Papers by Keyword: Nanocrystalline Metals

Abstract: Nanocrystalline nonferrous metals (Cu, Ag, and Al) were fabricated by room-temperature-molding method and vacuum-warm-compaction method, respectively. X-ray diffraction, Positron annihilation spectroscopy, and density test were utilized to characterize these as-prepared nanocrystalline specimens. The thermal stability and micro-void distribution of the as-prepared nanocrystalline metals were compared and discussed in detail. The experimental results show that the increasing of density is unremarkable while the microstrain reduces during warm-compaction process. The nanocrystalline nonferrous metals prepared by VWC have better thermal stability. Positron annihilation spectroscopy analysis indicates that, compared with the specimens prepared by RM, the average size of micro-void and proportion of single vacancy is a little larger in nanocrystalline copper fabricated by VWC.

Abstract: In order to synthesize morphology and structure-controlled nano-copper, chemical reduction synthesis method was used coalescing microwave-assisted and modification. Cu nanowires with diameters of 20 to 25nm and lengths of 1.2 to 2.1μm were yielded by reduction of copper acetate tetrahydrate of (1, 2)-propanediol system using ascorbic acid as a reductant, polyethylene sorbitan monooleate (Tween 80) as a modifier and by microwave-assisted heating. The microstructure of these nanowires has been characterized by XRD and TEM. The XRD patterns indicate that the lattice parameters of these Cu nanowires are enlarged about 1.162% while the lattice parameters of Cu nanospheres are reduced about 1.024%. The growth mechanism of Cu nanowires is also suspected. The anti--wear and reducing friction performance of liquid paraffin with either Cu nanowires or Cu nanospheres has been measured by ball--on--disk UMT--II tribometer. It is found that the tribological performance of liquid paraffin with nano-Cu is improved, and the enhancement of improvement with Cu nanowires is better than with Cu nanospheres.

Abstract: A nanocrystalline (NC) layer with the thickness of 30 µm was produced on pure titanium surface by surface mechanical attrition treatment (SMAT). Microstructure observation indicated that the grain size increases with depth from the treated surface. The friction coefficient decreases and the wear resistance increases with the SMAT sample as compared to its coarse-grained counterpart. The improvement of the wear properties could be attributed to the higher hardness of SMAT sample.

Abstract: Shear punch test (SPT) has been used to study the mechanical properties of Cu, Cu–10 wt.% Zn, Cu–20 wt.% Zn and Cu–30 wt.% Zn after ball milling with an average grain size in the range of 33-12nm. The strain rate sensitivity (SRS) and physical activation volume have been determined. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. This suggests that the thermally activated process in nanocrystalline (nc) metal/alloys is different from the conventional forest dislocation cutting mechanism. The stacking fault energy (SFE) of Cu-Zn alloys decreased with the adding of Zn, and deformation twins are anticipated to introduce into the nc Cu-Zn alloys during process of ball milling. Dislocations could accumulate along the TBs and carry the plastic strain, so the ductility of nc Cu-Zn alloys could be improved.

Abstract: Ultra-large compressive plasticity at room temperature has recently been observed in electrodeposited nanocrystalline nickel (nc-Ni) under micro-scale compression (Pan, Kuwano, Fujita and Chen: Nano Lett. Vol. 7 (2007), p. 2108). With aid of a TEM sample preparation technique employing focused ion beam (FIB), TEM observations on deformed nc-Ni evidenced deformation-induced microstructural evolution of nc-Ni at a variety of strain levels: Whilst the deformation increases, substantial grain growth is uncovered in the nc-Ni. No apparent ex situ evidence of intragranular dislocation activities is found in the deformed sample. As thermal diffusion plays an insignificant role in the deformation in nc-Ni at room temperature (~0.17Tm), this premium plasticity is achieved in accommodation with the grain-boundary-mediated deformation, with assistance of extensive grain growth that is mainly driven by high stresses at steady plastic flow.

Abstract: This paper reviews a method, “in situ consolidation ball milling” that provides artifactfree bulk nanocrystalline samples for several ductile metals such as Zn, Al and Al alloys, and Cu and Cu alloys. The preparation method is described in this paper and examples of the mechanical behavior of nanocrystalline materials made by this technique are given. It is found that in such artifact-free metals, combinations of both high strength and good ductility are possible.

Abstract: A computation sample of nanocrystallion metal with a crack is proposed in this work. The structural evolution scheme is employed to model the mass flow, the grain boundary cavitation, and the crack growth of the specimen under remote loading. The scheme leads to a system of ordinary differential equations that can be solved by Euler integration. The simulation validates the proposal that the ductile versus brittle transition of nano-grained metals is dictated by the competition of creep deformation promoted by grain-boundary kinetics, and the decohesion of grain boundaries.

Abstract: In recent years, nano-crystalline materials have attracted many researchers’ attention, but the fracture mechanism has not been fully clarified. In a molecular dynamics (MD) simulation, grain size and crystal orientation can be chosen, and their effects on the mechanical properties of nano-crystalline materials can be evaluated clearly. This research first compares the results of crack growth behavior in single crystalline Fe for three typical interatomic potentials (Embedded Atom Method (EAM), Finnis Sinclair (FS), and Second Nearest Neighbor Modified EAM (2NNMEAM) potentials) and a Hybrid potential method, which uses FS potential for bcc structure atoms and 2NNMEAM potential for non-bcc structure atoms. The 2NNMEAM potential is accurate, but the computation time is dozens of times that of FS potential, which is the simplest of the three interatomic potentials. Therefore, the 2NNMEAM potential requires too much calculation for the purpose of this research that analyzes the crack growth behavior in nano-crystalline metals. However, Hybrid potential is able to give results similar to those of the 2NNMEAM potential, and the calculation time is close to that of the FS potential. From these results, the crack extension behavior in relatively large nano-crystalline models is analyzed using the Hybrid potential, and we demonstrate the grain-size dependency of the fracture behavior.

Abstract: Fine-grained polycrystalline metals have a very high yield stress and excellent workability. Hence, numerous researchers are trying to develop an efficient process to obtain such materials. Our goal is to develop an efficient severe plastic deformation (SPD) process through investigating grain-refinement mechanisms in Equal Channel Angular Pressing (ECAP). In this paper, a series of molecular dynamics (MD) simulations of severe simple-shear deformations, which are ideally equivalent to SPD applied by typical ECAP processing routes, is performed using three-dimensional models that are thin and have a square shape with a periodic-boundary condition. We analyze the influences of the processing route and initial texture on the microstructural evolution. It is shown that twinning deformations are dominant under the calculated conditions, and that the structural evolution is notably affected by the relationship between the applied simple-shear direction and the characteristic crystal orientation, which can easily cause a twinning deformation. We conclude that Route A, without a rotation of the billet between processes, is the most efficient route. This is because twinning deformations along the simple-shear direction interact with the twin boundaries developed by the stress-component conjugate to the simple-shear. Furthermore, we demonstrate that the influence of the initial texture difference remains in force during multiple processes that have the same sliding plane.

Abstract: Current and prospective trends in application of metallic nanomaterials have been studied. The study has been conducted within the Nanoroad SME European project – as the first step for a roadmap for industrial application of nanomaterials. The web page of the project is http://www.nanoroad.net/. The present report presents an analysis of patents, papers, national and European projects in the field of nano-metals, and also an analysis of the present state of research and expected trends in this domain. Based on the performed analysis a data base of nanomaterials has been developed as well as roadmaps with expected time to applications. It can be found under http://bourgogne.arist.tm.fr/nanoroadsme/home/. The roadmap is mainly addressed to SMEs to help them to decide about applications or production of nanomaterials.