Grain Boundaries

High-temperature creep tests were conducted on polycrystalline copper of commercial purity in order to investigate the inter-relationship between the extent and the rate of grain boundary sliding (GBS) and the development of internal cavitation. An image processing technique was used to provide quantitative information on the size and shape of the cavities formed over a range of stresses and at temperatures from 673 to 873 K. The results demonstrate that cavity development is inhomogeneous within any specimen, such that there is an increase in the level of cavitation in regions of higher local strain. It is shown by quantitative measurements that the total cavitated area increases at the faster sliding rates, but the number density of cavities decreases at the highest sliding rates because of cavity interlinkage. When interlinkage is taken into account, the results confirm that the overall level of cavitation is related to the occurrence of GBS.

Alumina bicrystals with low-angle and near-⌺3 <0001> tilt grain boundaries were fabricated using diffusion bonding to study the dislocation structures in alumina grain boundaries. The resulting grain-boundary structures were investigated using high-resolution transmission electron microscopy, and the grain-boundary energies were analyzed using theoretical calculations. It was found that partial dislocations with Burgers vectors of the type 1 3 <101 0> were periodically located in the boundaries and that a stacking fault between pairs of partials was formed in such boundaries. The length of the stacking fault decreased with increased misorientation angles, which was reasonably predicted by the theoretical calculation. In the case of a near-⌺3 grain boundary, an array of displacement shift complete dislocations with the Burgers vector of 1 3 <11 00> was periodically formed along the boundaries. These boundaries did not have stacking faults. The spacing between the dislocations decreased with increased deviation angle from the exact-⌺3 boundary with the tilt angle of 60°. I. Introduction M. Rühle-contributing editor Manuscript No. 186773.

p~c Mopoftv b~dt fo tt" CONOCdOf a Of ,M~mou is e.tmmd to *wage I ftwxm pW rospon. uýinii i hm titi nr wie i wa mm"t~VW ow"MI ,U dw 0giNvu gaiwvl amd mutita"i Ut. d4M tlemadmp.gadrwihe COisclofi at rdOmi46n. $amd wmoaW.W "udtigW bme exrom O N ory &WGu"lw m~nof to 00. ; a, l$e 12ý0= VA 2220-4302. an• d0 0410 o MeIgpmtS .

Nanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nano-sized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials.

The objective of this review article is to provide a concise discussion of atomistic modeling efforts aimed at understanding the nanoscale behavior and the role of grain boundaries in plasticity of metallic polycrystalline materials. Atomistic simulations of grain boundary behavior during plastic deformation have focused mainly on three distinct configurations: (i) bicrystal models, (ii) columnar nanocrystalline models, and (iii) 3D nanocrystalline models. Bicrystal models facilitate the isolation of specific mechanisms that occur at the grain boundary during plastic deformation, whereas columnar and 3D nanocrystalline models allow for an evaluation of triple junctions and complex stress states characteristic of polycrystalline microstructures. Ultimately, both sets of calculations have merits and are necessary to determine the role of grain boundary structure on material properties. Future directions in grain boundary modeling are discussed, including studies focused on the role of...

The current components associated with the grain boundaries of diffused p/n junction polysilicon solar cells made on nand ptype Wacker substrates are analyzed and experimentally identified. New electrical methods for determining the presence

Thanks are due to Peter Qumbsch, who assisted in the preliminary planning of the symposium; to Christine Townsley, who provided invaluable administrative support at UCSB; to Margherita Chang and Lisa Fischer, who assembled the table of contents and indices; to the MRS staff, who offered excellent logistical support; and to the 1998 MRS Fall Meeting Chairs,

The predominance of phenomenological power laws in creep of crystalline materials indicates that the dislocation mechanics of inelastic deformation of crystalline materials has not yet been fully understood. We review the progress towards a general and comprehensive model. In general, dislocation-mediated plasticity leads to generation of dislocations in the crystal interior. Creep, i.e. plasticity at constant stress, continues only if dislocations are able to disappear again (dislocation recovery). A simple model of creep of subgrain-free materials reproduces characteristic features of steady-state creep, but shows significant quantitative deficiencies, indicating that subgrain formation must not be neglected. It is proposed that migration of low-angle subgrain boundaries constitutes the process controlling creep in most cases of single-and multi-phase materials with conventional grain size. High-angle boundaries begin to play a significant role when their spacing d approaches the steady-state subgrain size w ∞ developing in coarsegrained materials. Depending on the w ∞ /d-ratio and deformation conditions, high-angle boundaries may harden or soften the material in the steady state of deformation.

