Grain Boundaries (grain + boundary)

Distribution by Scientific Domains
Distribution within Polymers and Materials Science

Kinds of Grain Boundaries

  • tilt grain boundary

  • Terms modified by Grain Boundaries

  • grain boundary diffusion
  • grain boundary migration
  • grain boundary sliding
  • grain boundary structure

  • Selected Abstracts

    Microtexture and Grain Boundaries in Freestanding CVD Diamond Films: Growth and Twinning Mechanisms

    Tao Liu
    Abstract Three groups of free-standing chemical vapor deposition (CVD) diamond films formed with variations in substrate temperature, methane concentration, and film thickness are analyzed using high-resolution electron back-scattering diffraction. Primarily {001}, {110}, and {111} fiber textures are observed. In addition, corresponding primary and higher order twinning components are found. As interfaces, high angle, low angle, primary twin, and secondary twin boundaries are observed. A growth and a twinning model are proposed based on the sp3 hybridization of the bond in the CH4 molecule that is used as the deposition medium. [source]

    Polymer Charge Transport: Charge-Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regioregular Poly(3-hexylthiophene) (Adv. Mater.

    ADVANCED MATERIALS, Issue 16 2009
    Grain boundaries can be engineered in directionally oriented thin films of poly(3-hexylthiophene) report Alberto Salleo and co-workers on p. 1568. Charge-transport studies coupled with X-ray and AFM characterization indicate that intergrain transport is greatly facilitated when neighboring grains can be bridged by relatively straight polymer chains. [source]

    Charge-Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regioregular Poly(3-hexylthiophene)

    ADVANCED MATERIALS, Issue 16 2009
    Leslie H. Jimison
    P3HT films that are highly anisotropic in-plane are produced using a directional crystallization technique, and the charge-transport properties of grain bourdaries between different orientations of crystallites are studied. Boundaries along the fiber provide a small barrier to charge transport when compared to fiber-to-fiber grain boundaries. The films allow a correlation to be drawn between the grain-boundary type and charge-transport behavior in P3HT. [source]

    Simulation of Thermoelectric Performance of Bulk SrTiO3 with Two-Dimensional Electron Gas Grain Boundaries

    Rui-zhi Zhang
    Density functional theory calculations and Boltzmann transport theory are used to simulate the thermoelectric properties of SrTiO3 ceramics with two-dimensional electron gas grain boundaries (GBs). This material can achieve a large thermoelectric figure of merit (ZT>1 at room temperature) by utilizing quantum confinement and energy filtering at GBs. The latter causes the ZT value to reach a maximum before decreasing with an increasing GB barrier height. The optimum barrier height was approximately 0.06 eV higher than the Fermi energy of the grain interior. Our results may aid the design of materials with environmentally benign thermoelectric oxides. [source]

    Direct Observation of Multilayer Adsorption on Alumina Grain Boundaries

    Shen J. Dillon
    Grain-boundary films 0.6 nm in size have been observed on the grain boundaries of neodymia (Nd2O3)-doped alumina (,-Al2O3) sintered at 1800°C. Direct observation by high-angle annular dark-field imaging in the aberration-corrected scanning transmission electron microscope shows that this type of grain-boundary structure is the result of multilayer adsorption. Neodymium cations adsorb onto the faces of each of the two grains that comprise the grain boundary by substituting for aluminum cations. The positions of these cations are slightly distorted relative to the perfect lattice, and a third atomic layer in the core of the grain-boundary resides between these two layers. The measurements also confirm that the thickness deduced from high-resolution transmission electron microscopy lattice images are accurate. [source]

    Atomic Structures and Electrical Properties of ZnO Grain Boundaries

    Yukio Sato
    Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain-boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic,structure,property relationships, well-defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain-boundary orientation relationship and grain-boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current,voltage (I,V) characteristics across ZnO grain boundaries. High-resolution transmission electron microscopy (HRTEM) observations and lattice-statics calculations revealed the atomic structures of the undoped ZnO [0001] ,7 and ,49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The ,49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I,V measurements of the undoped ZnO bicrystals showed linear I,V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I,V characteristics of ZnO ceramics. On the other hand, the nonlinear I,V characteristic appeared when doping the boundaries with Pr. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Pr-doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I,V characteristics, as the Pr existed in the three-plus state and would not produce acceptor states in the boundary. First-principles calculations revealed that Pr doping instead promotes the formations of acceptor-like native defects, such as Zn vacancies. We believe that such acceptor-like native defects are microscopic origin of the nonlinear I,V characteristics. Investigations of various types of grain boundaries in the Pr and Co-codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I,V characteristics significantly depend on the grain-boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I,V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I,V characteristics. Finally, a Pr and Co-codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I,V characteristic. This result indicates that ZnO single-grain-boundary varistors can be designed by controlling grain-boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I,V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single-grain-boundary varistors. [source]

