Microstructure characterization of gradient structured surface layer in pure zirconium

A gradient nanostructure with thickness of 150 μm at surface layer was produced on pure zirconium (Zr) by ultrasonic shot peening (USSP). The microstructure of the gradient layer was characterized by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that multiple gradient features that consist of grain size gradient, strain gradient (low angle grain boundaries (LAGBs) gradient and geometrically necessary dislocation (GND) density gradient) and deformation twinning gradient are observed. By high resolution EBSD analysis, it is found that 101¯2<101¯1>/112¯1<1¯1¯26> twinning and dislocation slip are observed in the highly deformed surface layer. The deformation twins are parallel or intersected with each other and subdivide the coarse grains into small blocks. The movement and accumulation of dislocations gradually transform into LAGBs which finally forms finely distributed equiaxed grains. The mutual promotion between slip and twinning accelerates the process of grain refinement. The combination of twin-twin intersections and twin-LAGBs interactions ultimately leads to the formation of nano grains in pure Zr during USSP process