SUPERPLASTICITY AND SOLID STATE BONDING OF TITANIUM ALLOYS

Experimental results related to solid state weldability of superplastic titanium alloys are presented. A correlation between superplastic flow and enhanced solid state weldability was established. It has been experimentally shown that a drop in the lower superplastic flow temperature with decreasing mean grain size provides an opportunity to decrease the temperature at whicmethods for titanium alloys.

Isothermal superplastic solid state bonding of 40Cr and Cr12MoV steels based on surface modification

Based on the feasibility of isothermal superplastic solid state bonding of 40Cr and Cr12MoV steels, the surfaces of both steels to be bonded were ultra-fined through high frequency hardening, then the superplastic solid state bonding were conducted, the microstructure and fracture surface of bonded joint were observed and analysed, and bonding mechanisms was researched. The experimental results show that with the sample surfaces of 4OCr and Cr12MoV steels after the high frequeney hardening, under the prepressing stress of 56. 6 MPa, initidl strain rate of 1.5 × 10^ -2 min^-1 and at the bonding temperature of800 -820℃, the superplastic solid state bonding can be carried out in about 3.5 min, and the joint strength is up to that of 40Cr steel base metal and the radial expansion ratio of the joint does not exceed 6%. The saperplastic solid state bonding parameter of both steels is within the ranges of the isothermal compressive superplastic deformation of Cr12MoV steel, and the deformation in Cr12MoV steel side near the interfacial zone of joint presents the characteristic of superplasticity. In bonding process, the atoms in two sides of joint interface have diffused each other.

Solid-liquid state pressure bonding of Si_3N_4 ceramics with aluminum based alloys and its mechanism

Solid liquid state pressure bonding of Si 3N 4 ceramics with aluminum based alloys, which contain a small amount of intermetallic compounds Al 3Ti or Al 3Zr, was investigated. With this new method, the heat resistant properties of the bonding zone metal are improved, and the joints’ strengths at high temperature is increased. The joints’ shear strength at room temperature and at 600 ℃ reach 126～133 MPa and 32～34 MPa, respectively, with suitable bonding pressure. The reaction between aluminum and Si 3N 4 ceramics, which produces Al Si N O type compounds is the dominant interfacial reaction, while the reactions between the second active element Ti or Zr in the aluminum based alloys and Si 3N 4 ceramics also occur to some extend. [

Chemical Composition and Crystal Structure of the Eggshell of the Green Crested Lizard Bronchocela cristatella(Agamidae)

The majority of extant reptiles are oviparous and produce eggs with three major components:embryo,yolk,and eggshell.The eggshell is species-specific and more diverse in squamate reptiles than in other amniote taxa.Here,we study the crystal structure,chemical composition,and bonding states of the eggshell of the green crested lizard Bronchocela cristatella.X-ray diffractometer(XRD)analysis showed the existence of two clearly defined and distinguishable crystalline phases,aragonite and calcite.Using the XRD data and a unit cell refinement routine,we identified two sets of cell parameters:a=4.956 A,b=7.965 A,and c=5.734 A for the aragonite phase;a=4.987 A,b=4.987 A,and c=17.056 A for the calcite structure.We used x-ray photoelectron spectroscopy to examine detailed elemental composition and bonding states and found that the eggshell was composed primarily of elements C,N,Ca,and O,with C,N and O bonded to various types of hybridization in the protein of the eggshell membrane.The Ca:C:O ratio for the calcium carbonate yielded a value of-7:8:21,which is close to the expected stoichiometric value of CaCO3.

Wavelet analysis of ultrasonic A-scan signal of solid-state welded joints

In the ultrasonic nondestructive evaluation of the quality of solid state welded joints, such as friction bonding and diffusion bonding, the main difficulty is the identification of micro defects which are most likely to emerge in the welding process. The ultrasonic echo on the screen of a commercial ultrasonic detector due to a micro defect is so weak that it is completely masked by noise, and impossible to be pointed out. In the present paper, wavelet analysis (WA) is utilized to process A scan ultrasonic signals from weak bonding defects in friction bonding joints and porosity in diffusion bonding joints. First, perception of WA for engineers is given, which demonstrates the physical mechanism of WA when applied to signal processing. From this point of view, WA can be understood easily and more thoroughly. Then the signals from welding joints are decomposed into a time scale plane by means of WA. We notice that noise and the signal echo attributed to the micro defect occupy different scales, which make it possible to enhance the signal to noise ratio of the signals by proper selection and threshold processing of the time scale components of the signals, followed by reconstruction of the processed components.