    Geometry and Electrical Properties of Grain Boundaries in Manganese Zinc Ferrite Ceramics

    Jong-Sook Lee
    For large-grained manganese zinc (MnZn) ferrite ceramics, grain misorientation determined by electron backscatter diffractions and grain-boundary resistance measured using microcontact impedance spectroscopy have been correlated. The degree of oxidation of grain boundaries and, hence, the barrier height depends on the overall grain-boundary network as well as on the individual boundary structure; therefore, a statistical analysis has been performed based on several hundreds of local measurements. When the boundaries are divided into low- and high-resistance groups, statistically significant differences in rotation axis and angle distributions are found. The misorientation distribution of the low-resistance boundary group is suggested to reflect the low-energy configurations of boundary planes in MnZn ferrites. [source]

    Ab Initio Calculations of Pristine and Doped Zirconia ,5 (310)/[001] Tilt Grain Boundaries

    Zugang Mao
    The structure of the cubic-ZrO2 symmetrical tilt ,5 (310)/[001] grain boundary is examined using density functional theory within the local density and pseudopotential approximations. Several pristine stoichiometric grain-boundary structures are investigated and compared with Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy results. The lowest-energy grain-boundary structure is found to agree well with the experimental data. When Y3+ is substituted for Zr4+ at various sites in the lowest-energy grain-boundary structure, the calculations indicate that Y3+ segregation to the grain boundary is energetically preferred to bulk doping, in agreement with experimental results. [source]

    Local Viscosity of Si-O-C-N Nanoscale Amorphous Phases at Ceramic Grain Boundaries

    Giuseppe Pezzotti
    Internal friction characterization has been used to quantitatively assess the viscosity characteristics of Si-O-C-N glasses segregated to nanometer-sized grain boundaries of polycrystalline Si3N4 and SiC ceramics. A relaxation peak of internal friction, which arises with rising temperature from the viscous sliding of glassy grain boundaries, was systematically collected and analyzed with respect to its shift upon changing the oscillation frequency. As a result of such an analysis, both activation energy for viscous grain-boundary flow and inherent viscosity of the intergranular glass film could be quantitatively evaluated. Two main features are shown: (i) the presence of N and/or C greatly affects the viscosity characteristics of SiO2 phases at Si3N4 and SiC grain boundaries; and (ii) the internal friction method has potential as a unique experimental tool for understanding the local properties of nanoscale amorphous phases in new ceramic materials. [source]

    Exuding Liquid from Grain Boundaries in Alumina

    N. Ravishankar
    Bicrystals of alumina with anorthite glass in the boundary were prepared by hot pressing. Annealing of the bicrystals leads to the migration of the intergranular liquid to the free surface of the sample. It is proposed that the migration is driven by the difference in the wetting behavior of the free surface and the boundary. [source]

    Unraveling Deterministic Mesoscopic Polarization Switching Mechanisms: Spatially Resolved Studies of a Tilt Grain Boundary in Bismuth Ferrite

    Brian J. Rodriguez
    Abstract The deterministic mesoscopic mechanism of ferroelectric domain nucleation is probed at a single atomically-defined model defect: an artificially fabricated bicrystal grain boundary (GB) in an epitaxial bismuth ferrite film. Switching spectroscopy piezoresponse force microscopy (SS-PFM) is used to map the variation of local hysteresis loops at the GB and in its immediate vicinity. It is found that the the influence of the GB on nucleation results in a slight shift of the negative nucleation bias to larger voltages. The mesoscopic mechanisms of domain nucleation in the bulk and at the GB are studied in detail using phase-field modeling, elucidating the complex mechanisms governed by the interplay between ferroelectric and ferroelastic wall energies, depolarization fields, and interface charge. The combination of phase-field modeling and SS-PFM allows quantitative analysis of the mesoscopic mechanisms for polarization switching, and hence suggests a route for unraveling the mechanisms of polarization switching at a single defect level and ultimately optimizing materials properties through microstructure engineering. [source]

    Polymer Charge Transport: Charge-Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regioregular Poly(3-hexylthiophene) (Adv. Mater.

