Local migration of grain boundaries in polycrystalline materials under plastic deformation conditions

We propose a theoretical model describing the local migration of grain boundaries (GBs) near triple junctions according to the new mechanism stimulated by the GB slip. Within the framework of this model, a driving force for the local migration is due to the interaction between sliding and structural GB dislocations responsible for the GB slip and misorientation, respectively.     

薄膜厚度对PZT/STO铁电电容器隧穿电阻的影响

结合Landau-Devonshire自由能理论和晶格模型,研究了PbZr1-xTixO3(PZT)/SrTiO3(STO)复合薄膜中PZT和STO厚度对铁电隧道结极化强度、电导和隧穿电阻等的影响。模拟结果表明,随着层数增加,极化强度增大;平均极化强度随着PZT厚度增加而增强,随着STO厚度增加而减弱;随着PZT和STO厚度增加,电导减小,隧穿电阻率增加;当厚度变化相同时,PZT引起电导的变化大,隧穿电阻率变化小。考虑薄膜面积的影响,当薄膜面积从无限大变成有限大时,极化强度、电导和隧穿电阻均减小。     

Amelioration of weld-crack resistance of the M951 superalloy by engineering grain boundaries

Fusion weld is a portable and economical joining and repairing method of metals.However,weld cracks often occur during the fusion weld of Ni-base superalloys,which hinder the applications of fusion weld on this kind of materials.In this work,the effects of microstructures of grain boundaries(GBs)of the prototype M951 superalloy on its weldability were investigated.The precipitated phases,the elemental segregations on GBs,and the morphologies of GBs can be largely altered by regulating the cooling rates of pre-weld heat treatments.With decreasing the cooling rate,chain-like M23X6 phase precipitates along the GBs,accompanying segregations of B,and GBs becomes more serrated in morphology.During fusion weld,the engineered GBs in the M951 superalloy with a low cooling rate favor the formation of the continuous liquid films on GBs,which together with the serrated GB morphology significantly prevents the formation of weld cracks.Our findings imply that the weld-crack resistance of the superalloys can be ameliorated by engineering GBs.     

Study of grain boundaries in polycrystalline Cr,Ta and W using nuclear gamma-resonance spectroscopy: Formation enthalpies of vacancy-oxygen complexes and single vacancies in grain boundaries,and dynamic properties of the grain boundary core in polycrystals of BCC metals

The formation of complexes of vacancies with oxygen atoms (complexes-VacO) in the core of grain-boundary (GB) leads to a dependency of the isomer shifts δ₁ of components-1 in NGR emission spectra of ⁵⁷Co(⁵⁷Fe) atomic probes localized in the GB core on the annealing temperature of polycrystalline metals in a technical vacuum. The formation enthalpies of complexes-VacO, Q_{cmpl, 1}, and single vacancies, Q_{Vac, 1}, in the GB core (states-1) in Cr, Ta and W polycrystals have been measured for the first time. Dynamic contributions to formation enthalpies Q_{Vac, 1} of single vacancies localized in the GB core in the state-1 and dynamic contributions to formation enthalpies Q_{cmpl, i} of complexes-VacO, which were localized in the lattice regions adjacent to GB’s (Adjacent Lattice Regions, ALR’s), states-2, and in the GB core, states-1, have been separated. The dynamic contribution to the formation enthalpy Q_Vac,1 of vacancies and Q_{cmpl, 1} of complexes-VacO in the GB core was several times smaller than the contribution to the formation enthalpy Q_{Vac, vol} of vacancies in the bulk of the crystallites because of low-frequency resonance modes of local collective vibrations of intrinsic atoms in the GB core.     

Further refinements in the energy of grain boundaries

Earlier surface dislocation analysis of a grain boundary recognized the tendency of the grain-boundary surfaces to coalesce in order to reduce surface energy. The coalescence process is described by a distribution of surface dislocations on the grain-boundary surfaces. In the present paper, previous analysis is further refined. In particular, the sum of the Burgers vectors of the surface array of grain-boundary dislocations is not equal to the Burgers vector of the grain-boundary lattice dislocation. Instead, the Burgers vector of the surface array is determined as a function of the coalescence of the grain-boundary surfaces. The conservation of Burgers vectors of dislocations is used to predict the presence of a screening array of dislocations. The screening array of dislocations is determined by minimization of the total energy of the configuration. The distortion around the boundary is relaxed by the screening array. In general, the distribution of the screening array is two dimensional. This result has been proved by the presence of a minimum energy configuration for two sets of screening arrays of dislocations situated at different distances from the boundary.     

