P. Huang

Time dependent plasticity at real nanoscale deformation

Nanoindentation creep test has been carried out in displacement of 4–100nm regime at single grained (SG) Cu and ultrafine grained (UFG) Cu. The strain rate sensitivity m has been determined and exhibits strong indentation size effect. The magnitudes of m in UFG and SG Cu are comparable and can be simulated by the same curve below a critical depth. The authors experimental findings and analysis support that the tip-sample diffusion is the dominant mechanism below a critical indentation depth and the effect of grain boundary media mechanisms will emerge as the indenter penetrates deeper.

Fivefold annealing twin in nanocrystalline Cu

Fivefold annealing twin has been observed in nanocrystalline Cu at zero external stress. The microstructure of the multifold twins at various annealing temperatures has been examined by transmission electron microscopic in detail. Contrary to the well known sequential twinning mechanism, coherent twin boundaries formed via migration of grain boundary segment were proposed to be the dominant mechanism of fivefold annealing twin, which associated with grain growth during annealing. Moreover, the possibilities of the proposed mechanism may operate in forming fivefold deformation twin were discussed.

Morphological healing evolution of penny-shaped fatigue microcracks in pure iron at elevated temperatures

This letter reports a joint experimental and numerical investigation of high temperature morphological healing of micron-sized intragranular microcracks in pure iron. Irregular penny-shaped microcracks were first created by low-cycle fatigue and then subjected to annealing in vaccum at 1173K. It is shown theoretically that, depending on its initial aspect ratio, a penny-shaped microcrack may evolve via surface diffusion into an isolated spherical void, or a doughnut-shaped channel pore with or without a central spherical void. Subsequent evolution causes the doughnut-shaped channel pore to break up into a ring of spherical voids via Rayleigh’s instabilities. These results were confirmed with experimental observations of typical configurations of voids that result from the crack healing process. The experimentally observed evolution time is also in good agreement with the predictions of finite element simulations of the evolution process.

Hybrid Process of Microarc Oxidation and Hydrothermal Treatment of Titanium Implant

A duplex process was proposed with which a titania containing calcium and phosphate on the surface of titanium alloy was formed by microarc oxidation in aqueous electrolytes, and finally converted to bioactive layer containing hydroxyapatite by subsequent hydrothermal synthesis. The layer is porous in micro scale and composed of crystals in nanometer size. There was no distinct interface between the hydroxyapatite layer and the substrate.

Preparation of microarc oxidation layer containing hydroxyapatite crystals on Ti6Al4V by hydrothermal synthesis

Titanium alloys are widely applied in producing hard tissue implants (such as bone, joint, and teeth), but they are bioinert materials and fail to form chemical bond with host bone. Bioactive hydroxyapatite (HA) coatings can form bony contact with bone tissue and shorten the healing period, but their dissolution and fracture limit their wide application [1]. It is essential to prepare bioactively modified layer on titanium alloy with high bond strength. Microarc oxidation (MO) is a technique of ingrowing oxidation ceramic layer on nonferrous metals, which was firstly developed in 1970s [2]. The antiscuff and anticorrosion properties of titanium alloys were improved after MO treatment [3, 4]. As for biomedical application, Ca2+ and PO3− 4 ions in electrolyte could enter the ceramic layer during MO process and existed in amorphous form, so that the bioactivity potential of titanium alloys was increased [5]. But unfortunately, HA crystals were not formed on the surface of MO layer, which makes it difficult for bone tissue to attach on the implants. Hydrothermal synthesis (HS) can convert other calcium phosphates to HA [6]. Here, the MO layer containing Ca and P on Ti6Al4V was hydrothermally treated to obtain surface modified bioactive layer containing HA crystals on titanium alloy. The study also showed that the MO-HS layer has high bond strength (above 25 MPa) with biologically favorable roughness and porosity. 25 × 2.5 titanium alloy (Ti6Al4V) plates were used as substrates. They were polished with 600 and 1000# abrasive paper, ultrasonically washed with deionized water and acetone. The electrolyte was prepared by calcium salt and phosphate salt with a Ca/P mole ratio 3. Ti6Al4V plate was anode and stainless plate cathode. Pulse-direct power resource was used with voltage 350 V and occupancy/(occupancy + void) ratio 60%. The oxidation time was 3 min. The samples by MO was hydrothermally treated in an autoclave at 160, 180 for 2 or 4 h, and the pH of water was adjusted to 11–12 by adding ammonia. Scanning electron microscopy (SEM) was used to analyze the surface and profile morphology, and X-ray diffraction (XRD) and electron dispersive X-ray analysis (EDXA) were used to analyze the phase and element composition. Tensile bond strength test was carried out

