AlN在Hi-B钢铁素体相中析出的形核机制

通过理论计算的方法,系统分析了AlN在Hi-B钢铁素体中的析出形核机制。结果表明,第二相AlN粒子不同析出形核机制下的临界形核尺寸随着温度的降低而降低,形核机制不同,临界形核尺寸不相同,但均匀形核和晶界形核的临界形核尺寸较为接近,同一温度条件下,位错形核的临界形核尺寸最小。第二相AlN粒子以位错形核、均匀形核、晶界形核3种形核机制形核的最快析出温度分别为1 273、1 193、1 293K。同时,温度在1 293K以下时,取向硅钢中AlN在铁素体中以位错形核为主,温度高于1 293K后,AlN的形核机制以晶界形核为主。     

Theory and experiment of martensitic nucleation in ZrO2 containing ceramics and ferrous alloys

An in situ TEM experiment on martensitic nucleation was performed using sub-micron ZrO₂ particles which were initially defect free. Extrinsic, spontaneous and heterogeneous nucleation was induced by introducing dislocation loops with a strong shear component and Hertzian contact stresses into selected particles. The critical characteristics of these defects which were required to trigger barrierless nucleation, in terms of the loop radius, Burgers vector, contact stress, and contact area were in good agreement with an analytic elasticity model. Along with dislocation walls, these defects which have a short range stress field can also account for intrinsic nucleation in ZrO₂ containing ceramics and ferrous alloys. The essential features of nucleating defects, which may be classified into intrinsic or extrinsic ones depending on their origins, and short range or long range ones depending on their stress fields, are analyzed and contrasted in view of their experimental evidence.     

室温注氢Fe-Cr合金在不同温度退火后位错环的表征

利用透射电子显微镜, 通过构建位错环在不同晶带轴下的投影图结合位错环消光判据, 对室温注氢后Fe-9%Cr模型合金在400、500及550℃退火形成的1/2 〈111〉和〈100〉两种类型的位错环进行了表征. 实验结果表明, 室温注氢Fe-9%Cr合金中柏氏矢量为〈100〉型位错环的数量随着退火温度的升高而逐渐增加. 在400和500℃退火后, 〈100〉型位错环所占比例分别为16.48%、92.78%;当退火温度升高到550℃时, 位错环全部转变为〈100〉型位错环. Fe-9%Cr合金中位错环类型转变温度区间为400~500℃, 与纯铁相比, 添加Cr元素能够使位错环类型转变温度升高.      

Electromigration-induced dislocation climb and multiplication in conducting lines

Electromigration along dislocation cores is considered as a mass-transport mechanism, in conducting lines of bamboo-like grains, below one half of the melting temperature. Given that the dislocation density in annealed metals may be insufficient to account for the observed mass-transport rate, this paper focused on electromigration-induced dislocation motion and multiplication. A prismatic loop climbs like a rigid ring, as electromigration relocates atoms along the core, from one portion of the loop to the other. Each loop is therefore a mass carrier: a vacancy loop migrates towards the cathode and an interstitial loop towards the anode. Furthermore, a thread of an edge dislocation multiplies prismatic loops under a sufficiently high electric field. A bamboo grain-boundary catches loops on one side and emits on the other. Available empirical facts are discussed according to this picture, including lifetime, linewidth, stress gradient and alloying.     

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.     

Stacking fault energy measurement from diffusion

The annealing kinetics of dislocation loops has been considered from climb theory based on a model involving vacancy diffusion as the rate controlling mechanism. The theory has been applied to fee metals of high, intermediate, and low stacking fault energy to determine both the intrinsic and extrinsic fault energy using transmission electron microscopy. Reliable values are obtained from metals with γ ? 70 erg per sq cm but for low γ metals the rate controlling mechanism is shown to be one of jog nucleation and propagation rather than vacancy diffusion. The technique of loop annealing is also shown to be applicable to hep metals such as zinc, magnesium, and cadmium, even when the foil surfaces act as vacancy sources. Results of loop shrinkage and loop growth are analyzed to provide fault energy values for those metals with hep structure.     

Force relations applicable to dislocation-forest intersections

Force-distance curves are determined for three different dislocation interaction processes involving stages of intersection processes between dislocation loops and straight dislocation lines, both of which in general are kinked. For a given stress, the energy of the configuration is minimized with respect to loop size and kink size. The loops and kinks are approximated by configurations of finite straight segments, but the energy expressions are exact. The results differ from earlier estimates for dislocation intersection processes: the stress vs distance of separation relations had exponents differing by as much as an order of magnitude from the usual hyperbolic law.     

