Search

Titanium alloys are processed to develop a wide range of microstructure configurations and therefore material properties. While these properties are typically measured experimentally, a framework for property prediction could greatly enhance alloy design and manufacturing. Here a microstructure-sensitive framework is presented for the prediction of strength and ductility as well as estimates of the bounds in variability for these properties. The framework explicitly considers distributions of microstructure via new approaches for instantiation of structure in synthetic samples. The parametric evaluation strategy, including the finite element simulation package FEpX, is used to create and test virtual polycrystalline samples to evaluate the variability bounds of mechanical properties in Ti-6Al-4V. Critical parameters for the property evaluation framework are provided by measurements of single crystal properties and advanced characterization of microstructure and slip system strengths in 2D and 3D. Property distributions for yield strength and ductility are presented, along with the validation and verification steps undertaken. Comparisons between strain localization and slip activity in virtual samples and in experimental grain-scale strain measurements are also discussed.

During service, superalloy turbine components degrade over time by creep and fatigue deformation mechanisms due to a complex combination of stresses at high temperatures. The high cost of fabricating Ni-base superalloy components and, consequently, replacement components has encouraged the development of rejuvenation (restoration) procedures to extend useful service life. The processes that limit repeated rejuvenation of directionally solidified GTD444 and single-crystal Rene N5(SX) alloys have been studied in detail. A rejuvenation cycle includes a rejuvenation heat treatment and/or small-scale material removal, followed by creep or fatigue testing. With the application of multiple rejuvenation cycles, the total creep rupture life of Rene N5(SX) tested at 982 °C and 206 MPa was extended by a factor of 2.8 over the baseline rupture life. To produce this increase in rupture life, creep strains were limited to 2 or 3 pct prior to application of a rejuvenation cycle, involving solutioning at 28 °C below the $$\gamma ^{\prime }$$ solvus temperature for 2 hours and aging at 1079 °C for 4 hours. Rejuvenation of compressive hold-time fatigue damage was also successful with the use of small-scale material removal. Due to the tortuosity of the grain boundaries in GTD444(DS), some boundaries are initially oriented transverse to the growth direction. The enhanced plasticity near these grain boundaries may be the primary reason why the initially single-crystal Rene N5(SX) specimens were more amenable to rejuvenation than GTD444(DS). For both alloys, recrystallization after multiple rejuvenation cycles was responsible for early failure during the subsequent creep test. Resonant ultrasound spectroscopy was successfully implemented to detect the presence of recrystallized grains.

Additive manufacturing (AM) is a new and promising production methodology adept at producing complex geometries, which can be optimized for lower weight and enhanced capabilities. The material properties of these additive components are dictated by the microstructures developed during processing, with a high sensitivity to grain structure and associated anisotropy. With this new processing modality comes the added difficulty of understanding the thermodynamics and kinetic mechanisms that dictate the evolution of microstructure. This research addresses the unique thermal conditions present in AM and the pathways for grain refinement in nanofunctionalized aluminum alloys. The Al-Ta system, in which Al3Ta intermetallic compounds are demonstrated to have substantial grain refining capacity, are the focus of this study. The grain size is shown to be reduced relative to pure aluminum by 1000X when tantalum is added at 1 vol%. The effectiveness of the Al3Ta intermetallic is dictated by the crystallography and availability of the inoculant phase under AM conditions.

Background: Advancements in the Digitial Image Correlation (DIC) technique over the past decade have greatly improved spatial resolution. However, many processes, such as plastic deformation, have a temporal component spanning from fractions of a second to minutes that has not yet been addressed in detail, particularly for DIC conducted in-situ in the scanning electron microscope (SEM). Objective: To develop a methodology for conducting time-resolved digital image correlation in the SEM for analysis of time-dependent mechanical deformation phenomena. Methods: Microscope and electron beam scanning parameters that influence the rate at which time-resolved DIC information is mapped are experimentally investigated, providing a guide for use over a range of timescales and resolutions. Results: Time-resolved DIC imaging is demonstrated on a Ti-7Al alloy, where slip band propagation is resolved with imaging dwell times of seconds. The limits of strain resolution and strain collection speeds are analyzed. Conclusions: The new developed methodology can be applied to a wide range of materials loaded in-situ to quantify time-dependent plastic deformation phenomena.