Ferritic–martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (lDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within ‘‘hot zones’’ with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution lDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels.

The development of an intergranular stress corrosion crack initiation site in thermally sensitized type 304 austenitic stainless steel has been observed in situ in high temperature oxygenated water using digital image correlation of time-resolved optical observations.  The grain boundary normal stresses were calculated using the Schmid-Modified Grain Boundary Stress (SMGBS) model of Was et al, applying three-dimensional data for the grain boundary planes and grain orientations.  The initiation site coincided with the most highly stressed sensitised boundary, demonstrating the importance of the combined contributions to crack initiation of grain boundary structure and plastic strain incompatibility.

This paper reports on grain boundary (GB) roles in lead-free tin halide perovskite thin films. Nano scale spatial mapping of charge separation efficiency in methylammonium tin halide (MASn(I 1−x Br x) 3 , MA=CH 3 NH 3) thin films were constructed by Kelvin probe force microscopy and conductive atomic force microscopy (C-AFM). We observed downward band bending at GBs under dark conditions and higher surface photovoltage along the GBs, confirmed by C-AFM which showed high local current flows along the GBs. The band bending degree and local current intensity were affected by the Br/I ratio. Photo-generated carriers were more effectively separated and collected at GBs with increased Br content, and hysteresis was observed in Br-rich Sn-halide perovskite. Supplementary material for this article is available online

Cellular automata are algorithms that describe the discrete spatial and temporal evolution of complex systems by applying local (or sometimes longrange) deterministic or probabilistic transformation rules to the cells of a regular (or non-regular) lattice. The space variable in cellular automata usually stands for real space, but orientation space, momentum space, or wave vector space can be used as well. Cellular automata can have arbitrary dimensions.

High temperature superconductivity (HTS, discovered in 1986) remains an active area of research worldwide, because its higher T c and, thus, more economical cryogenic cooling have raised the prospects for electric power application. The discovery of MgB 2 has rekindled the search for new superconductors with higher T c . Recently, various acceleration programs have been launched in Europe, USA and Japan. The advance in HTS conductor has enabled the demonstration of various application prototypes, including, power cables, transformers, motors, and fault current limiters. However, full commercialisation of HTS application critically relies on the realisation of HTS conductors that are reliable, robust and low cost with low AC-losses. Worldwide activities are, therefore, focused on developing processing technologies to fabricate the so-called coated conductor based on YBCO to fulfil the stringent specifications. While a high critical current density of around 5 MA/cm 2 (77 K) has been achieved, the conductor cost is currently estimated to be 10-50 times higher than what would be accepted. #

The effect of Mn content on the microstructure and mechanical properties of two ultrafine grained0.2%C–Mn steels has been investigated. The ultrafine grained microstructure was produced by use of largestrain warm deformation and subsequent annealing. The final microstructure consists of fine cementite par-ticles within an ultrafine grained ferrite matrix. The increase in the Mn content leads to a decrease in the average ferrite grain size (from 1.3 to 0.8 um for an increase in the Mn content from 0.74 to  1.52mass%). This can be attributed to the enrichment of Mn in the cementite particles, which becomes finer in the steelwith a higher Mn content. The increase in the Mn content results in an increase in strength at equal ductility and toughness.

