Interactions between carbon solutes and dislocations in bcc iron

Carbon solute–dislocation interactions and solute atmospheres for both edge and screw dislocations in body-centered cubic (bcc) iron are computed from first principles using two approaches. First, the distortion tensor and elastic constants entering Eshelby’s model for the segregation of C atoms to a dislocation core in Fe are computed directly using an electronic-structure-based the total energy method. Second, the segregation energy is computed directly via first-principles methods. Comparison of the two methods suggests that the effects of chemistry and magnetism beyond those already reflected in the elastic constants do not make a major contribution to the segregation energy. The resulting predicted solute atmospheres are in good agreement with atom probe measurements.     

First-principles study on the mobility of screw dislocations in bcc iron

The fully two-dimensional Peierls barrier map of screw dislocations in body-centered cubic (bcc) iron has been calculated using the first-principles method to identify the migration path of a dislocation core. An efficient method to correct the effect of the finite size cell used in the first-principles method on the energy of a lattice defect was devised to determine the accurate barrier profile. We find that the migration path is close to a straight line that is confined in a {1 1 0} plane and the Peierls barrier profile is single humped. This result clarifies why the existing empirical potentials of bcc iron fail to predict the correct mobility path. A line tension model incorporating these first-principles calculation results is used to predict the kink activation energy to be 0.73 eV in agreement with experiment.     

Thermomechanical processing of iron,titanium, and zirconium alloys in the bcc structure

The body centered cubic (bcc) metals undergo a high level of dynamic recovery during elevated temperature straining so that the stress increases monotonically to a steady-state value σ_s. The strain rate and σ_s are related by means of the power, the exponential, or the sinh law with an Arrhenius temperature relationship. The activation energy for a iron has values of 250–280 kJ/mol, whereas for β titanium and β zirconium it is in the range 134–184 kJ/mol. The structure developed during hot working consists of elongated grains containing subgrains of dimension inversely proportional to σ_s. In warm working of α iron (limited to below 0.66T _m), the textures are similar to those for cold working. In working β titanium and β zirconium which is limited to above 0.6T _m except in β stabilized alloys or as matrix in α+ β processing, the bcc textures transform into α textures. The α iron relies principally on substructure strengthening in association with carbides. The β phases can be thermomechanically processed to provide equiaxed or lamellar a in a variety of dimensions and combinations, with or without substructure. Hot working of the bcc refractory metal alloys, principally molybdenum, is similar to hot working of α iron.     

Monitoring of materials, processes, and technologies by measuring acoustic emission

Conclusions Quantitative methods of AE monitoring of materials and processes are being advanced in the direction of creating computer “intellectual devices” that would unite the accumulated experience in analyzing materials by the method of AE under various test conditions and treatment processes. Such an expert system will be based on the accumulated professional data that make it possible to classify the results of measurements and draw conclusions on the quality of the material with recommendations to the researcher of optimum algorithms for acquiring, storing, and representing the information in correspondence with the fixed strategies of materials monitoring. The new intellectual system based on AE methods will provide a high level of diagnostics of the quality of materials, processes, and technologies. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 40–48, April, 1999.     

Nanoscale phase field microelasticity theory of dislocations: model and 3D simulations

The first Phase Field model of evolution of a multi-dislocation system in elastically anisotropic crystal under applied stress is formulated. The model is a modification and extension of our Phase Field Microelasticity approach to the theory of coherent phase transformations. The long-range strain-induced interaction of individual dislocations is calculated exactly and is explicitly incorporated in the Phase Field formalism. It also automatically takes into account the effects of “short-range interactions”, such as multiplication and annihilation of dislocations and a formation of various metastable microstructures involving dislocations and defects. The proposed 3-dimensional Phase Field model of dislocations does not impose a priori constraints on possible dislocation structures or their evolution paths. Examples of simulation of the FCC 3D system under applied stress are considered.     