International audience; Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip bands by high-resolution digital image correlation (DIC) in a polycrystalline nickel-based superalloy. Quantitative measurements of the slip irreversibility are challenging for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic strain accumulation in a nickel-based superalloy. The method detects bands with high levels of irreversibility early in cycling that ultimately form fatigue cracks upon further cycling. The local microstructural configurations that induce large amounts of plasticity and slip irreversibility are correlated to crack nucleation locations.

Nondestructive X-ray diffraction contrast tomography imaging was used to characterize the microstructure evolution in a polycrystalline bulk strontium titanate specimen. Simultaneous acquisition of diffraction and absorption information allows for the reconstruction of shape and orientation of more than 800 grains in the specimen as well as porosity. Three-dimensional microstructure reconstructions of two coarsening states of the same specimen are presented alongside a detailed exploration of the crystallographic, topological and morphological characteristics of the evolving microstructure. The overall analysis of the 3D structure shows a clear signature of the grain boundary anisotropy, which can be correlated to surface energy anisotropy: the grain boundary plane distribution function shows an excess of 〈100〉-oriented interfaces with respect to a random structure. The results are discussed in the context of interface property anisotropy effects.

A new class of $$\gamma ^{\prime }$$-containing Co-based superalloys that are promising for high temperature applications has been investigated under cyclic loading conditions. A series of single crystal variants of these Co-based superalloys have been cyclically loaded above their elastic limit at $$750\,^{\circ }\hbox {C}$$ in air to study their behavior in the low cycle regime. Interrupted testing was performed to observe the early stages of fatigue failure. Optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to characterize post-mortem specimens. Fatigue failure occurred due to surface cracks that developed during the early stages of cycling in the aluminide-coated sample gauge sections. Growth of these surface cracks into the substrate was associated with extensive oxidation and intermetallic phase precipitation, which accelerated crack propagation as compared to Ni-based superalloys. These observations suggest that improvements in the oxidation resistance and high temperature strength of $$\gamma ^{\prime }$$-containing Co-based superalloys will enhance their fatigue behavior.

Uniform strain relaxation and defect densities across the entire wafer are important aspects of lattice-mismatched or metamorphic epitaxy of semiconductors. Achieving this requires an understanding of the sources of dislocations that lead to relaxation. We show that wafer edges are a major source of misfit dislocations in InGaAs graded buffers on GaAs substrates. AlInP solar cells fabricated from regions on the wafer affected by edge sources have a markedly lower performance due to a local increase in the threading dislocation density. Spatially resolved x-ray reciprocal space maps and electron backscattered diffraction maps of the InGaAs compositionally graded buffer reveal anomalous epi-layer tilting on 2-inch substrates from these dislocation sources at the wafer edge. The tilting abruptly changes direction partially through the graded buffer growth as the dislocations from the wafer edge appear in the x-ray sampling areas. We speculate that anisotropic misfit dislocation glide in III-V materials in combination with non-uniform stress states during growth biases slip system activity. These experiments demonstrate that lattice-mismatched epitaxy requires additional controls over the growth environment to achieve uniform strain relaxation for wafer scale heterogeneous integration.

Micro-tensile specimens of nickel-based superalloy oligocrystals were tested in-situ in an scanning electron microscope in transmission mode (TSEM) enabling observation of dislocations. The dynamics of dislocation motion during tensile loading were captured and correlated with the measured intermittencies during plastic flow recorded by high load- and temporal-resolution sensors. This investigation in particular focused on the dislocation behavior near twin boundaries with different slip configurations. A multiplicity of deformation mechanisms at the dislocation scale were observed within individual slip bands, including precipitate shearing, dislocation decorrelation and antiphase boundary-coupled shearing. These processes affect strain localization near twin boundaries and provide new defect-level insights on plastic localization and fatigue crack initiation in these alloys.

