Interference Effects in Nanocrystalline Systems

Interference (cross-correlation) effects are present in the X-ray powder diffraction pattern of a polycrystalline aggregate. In an experimental diffraction pattern, the information is highly overlapped and can be confused with other effects. In this article, it is shown that the analysis of the patterns calculated from a cluster equilibrated via molecular dynamics simulation allows those effects to be separated. Extra intensity is observed, because of the presence of the grain boundaries contribution which is unexpectedly not that of an amorphous phase.     

A mathematical model for the solute drag effect on recrystallization

The model for the solute drag effect in phase transformations has been applied to recrystallization, i.e., moving grain boundaries. In this model, the total driving force is dissipated by the interfacial energy, the finite interfacial mobility, the solute drag in boundaries, and diffusion in the matrix ahead of the interface, of which all are taken into account consistently. The effects of the Gibbs energy of segregation and the diffusivity of impurity atoms in boundaries were investigated. The results show that the Gibbs energy of segregation mainly affects the critical composition at which the drastic change in the boundary velocity appears, and the diffusivity of impurity atoms in boundaries mainly affects the velocity reduced by the solute drag effect. In other words, the Gibbs energy of segregation and the diffusivity of impurity atoms in boundaries can be evaluated from experimental data by means of the present model. This model was applied to the Al-Mg system, and the Gibbs energy of segregation and the diffusivity of Mg in boundaries were evaluated from experimental data. The evaluated Gibbs energy of segregation agrees with the estimate based on elastic energy considerations. The diffusivity estimated from this model is smaller than that measured along the grain boundary.     

The detection of monolayer grain boundary segregations in steels using stem-eds X-ray microanalysis

The detection and measurement of monolayer embrittling segregations to prior austenite grain boundaries in ferritic steels using scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis are considered theoretically. The influence of experimental variables of electron probe size, electron accelerating voltage, and foil thickness on the detectability limits are considered in relation to electron probe spreading within the foil and the resulting X-ray counting statistics. The analysis predicts that a grain boundary coverage of 0.01 and 0.2 of a monolayer of tin and phosphorus, respectively, may be detected in a ferritic steel using a conventional STEM-EDS system.     

A mathematical model for the solute drag effect on recrystallization

The model for the solute drag effect in phase transformations has been applied to recrystallization, i.e., moving grain boundaries. In this model, the total driving force is dissipated by the interfacial energy, the finite interfacial mobility, the solute drag in boundaries, and diffusion in the matrix ahead of the interface, of which all are taken into account consistently. The effects of the Gibbs energy of segregation and the diffusivity of impurity atoms in boundaries were investigated. The results show that the Gibbs energy of segregation mainly affects the critical composition at which the drastic change in the boundary velocity appears, and the diffusivity of impurity atoms in boundaries mainly affects the velocity reduced by the solute drag effect. In other words, the Gibbs energy of segregation and the diffusivity of impurity atoms in boundaries can be evaluated from experimental data by means of the present model. This model was applied to the Al−Mg system, and the Gibbs energy of segregation and the diffusivity of Mg in boundaries were evaluated from experimental data. The evaluated Gibbs energy of segregation agrees with the estimate based on elastic energy considerations. The diffusivity estimated from this model is smaller than that measured along the grain boundary. ZI-KUI LIU, formerly with the Department of Materials Science and Engineering, Royal Institute of Technology     

A finite difference model for the kinetics of sensitization

This paper reports on the development of a finite difference model of the sensitization process. The model can be applied to alloy systems in which diffusion of one component to and within the grain boundary determines the rate of precipitation in the boundary. An example application of the model to sensitization of austenitic stainless steel is given. The model predicts the Cr-profiles in and normal to the grain boundaries and includes the effect of overlap of the Cr-profiles along the grain boundary. Because overlap is considered, the model is applicable in the short time range prior to the formation of a uniform Cr-level along the boundary, as well as at longer times corresponding to conditions for which error function type models available in the literature are applicable. Thus, the model provides a distinct advantage over existing models in that sensitization time can be calculated once the carbide spacing is known. Calculated Cr-profiles agree well with experimentally measured profiles and are consistent with the observation that grain boundaries can be sensitized over their entire length or only along segments of their length.     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel-II. Fine scale segregation behaviour