Ab initio investigation of the Peierls potential of screw dislocations in bcc Fe and W

The easy, hard and split core configurations of the 〈1 1 1〉 screw dislocation and the energy pathways between them are studied in body-centered cubic (bcc) Fe and W using different density functional theory (DFT) approaches. All approaches indicate that in Fe, the hard core has a low relative energy, close to or even below that of the saddle configuration for a straight path between two easy cores. This surprising result is not a direct consequence of magnetism in bcc Fe. Moreover, the path followed by the dislocation core in the (1 1 1) plane between easy cores, identified here using two different methods to locate the dislocation position, is almost straight, while the energy landscape between the hard core position and the saddle configuration for a straight path is found to be very flat. These results in Fe are in contrast with predictions from empirical potentials as well as DFT calculations in W, where the hard core has an energy about twice that of the maximum energy along the Peierls barrier, and where the dislocation trajectory between easy cores is curved. Also, the split core configuration is found to be unstable in DFT and of high energy in both Fe and W, in contrast with predictions from most empirical potentials.     

Measurements of experimentally grown 3-D xenon multiplets and comparative phase-field simulations

Experimentally observed growth morphologies (dendrites, doublons, and seaweed) of three-dimensional (3-D) xenon crystals during free growth into pure supercooled melt were investigated in this study. Measurements of the 3-D reconstruction of dendrites were compared to analytical predictions and scaling parameters were determined. For two different phase-field models morphology diagrams were derived and compared to analytical model calculations. By using special initial conditions it is possible to reproduce the growth morphologies found in these experiments.     

Activation volume and density of mobile dislocations in hydrogen-charged iron

In order to monitor the influence of hydrogen on the coupled evolution of dislocation velocity and mobile dislocation density, we applied repeated transients to pure iron under simultaneous hydrogen cathodic charging conditions. The effective activation volume and the thermal stress were determined at different hydrogen concentrations. The effective activation volume decreases immediately with cathodic charging. At high hydrogen concentrations, the activation volume decreases and the thermal stress increases rapidly. The density of mobile dislocations in the hydrogen-charged iron has a lower rate of exhaustion than the hydrogen-free one. The thermal activation energy decreases and the average dislocation velocity increases as a function of hydrogen concentration. Transmission electron microscopy reveals hydrogen-induced tangled dislocations, which indicates a weakening of the repulsive stress field between dislocations.     

基于原子尺度模拟研究HCP镁中的一次及二次孪生模式

通过分子动力学模拟（MD）,研究在HCP镁中的一个对称倾斜晶界与基面滑移的位错相互作用而激发的变形孪晶,也就是孪晶形核与长大的过程（或者是孪晶界迁移,TBM）。{1^-1^-21}孪晶在该过程中是最易被激发的孪生模式。一旦这样的孪晶形成了,它们就会不断长大。该种孪晶界迁移是由单纯的原子位置局域调整造成的。在模拟过程中同时也产生了二次孪晶{1^-1^-22}。该二次孪晶模型的孪晶形核与长大需要克服的能垒与{1^-1^-21}孪晶不同。同时,二次孪晶的孪晶界迁移过程是通过孪晶界上的锥形滑移而激发的。     

Pinning effect of second-phase particles on grain growth in polycrystalline films studied by 3-D phase field simulations

Three-dimensional simulations of grain growth in thin films containing finely dispersed second-phase particles were performed using a phase field model. The simulations show that although the growth behavior of the columnar grain structures in thin films is essentially two-dimensional, the interaction between the particles and the grain boundaries is three-dimensional. Grain boundaries can therefore more easily break free from the particles than in purely two-dimensional systems, resulting in fewer grain boundary–particle intersections and a larger final grain size. For a given volume fraction f_V and size of the particles r, the final grain size ${\overline{R}}_{\text{lim}}$ increases with film thickness. Moreover, it was found that particles located in the middle of the film are most efficient in pinning grain boundaries. A classical Zener type relation ${\overline{R}}_{\text{lim}}/r=K(1/f_{\text{V}}^b)$ cannot describe these effects.     