    ADVANCED MATERIALS, Issue 16 2009
    Grain boundaries can be engineered in directionally oriented thin films of poly(3-hexylthiophene) report Alberto Salleo and co-workers on p. 1568. Charge-transport studies coupled with X-ray and AFM characterization indicate that intergrain transport is greatly facilitated when neighboring grains can be bridged by relatively straight polymer chains. [source]

    Improvement in crystalline perfection, piezo-electric property and optical transparency of in-situ poled Fe,LiNbO3 single crystals by post growth annealing and poling

    G. C. Budakoti
    Abstract Crystalline perfection, piezoelectric response and optical transparency of in-situ poled Fe,LiNbO3 single crystals was studied. Very low angle grain boundaries and the variations in the piezoelectric charge constant d33 were observed in the as-grown crystals. Grain boundaries were successfully removed at higher annealing temperatures but the d33 value was decreased. Low crystalline perfection and d33 were observed after poling the annealed specimen. These parameters were improved by low temperature annealing followed by very slow cooling. FTIR spectra revealed that OH, and CO32, ionic defects were present in the as-grown crystals. The OH, ion concentration was reduced, CO32, ions were removed and optical quality was improved after annealing at higher temperatures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Rock thermal data at the grain scale: applicability to granular disintegration in cold environments

    Kevin Hall
    Abstract Consideration of the mechanisms associated with the granular disintegration of rock has been limited by available data. In most instances, both the size of the transducer and the nature of the study have negated any applicability of the resulting data to the understanding of grain-to-grain separation within rock. The application of microthermocouples (,0·15 mm diameter) and high-frequency logging (20 s intervals) at a taffoni site on southern Alexander Island and from a rock outcrop on Adelaide Island (Antarctica) provide new data pertaining to the thermal conditions, at the grain scale, of the rock surface. The results show that thermal changes (,T/t) can be very high, with values of 22 °C min,1 being recorded. Although available data indicate that there can be differences in frequency and magnitude of ,uctuations as a function of aspect, all aspects experienced some large magnitude (,2 °C min,1) ,uctuations. Further, in many instances, large thermal changes in more than one direction could occur within 1 min or in subsequent minutes. These data suggest that the surface grains experience rapidly changing stress ,elds that may, with time, effect fatigue at the grain boundaries; albedo differences between grains and the resulting thermal variations are thought to exacerbate this. The available data failed to show any indication of water freezing (an exotherm) and thus it is suggested that microgelivation may not play as large a role in granular breakdown as is often postulated for cold regions, and that in this dry, Antarctic region thermal stress may play a signi,cant role. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Optimization of the Magnetic Properties of FePd Alloys by Severe Plastic Deformation,

    Abdelahad Chbihi
    Abstract A FePd alloy was nanostructured by severe plastic deformation following two different routes: ordered and disordered states were processed by high pressure torsion (HPT). A grain size in a range of 50 to 150,nm is obtained in both cases. Severe plastic deformation induces some significant disordering of the long range ordered L10 phase. However, transmission electron microscopy (TEM) data clearly show that few ordered nanocrystals remain in the deformed state. The deformed materials were annealed to achieve nanostructured long range ordered alloys. The transformation proceeds via a first order transition characterized by the nucleation of numerous ordered domains along grain boundaries. The influence of the annealing conditions (temperature and time) on the coercivity was studied for both routes. It is demonstrated that starting with the disorder state prior to HPT and annealing at low temperature (400,°C) leads to the highest coercivity (about 1.8,kOe). [source]

    Plasticity and Grain Boundary Diffusion at Small Grain Sizes,

    Gerhard Wilde
    Bulk nanostructured,or ultrafine-grained materials are often fabricated by severe plastic deformation to break down the grain size by dislocation accumulation. Underlying the often spectacular property enhancement that forms the basis for a wide range of potential applications is a modification of the volume fraction of the grain boundaries. Yet, along with the property enhancements, several important questions arise concerning the accommodation of external stresses if dislocation-based processes are not longer dominant at small grain sizes. One question concerns so-called "non-equilibrium" grain boundaries that have been postulated to form during severe deformation and that might be of importance not only for the property enhancement known already today, but also for spectacular applications in the context of, e.g., gas permeation or fast matter transport for self-repairing structures. This contribution addresses the underlying issues by combining quantitative microstructure analysis at high resolution with grain boundary diffusion measurements. [source]

