间隙原子对高熵合金组织及性能影响的研究现状

高熵合金作为一种新型的合金体系,虽然其组成元素复杂,但能形成简单的固溶体,具有许多异于传统合金的结构和性能特征,其研究近年来成为热点。间隙原子可以溶入基体晶格间隙产生固溶强化,与合金元素结合形成细小弥散强化相,以及降低层错能,改变位错运动方式等,从而改善高熵合金性能。文章在论述高熵合金组织结构特性的基础上,分析了间隙原子C、N、O、B对高熵合金相形成规律、强化机理、塑性变形机制的影响,总结了间隙原子含量及其产生的固溶强化、晶粒细化、第二相强化作用对高熵合金组织性能等方面影响的研究进展,最后提出了含间隙原子的高强高韧高熵合金组织结构设计研究的新方向。      

On the distribution of carbon in martensite

The statistical-mechanics of a generalized perfect lattice gas is used to describe the distribution of interstitial solute atoms in martensite. In untempered martensite, partitioning of mobile interstitial carbon occurs between normal octahedral interstitial sites and those distorted sites around immobile dislocations. The statistics adopted acknowledge the finite number of each kind of site per unit volume of martensite. The dislocation density, fraction of twinned martensite, and the arrangement of dislocations are all input variables in the calculations. The principal quantities calculated are the fraction of carbon atoms segregated to dislocations and the fraction of distorted sites occupied as functions of the carbon content and substructure. The equilibrium distribution of carbon is also determined for tempering conditions where either ∈-carbide or cementite may precipitate. Here, the change in the solubility limit of ferrite with dislocation density is predicted. In untempered low carbon martensites (at 300°K) 85 pct of the carbon will be segregated to dislocations at equilibrium. This value decreases to 60 pct in an 0.80 wt pct C steel. Less than 5 pct of the distorted sites are filled when the dislocation distribution is uniform. Much higher concentrations occur when the long range stresses of the dislocations are relaxed and the mean carbon/dislocation interaction energy increases. Analogous results are presented for the equilibrium among carbides, normal sites, and distorted sites. The predictions of the lattice gas model are in agreement with numerous independent experimental observations. David Kalish, formerly withLockheed-Georgia Co. E. M. Roberts, formerly with Lockheed-Georgia Co.     

再结晶钼合金的晶界偏析及其对脆性晶间断裂的影响

业已确定，再结晶钼合金的碳和氧偏析与温度的关系并不是单调的，这是一系列组织结构变化的结果，位错亚组织破碎，晶粒长大和晶界迁移，晶粒边界粒子的析出和溶解，近边界区内杂质元素之间的化学相互作用。建立了晶界化学和冷脆性温度之间的关系，造成晶间脆性的因素是一定温度下的氧偏析增加，杂质原子导致的近边界区强化，沿着晶界的扁平碳化物形成。     

Influence of interstitial carbon,nitrogen, and hydrogen on the plasticity and brittleness of steel

The introduction of carbon, nitrogen, and hydrogen in steel is analyzed in terms of the electron structure, dislocation properties, hardening, and failure of the steel. The similarity and differences in the mechanical properties of the corresponding solution solutions are discussed in relation to the influence of these elements on the density of electron states at the Fermi level of iron and correspondingly on the concentration of free electrons. Carbon reduces the concentration of free electrons, while nitrogen and hydrogen have the opposite effect. Hence, the atomic interaction is changed: specifically, its covalent or metallic component will be intensified. The dislocation rate in deformation is analyzed in the approximation of mobile and immobile interstitial atoms. In the first case, the interstitial atoms obstruct dislocational slip; the mobility of the dislocations is determined by the binding enthalpy of the dislocations with impurity atoms. If the interstitial atoms may accompany dislocations, the atomic bond is locally changed in dislocational atmospheres. That affects the unit energy of the dislocations and the distance between them in the slip planes. On the basis of the research results, the significant similarity between the hydrogen brittleness of austenitic steel and the ductile–brittle transition in alloying with nitrogen is explained.     