Grain boundaries (GBs) are planar lattice defects that govern the properties of many types of polycrystalline materials. Hence, their structures have been investigated in great detail. However, much less is known about their chemical features, owing to the experimental difficulties to probe these features at the atomic length scale inside bulk material specimens. Atom probe tomography (APT) is a tool capable of accomplishing this task, with an ability to quantify chemical characteristics at near-atomic scale. Using APT data sets, we present here a machine-learning-based approach for the automated quantification of chemical features of GBs. We trained a convolutional neural network (CNN) using twenty thousand synthesized images of grain interiors, GBs, or triple junctions. Such a trained CNN automatically detects the locations of GBs from APT data. Those GBs are then subjected to compositional mapping and analysis, including revealing their in-plane chemical decoration patterns. We applied this approach to experimentally obtained APT data sets pertaining to three case studies, namely, NiP , Pt-Au, and Al-Zn-Mg-Cu alloys. In the first case, we extracted GB specific segregation features as a function of misorientation and coincidence site lattice character. Secondly, we revealed interfacial excesses and in-plane chemical features that could not have been found by standard compositional analyses. Lastly, we tracked the temporal evolution of chemical decoration from early-stage solute GB segregation in the dilute limit to interfacial phase separation, characterized by the evolution of complex composition patterns. This machine-learningbased approach provides quantitative, unbiased, and automated access to GB chemical analyses, serving as an enabling tool for new discoveries related to interface thermodynamics, kinetics, and the associated chemistry-structure-property relations.

Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.

We study grain boundary embrittlement in a quenched and tempered Fe–Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper-embrittlement at 450 °C. Tempering at 600 °C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth.

Dielectric Ceramics (DRs) have revolutionized the microwave wireless communication industry globally. The three key properties of ceramic dielectrics that determine their functionality at microwave and millimetrewave frequencies include relative permittivity (ε r ), unloaded quality factor Quthe inverse of the dielectric loss (tanδ) and temperature coefficient of resonant frequency (τ f ). These microwave dielectric properties are affected by a number of factors. These includes tolerance factor, onset of structural phase transitions, dark core formation, processing conditions, raw materials and impurities, order/disorder behavior, compositional ordering, porosity, humidity, grain size, orientation of the crystallites, and grain boundaries. The data related to these factors is scattered. The main purpose of this review is to bring these together and present the effects of these factors on the microwave dielectric properties. Control of these factors is important for improvement in the microwave properties. This review would be very helpful to the novice researchers and technologists in the field.

The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislo-cation density (~10 14 m −2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE mi-cromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between fer-rite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels.

The densification behaviour, microstructural development and mechanical properties of a 5 vol.% silicon carbide dispersed alumina nanocomposite were studied by incorporating six different oxide dopants (1 wt.%). It was found that MgO, Y 2 O 3 and CeO 2 enhanced the densification of the nanocomposite, which can be explained by a liquid phase assisted sintering process. The yttriadoped nanocomposite could be pressureless sintered at 1550 C. Doping with MgO or CeO 2 refines the grain size of the matrix alumina whereas yttria addition induces exaggerated grain growth. The Young's modulus, hardness, fracture toughness and erosive wear resistance were evaluated. Doped nanocomposites exhibit slightly lower Young's modulus and higher hardness values than that of the undoped nanocomposite. CeO 2 doping improves hardness and fracture toughness slightly whereas the improvement in erosive wear resistance was 2.5 times higher than the other nanocomposites. Improvements in properties are explained in terms of residual compressive stresses, grain refinement and grain boundary strengthening. #

Here we report our recent progress in the development, optimization, and application of a technique for the three-dimensional (3-D) high-resolution characterization of crystalline microstructures (3D EBSD; EBSD tomography). The technique is based on automated serial sectioning using  a focused ion beam (FIB) and characterization of the sections by orientation microscopy based on electron backscatter diffraction (EBSD) in a combined FIB–scanning electron microscope (SEM). On our system, consisting of a Zeiss–Crossbeam FIB-SEM and an EDAX-TSL EBSD system, the technique currently reaches a spatial resolution of 100 100 100 nm3 as a standard, but a resolution of 50x50x50 nm^3 seems to be a realistic optimum. The maximum observable volume is on the order of 50x50x50um^3. The technique extends all the powerfulfeatures of two-dimensional (2-D) EBSD-based orientation microscopy into the third dimension of space. This allows new parameters of the microstructure to be obtained—for example, the full crystallographic characterization of all kinds of interfaces, including the morphology and the crystallographic indices of the interface planes. The technique is illustrated by four examples,including the characterization of pearlite colonies in a carbon steel, of twins in pseudo nano-crystalline NiCo thin films, the description of deformation patterns formed under nanoindents in copper single crystals, and the characterization of fatigue cracks in an aluminum alloy. In view of these examples, we discuss the possibilities and limits of the technique. Furthermore, we give an extensive overview of parallel developments of 3-D orientation microscopy (with a focus on the EBSD-based techniques) in other groups.