    Ultra-Fast Atomic Transport in Severely Deformed Materials,A Pathway to Applications?,

    Sergiy Divinski
    Abstract Severe plastic deformation of pure Cu and Cu-rich alloys was found to create a hierarchical combination of fast and ultra-fast diffusion paths ranging from non-equilibrium grain boundaries to non-equilibrium triple junctions, vacancy clusters, nano- and micro-pores, and finally to general high-angle grain boundaries. Under certain conditions, a percolating network of porosity can be introduced in the ultra-fine grained materials by a proper mechanical and thermal treatment. This network may offer promising opportunities for creating materials with tailor-made properties, including combinations of improved mechanical performance with a possibility of self repair using "vascular structures" for atom transport. Applications in such areas as drug eluting bioimplants and lead or polymer eluting materials for reduction of friction based on impregnation of porosity networks with these agents are also envisaged. [source]

    Processing of Carbon Nanofiber Reinforced ZrB2 Matrix Composites for Aerospace Applications,

    Jorge Barcena
    Ceramic matrix composites (CMCs) based on zirconium diboride (ZrB2) reinforced by vapor grown carbon nanofibers are a potential constituent of reusable thermal protection systems. A manufacturing procedure was devised that involved the fabrication of thin films by tape casting to obtain a layer that could be integrated into a more complex system. Higher thermal conductivities and improved toughness can be expected for nanofiber additions, as compared to the matrix alone. Consolidation by hot-pressing was more effective than pressureless sintering, in terms of the final relative density and flatness of specimens. Examination of microstructures showed that few carbon nanofibers were present in the matrix after consolidation by sintering, which was attributed to a reaction between the nanofibers and zirconium oxide present on the surface of the ZrB2 powder. As a solution, oxygen impurities from the boride powders were removed by reduction with carbon coatings derived from phenolic resin. The deleterious reaction was avoided, but residual carbon remained at the grain boundaries, likely from decomposition of the binder. The use of an alternative binder (PMMA vs. PVB) will be used in future studies to reduce the residual carbon content. Further, consolidation by Spark Plasma Sintering (SPS) will be explored to further reduce the reaction of surface oxides with the nanofibers. Finally, characterization of the microstructure at the nanometric level and further determination of the mechanical and thermal properties will be conducted as part of future studies. [source]

    Microstructural Characterization of Lamellar Features in TiAl by FIB Imaging,

    Dennis Peter
    A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl-based two-phase alloys using the focused ion beam (FIB) technique. Two , -titanium aluminide alloys with a fine-grained duplex and a nearly lamellar microstructure are examined. The special FIB-based preparation procedure results in high contrast ion beam-induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two-phase structures, e.g. lamellar colonies in , -TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB-derived data, we compare them to results obtained with another method to determine phase fractions, electron back-scatter diffraction (EBSD). This direct comparison shows that the FIB-based technique generally provides slightly higher ,2 -fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB-based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two-phase TiAl alloys and other such materials with fine, lamellar microstructures. [source]

    The Precipitation Behavior of Superalloy ATI Allvac 718Plus

    Gerald A. Zickler
    Abstract ATI Allvac 718Plus is a novel nickel-based superalloy, which was designed for heavy-duty applications in aerospace gas turbines. The precipitation kinetics of the intermetallic , (Ni3Nb) and ,, (Ni3(Al,Ti)) phases in this alloy are of scientific as well as technological interest because of their significant influence on the mechanical properties. Important parameters like grain size are controlled by coarse , precipitates located at grain boundaries, whereas small ,, precipitates are responsible for strengthening by precipitation hardening. In the present study, the microstructure is investigated by three-dimensional atom probe tomography and simulated by computer modeling using the thermo-kinetic software MatCalc. The results of numerical simulations and experimental data are compared and critically discussed. It is shown that the chemical compositions of the phases change during isothermal aging, and the precipitation kinetics of , and ,, phases interact with each other as shown in a time temperature precipitation (TTP) plot. The TTP plot shows C-shaped curves with characteristic discontinuities in the temperature range, where simultaneous and concurrent precipitation of the , and ,, phases occurs. This leads to a competition in the diffusion of Nb and Al, which are partly present in both phases. Thus, the present study gives important information on heat treatments for ATI Allvac 718Plus in order to achieve the desired microstructure and mechanical properties. [source]

    Back Cover: Fundamentals of Metal-induced Crystallization of Amorphous Semiconductors (Adv. Eng.

