However, the final textures of the two types of rolled samples at 30% thickness reduction were similar.