Science in China Series E-Technological Sciences
An analysis model has been established according to the structure feature of high porosity metals, and the mathematical relationship between the tensile strength and porosity for this material has been derived from the model. Moreover, the corresponding theoretical formula has been proved good to reflect the variation law of tensile strength with porosity for high porosity metals by the example experiment on nickel foam.
关键词:
high porosity metal;tensile strength;porosity;foams
Philosophical Magazine a-Physics of Condensed Matter Structure Defects and Mechanical Properties
A first-principles method based on the local-density approximation using discrete variational clusters has been used to study the electronic structure of the hcp metals, Be, Mg, Sc, Y, Ti, Zr, Co, Zn and Cd. The binding energy of these metals was calculated in relation to the volume of a unit cell. The variation in the binding energy with the unit cell volume was obtained by means of a polynomial fit. The theoretical tensile strength and bulk modulus of these metals were estimated from the electronic structure and binding energy calculations. The predicted bulk moduli for these metals are in good agreement with experimental findings and other available theoretical data. A linear relationship between the calculated and the experimental strengths is observed.
关键词:
potentials;density
Applied Physics Letters
Bulk Cu foils have been synthesized via magnetron sputtering with an average twin spacing of 5 nm. Twin interfaces are of {111} type and normal to the growth direction. Growth twins with such high twin density and preferred orientation have never been observed in elemental metals. These Cu foils exhibited tensile strengths of 1.2 GPa, a factor of 3 higher than that reported earlier for nanocrystalline Cu, average uniform elongation of 1%-2%, and ductile dimple fracture surfaces. This work provides a route for the synthesis of ultrahigh-strength, ductile pure metals via control of twin spacing and twin orientation in vapor-deposited materials. (c) 2006 American Institute of Physics.
关键词:
nanocrystalline copper;ductility;metals;size
Nature
In conventional metals, there is plenty of space for dislocations-line defects whose motion results in permanent material deformation-to multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries(1,2) and other obstacles(3,4). For nano-structured materials, in contrast, dislocation multiplication is severely confined by the nanometre-scale geometries so that continued plasticity can be expected to be source-controlled. Nano-grained polycrystalline materials were found to be strong but brittle(5-9), because both nucleation and motion of dislocations are effectively suppressed by the nanoscale crystallites. Here we report a dislocation-nucleation-controlled mechanism in nano-twinned metals(10,11) in which there are plenty of dislocation nucleation sites but dislocation motion is not confined. We show that dislocation nucleation governs the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, occurring at a critical twin-boundary spacing for which strength is maximized. At this point, the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes switches to a dislocation-nucleation-controlled softening mechanism with twin-boundary migration resulting from nucleation and motion of partial dislocations parallel to the twin planes. Most previous studies(12,13) did not consider a sufficient range of twin thickness and therefore missed this strength-softening regime. The simulations indicate that the critical twin-boundary spacing for the onset of softening in nano-twinned copper and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twin-boundary spacing, and the higher the maximum strength of the material.
关键词:
molecular-dynamics simulation;nanocrystalline materials;mechanical-properties;nanotwinned copper;deformation;plasticity;nanoscale;ductility;crystals;nickel
Acta Materialia
A statistical analysis is employed to investigate the mechanical performance of nanostructured metals with bimodal grain size distribution. The contributions of microcracks in the plastic deformation are accounted for in the mechanism-based plastic model used to describe the strength and ductility of the bimodal metals. The strain-based Weibull probability distribution function and percolation analysis of microcracked solids are applied to predict the failure behavior of the bimodal metals. The numerical results show that the proposed model can describe the mechanical properties of the bimodal metals, including yield strength, strain hardening and uniform elongation. These predictions agree well with the experimental results. The stochastic approaches adopted in the proposed model successfully capture the failure behavior of bimodal coppers that are sensitive to grain size and the volume fraction of coarse grains in addition to the corresponding threshold for percolation. These results will benefit the optimization of both strength and ductility by controlling constituent fractions and the size of the microstructures in materials. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
关键词:
Bimodal grain size distribution;Strength;Ductility;Weibull;probability distribution;Percolation model;high-tensile ductility;plastic-deformation;high-strength;nanocrystalline metals;enhanced plasticity;boundary diffusion;al-alloy;copper;behavior;cracks
Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing
Three-point bending test and acoustic emission technique are used to determine the fracture toughness and fracture process of three types of modified 9Cr-1Mo steel weld metals. Scanning electron microscopy is used to accomplish fractography analysis of fracture specimens. Microstructure of weld metals is investigated using optical microscopy and transmission electron microscopy. The fracture process and factors which affect fracture of the 9Cr-1Mo steel weld metals in post-weld heat treated condition are studied. Experimental results show that the modified 9Cr-1Mo steel weld metals fracture by a quasi-cleavage mechanism at ambient temperature. The microstructure of the weld metals is composed of mainly tempered martensite with M23C6 precipitates. In weld metals, microcracks nucleate from non-metallic inclusions. Fractures develop very quickly when cracks started to propagate. Comparatively, in weld metals with low strength, microcracks initiate at a low stress, but propagation of cracks is limited by plastic deformation. In weld metals with high strength, microcracks nucleate at high stresses, but cracks propagate very quickly and lead to almost immediate fracture of the specimens. As a result, weld metals with the low strength have a higher fracture toughness, while weld metals with higher strengths has a lower fracture toughness. (C) 1999 Elsevier Science S.A. All rights reserved.
关键词:
9Cr-1Mo steel;weld metal;fracture toughness;microstructure;acoustic;emission
Tribology International
This paper describes evaluations of dynamic mechanical properties of typical metals (Al, Cu, Ti, Ni, Co, and Mo) with single pendulum scratch tests. The: curve fitting method with a polynomial has been used to calculate the geometrical parameters of the scratch track during the scratching process. The methodology and analytical modeling are developed for determining a series of mechanical properties, such as elastic modulus, shear modulus and cohesive strength of materials, with the help of calibrations to those of these metals. These parameters are effective to describe the dynamic mechanical properties of massive materials and modified surfaces of materials. Using these methods, the mechanical properties of plasma sprayed Ni-ZrO2 coatings are determined. (C) 1999 Elsevier Science Ltd. All rights reserved.
关键词:
single pendulum scratch test;dynamic mechanical property;analytical;modeling;coating;abrasive behavior;wear behavior;thin-films;coatings;composites;hardness;modulus;alloys