Additive manufacturing (AM) of metallic components promises many advantages over conventional manufacturing processes through high design flexibility across multiple length scales and precision coupled with an astonishing combination of mechanical properties. Characterizing the relationship between microstructure and mechanical properties remains one of the major challenges for this novel technology. A natural precursor is identifying the influence of the processing path on the developing microstructure. We combine experimental studies of single-track laser powder bed fusion (LPBF) scans of AISI 316L stainless steel, finite element analyses, and large-scale three-dimensional discrete dislocation dynamics simulations to provide a unique understanding of the underlying mechanisms leading to the formation of heterogeneous defect structures in additively manufactured metals. Our results show that the interruption of dislocation slip at solidification cell walls is responsible for the formation of cellular dislocation structures, highlighting the significance of solute segregation for plastic deformation of additively manufactured components. This work provides a mechanistic perspective on heterogeneous microstructure formation and opens the potential for a reliable prediction of the resulting mechanical properties of additively manufactured parts., Comment: Accepted manuscript

We have directly written nanoscale patterns of magnetic ordering in FeRh films using focused helium-ion beam irradiation. By varying the dose, we pattern arrays with metamagnetic transition temperatures that range from the as-grown film temperature to below room temperature. We employ transmission electron microscopy, X-ray diffraction, and temperature-dependent transport measurements to characterize the as-grown film, and magneto-optic Kerr effect imaging to quantify the He+ irradiation-induced changes to the magnetic order. Moreover, we demonstrate temperature-dependent optical microscopy and conductive atomic force microscopy as indirect probes of the metamagnetic transition that are sensitive to the differences in dielectric properties and electrical conductivity, respectively, of FeRh in the antiferromagnetic (AF) and ferromagnetic (FM) states. Using density functional theory, we quantify strain- and defect-induced changes in spin-flip energy to understand their influence on the metamagnetic transition temperature. This work holds promise for in-plane AF-FM spintronic devices, by reducing the need for multiple patterning steps or different materials, and potentially eliminating interfacial polarization losses due to cross material interfacial spin scattering.

Based on simulations of tensile tests, the macroscopic stiffness and strength of the α phase of Ti-6Al-4V are estimated. Sets of virtual samples are instantiated using characterization data from three-dimensional electron back-scattered diffraction (EBSD) scans collected with the TriBeam microscope and local properties extracted from in situ loading high-energy x-ray diffraction (HEXD) experiments. The simulations use a crystal-scale finite element framework to compute the mechanical response of the virtual samples under tensile loading. From the simulation data, mechanical properties are extracted from volume elements ranging in size from a fraction of the gage section to its entire length. To detect macroscopic yield, a flood-fill algorithm is used to identify a zone of plastically deformed finite elements extending through a volume element. Trends in the estimated properties as functions of the volume element size are examined. The lower bound of volume element size necessary to replicate experimentally measured macroscale properties is indicated. The technique and results provide a guideline for estimating macroscale property values in components designed with size smaller than the lower bound due to constraint.

Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a Rene 88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400 °C to 650 °C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.

First-principles DFT methods are combined with an experimental approach to characterize the creep behavior of quinary Co-based L1 $$_2$$ -containing superalloys at elevated temperature conditions. Temperature-dependent SISF energies have been modeled, combining 0 K formation energies with vibrational free energy calculations to assess deformation mechanisms at finite temperature. Two different Co-Al-W alloys, containing the maximum possible amount of DFT-identified d-block alloying additions, were identified and cast as single crystals via the Bridgman process. Creep tests have been performed at two primary testing conditions, one at 900 $$^\circ $$ C and the other at 982 $$^\circ $$ C. Transmission scanning electron microscopy (TSEM) was performed at 30 kV in a scanning electron microscope to rapidly characterize the defect substructures. We observe a coupled APB/SISF/APB defect structure in Co-based superalloys at the low-temperature condition, similar to the defect structure observed in CoNi, in spite of containing no Ni. At 982 $$^\circ $$ C, there is no evidence of faults and precipitates instead contain antiphase boundaries. The role of composition and temperature-dependent fault energies in the deformation process is addressed.

