Grain Refinement And Mechanical Property Of Zirconium Subjected To Surface Mechanical Attrition Treatment

Nanostructured metallic materials have been the subject of considerable research in recent years due to their unique microstructure and appealing mechanical property. Severe plastic deformation techniques are capable of producing perfect nanocrystalline materials (i.e. bulk density, artifacts-free and contamination-free) by inducing ultra-large strains.The hexagonal-close-packed (HCP) metals have wide application in industry. In particular, HCP materials possess exceptional mechanical properties with grain refinement down to nano-scale. Hence, to study the mechanism of grain refinement and mechanical response is of great significance from both scientific and technical view.In the present work, the deformed layer consisted of nanocrystalline and ultra-fine crystallites was fabricated in the surface of Zr by means of SMAT technique. The microstructure was studied at various strain levels in order to establish themechanism of both strain-induced grain refinement and strain accommodation. The microstructure was characterized by X-ray diffraction, optical microscopy, scanning and transmission electron microscopy. The tensile property and hardness of the surface nanocrystallization zirconium were also studied. The main conclusion can be drawn as follows.(1) The grain refinement down to nanoscale was realized during the SMAT process. The discontinuous migration recrystallization and continuous dynamic recrystallization occurred successively with increasing strain levels, leading to the grain refinement.(2) The nanosized FeCr intermetallic compounds were synthesized on the treated surface of ~ 25 μm thick. The FeCr grains had an average grain size of 25±10 nm and exhibited an ultrahigh nanohardness of 10.2 GPa. The deformation-induced fast diffusion of Fe and Cr from the steel shots into Zr occurred during SMAT, leading to an in situ formation of intermetallic compounds.(3) The results of tensile tests indicated that the surface nanocrystallization leads to an increase in yield strength and ultimate tensile strength and to a decrease in uniform elongation, compared with as-received ones. The tensile strength increased with strain rate in the range of 10~-4-10~-3 s~-1 whereas decreased when strain rate was 10~-2 s~-1 due to the change of deformation mode. The dimple character appeared on the fracture surface. Surfacenanocrystallization enhanced the hardness of the treated surface.