    The Backcover shows a covering layer of aluminum lowers the crystallization temperature of amorphous silicon (a-Si). First the a-Si covers ("wets") the grain boundaries in the aluminum layer (Al). Once the wetting a-Si film has reached a critical thickness, crystallization starts at the grain boundaries. More details can be found in the article by E. J. Mittemeijer on page 131. [source]

    Thermodynamic Investigation of Alkali-Metal-Induced High Temperature Embrittlement in Al-Li Alloys,

    S. Zhang
    Alkali metals are undesirable impurity elements in aluminum-lithium alloys. Despite their trace amount, they lead to high temperature embrittlement (HTE). In the present work, the results of a thermodynamic investigation are presented to elucidate its mechanism and compared with available experimental data. HTE arises from an intergranular alkali-metal-rich liquid phase that segregates into grain boundaries from the matrix and significantly weakens their strength. A new model is developed to describe the tendency of HTE, which shows grain refinement can decrease the tendency. [source]

    Diffusion in Nanocrystalline Metals and Alloys,A Status Report,

    R. Würschum
    Abstract Diffusion is a key property determining the suitability of nanocrystalline materials for use in numerous applications, and it is crucial to the assessment of the extent to which the interfaces in nanocrystalline samples differ from conventional grain boundaries. The present article offers an overview of diffusion in nanocrystalline metals and alloys. Emphasis is placed on the interfacial characteristics that affect diffusion in nanocrystalline materials, such as structural relaxation, grain growth, porosity, and the specific type of interface. In addition, the influence of intergranular amorphous phases and intergranular melting on diffusion is addressed, and the atomistic simulation of GB structures and diffusion is briefly summarized. On the basis of the available diffusion data, the diffusion-mediated processes of deformation and induced magnetic anisotropy are discussed. [source]

    Numerical modelling of fatigue crack initiation and growth of martensitic steels

    ABSTRACT This paper presents a numerical simulation of micro-crack initiation that is based on Tanaka-Mura micro-crack nucleation model. Three improvements were added to this model. First, multiple slip bands where micro-cracks may occur are used in each grain. Second improvement deals with micro-crack coalescence by extending existing micro-cracks along grain boundaries and connecting them into a macro-crack. The third improvement handles segmented micro-crack generation, where a micro-crack is not nucleated in one step like in Tanaka-Mura model, but is instead generated in multiple steps. High cycle fatigue testing was also performed and showed reasonably good correlation of proposed model to experimental results. Because numerical model was directed at simulating fatigue properties of thermally cut steel, edge properties of test specimens were additionally inspected in terms of surface roughness and micro-structural properties. [source]

    Influence of bulk damage on crack initiation in low-cycle fatigue of 316 stainless steel

    ABSTRACT To investigate the effect of bulk damage on fatigue crack initiation, crack initiations due to low-cycle fatigue of Type 316 stainless steel were observed by electron backscatter diffraction (EBSD) and scanning electron microscopy. The EBSD observations showed that local misorientation developed inhomogeneously due to the cyclic strain, and many cracks were initiated from the slip steps and grain boundaries where the local misorientation was relatively large. The crack initiations could be categorized into two types: enhancement of the driving force by geometrical discontinuity (slip steps and notches), and reduction of material resistance against crack initiation caused by accumulated bulk damage at grain boundaries. In particular, more than half of the cracks were initiated from grain boundaries. However, in spite of the significant bulk damage, the fatigue life was extended by removing the surface cracks under strain of 1 and 2% amplitude. The stress state at the microstructural level was changed by the surface removal, and the damaged portion did not suffer further damage. It was concluded that although bulk damage surely exists, the fatigue life can be restored to that of the untested specimen by removing the surface cracks. [source]

    Fatigue damage analysis in a duplex stainless steel by digital image correlation technique

