Ultrafine-grain-sized zirconium by dynamic deformation

A polycrystalline zirconium alloy (Zircadine 702, containing 0.7% Hf) was subjected to high plastic strains (shear strains of 25–100) at a high strain rate (∼104 s−1) in an experimental setup comprising of a hat-shaped specimen deformed in a split Hopkinson bar. A narrow region of intense plastic deformation (10–25 μm thick) is produced which was analyzed by scanning (electron backscattered diffraction) and transmission electron microscopy. The microstructure within the shear band is characterized by equiaxed grains with an average size of 200 nm. The temperature excursion undergone by the deforming region is calculated on the basis of a Zerilli–Armstrong constitutive equation. The calculated temperature for a shear strain of 100 is equal to 930 K, corresponding to 0.43Tm. Electron backscattered diffraction reveals that plastic deformation by shear leads to a strong 〈112¯0〉 fiber texture prior to the breakup of the existing grain structure. The ultrafine-grain structure observed is similar to that obtained in conventional severe plastic deformation processes such as equal channel angular extrusion, suggesting that the mechanism of grain refinement is the same in both processes, in spite of the differences in strain rate and thermal excursion. A mechanism is proposed for the breakup of the existing equiaxed microstructure (with grain size ∼14 μm) into an ultrafine structure. It has three stages: (1) formation of elongated cells and subgrains; (2) increased misorientation between neighboring grains and breakup of elongated grains into smaller units; and (3) rotation of boundaries by grain boundary rotation and formation of equiaxed structure.