Influence of austenite and martensite strength on martensite morphology

The martensite morphology and austenite flow strength have been determined in a variety of ferrous alloys chosen so that the austenites were paramagnetic, ferromagnetic, substitutional strengthened, and interstitial strengthened. It is demonstrated that two of the most important variables in determining the habit plane (and thus morphology) of martensite in a given alloy are the resistances to dislocation motion in austenite and in ferrite (i. e., martensite). In the wide variety of alloys where martensite with a {259}_γ habit plane was observed, the austenite flow strength atM _s is greater than 30,000 psi. At lower austenite strengths, either {225}_γ or {111}_γ habit planes are found depending on the resistance to dislocation motion in ferrite. Thus, {225} martensites are not always found as part of the spectrum between {111} and {259} martensites but only in the cases (e. g., interstitial strengthening) where ferrite is preferentially strengthened relative to austenite. All of the observations are consistent with the idea that the habit plane observed in a given alloy is the one involving the minimum plastic work for the lattice invariant shear.     

Solid state amorphization by mechanical alloying—An atomistic model

A model that provides a quantitative criterion for the ability to form an amorphous phase of a bi-elemental system by Mechanical Alloying (MA) and the concentration range where the amorphization occurs is presented. It is assumed that amorphization by MA is obtained when impurity atoms penetrate into interstitial sites and form local distortions of the lattice. When the local distortions reach some critical value, the long range order of the lattice is destroyed and the amorphous phase is obtained. The model has been applied to various experimental data reported in the literature, and the results are discussed.     

Study of Fatigue in Metals Using Ultrasonic Technique

Abstract

An investigation of the basic mechanisms associated with cyclic and fatigue stressing was undertaken employing as a research tool a megacycle frequency internal friction method (ultrasonic wave attenuation). The materials used were plain high-carbon and alloy steels (SAE 1095 and SAE 4340). The behaviour of lattice imperfections, dislocations and interstitial atoms (carbon) in particular, was studied in more detail, since these are reported to have a strong influence on the fatigue characteristics of iron-base alloys.

Initially, static stressing conditions were studied. Using a dislocation loop pinning model, the conditions for the break-away of a dislocation loop from interstitial atoms in annealed and in cold-worked material were investigated. These findings were applied to the experimental results obtained on cyclic stressing and an interpretation is offered in terms of the formation of active dislocation loops and their immobilization by interstitial atoms.

The results are discussed with reference to improving fatigue characteristics of iron-base alloys and a similarity with the work of Mason (19) on non-ferrous metals is noted.     

Hydrogen retrapping after thermal charging of hydrogen in iron single crystal

A theory to estimate and describe the behavior of supersaturated hydrogen in interstitial sites of a normal lattice and in trap sites is suggested, and the experimental proof is provided by thermal analysis. In this theory, variation with temperature changes of the equilibrium state between hydrogen in trapping sites and in normal lattice sites, which occurs during rapid cooling after hydrogen charging at high temperature, is considered. Two evolution rate peaks of hy-drogen corresponding to a reversible trap, a dislocation, and to an irreversible trap, a microvoid, are observed, respectively, at 388 and 538 K in a thermal analysis plot. The hydrogen amount released from the reversible trap is increased with decreasing microvoid concentration, even though the reversible trap density is maintained at the same level. According to the theoretical analysis, supersaturated hydrogen dissolved in a normal lattice site by a rapid cooling of hydrogen-charged iron from high temperature is predominantly retrapped into the vacant irreversible trap-ping sites. The remaining hydrogen exists in the normal lattice interstitial sites and will maintain local equilibrium with hydrogen in the reversible trap sites. The apparent hydrogen diffusivities at 293 K with each type of trap are estimated to be 1 × 10^-6 cm²/s for reversible traps and 4 × 10^-8 cm²/s for microvoid traps, based on changes in the hydrogen amount released from each type of trap with the room-temperature anneal time in vacuum. Formerly Graduate Student, Korea Advanced Institute of Science and Technology     

Calculations of the binding of hydrogen to fixed interstitial impurities in nickel

We have calculated the binding energy of hydrogen to inert, fixed interstitial impurity atoms He, Ne, Ar, Kr, and Xe and also the chemically active impurities C, O, and N in Ni using local density functional theory including lattice relaxation.     

Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 × 10⁻⁴ oxygen equivalent) and commercial purity (6 × 10⁻³ oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the(c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a student at the University of Florida.     

