Tuning Constituent Layer Scale For Mechanical Properties Of Copper-Based And Nickel-Based Laminated Composites

The strength and ductility of metal materials have always been a competitive relationship.Obtaining higher strength of materials is at the expense of better ductility.Ultra-fine grained(UFG)metals prepared by severe plastic deformation technique,nanocrystalline(NC)and amorphous materials have exhibited higher strength but unfavorable ductility,toughness,work hardening capacity and unstable microstructures.Therefore,issues on how to improve the strength of metal materials without sacrificing or less sacrificing ductility have been a hot research topic.Lamellar structural strengthening technique might be a better selection for the strengthening and toughening of metal materials.Different structural Cu/Ni laminated composites and two typical sandwich-structured composites of ED-Cu/CR-Cu/ED-Cu and Ni/Fe78B13Si9/Ni were prepared by electrodeposited technique and cold rolled method.Effects of strain rate,monolayer thickness of constituent layers(?),gradient structure and thickness ratio on mechanical properties of Cu/Ni laminated composites were investigated systematically through tensile and fatigue tests combined with the characterization of microstructures and fracture morphologies.In addition,interface effects on the tensile properties and the fracture behaviors of the two sandwich-structured composites were also investigated systematically.The experimental results were obtained as follows:UFG Cu/Ni laminated composites prepared using a dual-bath electrodeposited technique exhibit the increase in both strength and ductility with increasing strain rate from 1×10-5 s-1 to 1×10-2 s-1.Fracture of the laminated composites changes from the brittle mode to the ductile mode with evident necking with increasing strain rate.The analysis of the stress gradient existed in the laminated composites with heterogeneous interfaces reveals that the abnormal increase in ductility may be attributed to the higher stress gradient established at higher strain rate,which eventually improves the strain hardening ability of the UFG laminates.Three different structural Cu/Ni laminated composites of ?=3 ?m,?=32 ?m(?,the sum of monolayer thickness of constituent layers)and gradient structure with the same total thickness were prepared by controlling electrodeposited technology.The?=3 ?m Cu/Ni laminated composites exhibit not only the higher tensile strength and the better strain hardening rate,but also the stronger resistance to post-necking compared with that of the ?=32 ?m and the gradient structured composite.Stress gradient,global flow stress and geometrically necessary dislocations(GNDs)increased with decreasing the value of ?.The results of post-necking plastic deformation indicated that a high density of layer interfaces in the ultrathin laminated composite may play a key role in delaying the development of unstable plastic deformation along the layer thickness direction through periodical strain gradient and a number of GNDs near the interfaces.Thus,the fatal localized plastic deformation along the through thickness direction has to be spread over along the loading direction,leading to the large neck breadth in the ultrathin laminated composites.The enhanced yield strength and improved ductility of gradient structured composites was attributed to the macroscopical gradient structure from outside to inside.Five kinds of Cu/Ni laminated composites with the thickness ratios(tNi/tCu)of 1.2,2.4,5.2,9.6 and 20 were prepared by only adjusting the electrodeposited time.Cu/Ni laminated composites with the thickness ratios of 5.2,9.6 and 20 exhibited much better strength and ductility,and the volume ratio of Ni and the number of interlayered interfaces played an important role.The results of fatigue tests showed that the laminated composites had the higher fatigue strength compared with the Ni sheets.The ultrathin Cu layers in the composite played an important role in resisting the primary crack propagation and the development of the secondary cracks by several crack-tip blunting mechanisms including the release of the stress concentration of the crack tip,crack deflection and delamination of the Cu/Ni layer interface regions.Sandwich-structured ED-Cu/CR-Cu/ED-Cu composites were successfully prepared by electrodeposited technology and rolling method.The hardness and tensile strength of CR-Cu and sandwich-structured ED-Cu/CR-Cu/ED-Cu composites increased first then decreased,with the rolling passes of CR-Cu ranged from 1-pass to 6-pass,and reached the maximum value at 5-pass.Compared with CR-Cu materials,the tensile strength of ED-Cu/CR-Cu/ED-Cu composites had a decrease,but the ductility had a larger improvement.The work hardening rates of the composites were higher than those of the CR-Cu,which was caused by the inherent better work hardening capacity of ED-Cu and the extra work hardening rate generated from stress mismatch existed in the interlayered interfaces of CR-Cu and ED-Cu.Sandwich-structured Ni/Fe78B13Si9/Ni composites with UFG Ni layers of 3?30 ?m on Fe-based amorphous alloys were prepared through electrodeposited technology.Tensile results indicated that the strength decreases gradually,while the ductility of the sandwich composite increases with increasing the UFG-Ni layer thickness,following a typical trade-off tendency between strength and ductility.Meanwhile,the fracture mode of the sandwich-structured composite varies with changing the,constraint layer thickness of the UFG-Ni.Normal fracture occurred in the tNi=3 ?m composite,and shear fracture became dominated when the UFG-Ni layer thickness became thicker.Residual stress induced by electrodeposition and the local necking of the UFG-Ni layers,which generates an out-of-plane tensile stress normal to the layer plane,plays an important role in fracture mode of the sandwich-structured Ni/Fe78B13Si9/Ni composites.