Study on Surface Grain Refinement of Zircaloy-4

A refined structure was obtained on Zircaloy-4 through ultra-sonic shot peening （USSP） treat-ment and characterized by optical microscopy （OM）, X-ray diffraction （XRD） and field-emission gun scan-ning electron microscopy （FEG-SEM）. It has been found that the treated sample consists of three grain struc-tures: surface nanocrystalline （60μm thick）, ultrafine grain （160μm thick） and the matrix. The grain refine-ment mechanism is attributed to severe plastic deformation （SPD） during the USSP treatment, which includes: formation of twins and dislocations, formation of dislocation tangles, subdivision by dislocation cells, orientation randomization.