The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 °C–650 °C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a Rene 88DT polycrystalline nickel-base superalloy at temperatures up to 650 °C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries.

In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elastic-plastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickel-base superalloys, IN100 and Rene 88DT. The material element that captures the fatigue crack-initiating features in Rene 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.

This paper applies the three-dimensional visualization techniques explored theoretically by Callahan, Echlin, Pollock, Singh & De Graef [J. Appl. Cryst.(2017),50, 430–440] to a series of experimentally acquired texture data sets, namely a sharp cube texture in a single-crystal Ni-based superalloy, a sharp Goss texture in single-crystal Nb, a random texture in a powder metallurgy polycrystalline René 88-DT alloy and a rolled plate texture in Ti-6Al-4V. Three-dimensional visualizations are shown (and made available as movies as supplementary material) using the Rodrigues, Euler and three-dimensional stereographic projection representations. In addition, it is shown that the true symmetry of Euler space, as derived from a mapping onto quaternion space, is described by the monoclinic color space groupPccin the Opechowski and Guccione nomenclature.

Crystallographic textures are commonly represented in terms of Euler angle triplets and contour plots of planar sections through Euler space. In this paper, the basic theory is provided for the creation of alternative orientation representations using three-dimensional visualizations. The use of homochoric, cubochoric, Rodrigues and stereographic orientation representations is discussed, and illustrations are provided of fundamental zones for all rotational point-group symmetries. A connection is made to the more traditional Euler space representations. An extensive set of three-dimensional visualizations in both standard and anaglyph movies is available.

International audience; The recent development of the high resolution and discontinuity-tolerant digital image correlation technique enables the extraction of discontinuities within a displacement field. The technique provides quantitative analysis of discontinuities arising from slip, shear bands, cracks, and grain boundary sliding in a variety of material systems, including polycrystalline metallic materials. The discontinuity-tolerant digital image correlation method can be implemented to retrieve not only quantitative discontinuity analysis but also the local strain and rotation fields that operate near these discontinuities. The present implementation includes high-resolution digital image correlation (HR-DIC) measurements collected in a scanning electron microscope for analysis of both the plastic and elastic fields that develop during deformation of polycrystalline metallic materials. The combination of the discontinuity-tolerant DIC technique with the computation of internal gradients enables extraction of non-localized strain and rotation fields during plastic deformation of a nickel-based superalloy. Therefore the lattice rotation/expansion and plastic localization that occur during deformation can be determined in a single experiment. This method is validated using synthetic images with preset deformation, and experimental measurements using the electron back scatter diffraction (EBSD) technique.

The evolution of damage, texture, and strain in additive manufactured (AM) 316L stainless steel produced via laser powder bed fusion was investigated during in-situ tensile loading using high energy X-rays. Synchrotron X-ray computed tomography (XCT) measurements were performed to determine the initial porosity and monitor the evolution of porosity during tensile loading as well as detect the initiation and growth of voids from pre-existing pore defects in the specimens. The as-built tensile specimens had a cross-sectional area of 1 mm2, which was chosen in order to understand damage behavior in thin-walled structures. Far-field X-ray diffraction measurements were performed to quantify crystallographic texture and the distribution of internal elastic strains during loading. The initial texture from the AM build process had a weak {220} texture aligned parallel to the build direction. As a result of tensile deformation, a strong {111} + {200} double fiber texture develops at high tensile strains and remains until fracture. XCT results confirmed that the inhomogeneous distribution of porosity near the surface played a significant role in damage evolution during tensile loading where voids and cracks initiated at pre-existing pores located within the contour zone. These pores were found to have asymmetric or irregular morphology. At high tensile strains, the massive accumulation of internal damage at these pores eventually connected to the surface reducing the ductility in these thin-walled AM samples and resulting in final failure.