Study on the C_(60)-Caged States of H_2 and N_2

The states of small molecule H2 and N2 in the Coo cage have been studied. It is shown that the molecular stable configUrations of the caged states exhibits little change compared with the free states, but the bond intensities of the caged states have become much stronger.

Preparation of silicon carbide nitride films on Si substrate by pulsed high-energy density plasma

Thin films of silicon carbide nitride （SiCN） were prepared on （111） oriented silicon substrates by pulsed high-energy density plasma （PHEDP）. The evolution of the chemical bonding states between silicon, nitrogen and carbon was investigated as a function of discharge voltage using X-ray photoelectron spectroscopy. With an increase in discharge voltage both the C 1s and N 1s spectra shift to lower binding energy due to the formation of C--Si and N--Si bonds. The Si--C--N bonds were observed in the deconvolved C ls and N ls spectra. The X-ray diffractometer （XRD） results show that there were no crystals in the films. The thickness of the films was approximately 1-2 μm with scanning electron microscopy （SEM）.

Solid-Liquid State Bonding of Si3N4 Ceramics with Ceramic-Modified Brazing Alloy

Solid-liquid state bonding of Si3N4 ceramics with TiN-modified Ag-Cu-Ti brazing alloy was used'- to enhance joint strength. The effects of the TiN particles on the microstructures, interfacial reactions, and room-temperature properties of the joints were investigated. The results show that the TiN particles are gen- erally well dispersed in the Ag-Cu eutectic base and the interface between them is both clean and com-pact. Changes in the TiN volume fractions from 0 to 20% exert no noticeable effect on the interfacial reac-tion between Ag-Cu-Ti and the substrates. Other bonding parameters being constant, the TiN volume frac-tion in the filler material plays a key role in the joint properties. For TiN volume fractions below 20%, the joints are reinforced, especially joints with 5% and 20% TiN. The average shearing strength of joints with 5% TiN is 200.8 MPa, 30% higher than that of joints with no TiN (154.1 MPa). However, for TiN volumes frac- tions above 20%, the joint strengths decrease.

The mechanism of hydrogen abstraction by high valence transition metal oxo compounds

We present here a systematic theoretical study to explore the underlying mechanisms of the H abstraction reaction from methane. Various abstracting agents have been modeled, using oxygen radicals and a set of high valence metal oxo compounds. Our calculations demonstrate that although H abstraction from CH3-H by metal oxoes can be satisfactorily fitted into the Polanyi correlation on the basis of oxygen radicals, the mechanisms behind are significantly different. The frontier orbital analyses show that there are three electrons and three active orbitals (3e, 3o) involved in H abstraction by oxygen radicals; whereas an additional orbital of pi(M-O)* is involved in H abstraction by M = O, resulting in a (4e, 4o) interaction. In terms of valence bond state correlation diagram, we find that H abstraction by a metal oxo may benefit from the contribution of ionic resonance structures, which could compensate the penalty of opening the M-O pbond. We believe that these findings can help to design more effective catalysts for the activation of light alkanes. (C) 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B. V. and Science Press. All rights reserved.

Lagged strain of laminates in RC beams strengthened with fiber-reinforced polymer

Based on the theory of concrete structure, a new expression was derived for lagged strain of fiber-reinforced polymer （FLIP） laminates in reinforced concrete （RC） beams strengthened with FRP. The influence of different preloaded states and nonlinear stress-strain relationship of compressed concrete were both taken into account in this approach. Then a simplified expression was given by ignoring tensile resistance of concrete. Comparison of analytical predictions with experimental results indicates satisfactory accuracy of the procedures. The errors are less than 8% and 10% respectively when the tensile resistance of concrete is or not considered. While the maximum error of existing procedures is up to 60%.

Maintaining nano-lamellar microstructure in friction stir welding (FSW) of accumulative roll bonded (ARB) Cu-Nb nano-lamellar composites (NLC)

Accumulative roll bonded （ARB） Copper Niobium （Cu-Nb） nano-lamellar composite （NLC） panels were friction stir welded （FSWed） to evaluate the ability to join panels while retaining the nano-lamellar structure. During a single pass of the friction stir welding （FSW） process, the nano-lamellar structure of the parent material （PM） was retained but was observed to fragment into equiaxed grains during the second pass. FSW has been modeled as a severe deformation process in which the material is subjected to an instantaneous high shear strain rate followed by extreme shear strains. The loss of the nano-lamellar layers was attributed to the increased strain and longer time at temperature resulting from the second pass of the FSW process. Kinematic modeling was used to predict the global average shear strain and shear strain rates experienced by the ARB material during the FSW process. The results of this study indicate that through careful selection of FSW parameters, the nano-lamellar structure and its associated higher strength can be maintained using FSW to join ARB NLC panels.