Grain boundaries play an important role in determining the mechanical properties of metallic materials. The impedance of dislocation motion at the boundary results in a strengthening mechanism. In addition, dislocations can pile-up, be transmitted or be absorbed by the grain boundaries based on the local stress state and grain boundary character. In this study, a dislocation density based crystal plasticity finite element model is applied to incorporate the interaction between the dislocations and the grain boundaries, and a simulation is conducted on polycrystalline alpha iron deformed to 12% in uniaxial tension. The results indicate that the geometrically necessary dislocation density is generally higher near the grain boundary than within the grain interior. Taylor factor mismatch sometimes reveals strong localization effects near the grain boundaries.

We report the fabrication of polycrystalline silicon piezoresistive cantilevers with submicron width and thickness for static force measurements, using electron beam lithography (EBL) and silicon micromachining. For cantilevers with length of 150 µm, leg width of 500 nm and thickness of 320 nm, a force sensitivity of 97 µV/pN and a resolution of 30 pN have been obtained. Bigger cantilevers (leg width 6 µm, thickness 650 nm) fabricated with nominally identical processing conditions have shown a lower resistivity and a higher gauge factor. We show that the differences result from an increased effect of the grain boundaries for the smaller cantilevers. But these have a better force sensitivity and resolution due to their reduced lateral dimensions.

A physical-based analytical ON-state drain-current model was developed based on a mobility model including both grain boundary barrier-controlled carrier conduction and gate voltage dependent mobility degradation. Mobility variation along the conduction channel caused by both effects was taken into account. The derived drain-current can be approximated by a previously followed form, however, with mobility term modified and saturation factor included. Our experimental effective channel mobility and above-threshold drain-current data from both lowtemperature and high-temperature processed polycrystalline silicon thinfilm transistors can be accurately fitted by the model, without introducing any empirical or artificial factors.

The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 C in a polycrystalline nickel-based superalloy was studied. Primary MC car-bides, enveloped within intergranular g 0 layers, decomposed resulting in the formation of layers of the undesirable h phase. These layers have a composition corresponding to Ni 3 Ta as measured by atom probe tomography and their structure is consistent with the D0 24 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various mis-orientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M 5 B 3 borides were found to be micro-structurally stable after creep and without substantial compositional changes. Borides at 850 C were found to coarsen, resulting in some cases into g 0-depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed.

The polycrystalline ceramic samples of Pb 1−x Sm x (Zr 0.55 Ti 0.45 ) 1−x/4 O 3 (x = 0.00, 0.03, 0.06 and 0.09) were prepared by solid-state reaction technique at high temperature. Electric impedance (Z) and modulus (M) properties of the materials have been investigated within a wide range of temperature and frequency using complex impedance spectroscopy (CIS) technique. The complex impedance analysis has suggested the presence of mostly bulk resistive (grain) contributions in the materials. This bulk resistance is found to decrease with the increase in temperature. It indicates that the PSZT compounds exhibit a typical negative temperature coefficient of resistance (NTCR) behavior. The bulk contribution also exhibits an increasing trend with the increase in Sm 3+ substitution to PZT. The complex modulus plots have confirmed the presence of grain (bulk) as well as grain boundary contributions in the materials. Both the complex impedance and modulus studies have suggested the presence of non-Debye type of relaxation in the materials.

