Interaction of dislocations with grain boundaries in Ni3Al

The interaction between slip dislocations and grain boundaries in hypo-stoichiometric Ni₃Al, with and without boron, has been investigated by using the in situ TEM deformation technique. In both alloys, the slip dislocations were incorporated into the grain boundaries and remained at the point of entry. The difference between the alloys was in the dominant response mode of the grain boundary to the stress concentration associated with a dislocation pileup. In the boron-free material, the stress was relieved primarily by the nucleation and propagation of a crack along the grain boundary. In contrast, in the boron-doped material, relief occurred by the emission of dislocations from the grain boundary. These results are consistent with boron increasing the cohesive energy of the grain boundary. The slip system activated at grain boundaries in the ductile ordered alloy was shown to satisfy the same slip transfer criteria that operate in f.c.c. disordered alloys.     

Segregation to and structure of [001] twist grain boundaries in CuNi alloys

The segregation, thermodynamic, and structural properties of [001] twist boundaries in CuNi alloys have been examined within a wide range of misorientations and temperatures. Cu always segregates to the boundary. The concentration of the first layer adjacent to the boundary increases monotonically with misorientation and no obvious cusps are observed. All other thermodynamic properties vary smoothly with the misorientation, with the exception of the vibrational entropy of the boundaries without segregation. The unsegregated vibrational entropy shows a large peak at the misorientation corresponding to the Σ17 boundary and two minima around the Σ13 and Σ5 boundary orientations. The concentration distribution within the plane of the grain boundaries can be described by the same structural unit model established for [001] twist boundaries in pure materials. Regions of large tensile stress show greater segregation than do regions of compressive stress. Regions of large shear stress tend to show reduced segragation compared with regions of small shear stress.     

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.     

Grain boundary segregation of antimony in iron base alloys and its effect on toughness

The equilibrium grain boundary segregation of antimony was investigated in iron base alloys (Fe-Sb, Fe-C-Sb, Fe-Ni-Sb) after annealing at temperatures between 550 and 750°C. Utilizing Auger electron spectroscopy (AES) the concentration of antimony at intergranular fracture faces was determined as a function of bulk concentration and equilibration temperature. The segregation of antimony in Fe-Sb alloys with mass contents of between 0.012 and 0.094 % Sb was described by the Langmuir-McLean equation. The evaluation leads to the free enthalpy of segregation ΔG_segr = ?19 kJ/mol - T 28 J/mol K. The relatively low value for the segregation enthalpy ΔH = ?19 kJ/mol indicates a rather small tendency for grain boundary segregation of Sb. However, its embrittling effect is strong, scanning electron micrographs (SEM) of fractured samples show that the percentage of intergranular fracture strongly increases with an increasing coverage of antimony at the grain boundaries. The data for Fe-0.93% Sb and Fe.1.91% Sb (mass contents) do not fit in the thermodynamic evaluation obviously due to formation of antimonide precipitates in the grain boundaries. The addition of carbon to Fe-Sb alloys results in a higher grain boundary cohesion which is caused by two effects of carbon, displacement of antimony from the grain boundaries by carbon and enhanced grain boundary cohesion. In the Fe-Ni-Sb alloys additional segregation of nickel was found at the grain boundaries but no enhanced antimony segregation, as expected from previous models of other authors, assuming Ni-Sb cosegregation.     

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。     

On Homogeneous Nucleation of Dislocation Loops in Nanocrystalline Materials

In this article, we readdress the question of homogeneous nucleation of dislocation loops in the context of nanocrystalline materials. In this case, the commonly adopted assumption of an infinite medium is no longer valid, and image forces on dislocations must be accounted for in the analysis. An additional energy term associated with the presence of finite boundaries may act to promote homogeneous nucleation and growth of dislocation loops. Based on a simplified consideration of a circular dislocation loop in a spherical nanoparticle or nanosized grain in a polycrystal, energy calculations are carried out to estimate the activation energy for homogeneous nucleation of a dislocation loop in such a system. Two different cases are considered: (1) a single nanoparticle and (2) a grain in a polycrystalline nanomaterial. Based on simulations for aluminum, it is shown that this mechanism may be plausible in both cases, albeit only for small particles and grains in the nanometer range.     

Solute-atom segregation at (002) twist boundaries in dilute NiPt alloys: Structural/chemical relations

Monte Carlo simulations, utilizing embedded atom method (EAM) potentials, are used to investigate solute-atom segregation behavior at symmetrical (002) twist boundaries, at T = 850 K, in Pt-3 at.% Ni and Ni-3 at.% Pt alloys. The results show that, unlike the previously investigated AuPt system, the boundaries are enhanced in solute atoms on both sides of the phase diagram. For low-angle boundaries on the Pt-rich side the atomic sites enhanced in solute concentration are arranged in hourglass-like structures centered on the square grid of primary grain boundary dislocations. While for the same boundaries on the Ni-rich side the atomic sites enhanced in solute concentration are located in bipyramidal regions based on the square cells of the same grain boundary dislocations. Thus, the atomic sites that are enhanced on one side of the phase diagram are not affected on the other side and vice versa.     