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.     

Finding Critical Nucleus in Solid-State Transformations

Based on the phase-field total free energy functional and free-end nudged elastic band (NEB) algorithm, a new methodology is developed for finding the saddle-point nucleus in solid-state transformations. Using cubic → tetragonal transformations in both two and three dimensions as examples, we show that the activation energy and critical nucleus configuration along the minimum energy path (MEP) can be determined accurately and efficiently using this new approach. When the elastic energy contribution is dominant, the nucleation process is found to be collective with the critical nucleus consisting of two twin-related variants. When the elastic energy contribution is relatively weak, the critical nucleus consists of a single variant, and the polytwinned structure develops during growth through a stress-induced autocatalytic process. A nontrivial two-variant critical nucleus configuration is observed at an intermediate level of the elastic energy contribution. This general method is applicable to any thermally activated process in anisotropic media, including nucleation of stacking faults and dislocation loops, voids and microcracks, and ferroelectric and ferromagnetic domains. It is able to treat nucleation events involving simultaneously displacive and diffusional components, and heterogeneous nucleation near pre-existing lattice defects. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASM Phase Transformations Committee.     

Effect of elastic distortions caused by Orowan loops in an infinite anisotropic medium on Moiré fringes

The effect of the elastic distortions caused by the discrete Orowan loops surrounding α-Fe particle embedded in a Cu matrix on the rotation angles of the dislocation loop plane and Moiré fringes was examined in terms of the anisotropic elasticity by using the numerical methods. For both isotropic and anisotropic cases, these angles were calculated depending on the shape of the dislocation loops. We arrived at the conclusion that, in order to explain the experimental rotation angles of Moiré fringes, the anisotropic properties and the shape of the discrete Orowan loops should be taken into account.     

Strain-induced Precipitation Behavior ofδPhase in Inconel 718 Alloy

To study the precipitation dynamics ofδphase in Inconel 718alloy,two-stage interrupted compression method was used in the region of cold deformation temperatures and the temperatures range from 875to 975℃.The precipitation-time-temperature(PTT)curve ofδphase was obtained by analyzing the softening kinetics curves.For verifying the type of the precipitates and confirming the validity of the test,the transmission electron microscopy(TEM),scanning transmission electron microscopy(STEM)and energy dispersion spectrum(EDS)were employed.Experimental results indicated that the PTT curve forδprecipitation exhibited a typical"C"shape and the nose points of start and finish precipitation were about 5sat 920℃and 2 815sat 940℃,respectively.In addition,the nucleation ofδwas heterogeneous.The nucleation sites varied with temperatures,including dislocation,grain boundary and stacking fault withinγ″phase.Andδparticles grew quickly at higher temperature with lower density.Moreover,the driving force of nucleation was mainly including chemical free energy,interfacial energy and dislocation distorted energy.And the dislocation distorted energy could decide the density of nucleation in the strain-induced process.     

Forecasting Low-Cycle Fatigue Performance of Twinning-Induced Plasticity Steels: Difficulty and Attempt

We find the existing empirical relations based on monotonic tensile properties and/or hardness cannot satisfactorily predict the low-cycle fatigue (LCF) performance of materials, especially for twinning-induced plasticity (TWIP) steels. Given this, we first identified the different deformation mechanisms under monotonic and cyclic deformation after a comprehensive study of stress–strain behaviors and microstructure evolutions for Fe-Mn-C alloys during tension and LCF, respectively. It is found that the good tensile properties of TWIP steel mainly originate from the large activation of multiple twinning systems, which may be attributed to the grain rotation during tensile deformation; while its LCF performance depends more on the dislocation slip mode, in addition to its strength and plasticity. Based on this, we further investigate the essential relations between microscopic damage mechanism (dislocation–dislocation interaction) and cyclic stress response, and propose a hysteresis loop model based on dislocation annihilation theory, trying to quickly assess the LCF resistance of Fe-Mn-C steels as well as other engineering materials. It is suggested that the hysteresis loop and its evolution can provide significant information on cyclic deformation behavior, e.g., (point) defect multiplication and vacancy aggregation, which may help estimate the LCF properties.     