Mechanical spectroscopy is a non-destructive technique that is very well suited for studying the dynamics of singularities such as structural defects in solids. It has been successfully applied in solid state physics and materials science for more than fifty years, and the present textbook aims at summarising the state-of-the-art in this field by presenting recent results obtained in Western European laboratories. The contents are divided into nine chapters. Introduction to mechanical spectroscopy (Ch.1) is based on a complete description of the elastic, viscoelastic, and viscoplastic behaviours of solids. The anelastic response is analysed from three different viewpoints: phenomenology, rheology and thermodynamics.

Two approaches in materials physics have proven immensely successful in alloy design: First, thermodynamic and kinetic descriptions for tailoring and processing alloys to achieve a desired microstructure. Second, crystal defect manipulation to control strength, formability and corrosion resistance. However, to date, the two concepts remain essentially decoupled. A bridge is needed between these powerful approaches to achieve a single conceptual framework. Considering defects and their thermodynamic state holistically as 'defect phases', provides a future materials design strategy by jointly treating the thermodynamic stability of both, the local crystalline structure and the distribution of elements at defects. Here, we suggest that these concepts are naturally linked by defect phase diagrams describing the coexistence and transitions of defect phases. Construction of these defect phase diagrams will require new quantitative descriptors. We believe such a framework will enable a paradigm shift in the description and design of future engineering materials.

Niobium pentoxide (Nb2O5) was added to cobalt spinel ferrite (CoFe2O4) powders for the first time, at varying amounts of 0, 5, 10 and 15 wt%. The purpose was to evaluate the effect of niobia on the crystalline phases, microstructure and complex electromagnetic behaviour (complex permittivity and permeability between 300 MHz and 10 GHz) of CoFe2O4. The samples were prepared by conventional ceramic methods and sintered at 1475 °C, as potential applications are as aerospace materials (radomes) which have to survive at such temperatures upon re-entry. The only crystalline phase observed in all samples was CoFe2O4 , but microstructural evaluation showed that a non-crystalline, niobium-rich intergranular region was formed between the grains with niobium addition, and this apparent liquid/glassy phase aided sintering as considerable grain growth was also observed. It was shown by Raman spectroscopy that this niobium-rich amorphous intergranular phase was FeNbO4. The electromagnetic measurements of complex permittivity (ε*) and permeability (μ*) measurements indicated a steady decrease in both permittivity and permeability with increasing niobium oxide addition, although the values for each sample were relatively stable between 300 MHz and 10 GHz. The real permittivity, ε′, decreased from ~ 12 in the pure CoFe2O4 to ~ 5.5 with increasing addition, while its imaginary part assumed values very close to zero. At the same time, the real permeability, μ′, decreased from ~ 1.4 to ~ 1.1, and a similar effect can be observed in the permeability curves. The results of the complex measurements also allowed us to obtain reflectivity graphs representing the energy loss of the incident electromagnetic wave when crossing the layer of the evaluated composition. The graphs are presented in the frequency domain, and indicate that the reflection loss increases with the addition of niobia.

Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (l-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized l-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels.

An overview on recent progress in grain boundary (GB) diffusion study is presented with emphasis on physical phenomena encountered in GB diffusion experiments such as the linear and non-linear segregation effects. Systematic investigations on pure and alloyed poly-and bicrystals gave conclusive information on the solute segregation behavior and allowed to extract the segregation isotherm solely from GB diffusion studies. Recent progress in fundamental understanding of diffusion processes in two-scale materials with two types of short-circuit diffusion paths is discussed on the basis of GB self-diffusion experiments and Monte-Carlo simulations in powder sintered nanocrystalline material.

Molecular-dynamics (MD) simulations of fully three-dimensional (3D), model nanocrystalline face-centered cubic metal microstructures are used to study grain-boundary (GB) diffusion creep, one mechanism considered to contribute to the deformation of nanocrystalline materials. To overcome the well-known limitations associated with the relatively short time interval used in our MD simulation (typically Ͻ10 Ϫ8 s), our simulations are performed at elevated temperatures where the distinct effects of GB diffusion are clearly identifiable. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed, our input microstructures were tailored to (1) have a uniform grain shape and a uniform grain size of nm dimensions and (2) contain only high-energy GBs which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogeneous, with a strain rate that agrees quantitatively with that given by the Coble-creep formula. The grain-size scaling of the Coble creep is found to decrease from d Ϫ3 to d Ϫ2 when the grain diameter becomes of the order of the GB width. For the first time a direct observation of the grainboundary sliding as an accommodation mechanism for the Coble creep, known as Lifshitz sliding, is reported. 