Nucleation of grain boundary cavities under the combined influence of helium and applied stress

Modelling cavity nucleation at grain boundaries in structural alloys under the combined influence of helium and stress is the primary objective of this paper. The role of stress in cavity nucleation is analyzed using an extension of classical theory by taking into account grain boundary sliding to describe stress concentration buildup and relaxation at particles and triple-point junctions. Helium clustering in the matrix is modeled using rate theory. The helium flux to grain boundaries is determined by the application of sink strength theory which takes into account the various competing clustering mechanisms in the matrix. Helium clustering on grain boundaries is also theoretically investigated using rate theory. The work agrees with experimental observations showing that irradiation results in grain boundary bubble densities which are orders of magnitude larger than cavity populations observed in conventional creep experiments. It is shown that even if the total injected helium is as little as one part per million, it can result in grain boundary bubble densities on the order of 10¹³ m⁻². Such cavity population exceeds typical grain boundary cavity densities associated with creep experiments. Grain boundary bubble densities are shown to reach steady state for injected helium amounts on the order of 10 parts per million.     

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.     

Mechanisms of strain accumulation and damage development during creep of prestrained 316 stainless steels

The effects of prestraining at room temperature and at the creep temperature of 848 K, as well as the responses to stress reductions during creep, have been studied for 316 stainless steels varying in composition and initial microstructure. The results are analyzed by contrasting the strengthening effects achieved by introducing high dislocation densities prior to creep exposure with the deleterious effects, which can occur when prestraining causes premature void nucleation at grain boundaries. In addition, by recognizing the differing contributions made by the grain interiors and the grain boundary zones to the overall rates of creep strain accumulation, a consistent explanation is provided for the diverse creep behavior patterns reported for different metals and alloys after various prestraining treatments.     

Atomistic Simulations of the Plasticity Behavior of Shock-Induced Polycrystalline Nickel

Shock loading single crystalline nickel creates a defected nanostructure dominated by stacking faults and twins. This transformation is caused by a complex interplay between the incident waves, the waves reflected from sample-free surfaces, and the interference between reflected waves. The plasticity behavior of this shock-induced defected nickel was studied using molecular dynamics (MD) simulations. Compared to a perfect single-crystal nickel sample of the same size, the twinned sample has significantly less yield stress in compression, a slightly lower yield stress in tension, and a yield stress about 30 pct higher in shear. Importantly, our simulations reveal the underlying atomistic mechanisms of dislocation nucleation and twin growth. We observe that while strengthening under shear loading involves lattice dislocations cutting through twins, weakening arises from nucleation of dislocations on the twins under tensile and compressive loading. Also, we have discovered precursors to dislocation loop nucleation in these simulations. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

优质GH738合金热变形过程中的再结晶机制

结合MTS压缩实验,分析了不同变形温度、应变速率、变形量及变形后保温时间对优质GH738合金再结晶的影响规律;进而,利用光学金相显微镜(OM)、透射电子显微镜(TEM)和电子背散射衍射(EBSD)分析,系统研究了该合金热变形前后的合金的晶粒组织、晶内亚结构、晶粒取向差异和弯曲晶界.结果表明:在实验参数范围内,优质GH7...     

Trapping of hydrogen and helium at grain boundaries in nickel: An atomistic study

Atomistic computer simulations of the trapping of hydrogen and helium at defect free grain boundaries in nickel are presented. Three symmetrical tilt boundaries that encompass a number of compact polyhedra of atoms are considered as regions of potential trapping sites. By employing the structural unit model, these boundaries are shown to be representative of a wide range of grain boundary structures. A general correspondence of trap locations in regions of expansion for both hydrogen and helium has been found; however, the binding energy for helium trapping is much greater than that for hydrogen. Consequently, clean grain boundaries in nickel appear to be important trapping sites for helium, but not significant sites for hydrogen binding. These results are consistent with experimental autoradiography, thermal desorption, and transmission electron microscopy observations. They imply that grain boundary trapping plays an important role in mechanisms of helium embrittlement, but not in hydrogen embrittlement of nickel.     