Computer modeling of grain boundaries in Ni3Al

Computer simulation of symmetric tilt grain boundaries (GB) Σ=5 [1 0 0] (0 1 2) and Σ=5 [1 0 0] (0 1 3) in alloy Ni₃Al was carried out. The energy of GB was calculated out by a method of construction of γ-surface with using Morse's empirical central-force potentials. Investigations shown that GB have several steady states: one is stable and other – metastable. These states differ by energy and atomic structure GB. It is shown, that GB in model CSL are unstable, the stabilization is achieved by additional displacement on some vector along plane of defect. Directions and value of potential barriers of GB slippage are calculated.     

Computer modeling of grain boundaries in Ni3Al

Computer modeling of symmetric tilt boundaries Σ = 5 [1 0 0] (0 1 2) and Σ = 5 [1 0 0] (0 1 3) in alloy Ni₃Al was carried out. The energy of grain boundaries (GBs) was calculated out by a method of construction of γ-surface by using Morse's empirical centralforce potentials. Researched GBs have four steady states: one is stable and three — metastable. These states of GBs differ by energy and atomic structure. It is shown that GBs in model of coincidence site lattice are unstable, the stabilization is achieved by additional displacement on some vector along plane of defect.     

Influence of grain boundaries and voids on the saturated magnetization in Fe/sub 3/O/sub 4/ films at a low magnetic field

We report the magnetic behaviors of Fe/sub 3/O/sub 4/ thin films grown by zero field growth (ZFG) and field growth (FG) techniques during the sputtering process. In FG conditions, an in situ 300 Oe field during growth is applied to a substrate, inducing an easy axis of magnetization. Structural observations obtained by high-resolution transmission electron microscopy measurements clearly depicted a significant reduction of the grain boundaries and voids in the Fe/sub 3/O/sub 4/ films grown under FG conditions, thus explaining the saturated magnetization of the Fe/sub 3/O/sub 4/ films at about 0.01 T. This behavior was expected due to a remarkable reduction of the antiferromagnetic exchange couplings between grains for FG conditions. In addition, the zero-field-cooled magnetization of the ZFG samples showed an abrupt change at about 285 K, confirming the existence of defects or other phases in the ZFG films.     

High spatial resolution imaging of the segregation of reactive elements to oxide grain boundaries in alumina scales

Abstract

Although it is well established that reactive elements such as yttrium and hafnium can segregate to oxide/metal interfaces and oxide grain boundaries in thermally grown oxides, their distribution and role at these sites are less certain. For example, their effect on oxide growth, scale plasticity and spallation is still debated. It has also been reported that hafnium and yttrium rich oxide particles can be present within growing alumina scales and that the growth or shrinkage of these particles can affect the Y and Hf distributions in the aluminium oxide grain boundaries in their vicinity. Hence, we now report the use of very high spatial resolution imaging in the SuperSTEM electron microscope to investigate the distribution of Y and Hf in aluminium oxide grain boundaries at the atomic level.

The oxide scales studied were detached from Fe – 20Cr – 5Al alloy substrates doped with Y and Hf, which had been oxidised for up to 100 h at 1250°C in laboratory air. The scales were ion beam thinned prior to examination in the STEM, and a series of tilting experiments and through focal series were used to map out the distributions of the reactive elements. The influence of electron beam/sample interactions was also studied and some evidence for the movement of Hf and Y atoms along the grain boundaries to the surfaces of the thin oxide foils is also reported.     

Formation of lamellar M23C6 on and near twin boundaries in austenitic stainless steels

Thin foil electron microscopy studies were made on the precipitation of lamellar M₂₃C₆ during aging at 973 K and 1073 K in water-quenched specimens of two austenitic stainless steels. After the precipitation on incoherent twin boundaries M₂₃C₆ formed on coherent twin boundaries and in the regions adjacent to incoherent twin boundaries. These precipitates showed lamellar morphology and were aligned in a specific manner with respect to the twin boundaries. Such lamellar precipitates were absent in the specimens which were isothermally treated at 1073 K after being transferred from the solution treatment temperature. The lamellar morphology of M₂₃C₆ is suggested to be developed by the influence of residual specific stress field around twin boundaries resulted from quenching.     