    ABSTRACT Strain field measurements by digital image correlation today offer new possibilities for analysing the mechanical behaviour of materials in situ during mechanical tests. The originality of the present study is to use this technique on the micro-structural scale, in order to understand and to obtain quantitative values of the fatigue surface damage in a two-phased alloy. In this paper, low-cycle fatigue damage micromechanisms in an austenitic-ferritic stainless steel are studied. Surface damage is observed in real time, with an in situ microscopic device, during a low-cycle fatigue test performed at room temperature. Surface displacement and strain fields are calculated using digital image correlation from images taken during cycling. A detailed analysis of optical images and strain fields measured enables us to follow precisely the evolution of surface strain fields and the damage micromechanisms. Firstly, strain heterogeneities are observed in austenitic grains. Initially, the austenitic phase accommodates the cyclic plastic strain and is then followed by the ferritic phase. Microcrack initiation takes place at the ferrite/ferrite grain boundaries. Microcracks propagate to the neighbouring austenitic grains following the slip markings. Displacement and strain gradients indicate probable microcrack initiation sites. [source]

    The effect of cerium on high-cycle fatigue properties of die-cast magnesium alloy

    Y. YANG
    ABSTRACT The effect of cerium (Ce) on high-cycle fatigue behaviour of die-cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio, R= 0.1 on specimens of AZ91D alloys with different Ce additions. The microstructure and fatigue fracture surfaces of specimens were examined using a scanning electron microscope (SEM) to reveal the micromechanisms of fatigue crack initiation and propagation. The results show that the grain size of AZ91D is refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D evaluated by the up-and-down load method increases from 96.7 MPa to 116.3 MPa (1% Ce) and 105.5 MPa (2% Ce), respectively. The fatigue cracking of AZ91D alloy initiates at porosities and inclusions of the alloy's interior, and propagates along the grain boundaries. The fatigue fracture surface of test specimens shows the mixed fracture characteristics of quasi-cleavage and dimple. [source]

    Molecular dynamics simulation of crack tip blunting in opposing directions along a symmetrical tilt grain boundary of copper bicrystal

    A. LUQUE
    ABSTRACT Mode I crack growth along some grain boundaries of copper embrittled by solute segregation shows strong anisotropy. For instance, growth along the direction on the symmetrical tilt boundary has been reported to occur by intergranular brittle fracture, whereas growth along the opposite sense occurs in a ductile manner. In this paper, we simulate such crack configurations using molecular dynamics (embedded atom method [EAM]) in 3-dimensional perfect bicrystalline samples of pure copper of the aforementioned orientation at room temperature. In both cases the response is ductile, crack opening taking place by dislocation emission from the crack tip. The critical stress intensity factors (SIFs) for dislocation emission have been calculated by matching the displacement fields of the atoms in the tip neighbourhood with the continuum elastic fields. They are of the same order of magnitude for both growth senses despite the different morphology of their respective blunted crack tips and of the patterns of dislocations constituting their plastic zones. Thus, it seems that, in agreement with published results of continuum crystalline plasticity for the same problem, the plastic anisotropy associated with the different orientation of the slip systems with respect to the crack cannot in this case explain the experimental behaviour observed with solute embrittled bicrystals. [source]

    Initiation and early growth of fatigue cracks in an aerospace aluminium alloy

    S. A. Barter
    Abstract Material imperfections usually play a substantial role in the early stages of fatigue cracking. This article presents some observations concerning fatigue crack initiating flaws and early crack growth in 7050-T7451 aluminium alloy specimens and in full-scale fatigue test articles with a production surface finish. Equivalent initial flaw size (EIFS) approaches used to evaluate the fatigue implications of metallurgical, manufacturing and service-induced features were refined by using quantitative fractography to acquire detailed information on the early crack growth behaviour of individual cracks; the crack growth observations were employed in a simple crack growth model developed for use in analysing service crack growth. The use of observed crack growth behaviour reduces the variability which is inherent in EIFS approaches which rely on modelling the whole of fatigue life, and which can dominate EIFS methods. The observations of realistic initial flaws also highlighted some of the significant factors in the fatigue life-determining early fatigue growth phase, such as surface treatment processes. Although inclusions are often regarded as the single most common type of initiating flaw, processes which include etching can lead to etch pitting of grain boundaries with significant fatigue life implications. [source]

    Fatigue crack growth threshold conditions at notches.

    Part I: theory
    A micromechanical description of the fatigue crack growth process at notches is presented. Crack interaction with the plastic slip barriers of the material (e.g. grain boundaries) and the influence of the notch stress gradient are intrinsically taken into account in the model. Both the notch fatigue crack initiation limit and the limit for propagation up to failure (i.e. the conventional notch fatigue limit) are clearly identified and calculated. The formation of non-propagating cracks is also explained. [source]