Microstructural Features Leading to Enhanced Resistance to Grain Boundary Creep Cracking in ALLVAC 718Plus

This study focuses on the microstructural features that enhance the resistance of ALLVAC 718Plus to grain boundary creep cracking during testing of samples at 704 °C in both dry and moist air. Fully recrystallized structures were found to be susceptible to brittle grain boundary cracking in both environments. Detailed transmission electron microscopy (TEM) microstructural characterization reveals features that are believed to lead to resistance to grain boundary cracking in the resistant microstructures. It is suggested that dislocation substructures found within the grains of resistant structures compete with the high-angle grain boundaries for oxygen, thereby reducing the concentration of oxygen on the grain boundaries and subsequent embrittlement. In addition, electron backscatter diffraction (EBSD) misorientation maps reveal that special boundaries (i.e., Σ3 boundaries) resist cracking. This is in agreement with previous findings on the superalloy INCONEL 718. Furthermore, it is observed that cracks propagate along high-angle boundaries. This study also shows that in this case, the presence of delta phase at the grain boundaries does not by itself produce materials that are resistant to grain boundary cracking.     

Dislocation and grain boundary interactions in metals

The passage of dislocations through grain boundaries in face centered cubic and body centered cubic polycrystalline metals was studied using dynamic in situ high voltage electron microscopy (HVEM), static transmission electron microscopy (TEM), and anisotropic elastic stress analysis. Several conclusions were reached: (1) when dislocations propagate across grain boundaries, the activated slip system can be predicted from pile-up properties and grain boundary orientation using a combined criterion based on boundary geometric factors and internal stresses; (2) different grain boundaries impede dislocation slip propagation to different degrees, the calculated value of the pile-up obstacle stress varying from 280 to 870 MPa for dislocation transmission through a grain boundary in 304 stainless steel; (3) dynamic in situ straining of miniature tensile specimens reveals additional modes of dislocation and grain boundary interactions that were hidden from static TEM observations. In connection with the last conclusion, simultaneous dislocation transmission and reflection was activated by a stressed pile-up and a complex mechanism involving coordinated movements of four sets of dilocations in and near a grain boundary was observed.     

A classification of symmetrical grain boundaries

A classification of all symmetrical grain boundaries and their tilt rotation axes based on a geometrical construction expressing the relationship between the interplanar spacing and the orientation of the normal to the boundary plane is introduced for the f.c.c. and b.c.c. lattices. It is shown that the favoured grain boundaries found by computer simulations for different models of interatomic forces are in agreement with the boundaries which belong to low levels of the classification schemes. It is anticipated that this classification, which is different from the classification based on the planar density of atomic sites, is a useful guide-line on the search for symmetrical boundaries that possess special properties.     

Characterization of diffusion induced grain boundary migration in the Ag/Cu system

The phenomenon of diffusion induced grain boundary migration (DIGM) when silver diffuses along copper's grain boundaries was confirmed through the characterization of the microstructure and the morphology of the migrated boundaries, the discontinuity and asymmetric character of the concentration profile, the dislocation configuration and the kinetics of migration by means of SEM, EPMA, TEM and AEM. The experimental results were discussed to prove the existence and the characteristics of DIGM in Ag/Cu system.     

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni₃Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.     

High temperature ductility loss in carbon-manganese and niobium-treated steels

The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated. Tensile ductility was measured as a function of temperature and strain rate. Percent elongation and reduction in area were used to characterize the temperature dependence and severity of the ductility loss. The size, distribution, and composition of grain boundary precipitates were measured on extraction replicas. Grain boundary segregation was measured by AES on samples that were deformed at 900 °C before being fractured under ultra-high vacuum at room temperature. Segregation of impurity residual elements and grain boundary precipitation are the primary factors responsible for the observed ductility loss. The embrittlement results in a low ductility fracture which is largely intergranular through the austenite grain boundaries. Segregation of Cu, Sn, and Sb was found on the fracture surfaces of the embrittled samples. High temperature deformation was necessary to produce segregation as no segregation was detected in undeformed samples. Grain boundary precipitation, particularly AIN but also Nb (C,N), contributed to the embrittlement when there was a relatively fine distribution of precipitates along the austenite grain boundaries. The most severe ductility loss occurred when grain boundary precipitation combined with Cu, Sn, and Sb segregation. Formerly Graduate Student, Lehigh University     

Compositional effects on the creep ductility of a low alloy steel

The role which P plays in determining the creep ductility of 2.25Cr-1Mo steel is examined by notched bar creep rupture tests on high purity material selectively doped with combinations of Mn, Si and P. The impurity concentrations, hardness and grain size were carefully controlled. The ductility of as-tempered samples containing dopants was found to be higher than those without dopants; however the ductility of step cooled samples containing Mn and P was found to be lower than as-tempered samples. It is suggested that P, when segregated to the prior austenite grain boundaries, enhances the nucleation of grain boundary cavities while retarding their growth. Mechanisms for each process are proposed. Formerly a Postdoctoral Research Fellow at the University of Pennsylvania formerly a Visiting Scientist at the University of Pennsylvania Formerly a Graduate Student at the University of Pennsylvania