"We suggest a dislocation based constitutive model to incorporate the mechanical interaction between mobile dislocations and grain
boundaries into a crystal plasticity finite element framework. The approach is based on the introduction of an additional activation
energy into the rate equation for mobile dislocations in the vicinity of grain boundaries. The energy barrier is derived by using a geometrical model for thermally activated dislocation penetration events through grain boundaries. The model takes full account of the geometry of the grain boundaries and of the Schmid factors of the critically stressed incoming and outgoing slip systems and is formulated as a vectorial conservation law. The new model is applied to the case of 50% (frictionless) simple shear deformation of Al bicrystals with either a small, medium, or large angle grain boundary parallel to the shear plane. The simulations are in excellent agreement with the experiments in terms of the von Mises equivalent strain distributions and textures. The study reveals that the incorporation of the misorientation alone is not sufficient to describe the influence of grain boundaries on polycrystal micro-mechanics. We observe three mechanisms which jointly entail pronounced local hardening in front of grain boundaries (and other interfaces) beyond the classical kinematic
hardening effect which is automatically included in all crystal plasticity finite element models owing to the change in the Schmid
factor across grain boundaries. These are the accumulation of geometrically necessary dislocations (dynamic effect; see [Ma A, Roters F, Raabe D. A dislocation density based constitutive model for crystal plasticity FEM including geometrically necessary dislocations. Acta Mater 2006;58:2169–79]), the resistance against slip penetration (dynamic effect; this paper), and the change in the orientation spread (kinematic effect; this paper) in the vicinity of grain boundaries."

A quasi 2-D conduction model based on the thermionic emission of charge carriers over the energy barriers at discrete grain boundaries is formulated for a polycrystalline silicon thin-film transistor with an undoped body. Each grain boundary is characterized by an energy-dispersed density of trap states. The occupied trap states are assumed to form a "line" charge adjacent to the interface of the channel and the gate dielectric of a transistor. The electrostatic potential of a grain boundary is subsequently determined. This general approach allows the modeling of energy barriers in a transistor without deliberate channel doping, and the resulting conduction model is continuously applicable from the "pseudosubthreshold" to the "linear" regime of operation of a transistor. Good agreement between the experimental and the calculated transfer and output characteristics is obtained. The procedure for determining the density of trap states is described and demonstrated. It is found that the energy dependence of the trap states can be approximated by a simple exponential function.

The creep-fatigue properties at 700°C of cast and wrought Udimet 720Li superalloy have been investigated. As expected, the introduction of a dwell time as low as 1s at maximum stress induces a creep-fatigue life debit compared to pure fatigue conditions. It is demonstrated that the mechanical behavior (plastic strain rate, ductility) is greatly affected by the grain size and precipitation distribution. Moreover, the creep-fatigue life seems to be hardly affected by the alloy microstructure, due to the development of localized damage. Whatever the environmental conditions (i.e. in air or under vacuum), failure has been identified to be intergranular.

Keywords: Charpy impact testo Toughnesso Ultrafine-grained materialso Intercritical annealingo Medium Mn steelo Brittle-ductile transitiono a b s t r a c t The effects of prior austenite (g) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (g R) and ferrite (a) after intercritical annealing at 640 C for 30 min. The phase fractions and the chemical composition of g R were almost identical in both types of specimens. However, their micro-structural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to À196 C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior g grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine a and a 0 martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior g grains through the recrystallization of aʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel.