Peierls-Nabarro hardening in the presence of point obstacles

In light of the controversy that has existed between proponents of Peierls-Nabarro hardening and dispersed barrier hardening with regard to possible low temperature deformation mechanisms in bee and hep metals, this paper examines the effect of point obstacles on Peierls-Nabarro hardening. The statics of double kink nucleation over Peierls barrier on finite dislocation segments are studied in detail. Point obstacles such as interstitial impurity atoms are considered to limit the length of the dislocation segments. Adopting the rate theory approach, temperature dependence of the yield stress is then investigated as a function of the concentration of point obstacles. It is found that point obstacles have little or no effect on double kink nucleation processes, when the applied stress is near the Peierls stress or at low test temperatures. At lower applied stresses or at higher test temperatures, the present results are significantly different from predictions of the Dorn-Rajnak calculation. In the present theory, the applied stress on the dislocation, τ, is found to be finite and independent of test temperature in dilute solid solutions at elevated temperatures where the Dorn-Rajnak theory predicts vanishing τ. This apparent “athermal” component of τ increases linearly with the concentration of point obstacles, whereas temperature dependent part of τ decreases. These predictions are consistent with experimental observations on iron-base alloys.     

Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 x 10^-4 oxygen equivalent) and commercial purity (6 x 10^-3 oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the (c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. A. M. GARDE, formerly a Post Doctoral Fellow at the University of Florida. E. AIGELTINGER, formerly a Post Doctoral Fellow at the University of Florida. B. N. WOODRUFF, formerly a student at the University of Florida.     

<Emphasis Type="Italic">In Situ</Emphasis> Neutron-Diffraction Studies on the Creep Behavior of a Ferritic Superalloy

Precipitate strengthening effects toward the improved creep behavior have been investigated in a ferritic superalloy with B2-type (Ni,Fe)Al precipitates. In situ neutron diffraction has been employed to study the evolution of the average phase strains, (hkl) plane-specific lattice strains, interphase lattice misfit, and grain-orientation texture during creep deformation of the ferritic superalloy at 973 K (700 °C). The creep mechanisms and particle-dislocation interactions have been studied from the macroscopic creep behavior. At a low stress level of 107 MPa, the dislocation-climb-controlled power-law creep is dominant in the matrix phase, and the load partition between the matrix and the precipitate phases remains constant. However, intergranular stresses develop progressively during the primary creep regime with the load transferred to 200 and 310 oriented grains along the axial loading direction. At a high stress level of 150 MPa, deformation is governed by the thermally activated dislocation glide (power-law breakdown) accompanied by the accelerated texture evolution. Furthermore, an increase in stress level also leads to load transfer from the plastically deformed matrix to the elastically deformed precipitates in the axial direction, along with an increase in the lattice misfit between the matrix and the precipitate phases.     

Substructure and dispersion hardening in aged,cold worked,and annealed Al-4 wt pct Cu alloy

Electron microscopy and X-ray line profile analyses have been employed to define the microstructures and substructures of pure aluminum and an overaged Al-4 wt pct Cu alloy after various thermomechanical treatments. Tensile tests were performed on the same materials, and the results have been interpreted in terms of structure. A given cold rolling reduction of the aged Al-4 wt pct Cu alloy produced a much higher dislocation density and a less cellular substructure than the same treatment produced in pure aluminum of comparable initial grain size. Annealing after cold work produced similar responses in both the pure metal and the alloy. For the aged alloy in the as-rolled, or rolled-and-annealed condition, dispersion strengthening and substructure strengthening were found to be linearly additive, and they accounted for virtually all the observed tensile yield strength. Substructure strengthening has been discussed in terms of the relation between dislocation density and the spacing and nature of the substructure boundaries.     

钛微合金化高强钢的组织性能及强化机制

 为了掌握钛微合金化高强钢的组织性能、第二相粒子特性和析出规律以及强化机制，采用光学显微镜、扫描电镜、透射电镜、拉伸试验机等设备并结合热力学计算，对高强度汽车车厢板进行了系统研究。研究结果表明，试验钢的显微组织类型主要为多边形铁素体+针状铁素体+少量索氏体，平均有效晶粒尺寸约为3.5 μm。钢中存在大量的球形TiC和少量的不规则形状Ti4C2S2及方形TiN析出物，析出顺序为TiN→Ti4C2S2→TiC。第二相析出物以TiC的沉淀强化效果最为显著，TiN和Ti4C2S2的沉淀强化效果十分微弱。试验钢中所有强化方式对试验钢的强度贡献大小顺序为细晶强化＞沉淀强化＞位错强化＞固溶强化＞晶格点阵强化，其中细晶强化和沉淀强化的强化效果最为显著，对屈服强度的贡献超过50%。     