Modelling of the grain size probability distribution in polycrystalline nanomaterials

Theoretical analyses have always resulted in nanomaterials’ grain size probability distribution being of varied form: approximately either lognormal, Rayleigh, normal, Weibull, etc. The isotropic Hillert’s model of grain growth which is more suitable for soap froth has been frequently used to establish these distributions with the hope of approximating experimental observations. Observed grain growth in nanomaterials shows departures from the Hillert’s model.In the present paper, the probability distribution of grain size in nanomaterials is dealt with. Use is made of a modified model of grain growth in polycrystalline nanomaterials developed recently by the authors. The modified model accounts for grain growth caused by curvature driven grain boundary migration and grain rotation-coalescence mechanisms. Since the grain size in the aggregate is random, the stochastic counterpart of the expression governing the incremental change in individual grain size is obtained by the addition of two fluctuation terms.The integro-differential equation governing the development of the probability density function of the grain size is obtained which is the generalised Fokker–Planck–Kolmogorov equation. Numerical solution to the integro-differential equation is obtained.Results from analytical modelling of grain size probability distribution in polycrystalline nanomaterials are different if the effect of grain rotation-coalescence mechanism on grain growth process is taken into account and, further, due to the addition of the fluctuation terms. Results also depend on the nature of the fluctuation term, which is a material property as the fluctuation in grain sizes varies from one material to another. It is shown that many of the major attributes of grain growth, such as self similarity (probability density approaching a stationary one), can be predicted by the solution of the Fokker–Planck–Kolmogorov equation.     

Changes in the geometry of grain boundaries during recovery/continuous recrystallization in α-Fe

Samples of α-Fe (Armco) have been deformed by 50% and subsequently annealed at 400 °C which is considerably below the conventional recrystallization temperature. The geometry of the grain boundaries has been studied through measurements of the grain boundary traces revealed on longitudinal cross-sections of the specimens. Two etching conditions were employed, one of which revealed networks containing only grain boundaries (designated GB in the further text) whilst the other resulted in delineating the 2-dimensional regions characterized by the presence of sulfur (designated S-GB). Changes in the geometry of these two types of grain boundaries as a result of plastic deformation and annealing are discussed in terms of in-situ high angle grain boundary formation and annihilation. This revised version was published online in November 2006 with corrections to the Cover Date.     

A TEM study of grain boundaries in internal boundary layer capacitors based on donor-doped (Sr, Ca) TiO3 ceramics

Donor-doped (Sr, Ca)TiO₃ ceramics, either oxidized or infiltrated by Bi₂O₃ and Bi₂O₃-PbO molten mixtures, have been studied using a transmission electron microscope equipped with EDX facilities. Grain boundary morphologies of these different materials are investigated. In oxidized samples, grain surfaces exhibit a chemical contrast, interpreted as the result of a TiO2 segregation that occurs during the sintering stage. Moreover, the in-detail characterization of infiltrated specimens reveals that intergranular regions present a facies strongly influenced by the type of mixture used. During the infiltration step, grains are slightly dissolved by molten oxides and Pb noticeably enhances this process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation enthalpies of vacancy–oxygen complexes and single vacancies in the core of grain boundaries in polycrystalline Cr,Ta, and W

Atomic probes (APs) ⁵⁷Co(⁵⁷Fe) are localized in the grain boundary (GB) core and adjacent regions of the lattice in polycrystalline metals by means of GB diffusion. Spectra of nuclear gamma resonance (NGR) of radiation of APs ⁵⁷Co(⁵⁷Fe) were used for analysis of the nearest neighborhood of APs. Vacancies, which are formed in the GB core of polycrystalline metals during annealing in a technical vacuum over the temperature range (0.2–0.4)T_melt, are captured by traps (oxygen atoms) and form stable complexes with oxygen (complexes-VacO). For the first time, formation enthalpies Q_cmpl,1 of complexes-VacO and formation enthalpies Q_Vac,1 of single vacancies in the GB core in Cr, Ta, and W polycrystals have been determined from Arrhenius dependencies of the contribution δ_Vac,1.     

Microscopic characterization of individual grain boundaries in Cu-III-VI2 chalcopyrites

The role of grain boundaries in polycrystalline Cu-III-VI₂ absorber material for thin film photovoltaics has not been fully understood and is currently under discussion. Recently, intensive efforts have been devoted to the characterization of the properties of individual grain boundaries using microscopic techniques, including Kelvin probe force microscopy (KPFM). KPFM provides local electronic information by measuring the surface potential in addition to the topography. We introduce the KPFM method and present simulations assessing the technique's limitations with respect to spatial resolution regarding the measurement of grain boundary properties. KPFM studies of individual GBs in the Cu(In,Ga)Se₂ materials system are reviewed and critically discussed, considering also results from other microscopic characterization techniques.