Doping effect and vacancy formation on ionic conductivity of ZrO 2 ceramics doped with RENbO 4 (RE ¼ Yb, Er, Y, Dy) were investigated using X-ray diffractometry, scanning electron microscope and corresponding ionic conductivity were evaluated using impedance spectroscopy in this work. The results show that defect distribution can be correlated with the phase transformation behavior modified by ionic radius of dopants. The total conductivity of 5 mol% RENbO 4 -doped ZrO 2 (3Y) comprises the intragrain and grain boundary (GB) conductivity. The intragrain conductivity of 5 mol% RENbO 4 -doped ZrO 2 (3Y) are lower than 3 mol% Y 2 O 3 -doped ZrO 2 (3Y-TZP) and 8 mol% Y 2 O 3 -doped ZrO 2 (8YSZ). The additions of Nb 2 O 5 to ZrO 2 (3Y) increase average lattice binding energy and activation energy, and the amount of oxygen vacancies was decreased. The average radius of oxygen vacancies of 5 mol% RENbO 4doped zirconia (3Y) were smaller than that of 8YSZ identified using hard-sphere model. The results imply that a specific doping content in zirconia which contributes a maximum content of non-interfering oxygen vacancies, the average radius of doping ions close to that of Zr 4+ and average binding energy as smaller as possible help obtain the highest conductivity of zirconia. To acquire an appropriate operation condition in the application of solid oxide fuel cell, electrical properties, phase transformation behavior and related mechanical properties need to be compromised. r

This article focuses on four topics that demonstrate the importance of atom probe tomography for obtaining nanostructural information that provides deep insights into the structures of metallic alloys, leading to a better understanding of their properties. First, we discuss the microstructure–coercivity relationship of Nd-Fe-B permanent magnets, essential for developing a higher coercivity magnet. Second, we address equilibrium segregation at grain boundaries with the aim of manipulating their interfacial structure, energies, compositions, and properties, thereby enabling benefi cial material behavior. Third, recent progress in the search to extend the performance and practicality of the next generation of advanced high-strength steels is discussed. Finally, a study of the temporal evolution of a Ni-Al-Cr alloy through the stages of nucleation, growth, and coarsening (Ostwald ripening) and its relationship with the predictions of a model for quasi-stationary coarsening is described. This information is critical for understanding high-temperature mechanical properties of the
material.

Microstructural evolution and martensitic transformation characteristics of Co 46 Ni 27 Ga 27 and Co 44 Ni 26 Ga 30 high temperature shape memory alloys were investigated in as-cast and hot-rolled conditions as a function of heat treatment. Heat treatments were selected to introduce single-, two-, and three-phase structures, where the precipitate phases do not martensitically transform. The effects of these precipitates, and associated compositional changes, on transformation temperatures, thermal hysteresis, and microstructural evolution during thermal cycling, were revealed. It was found that martensite start temperature linearly depends on the valence electron concentration (e/a) of the matrix, if the Ga content is constant. For a given e/a, the higher the Ga content is, the higher the transformation temperatures become. The presence of c 0 precipitates and the volume fraction of c phase were shown to have strong influence on transformation thermal hysteresis. The most cyclically stable compositions with narrow hysteresis (<40°C) were identified. In these compositions, a room-temperature aging phenomenon, possibly mediated by point defects, was discovered, which recovers the transformation temperature changes upon thermal cycling. They also demonstrate reversible martensitic transformation in constant-stress thermal cycling experiments. However, their crystallographic texture should be engineered to increase the transformation strain, and ductile c-phase content should be reduced to improve cyclic reversibility.

The development of strongly cube textured Cu based substrates is important in the cost effective production of long lengths of high temperature superconducting cables. The present paper reports textures (deformation and recrystallisation) development in pure Cu, Cu–Al, Cu–Mn (with a solute content of 1–3 at.%) and Cu–35 at.% Ni alloys.

A series of nanocrystalline Fe–C alloys with different carbon concentrations (xtot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of a-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing xtot. Eventually the size reaches a lower limit of about 6 nm in alloys with xtot > 6.19 at.% (1.40 wt.%); a further increase in xtot leads to the precipitation of carbon as Fe3C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe3C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy.

We investigated the hydrogen embrittlement of a Fe–18Mn–1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7  10^6  /s1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe–Mn–C twining-induced plasticity steel.