Fabrication And Mechanical Properties Of Brick-and-mortar Structured Metals

Nacre exhibits excellent fracture toughness,which can be attributed to its hierarchical organization of the structure:brick-and-mortar or nacre-like structure.To date,the brick-and-mortar structure shows preferential uses in the design and the synthesis of new structural materials,such as high-performance ceramics and glasses.The metallic structure unit is proposed to bring potential improvements in toughening of the brick-and-mortar structure,as predicted by theoretical model.As a modification,the ceramic/metal composites with the brick-and-mortar structure were reported.However,the beneficial role of metallic mortar was weakened due to the poor interfacial bonding.To provide evidence and guidance for the development of novel structural metals with high strength and high toughness,starting from the study on layered structure,this thesis focuses on the architecture of the nacre-like structure in which metallic materials,for instance,pure Ni and Ni-P alloys serve as structure units;on the basis of this,a serial investigation on the effects of the layered structure and the nacre-like structure on the mechanical properties were performed,and the crack growth behavior as well as the underlying toughening mechanisms were revealed.We synthesized the amorphous Ni-P/crystalline Ni laminates,in which the crystalline Ni layer is?20 ?m thick and the Ni-P layer thickness varies from 0.1 to 6.0?m,by periodically modifying electrolyte and regulating deposition time during electroplating.A strong dependence of mechanical properties on the thickness of Ni-P(i.e.,tNi-P)layer was revealed.With an increasing tNi-P,the tensile strength and the uniform elongation,respectively,increase and decease monotonously,but the yield strength first increases and then decreases.The yield strength peaks at 724 MPa,when the tNi-P is 1.0 ?m.Moreover,when the tNi-P is 0.1 ?m,the ultimate strength and elongation are 884 MPa and 10%,respectively,which were 6%and 43%higher than that of the monolithic crystalline Ni,i.e.,indicative of a simultaneous enhancement in strength and ductility.As revealed by analyzing fractured specimens,fracture mechanism of the layered structure transformed with an increase in tNi-P.At the case of a thin amorphous layer(i.e.,tNi-P<1.0?m),the geometric constraint caused by Ni layers inhibits shear banding in Ni-P layers.Thus,Ni-P layers can bear large tensile plastic strain accompanied with Ni layers.Cracks initiate in the crystalline layer interior,showing a ductile fracture.At the case of a thick amorphous layer(i.e.,the tNi-P>1.0 ?m),shearing dominates the deformation of the Ni-P layer under tension,and enables intensive strain localization at a low strain.Cracks initiate in the Ni-P layer interior and rapidly penetrate through the whole layer,leading to a brittle fracture.For the first time we achieved the architecture of the brick-and-mortar structure in metals,coupling with lithography and electrodeposition.Lithography provides a method of replicating and transferring the precise 2-dimentional pattern of the nacre-like structure;and electrodeposition offers a feasible approach to build and stack this 3-dimentional structure.Ultimately,three different kinds of metallic nacre-like hybrids:amorphous Ni-P/crystalline Ni,nano-grained Ni/crystalline Ni,nano-grained Ni/crystalline Cu.As demonstrated by microstructure characterization,the contiguous metallic mortar,with a thickness of approximately 0.5 ?m,acts as bridges to glue or connect the separated bricks(with the dimensions of?2.5×20×40 ?m3)orderly organized in space.Combination of lithographic and electrodeposition allows us to replicate the complex,fine hierarchical organization which is highly similar to brick-and-mortar structure in nacre.The deformation ability of the nacre-like structure is superior to that of the layered structure:the nacre-like structure can be folded in half,while the layered structure shows a catastrophic failure under the bending load.The aforementioned metallic hybrids having a nacre-like structure get toughened up via extrinsic toughening mechanisms:crack deflection and brick pull-out,according to the analysis on crack growth behavior.The dominant extrinsic toughening mechanism and the final toughening ability was related to the material of structure unit and the interface strength.For nano-grained Ni/crystalline Cu,its low-strength,soft,metallic mortar(crystalline Cu)is easy to fail,and its interface bonding is poor,causing pull-out of the hard metallic bricks;the poor interface bonding,however,is insufficient to transfer load effectively,and high-angel crack deflection and significant energy dissipation can no longer be achieved,and resulting in a low fracture toughness value(8.94 MPa·m1/2).For nano-grained Ni/crystalline Ni,the soft,metallic mortar(crystalline Cu)has a relative high strength,and the interface is strong;the shear stress generated at interface,thus,is high enough to deform the hard,metallic bricks(nano-grained Ni),and to cause apparent necking and failure as the soft phase was broken.This is the main reason which contributes to a high toughness value(18.17 MPa·m1/2)of this specimen.In this case,the pull-out of brick and crack deflection become less pronounced,manifesting an inadequate toughening capability.For amorphous Ni-P/crystalline Ni,the hard,metallic bricks possess a highest strength and a limited plastic deformation ability,and the soft metallic mortar was plasticly deformed upon loading;the metallic bricks will serve high shear stress due to a desirable interface bonding,leading to the pull-out of the bricks and significant crack deflection ahead of crack tip.This kind of nacre-like metal exhibits an optimal extrinsic toughening efficiency and a high toughness value approximating to that of the NC-Ni/UFC-Ni sample,up to 18.06 MPa·m1/2.The crack propagation behavior in brick-and-mortar structure has obvious anisotropy.For the structure notched in 90° orientation(i.e.,the initial notch orientation is perpendicular to the brick length direction),the specimen exhibits an overall winding crack path and a high toughness value(20.78 MPa·m1/2).In this case,the crack advance will encounter the wide metallic brick/mortar interface in long-axis section that can serve as an effective "crack arrester",which effectively contributes to the brick pull-out or crack deflection along interface induced by interfacial failure,and to increase crack growth resistance.For the structure notched in 0° orientation(i.e.,the initial notch orientation brick is parallel with the brick long edge),the interface between an array of bricks and the interconnected mortar provides the easy access for crack extension,and the crack front can hardly be blocked/deflected by the bricks,leaving an inferior crack growth resistance and a lower toughness value(18.01 MPa·m1/2).Thus,the nacre-like structure notched in 90° orientation would be more damage-tolerant than that notched in 0° orientation.