Evaluation of fatigue damage in reinforced concrete slab by acoustic emission

The applicability of acoustic emission (AE) technique for evaluation of fatigue damage in reinforced concrete (RC) slabs under cyclic loadings in both laboratory and as a structure in service is studied. The fundamental test performed in laboratory shows that the cracking process can be practically monitored by the measurement of AE signals. Analysis of the relationship between loading phase and AE activity indicates that the final stages of the fracture process can be evaluated by detecting AE signals generated near the minimum loading phase. Comparison of the results from the structure and that from the laboratory specimen demonstrates that AE energy can be an effective parameter for the evaluation of fatigue damage in RC slabs in service.     

Correlation between microstructure,strain behavior,and acoustic emission of soft PZT ceramics

The influence of grain size and domain configuration on the microscopic and macroscopic electromechanical properties of soft lead zirconate titanate (PZT) ceramics were studied. Fine- and coarse-grained PZT ceramics with an average grain size of 1.5 and 3.5 μm were prepared using the conventional mixed oxide route and different sintering conditions (1225°C/2 h and 1300°C/10 h). The experiments were performed on PZT with different Zr/Ti-ratios in order to obtain pure tetragonal (45/55), pure rhombohedral (60/40), and morphotropic (54/46) compositions. To distinguish between the intrinsic and extrinsic effect microscopic strain measurements (Synchrotron X-ray diffraction) were compared with macroscopic measurements of the mechanical hysteresis loop. The results were correlated with acoustic emission measurements which microscopically reflected the intrinsic and extrinsic contributions to the macroscopic strain of the different crystallographic modifications.     

Evaluation of stress corrosion cracking phenomenon in an AISI type 316LN stainless steel using acoustic emission technique

This paper deals with the analysis of the acoustic emission (AE) signals to determine the micro-process during stress corrosion cracking (SCC) of AISI type 316LN stainless steel that cause the AE, and thus the mechanism of the SCC process. AE with amplitudes ranging from 27.6 to 46.5 dB with different counts, energy and rise times occurred during SCC of type 316LN stainless steel in 45% MgCl₂ at 413 K. The analysis of the AE signals in conjunction with fractography indicated that a surge in the AE counts and energy indicated initiation of SCC. AE was found to be continuous prior to the initiation. The time gap between AE events increased during initiation. AE events occurred in bursts during crack growth. Plastic deformation ahead of the crack tip was determined to be the major source of AE during propagation of SCC in type 316LN stainless steel. The cracking was found to initiate and propagate in the transgranular mode.     

Characterization and control of selective corrosion of α,β′-brass by acoustic emission

Acoustic emission (AE) associated with electrochemical measurements (electrochemical impedance spectroscopy and polarization curves) are used to study the selective corrosion of α,β′-brass in an ammonia buffer solution. AE reveals, in this work, three different populations of events during the corrosion of this alloy. The corrosion process was found to proceed via oxygen reduction following a diffusion-controlled mechanism and selective dissolution of α,β′-brass was controlled by the zinc atoms diffusion through oxide film. The diffusion coefficient obtained for zinc in α,β′-brass following Cottrell law is of the order of 9.28×10⁻¹² cm²/s. An equivalent circuit modelling the reactions taking place at the electrode/electrolyte interface was suggested. The analysis by X-ray diffraction of residual stresses, before and after corrosion, showed that the dezincification of metal produces two types of residual stresses: macro and microstresses. Their evolution correlates perfectly with the measured acoustic activity. These stresses are then the sources of AE. This shows the important potential of AE in the detection and the control of selective corrosion phenomena.     

利用光致等离子体声信号监测激光焊缝的熔透性

介绍了CO2 激光深熔焊接时 ,利用光致等离子体声信号来监测焊缝熔透性的一种方法。文中通过试验研究 ,获得了焊缝熔透性变化时 ,光致等离子体声信号的变化规律 ,从而建立了一种可正面监测焊缝熔透性的方法     

Ti and Cr nitride coating/steel adherence assessed by acoustic emission wavelet analysis

Wavelet Transform (WT) was applied to Acoustic Emission (AE) signals from scratch tests on stainless steel samples with Cr or Ti nitride coatings obtained by physical vapor deposition, in order to characterize the coating adherence failure mechanisms. The WT, employed to analize the essentially non-stationary AE signals, was adequate to identify different stages of failure (transversal coating microfractures and coating/matrix debonding microfractures), which could be precisely delimited by a unique parameter.