Influences Of Network Connectivity On The Mechanical Properties Of Bicontinuous-structured Metals

Nanoporous metals prepared by dealloying exhibit a nanoscale open network structure.These materials combine large surface area and macroscopic sample dimension,which have enabled many novel applications and functionalities.Because of a smaller is stronger effect,similar to nanowires reported previously,the nano-ligaments in nanoporous metals gain a strength,which is much larger than that of bulk counterparts and even approach GPa-scale.However,the structure of dealloyed nanoporous metals often contains a large number of dangling ligaments,which reduce the volume fraction of bear-loading ligaments,thus decrease the macroscopic strength and elastic modulus of nanoporous metals.For conventional open cell foams or porous materials,their strength and Young's modulus can be well described by Gibson-Ashby scaling equations.However,previous experimental observations indicate that both strength and Young's modulus of nanoporous gold prepared by dealloying are lower than that predicted by Gibson-Ashby equations.Question then arises as to whether the Gibson-Ashby scaling equations are valid for nanoporous metals.Several modified scaling laws have been proposed,based on structural feature of nanoporous metals,particularly the network connectivity.However,it is difficult to verify these models experimentally because of the involvement of the size effects.Moreover,infiltrating a nanoporous or porous metal with second phase would lead to an interpenetrating phase composite,which might exhibit excellent mechanical properties.Such composites can be simply prepared by liquid metal dealloying(LMD).Again,their mechanical properties may be relevant to the network connectivity of each phase,which has not been explored before.In this study,we prepared high quality porous FeCr samples with coarse structure size,and systematically studied their mechanical properties at different relative densities.In addition,by detailed structural characterization and mechanical characterization of bi-continuous Ni/Ag composite prepared by LMD,we explore the influence of the bi-continuous structure on its mechanical properties.Furthermore,we explore the influence of the network connectivity on mechanical properties of bi-continuous FeCr/Mg composite by adjusting precursor alloy's Cr content to change the network connectivity of porous FeCr phase.The main results are summarized as follows:1.Preparation of high quality bi-continuous FeCr/Mg composites and porous FeCr materialsBy adjusting precursor alloy composition and optimizing LMD process,we successfully prepared high quality bulk bi-continuous FeCr/Mg composites.The formation of plate-like defects,which were previously observed at grain boundaries,have been avoided.Porous FeCr samples were then prepared by dissolving Mg from these composites.The ligament size of porous phase or porous FeCr is approximately 4 ?m.The grain size of FeCr is tens of microns,which is much larger than their ligament size.2.Mechanical properties of Porous Fe0.8Cr0.2 and the validity of modified scaling laws1)Mechanical properties of porous Fe0.8Cr0.2 samples with different relative density have been studied systematically.The ligament size of these samples is coarse and very similar(?4 ?m),so that the influence of size effect on ligament strength can be ignored.2)The strength and Young's modulus of porous Fe0.8Cr0.1 monotonously decrease with the reduction of relative density.Their variations can not be fitted by using G-A scaling laws in the full range.However,in log-log plots,the strength and Young's modulus show a linear relationship,i.e.F??3/4.This correlation is fully compatible with the G-A predictions,although the standard G-A scaling laws have failed to predict the strength and Young's modulus of dealloyed porous materials.3)These seemingly conflicting observations can be understood in terms of a scenario that involves dangling ligaments and decreased network connectivity in dealloyed porous structures.The existence of dangling ligaments would lower both the strength and the elastic modulus of the macroscopic porous materials,compared with data predicted by using the standard G-A equations.Meanwhile,the load-bearing network,whose relative density is lower than the true relative density,is self-similar and follows the G-A scaling laws.3.The influence of bi-continuous structure on mechanical properties1)Bi-continuous Ni/Ag composite was prepared by LMD.Ag phase and Ni phase exhibit large grains,which interpenetrate each other in three-dimensional space,forming a double interlocking structure.The boundaries between Ni phase and Ag phase are noncoherent,and there are high-density geometrically necessary dislocations in Ag phase caused by thermal stress.In addition,plate-like defects have been avoided,which used to be located on the grain boundaries.2)The influence of bi-continuous structure on the elastic modulus and yield strength of this material is not very significant.However,under certain conditions,the bi-continuous structure can improve the tensile plasticity,because it is beneficial to the coordinated deformation of Ag phase and Ni phase.3)Owing to the curled ligaments and double interlocking structure of this composite,geometrically induced strain hardening leads to enhance strain hardening rate.4.The influences of network connectivity on mechanical properties of bi-continuous metal composites1)With increasing Cr content,the porous phase morphology of bi-continuous FeCr/Mg composite gradually change from rod-like shape to sphere-like shape.In consequence,the network connectivity decreases continuously.For samples with Cr content in porous phase below 40 at.%or greater than 80 at.%,the network connectivity changes little.For samples with Cr content in porous phase between 40 at.%and 80 at.%,the network connectivity changes significantly.2)The change of network connectivity has no significant influence on the strength of bi-continuous FeCr/Mg composite,while the plasticity is significantly reduced with the decrease of network connectivity.