A Nanoscale Study of Dislocation Nucleation at the Crack Tip in the Nickel-Hydrogen System

Strengthening and embrittlement are controlled by the interactions between dislocations and hydrogen (H)–induced defect structures that can inversely affect the deformation mechanisms in materials. Here we present a simulation framework to understand fundamental issues associated with H-assisted dislocation nucleation and mobility using Monte Carlo (MC) and molecular dynamics (MD). In order to study the interaction between H and dislocations and its effect on material failure, we extensively examined mode I loading of an edge crack using MD simulations. The MD calculations of the total structural energy in the nickel (Ni)–H system shows that H atoms prefer to occupy octahedral interstitial sites in the fcc Ni lattice. As H concentration is increased, the Young’s modulus and the energy required to create free surface decreased, resulting in H-enhanced localized plasticity. The MD simulations also indicate that H not only facilitates dislocation emission from the crack tip but also enhances dislocation mobility, leading to softening of the material ahead of the crack tip. While the decrease in surface energy suggests H embrittlement, the increase in local plasticity induces crack blunting and prohibits crack propagation. The mechanisms responsible for transitioning from a ductile to brittle crack behavior clearly depend on the H concentration and its proximity to the crack tip. Enhanced plasticity will occur within a general field of H atoms that results in lower stacking fault and surface energies, yet H interstitials on preferential slip planes can inhibit dislocation nucleation.     

Electrotransport purification and some characterization studies of vanadium metal

High-purity vanadium metal having an interstitial impurity content of <5 wt ppm and a resistance ratio,R _{300 K}/R _{4.2 K}, of >1880 has been prepared by an electrotransport technique. The lattice parameter of vanadium of this purity is 3.0269 +- 3Å. An extrapolated value for the room temperature resolved flow stress of vanadium free of C, O, and N was determined to be 0.41 kg/mm² on this material.     

半绝缘砷化镓晶片中碳的微区分布

通过化学腐蚀(AB腐蚀液)、金相显微镜观察、透射电镜(TEM)及能谱分析(EDX)，对LEC法生产的半绝缘砷化镓(SI-GaA8)单晶中碳的傲区分布进行了分析。其结果表明：碳的微区分布与晶片中位错密度及分布存在对应关系。高密度位错区位错形成胞状结构，该结构的胞壁区碳含量高，近胞壁区次之，剥光区碳含量低于检测限。     

Surface strengthening produced by an inherent polycrystalline layer

Inherent polycrystalline layers 20 to 40 μm thick were produced on Cu single crystal substrates by a controlled surface abrasion and annealing technique. A surface zone produced in this fashion theoretically minimizes certain parameters associated with film strengthening such as, film-substrate modulus differences, chemical composition gradients, film decohesion, lattice parameter misfit and surface energy variations. Investigations of the mechanical behavior of Cu single crystals surrounded with such polycrystalline layers were compared with uncoated single crystals and the results correlated with etch-pit observations and mechanical relaxation measurements. The results indicated that plastic constraints were the principle mechanism responsible for the excess hardening of crystals with a polycrystalline surface zone. The constraint effect was anisotropic with enhanced higher order slip activity, observed in the substrate as an excess forest dislocation density with depth, being principally associated with primary screw dislocation accumulations at the coating-substrate interface. Correlation of strain recovery studies with observations of slip development in the surface layer showed that initially the polycrystalline zone acted as an impenetrable barrier which eventually degenerated when slip activated in the surface grains at strains ≈ 3 x 10^-4.     

An Assessment of Milling Time on the Structure and Properties of a Nanostructured Ferritic Alloy (NFA)

The tensile properties of a 14 wt pct chromium nanostructured ferritic alloy (NFA) are assessed as a function of attrition time. Small angle X-ray scattering results show quantitatively that the number density of precipitated oxides increases as a function of milling time. This difference in oxide density alone is not enough to describe the tensile behavior of the NFA as a function of temperature. As a result, a previously proposed root mean square strengthening model is applied to the current study where direct dispersion strengthening, grain boundary strengthening, dislocation forest hardening, and matrix hardening are all considered. When an optimization routine is conducted, the fitting results suggest that the precipitated oxides are soft obstacles to dislocation motion.