Dynamic restoration mechanisms and microstructural refinement of alpha-zirconium with large strain deformation at elevated temperatures

The extensive use of zirconium as an elevated-temperature structural material in the nuclear industry since the 1950's has led to many investigations on its behavior during creep. A review of the different creep investigations of alpha zirconium is presented in the first part of this study. No agreement for the mechanism governing creep of zirconium was found among the different creep mechanisms considered for the relevant stress range. However, a review article recently showed that alpha-zirconium follows five-power law creep at intermediate modulus-compensated stress levels. This indicates that dislocation-climb is the deformation mechanism, with dynamic recovery as the restoration mechanism. This implies that the creep activation energy should be equal to the self-diffusion activation energy. However, the discrepancy between the published data of creep and self-diffusion activation energies suggests a different creep mechanism and/or an extra restoration mechanism could be occurring.; Constant strain-rate tensile and torsion and constant stress tests were performed between 400 and 800°C. Constant strain-rate torsion tests were utilized to calculate the variation of the activation energy with temperature. It appears that the apparent activation energy followed the same trend as the self-diffusion activation energy, suggesting dislocation climb as the rate-controlling mechanism and dynamic recovery as the restoration mechanism. The grain refinement observed during constant strain-rate tests was characterized by polarized optical microscopy, transmission electron microscopy and convergent beam electron diffraction, along with electron backscattered diffraction (EBSD). No evidence of discontinuous dynamic recrystallization was observed and dynamic recovery, via dislocation climb, appears to be the sole restoration mechanism. Also, it appears that the formation of small, equiaxed (sub)grains leading to a significant grain refinement and a bimodal distribution of boundary misorientation are due to geometric dynamic recrystallization. Texture analysis by X-ray diffraction and EBSD revealed that the softening observed above 650°C is likely caused by the development of a deformation texture. Finally, a study is proposed to assess the mechanism of formation of high-angle boundaries in single crystals. This could bring insights on the controversy regarding the existence of continuous dynamic recrystallization and help develop a reliable technique to study the evolution of high-angle boundaries with plastic deformation.