Dislocations interaction induced structural instability in intermetallic Al 2 Cu

Intermetallic precipitates are widely used to tailor mechanical properties of structural alloys but are often destabilized during plastic deformation. Using atomistic simulations, we elucidate structural instability mechanisms of intermetallic precipitates associated with dislocation motion in a model system of Al2Cu. Interaction of non-coplanar <001> dislocation dipoles during plastic deformation results in anomalous reactions—the creation of vacancies accompanied with climb and collective glide of <001> dislocation associated with the dislocation core change and atomic shuffle—accounting for structural instability in intermetallic Al2Cu. This process is profound with decreasing separation of non-coplanar dislocations and increasing temperature and is likely to be operative in other non-cubic intermetallic compounds as well.Intermetallics: dislocation climb promotes precipitate dissolutionInstead of gliding, dislocations in intermetallic precipitates interact to climb, leaving defects behind. Qing Zhou and colleagues at Xi’an Jiaotong University in China and their collaborators in the United States of America used molecular dynamics simulations to investigate the interactions of dislocations in intermetallic precipitates in aluminium-copper during deformation. They showed that instead of traditional glide, dislocations that do not lie on the same plane can interact by climbing then gliding along a new atomic plane without cancelling each other out, leaving vacancies behind. This defect creation happened faster at higher temperatures, creating extended dislocation cores and vacancy clusters that could facilitate precipitate dissolution. Research into intermetallic stability during deformation may thus help us avoid failure of alloys strengthened with precipitates.

Microstructure and flow stress of nanoscale Cu/Nb multilayers

Nanoscale Cu/Nb multilayers with individual layer thicknesses of 2, 5, and 15nm were prepared by d.c. magnetron sputtering. The cross-sectionalmorphologies of the multilayers were examined under transmission electron microscopy (TEM) as well as high resolution TEM, whilst the flow stresses weremeasured with nanoindentation. A unique cross-sectional microstructure comprising well-modulated and mixed regions was observed, causing length-scale-independent flow stresses not found in existing studies, and shear bandswere absent upon plastic deformation. Built upon this unique microstructure, possible mechanisms underlying the high plastic stability and length-scale-independent flow stresses of Cu/Nbmultilayers were discussed in terms of amorphous-crystalline interface and its interaction with both mixed and well-modulated regions.

Two-Dimensional X-Ray Diffraction for Structure and Stress Analysis

This paper introduces the recent progress in two-dimensional X-ray diffraction as well as its applications in microstructure and residual stress analysis. Based on the matrix transformation between diffraction space, detector space and sample space, the unit vector of the diffraction vector can be expressed in the sample space corresponding to all the geometric parameters and Bragg conditions. The same transformation matrix can be used for texture and stress analysis. The fundamental equations for both stress measurement and texture measurement are developed with the matrix transformation defined for the two-dimensional diffraction. Stress measurement using twodimensional detector is based on a direct relationship between the stress tensor and the diffraction cone distortion. The two-dimensional detector collects texture data and background values simultaneously for multiple poles and multiple directions.

An Investigation of Residual Stress of Porous Titania Layer by Micro-Arc Oxidation under Different Voltages

The surface modification of titanium by micro-arc oxidation under different voltages was processed to achieve good direct oseointegration. The new technique of two-dimensional X-ray diffraction was used to measure the residual stress of the layer. The results show that a porous titania layer containing Ca and P is obtained by micro-arc oxidation. The pore size and Ca/P of the layer are affected by the voltage. The high voltage can induce forming CaTiO3. The residual stress under different voltage is compressive stress and increases with the improvement of the voltage.

Effects of free surface and heterogeneous residual internal stress on stress-driven grain growth in nanocrystalline metals

By reevaluating the experimental study of Zhang et al. (2005), here we demonstrate that the extent of grain growth, previously proposed to be solely driven by external stress, may have been significantly overestimated. A new physical mechanism, termed as free surface assisted stress-driven grain growth (or self-mechanical annealing), is proposed and discussed in detail. Representing the cooperative effect of free surface and heterogeneous residual internal stress, the proposed mechanism is considered more favorable than the traditional pure stress-driven mechanism for interpreting the abnormal grain growth widely observed in deforming nanocrystalline metals at room temperature.