Fabrication And Properties Of Bulk Cu/Ta And Cu/V Nanolayered Composites

As novel nanostructured materials designed via interface engineering,the nanolayered composites have garnered significant interest due to its desirable properties such as high strength,high thermal stability,and excellent irradiation tolerance.Such outstanding properties make this class of materials promising candidates for next generation nuclear applications.However,bottom-up synthesis technique,such as physical vapor deposition(PVD)or chemical vapor deposition(CVD)method,only produce nano layered composites in the thin films,limiting its applications in engineering,especially in future nuclear fusion power reactors.Hence,using a top-down severe plastic deformation(SPD)technique,called cross accumulative roll bonding(CARB),we developed high performance bulk Cu/V and Cu/Ta nanolayered composites,which simultaneously achieve high-strength and high-stability.Compared with traditional ARB technique,CARB fabrication technique allowed to effectively suppress the formation of plastic instabilities and edge cracks during the repeated rolling process.In contrast to the sample prepared via the ARB process,where the rolling strain accumulate in a single rolling direction(RD),resulting in severe strain incompatibility between Cu layers and V(Ta)layers,which significantly promoted the formation of plastic instabilities and edge cracks,the CARB process could effectively lower the strain incompatibility by distributing the rolling strain evenly in the transversal direction(TD)and the RD.Cu/V and Cu/Ta nanolayered composites in the form of bulk sheet with layer thickness h ranging from micrometer down to nanoscale were successfully developed for the first time via CARB technique.With this fabrication methods,the minimum individual layer thicknesses can be controlled at h = 25 nm and h = 50 nm for the Cu/V and Cu/Ta nanolayered composites,respectively.For the different layer thickness h,the continuous lamellar structure was maintained and the Cu/Ta(V)interfaces were relatively flat,planar,and sharp.Specially,for layer thickness in the nanoscale,CARB Cu/Ta(V)nanolayered composites contain high density of bimetal interfaces accompanied with extremely high-aspect-ratio grains and hence a very low density of grain boundaries.Mechanical and thermal stability test results revealed that CARB Cu/Ta(V)nanolayered composites simultaneously achieve high-strength and high thermal stability,breaking down the typical trade-off between strength and thermal stability for the tradition nanostructured materials fabricated by SPD methods.For minimum layer thickness,CARB Cu/Ta(V)nanolayered composites achieve 3?5 times increase in strength and hardness when compared with their bulk counterparts.The strengthening of the 'CARB Cu/Ta(V)nanolayered composites follows the apparent Hall-Petch relationship,in accord with the general strengthening mechanism for deformation microstructure.Annealing results shown that the hardness of bulk CARB Cu/Ta(V)nanolayered composite maintained unchanged after exposure to 500 ? for 1 h,indicating that these CARB nanolayered composites possess outstanding thermal stability.It owes this extraordinary property to following three aspects:i)specific nanolayered structure comprised of two phases that do not chemically mix;ii)the unusually high density of heterophase interface;iii)the extremely high-aspect-ratio grains accompanying with a very low density of grain boundaries emerge from extreme rolling strains.Although outstanding thermal stability is achieved in bulk CARB Cu/Ta(V)nanolayered composites,the thermally induced mechanical and microstructural instabilities are obvious when annealing temperature exceeded critical value.Rayleigh instability accompanied with grain boundary grooving are thermal instability mechanisms responsible for the obvious instabilities after annealing to 800 ?.According to the He ions irradiation results of bulk nanolayered Cu/V composites with layer thickness h = 50 nm,irradiation fluence have a significant impact on the mechanical property and microstructure stability.Bulk Cu/V nanolayered composite remained stable with respect to mechanical property and microstructure after exposed to irradiation fluence of 2×1021 ions/m2.In contrast,for the case of sample irradiated with high fluence of 7×1022 ions/m2,significant radiation hardening is present in the mechanical property;obvious surface blistering accompanied with elongated He voids and numerous visible He bubbles are developed in the microstructure.