Preparation And Mechanical Properties Of Multiscale Composite Layered Nanocrystalline Ni

Nanocrystalline materials exhibit high strength,but low plasticity,however,in the process of deformation,the low ductility is usually exhibited which seriously limits the application of nanocrystalline materials in engineering.The low plasticity of nanocrystalline materials is attributed to the lack of effective dislocation motion,which makes it impossible for the nanocrystalline materials to form a larger work hardening ability to maintain plastic deformation during the deformation process.It is reported that the strain hardening ability of nanocrystalline materials can be effectively improved by constructing a composite structure with bimodal or multimodal grain size distribution.Based on this,in this paper,the layered bulk nanocrystalline Ni with large grain layer and small grain layer was prepared by the direct current electrodeposition and with alternating current density(1.5A/dm2 and 23A/dm2).The crystal structure and morphology of the coatings were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results show that in the current density of 1.5A/dm2,the coating preferred(200)texture and the grain size is about 267 nm in the ultrafine grained category;in the current density of 23A/dm2,the coating has no growth texture and its grain growth is random with the grain size about 45 nm in the category of nanocrystal.Under continuous tensile mode,the tensile strength of layered nanocrystalline Ni(ζUTS)and elongation(δETF)reached 1290 MPa,12.5% at the strain rate of 4.17×10-2s-1,respectively.Compared with the nanocrystalline and ultrafine grained Ni,the elongation improved remarkably with a moderate strength.The work hardening rate(WHR)was kept to 8% of the plastic strain,indicating that the processing hardening ability of the layered nanocrystalline Ni was strengthened.Scanning electron microscopy(SEM)results showd that the morphology of the layered nanocrystalline Ni was corresponding to the morphology of the ultrafine and nanocrystalline Ni,which showed the lamellar dimples and bulge structure.In order to research the effect of layer thickness ratio on mechanical properties,we prepared wide distribution ratio Nis.The results showed that when the ratio of hard and soft layers is 1:1(interval 1h-4min),we got the best mechanical properties(tensile strength and elongation about 1415 MPa,12.3%)and WHR(true strain:9.2%).Under continuous tensile mode,layered nanocrystalline Ni(interval 1h-4min)exhibited a strong strain rate sensitivity(m:0.018,V:19b3)at a wide strain rate(0.417s-1-4.17×10-5s-1).The δETF of layered nanocrystalline Ni increased to 14.6% at the strain rate of 4.17×10-2s-1 under tension relaxation mode indicating that different deformation modes had great influence on the mechanical properties of nanocrystalline materials.