A combination of TEM, FIM, AP and IAP has been used to study boron grain boundary segregation in austenitic stainless steels of the types 316L (with 40 ppm or with <1 ppm boron) and “Mo-free 316L” (23 ppm boron). High resolution segregation profiles were determined for cooling rates from 0.29 to 530°C/s for three starting temperatures: 800, 1075 and 1250°C. Boron grain boundary segregation was found after all heat treatments. The segregation behaviour was mainly of the nonequilibrium type after cooling from 1075 or 1250°C whereas equilibrium segregation dominated after rapid cooling from 800°C. The influence of the relative grain orientation on the amount of non-equilibrium segregation was small for general boundaries. However, no segregation was detected at coherent twin boundaries. The binding energy of boron to austenite grain boundaries was estimated at 0.65 ± 0.04 eV for both types of steels. The influence of the composition and boron content of the steels on the segregation behaviour is discussed and the experimental techniques used are presented.     

Behavior of grain boundary chemistry and precipitates upon thermal treatment of controlled purity alloy 690

Grain boundary composition and carbide composition and structure were characterized for various microstructures of controlled purity alloy 690. Heat treatments produced varying degrees of grain boundary chromium depletion and precipitate distributions which were characterizedvia scanning transmission electron microscopy (STEM). Convergent beam electron diffraction revealed that the dominant carbide is M₂₃C₆, and energy dispersive X-ray analysis (EDAX) determined that the metallic content was about 90 at. pct chromium. A discontinuous precipitation reaction was observed and is attributed to a high degree of carbon supersaturation. Grain boundary composition measurements confirm that chromium depletion is controlled by volume diffusion of chromium to chromium-rich grain boundary carbides in the temperature range of 873 to 1073 K. Grain boundary chromium levels as low as 18.8 at. pct were obtained by thermal treatment at 873 K for 250 hours and 973 K for 1 hour. A thermodynamic and kinetic model developed for alloy 600 was modified to describe the development of the chromium depletion profile in alloy 690 during thermal treatment. Experimentally measured chromium profiles agree well with the model results for the dependence of the chromium depletion zone width and depth on various input parameters. The establishment of the model for alloy 690 allows the chromium depletion zone width and depth to be computed as a function of alloy composition, grain size, and temperature. The chromium depletion profiles and the precipitate structure and composition of controlled purity 690 are compared to those of controlled purity 600. A thermodynamic analysis of the carbide stability indicates that other factors, such as favorable orientation relationships, play an important role in controlling the precipitation of Cr₂₃C₆ in nickel-base alloys.     

Analysis of intergranular impurity concentration and the effects on the ductility of copper-shaped charge jets

Shaped charge liners (SCLs) are thin-walled metallic cones that are explosively driven to high pressures and strain rates. From a metallurgical point of view, the SCLs provide an excellent experimental test bed to evaluate the influence of microstructure on high strain rate deformation and failure. In this work, a geometrical analysis based on an assumed tetrakaidecahedron grain shape is applied to determine the relationship between grain size, overall impurity content, and ductility of the liners. The measured parameter for ductility in this case is the break-up time for sulfur-doped, oxygen-free electronic (ofe) copper SCLs after they are explosively driven. The calculations determine the number of impurity atoms as a function of grain size, the number of available sites at the intercrystalline defects, and the intercrystalline impurity concentration. Recent experiments have shown that larger grain size liners with low impurity contents exhibit better ductility than smaller grain size liners with higher impurity concentrations, which is contrary to conventional wisdom that the liner ductility scales directly with grain size or impurity content alone. Within the range of grain sizes and bulk impurity contents in this study, the analysis suggests that the quadruple nodes and triple lines are saturated with impurities. Over this same range of impurities and grain sizes, only a fraction of a monolayer of impurities exists at the grain boundaries if all boundaries are assumed to be equally susceptible to sulfur segregation. Modification of the analysis by assuming partitioning of the sulfur only to crystallographically random boundaries, however, suggests that there is a correlation between the jet break-up time and the transition to complete monolayer coverage of such boundaries. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

STEM-EDS X-ray microanalysis of small precipitates in iron base thin foils

The experimental conditions for detection of segregated elements in the X-ray spectrum recorded from small precipitates ≦ 100 nm diameter contained within a thin foil of an iron base alloy using STEM-EDS X-ray microanalysis are described. The influence of precipitate size, position and composition, foil thickness, electron accelerating voltage, and X-ray emission intensity on its detectability are evaluated. Optimum experimental conditions for microanalysis of these precipitates are established and the limitations of current techniques discussed.     

