Microstructure evolution and mechanical properties of nanocrystalline zirconium processed by surface circulation rolling treatment

In this work, the microstructure evolution and nanoindentation hardness of nanocrystalline zirconium processed by surface circulation rolling treatment at cryogenic temperatures were investigated in details. Experimental results indicated that the total deformation layer depth exceeds 600 μm. The average grain sizes vary from about 8 nm in the topmost surface to micrometers in the coarse grained matrix, corresponding to a gradient variation in hardness from about 6.0 to 2.86 GPa. The microstructure evolutions were found that the deformation bands form at the initial stage of the deformation. With increasing the strain, the dislocation cells form in interior of the deformation bands and finally transformed into nanograins. The Hall-Petch relationship between hardness and grain size is not linear due to the change of the deformation mechanism from dislocation pile-ups to grain-boundary sliding as the grain size becomes smaller.