Constitutive model of the TWIP effect in a polycrystalline high manganese content austenitic steel

The TEM study of our steel with a high manganese content reveals that mechanical twining (TWIP effect) occurs during the deformation at room temperature. Microtwins are organised into parallel stacks and two systems are sequentially activated in each grain. They participate to the deformation and they are strong obstacles for the dislocations and for other twins, leading to the decrease of the effective grain size. Thus, TWIP provides our alloy a very good ductility and a high hardening rate. Our constitutive modelling deduced from a model proposed by Bouaziz and Guelton [4] integrates this typical organisation of microtwins. Twinning is quantified in each grain by the partial volume fraction of twins in each system. A nucleation law for the microtwins is introduced which depends on the local stress and the stress relaxation due to pre‐existing twins. The flow stress is deduced from the dislocation density, which evolves with the dynamical recovery and the decrease of the mean free path (MFP). The MFP takes into account the grain and twin boundaries and the forest dislocations. The strain is calculated by adding the contributions of dislocation glide and twinning accounting the orientation of the grain. To treat the polycristal, the behaviours of different grain orientations are mixed by assuming at each strain step that the increment of elastic energy stored is the same in each grain. The model was successfully applied to describe the mechanical properties of our alloy, for two different grain sizes. Some microstructural parameters are yet fitted. This leads to an insufficient prediction of the evolution of the microstructure. In further developments, we expect to introduce numerical simulation results on local characteristics of microtwins (thickness, critical resolved shear stress for twinning) and experimental results on the rate of twin nucleation.     

Altering the time cycle of heat treatment by controlling grain boundary and subboundary structure

The spectrum of grain and subgrain boundary structures in the matrix phase can frequently be changed by processes such as texturing and polygonization. These changes can markedly alter the morphology, kinetics of nucleation and/or growth and even the identity of the precipitate formed during subsequent heat treatment. The numerous effects of grain and subgrain boundary structure upon precipitation processes are summarized and discussed. When the boundary structure is of the dislocation type many of the noncrystallographic effects are qualitatively explicable in terms of Cahn’s theory of nucleation at isolated dislocations or by simple extensions of this theory; but a more detailed study of the strain energy interactions between a plate-like nucleus and a dislocation boundary is needed for a quantitative understanding of these effects. Some anomalies which are presently unexplained are noted. At general high-angle grain boundaries, on the other hand, the effects of boundary structure are simpler and better understood. Grain boundary allotriomorphs are normally the only morphology present. A question remains about the detailed mechanism of nucleation, but the relative roles of volume diffusion directly to allotriomorphs and of volume diffusion aided by interfacial diffusion in the growth kinetics of allotriomorphs are becoming increasingly well established. This paper is based upon a presentation made at a symposium on Altering the Time Cycle of Heat Treatment, held at the Philadelphia meeting of The Metallurgical Society of AIME, October 14, 1969, under the sponsorship of the IMD Heat Treatment Committee.     

TEM study on the nucleation and growth of hydride in LaNi5 alloy

The nucleation and growth of hydride in LaNi₅ alloy studied by analyzing dislocation structures after a cycle of hydrogen absorption and desorption. Prismatic dislocation loops and misfit superdislocations were observed depending on the stages of nucleation and growth. From analysis on the morphology, Burgers vector, and line vector of these dislocations, a model for the nucleation and growth of hydride is suggested.     

Dislocation structures in fatigued polycrystalline copper

The dislocation structures in fatigued polycrystalline copper with small average grain size were investigated over a plastic strain range from 1.5 x 10⁻⁵ to 10⁻². It was found that the dislocation structures are arranged into three types of configurations, which correspond to the three regions in the cyclic stress-strain curve. Cylidirical loop patch structure are present in region A for low strain amplitudes, similar to those observed previously in coarse grained polycrystals. Moreover, irregular loop patches are also formed in this region for small grains polycrystals rather thanin region B at intermediate strain amplitudes for coarse grained polycrystals. In region B, persistent slip band (PSB) structures are formed but with a low volume content compared with the coarse grained polycrystals. In region C, at high plastic strains, the dislocation structures are dominated by dipolar walls. In addition, labyrinth structures are developed in region C instead of region B for coarse grained polycrystals. All the dislocation structures observed are viewed as forms of dipolized structures. A dipolized dislocation arrangement model is proposed to describe the formation process of dislocation structures. It is shown that all the dislocation configurations formed in cycled polycrystalline copper are low energy structures.     

Does conformational free energy distinguish loop conformations in proteins?