The stress corrosion susceptibility of a quenched and tempered 12 pct crmov martensitic stainless steel

The stress corrosion susceptibility of a martensitic 12 pct Cr 1 pct MoV stainless steel in alkaline chloride solution has been measured as a function of tempering heat treatment. The microstructures produced during tempering have been characterized by transmission electron microscopy and related to measured hardness values. In addition, scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis has allowed the distribution of alloying elements within the microstructure to be examined. Electron energy loss spectroscopy was used to establish fully precipitate compositions, and the microanalysis results have been explained in terms of a diffusion controlled growth of grain boundary precipitates. The overall stress corrosion cracking susceptibility has been correlated with the development of chromium solute depletion profiles about prior austenite grain boundaries.     

E690海洋平台用钢力学性能和海洋大气腐蚀行为

以传统的E36海洋平台钢为对比钢,研究三种E690海洋平台钢的组织和力学性能,以及模拟海洋大气环境下的腐蚀行为.通过失重法测得实验钢在不同腐蚀时间下的腐蚀速率,利用扫描电镜和X射线衍射仪观察并测定了锈层的形貌特征和相组成,采用电子背散射衍射技术对实验钢的晶界类型进行分析.结果表明:以贝氏体组织为特征的E690海洋平台钢具有优异的力学性能,-40℃的冲击值超过了200 J;晶界类型主要为3°~15°的亚晶界和大于50°的大角度晶界;E690海洋平台钢周浸16 d后的锈层致密且腐蚀速率已趋于稳定,最低腐蚀速率为0.84 mm·a-1,远低于组织为铁素体+珠光体钢的1.4 mm·a-1,实验钢的锈层主要由Fe₃O₄、α-FeOOH、β-FeOOH及γ-FeOOH四种晶态相和非晶无定形物组成.通过分析得出,热处理工艺和组织构成对材料的初期腐蚀行为有重要影响,而化学成分和锈层自身的致密性对材料后期腐蚀行为起决定作用.      

Cyclic deformation of pure aluminum bicrystals

The dynamic behavior of 〈112〉-tilt grain boundaries in aluminium bicrystals under the influence of cyclic stresses at elevated temperatures is reviewed. Bicrystals, containing low- and high-angle grain boundaries within a wide range of misorientation angles, were deformed at several combinations of stress, temperature, and number of cycles. The grain-boundary (GB) displacement and the deformed structure of bicrystals were framed using standard optical microscopy. The grain orientations were measured using the electron backscatter diffraction (EBSD) technique with a scanning electron microscope (SEM), before and after the deformation. There is distinct evidence of a sharp transition angle between low- and high-angle grain boundaries, with respect to the ability of the boundaries to move under the given parameters. The experimental observations lead to the conclusion that a difference in the dislocation structure in two grains causes the driving force for GB migration.     

Evaluation of Mean Velocity and Turbulence Measurements with ADCPs

To test the ability of acoustic Doppler current profilers (ADCPs) to measure turbulence, profiles measured with two pulse-to-pulse coherent ADCPs in a laboratory flume were compared to profiles measured with an acoustic Doppler velocimeter, and time series measured in the acoustic beam of the ADCPs were examined. A four-beam ADCP was used at a downstream station, while a three-beam ADCP was used at a downstream station and an upstream station. At the downstream station, where the turbulence intensity was low, both ADCPs reproduced the mean velocity profile well away from the flume boundaries; errors near the boundaries were due to transducer ringing, flow disturbance, and sidelobe interference. At the upstream station, where the turbulence intensity was higher, errors in the mean velocity were large. The four-beam ADCP measured the Reynolds stress profile accurately away from the bottom boundary, and these measurements can be used to estimate shear velocity. Estimates of Reynolds stress with a three-beam ADCP and turbulent kinetic energy with both ADCPs cannot be computed without further assumptions, and they are affected by flow inhomogeneity. Neither ADCP measured integral time scales to within 60%.     

Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called “special” boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called “special” boundaries.     

