| Due to the open three-dimensional porous structure,nano-scale ligament/pore size,large volume-specific surface area,and macroscale sample sizes,nanoporous(NP)metals prepared by dealloying have attracted great interest for many excellent mechanical,physical,and chemical properties.As a result,the formation mechanism,structural characteristics,properties and applications of nanoporous metals have attracted extensive attention of researchers.Mechanical properties of NP metal are critical for those practical applications;thus,the mechanical behavior of NP metals has been one of the research hotspots in this field.Researchers discovered that NP metals have a "size effect" of "smaller is stronger",which means that their strength increases as ligament size decreases.However,due to some "dangling ligaments" inside the porous structure which cannot contribute to the instant strength,the apparent strengths of the NP metals are lower than the theoretical values.The corrected strengths of ligaments are significantly higher than the uncorrected strengths,and agree well with the theoretical shear strength of metals.What’s more,under the advantage of high specific surface area,"surface effect" can be separated and explored its influence on the mechanical properties of nanoscale metal materials.So far,extensive research on the mechanical behavior of NP metals has been conducted,but some issues remain unresolved.Considering the above questions,the thesis has included thorough researches and the main results are summarized below:1.Surface triple junctions govern the strength of a nanoscale solidThe "surface effect" of the mechanical properties of NP metals is concentrated in surface-controlled mechanical properties,including strength,elastic modulus,and fracture toughness.Controlled by electrochemical modification,the surface-induced changes in the mechanical response of nanoscale ligaments can be detected by examining the NP metals at the macroscopic scale.In this context,the "surface triple junction effect" can be separated from nanoscale metal materials in this paper,focusing on the effect of mechanical response.1)Two types of millimeter-sized nanoporous gold(NPG)with similar morphology and ligament size were fabricated by electrochemical dealloying:nanocrystalline NPG(NC-NPG)containing large quantities of surfaces,grain boundaries,and surface triple junctions(STJs);and coarse-grained NPG(CG-NPG)with large surface area but very few grain boundaries and STJs.2)Due to the insensitivity of grain boundary to electrochemical modification,the "STJ effect" and "surface effect" can be measured in NC-NPG under electrochemical controlling the surface state and observing its influence on the strength,and "surface effect" can be measured in CG-NPG for comparison.The oxidation-induced relative change in the strength of NC-NPG coincide with that of similar ligament-sized CGNPG for ligament size(L)was greater than 100 nm,but significantly deviate from and exceed that of CG-NPG when the ligament size(L)was below 100 nm.3)Combined with the activation volume and transmission electron microscopy examinations,the plastic deformation mechanism of CG-NPG is controlled by the surface when the ligament size(L)was below 100 nm,and NC-NPG is controlled by the surface and STJs.It is suggested that the STJs may act as the preferred nucleation position of dislocations at a small size to affect the deformation behavior and mechanical properties of nanomaterials.2.Synthesis and mechanical properties of ordered nanoporous goldNanoporous metals prepared by dealloying generally exhibit a classical random and isotropic NP structure with a lower bearing efficiency and structure-connectivity compared with ordered oriented NP structures-this phenomenon further deteriorates the strength and stiffness,which are much lower than the theoretical values.In view of the disordered and poor connectivity of NP metals,in this paper,we propose to improve the mechanical properties of NP metal materials by constructing ordered and oriented nanoporous structures.1)Ordered nanoporous gold was prepared by chemical dealloying of Al2Au master alloy.The structure of the nanoporous gold is different from the previously reported threedimensional disordered network structure,and its pores are almost perpendicular to the outer surface.2)Ordered NPG exhibits excellent ductility under out-of-plane compression,with a"plateau region" that may be related to its non-uniform deformation mode.Under compression,ordered NPGs deform by the formation and expansion of kink bands.Ordered NPGs deform via the formation and expansion of kink bands under compression.Within this deformation band,ordered structure was essentially preserved except that the microstructure is misoriented from the rest of the undeformed sample,which is a characteristic feature of materials with pronounced anisotropic structure response.3)The out-of-plane mean strength(461±52 MPa)of ordered NPG is higher than all NPGs reported in previous studies.In addition to its special topology structure,the high strength of ordered NPG originates from the high strength of pore-wall material,which includes nano-size effect,nano-twin boundaries,and residual Al.The high strength,the ordered pore channels,and the excellent thermal and electrical conductivity of metallic pore walls might also enable novel applications of this new type of NP metal.However,the strength of ordered NPGs experimentally measured in this study is much lower than the value predicted for an ideal honeycomb-structured NPG according to the GibsonAshby scaling laws,which might be due to the topology imperfections such as the inclination of pore channels and the voids in pore walls.Ordered NPGs may be further strengthened by mitigating the topology imperfections.3.The hardness-to-strength ratio of nanoporous goldThe hardness properties of porous materials generally depend on compressibility,i.e.,plastic Poisson’s ratio and compressive strain hardening.Since the plastic Poisson’s ratio and the compressive strain hardening rate of conventional porous materials tend to increase or decrease simultaneously,it is difficult to clarify the relationship between these two and the hardness properties.The NP metal,on the other hand,has an abnormal uniform-non-uniform deformation transition,which means that as the relative density increases,the NP metal transforms from homogeneous deformation to localized densification and presents a stress plateau.That is,unlike conventional porous materials,NPG with a high relative density exhibits a high plastic Poisson’s ratio,but a low strain hardening rate.As a result,in this paper,given the unclear relationship between the plastic Poisson’s ratio,strain hardening rate,and hardness performance of conventional porous materials,we propose to clarify the relationship between the above two factors and hardness performance,as well as the influencing mechanism via the hardness performance of NP metals.1)The hardness-to-strength(H/σ)ratios of various NPGs were measured by microVickers hardness and compression tests.The plastic Poisson’ s ratio of nanoporous gold is in the range of 0.06～0.20,which differs from the plastic Poisson’s ratio of lowdensity porous material(near-zero),and much lower than solid metal materials’(～0.5).The experimental results revealed that the relationship between the hardness-to-strength ratio of nanoporous gold and the plastic Poisson’s ratio did not follow the empirical correlation of Shaw et.al.,and did not increase monotonically with the increase of plastic Poisson’s ratio.2)For nanoporous gold with plastic Poisson’s ratio less than 0.20 and compressive stress "plateau region",the hardness-to-strength ratio is about～1.0 when the relative density is less than～0.50.For the nanoporous gold with strain hardening,the hardnessto-strength ratio is linearly correlated with the normalized strain hardening rate and increases as the normalized strain hardening rate increases.According to the fitting expression,the value of plastic deformation～0.27 below the indenter is consistent with the experimental result. |
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