Limitations in protein homology modeling often arise from the inability to adequately model loops. In this paper we focus on the selection of loop conformations. We present a complete computational treatment that allows the screening of loop conformations to identify those that best fit a molecular model. The stability of a loop in a protein is evaluated via computations of conformational free energies in solution, i.e., the free energy difference between the reference structure and the modeled one. A thermodynamic cycle is used for calculation of the conformational free energy, in which the total free energy of the reference state (i.e., gas phase) is the CHARMm potential energy. The electrostatic contribution of the solvation free energy is obtained from solving the finite-difference Poisson-Boltzmann equation. The nonpolar contribution is based on a surface area-based expression. We applied this computational scheme to a simple but well-characterized system, the antibody hypervariable loop (complementarity-determining region, CDR). Instead of creating loop conformations, we generated a database of loops extracted from high-resolution crystal structures of proteins, which display geometrical similarities with antibody CDRs. We inserted loops from our database into a framework of an antibody; then we calculated the conformational free energies of each loop. Results show that we successfully identified loops with a "reference-like" CDR geometry, with the lowest conformational free energy in gas phase only. Surprisingly, the solvation energy term plays a confusing role, sometimes discriminating "reference-like" CDR geometry and many times allowing "non-reference-like" conformations to have the lowest conformational free energies (for short loops). Most "reference-like" loop conformations are separated from others by a gap in the gas phase conformational free energy scale. Naturally, loops from antibody molecules are found to be the best models for long CDRs (> or = 6 residues), mainly because of a better packing of backbone atoms into the framework of the antibody model.     

材料辐照损伤中的点缺陷团簇与一维迁移现象

材料辐照损伤是核反应堆材料、尤其是核聚变堆材料所面临的重要问题。高能粒子（中子、离子、电子）辐照在材料中会产生大量的点缺陷,即自间隙原子和空位。这些点缺陷聚集在一起会形成自间隙原子团簇和空位团簇,从而对材料结构和性能的演化产生重要影响。空位团簇包括有空洞、层错四面体、空位型位错环,而自间隙原子团簇则只有自间隙型位错环。本文介绍了两种点缺陷团簇的性质、及其对于以材料辐照肿胀为主要内容的材料辐照损伤性能的影响。作为空位团簇,比较详细介绍了具有本课题组特色的空位型位错环的研究,同时分析了合金元素和氢同位素对空位型位错环的影响。在铁试样中形成的这种空位型位错环尺寸可达100 nm左右,该空位型位错环具有两种柏氏矢量, b =<100> 和 b =1/2<111>,前者的数密度比后者高一个数量级。对于自间隙原子团簇,则重点介绍了与其相关的一维迁移现象及其研究动态,该一维迁移性能有可能是影响高熵合金辐照性能的重要因素。      

高磁感取向硅钢中A1N的析出研究

通过热力学和动力学计算相结合的方法,系统地分析了高磁感取向硅钢中A1N在均热过程中的析出机 制。计算结果表明,在高磁感取向硅钢的均热温度下AIN处在α+γ 两相区,同时具备热力学析出条件。在均匀形核、晶界形核和位错形核3种机制下,A1N的临界形核尺寸处在同一数量级且随温度降低而减小。相同温度下, A1N晶界形核的临界形核功最小,相对形核率最大,即较易发生晶界形核。A1N在α相和γ相中均匀形核、晶界形 核和位错形核的最快析出温度分别为1203 K、1303 K、1243 K和1213 K、1305 K、1233 K。 AIN在均热温度下以晶界形核为主。     

The topography of the cecum and proximal loop of the colon in cattle with cecal dilatation

The topography of the cecum and the proximal loop of the colon was studied in 32 cattle of different german breeds, female gender and different age (5.2 +/- 2.0 years), which were surgically treated for cecal dilatation in the Medical and Forensic Veterinary Clinic II of the Justus-Liebig-University in Giessen. In 13 cases (41%) dilatation and dislocation--without or with torsion (along a longitudinal axis, round to the left)--of the cecum, with dilatation of the proximal loop of the colon was diagnosed. In 3 cases (9%) dilatation, dislocation and torsion (along a longitudinal axis, round to the left) of the cecum without dilatation of the proximal loop was found. In 6 cases (19%) dilatation and dislocation of the cecum with dilatation, dislocation and torsion of the proximal loop of the colon (along a longitudinal axis) to the left, and in 10 cases (31%) dilatation and dislocation of the cecum with dilatation, dislocation and torsion of the proximal loop of the colon (along a longitudinal axis) to the right, was observed. By lining up single findings, models are presented how torsion of the cecum to the left, and torsion of the proximal loop of the colon to the left or to the right may develop.