Molecular Dynamics Simulations of Dislocation Activity in Single-Crystal and Nanocrystalline Copper Doped with Antimony

Recent experimental and simulation results have indicated that high-temperature grain growth in nanocrystalline (NC) materials can be suppressed by introducing dopant atoms at the grain boundaries. However, the influence of grain boundary dopants on the mechanical behavior of stabilized NC materials is less clear. In this work, molecular dynamics (MD) simulations are used to study the impact of very low dopant concentrations (<1.0 at. pct Sb) on plastic deformation in single-crystal and NC Cu. A new interatomic potential for low Sb concentration Cu-Sb solid-solution alloys is used to model dopant/host and dopant/dopant interatomic interactions within the MD framework. In single-crystal models, the strained regions around the Sb atoms act as heterogeneous sources for partial dislocation nucleation; the stress associated with this process decreases with increasing Sb concentration. In NC models, MD simulations indicate that Sb dopants randomly dispersed at the grain boundaries cause an increase in the flow stress in NC Cu, implying that Sb atoms at the grain boundaries retard both grain boundary sliding and dislocation nucleation from grain boundary regions.     

Nonequilibrium solute segregation to austenite grain boundaries in low alloy ferritic and austenitic steels

Nonequilibrium segregations of substitutional solute and impurity elements to austenite grain boundaries during cooling from the austenitizing temperature have been measured in thin foil specimens of ferritic 2.25 pct Cr 1 pct Mo, 2.25 pct Cr 1 pct Mo 0.08 pct Sn and austenitic Type 316 steels using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometry (STEM-EDS). The results are discussed and compared with the predictions of a theoretical analysis which describes the segregation in terms of a quench-induced diffusion of vacancy-solute atom pairs to the grain boundaries.     

Nonequilibrium solute segregation to austenite grain

Nonequilibrium segregations of substitutional solute and impurity elements to austenite grain boundaries during cooling from the austenitizing temperature have been measured in thin foil specimens of ferritic 2.25 pct Cr 1 pct Mo, 2.25 pct Cr 1 pct Mo 0.08 pct Sn and austenitic Type 316 steels using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometry (STEM-EDS). The results are discussed and compared with the predictions of a theoretical analysis which describes the segregation in terms of a quench-induced diffusion of vacancy-solute atom pairs to the grain boundaries.     

Shear band formation in plane strain compression

Shear band formation during plane strain compression of single crystals and polycrystals of an Al-3 wt% Cu alloy was studied. X-ray and electron diffraction, optical microscopy, and electron (TEM/STEM) microscopy were used to document the structure and micromechanisms of the localization process. Complementary finite element studies were performed using the measured single crystals' single slip system strain hardening data. The experimental observations and the computed deformation response are in very close agreement, and indicate that localization, through macroscopic shear band formation, occurs in continuously strain hardening, damage-free material. Shear band formation was preceeded by the development of very coarse slip which, like the shear bands, propagated across entire grains and, in single crystals, across the entire crystal. The computations and experiments showed that geometrical softening, caused by nonuniform lattice reorientation, is an important micromechanical influence on the localization process in both single crystals and polycrystals. Also, the propagation of shear bands across grain boundaries in polycrystals was looked at experimentally and computationally. Particular attention was paid to the crystallography of shear band transmission through grain boundaries.     

Influence of Grain Size and Age-Hardening on Dislocation Pile-Ups and Tensile Fracture for a Ti-AI Alloy

In an aged Ti-8.6 wt pct Al alloy macroscopic embrittlement occurs with increasing grain size and degree of age hardening. The influence of the grain sizeL on the true fracture strain can be described by εF ∼L-¹ Tensile crack nucleation is caused microscopically by strong dislocation pile-ups which crack the grain boundaries. Using transmission electron microscopy and equations from the dislocation theory, an experimental method was developed to determine quantitatively the shear stress concentrations at the grain boundaries which are produced by the dislocation pile-ups and cause crack nucleation. The experimental results show that for all investigated grain sizes and degrees of age hardening a critical local stress t* _C ≈ 38 GPa leads to crack nucleation. Based on this result equations were derived which describe the combined influence of grain size and age hardening on the true fracture strain and on the true fracture stress. These equations show a good agreement with the tensile test results.