Fatigue crack initiation at high temperatures occurs at microscopic fatigue shear bands that form near twin boundaries in polycrystalline nickel-base superalloys that contain minimal metallurgical defects. The associated dislocation sub-structure is complex and dependent on thermal and mechanical conditions. Here we show the formation of fine-scale carbide precipitates along these fatigue shear bands in a nickel-base superalloy during fatigue above 650 °C. The formation of carbide precipitates is studied using calculations of bulk and pipe diffusion. The contribution of pipe diffusion is observed to be critical in the formation of fine-scale carbides and occurs coincident with the delocalization of slip during fatigue.

The equiaxed microstructure of Ti-6Al-4V contains nominally 10µm to 15µm α -phase grains. Depending on processing, these fine grains may aggregate into large, millimeter-scale regions of similar crystallographic orientation. These so-called microtextured regions are detrimental to quasi-static and fatigue properties. Their presence may facilitate the formation of long-range strain localization—bands of plastic strain that traverse grain boundaries and terminate at the boundaries of the microtextured region which compromises strength and ductility—and can cause early fatigue crack nucleation. Furthermore, the low angle boundaries within microtextured regions offer little resistance to crack growth, and corresponding increases in crack growth rates have been observed in these regions. Despite significant research into the effects of microtextured regions on macroscopic properties, there is still a lack of rigorous definition of what constitutes a microtextured region. To this end, we present a computational study in which deformation simulations are performed to study the effect of the intensity and character of microtexture on the localization of plastic strain. Mean orientation and intensity of microtexture are parameterized in a suite of simulations. Simulations indicate that strain localization is facilitated both by the presence of some amount of orientation spread within a microtextured region, and further by the intensity of microtexture in neighboring regions. Simulations are further discussed with respect to high resolution experimental measurements of intragrain strain in a microtextured Ti-6Al-4V specimen, which exhibit similar mechanical trends.

The background of an electron backscattered diffraction pattern contains quantitative surface topography data. We propose a new approach to extract this topography from the location of the maximum background intensity in each diffraction pattern. Using specular reflection as a zero-th order approximation, we employ Monte Carlo simulations to determine corrections to the scattering angles, which then allow us to accurately determine the local surface normal. We derive the theoretical model and illustrate the approach experimentally using a β-Sn sphere, for which we determine the local surface normal orientation from the background intensity shifts in electron backscatter diffraction patterns.

Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip planes in a polycrystalline nickel base superalloy. Quantitative measurements of the slip irreversibility is challenging to be performed for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic accumulation in a nickel base superalloy. The local microstructural configurations that induces large amounts of plasticity and slip irreversibility are captured and correlated to crack nucleation locations.

We report a growth mechanism that produces in-plane $[1\overline{1}0]$ oriented ErSb nanowires formed during codeposition of $\mathrm{E}{\mathrm{r}}_{0.3}\mathrm{G}{\mathrm{a}}_{0.7}\mathrm{Sb}$ via molecular beam epitaxy (MBE). Nanowires are characterized by in situ scanning tunneling microscopy (STM), as well as ex situ transmission electron microscopy (TEM) and electron channeling contrast imaging. We show that complexes of macrosteps with step heights on the order of 7 nm form during nanowire growth. The macrosteps are shown to be part of the in-plane nanowire growth process and are directly responsible for the observed stratified distribution of in-plane nanowires. TEM indicates that initial growth results in out-of-plane nanowires transitioning to in-plane nanowires after a critical film thickness. A surface energy model is put forward that shows the critical thickness is due to minimization of the $\mathrm{GaSb}{110}$ surfaces formed during out-of-plane nanowire growth. Kinetics of the transition are discussed with respect to observed features in STM, along with the material parameters needed to achieve in-plane nanowire growth.

We use an electron beam to induce and directly observe dislocation glide in strained GaAs thin films on silicon substrates at room temperature using electron channeling contrast imaging (ECCI). Plastic behavior in this brittle material is kinetically facilitated by a lowering of the Peierls barrier for glide due to recombination of cathodogenerated electron-hole pairs at dislocations. The high residual strain in the GaAs film provides the overall driving force for motion. The dynamics of a variety of previously inaccessible dislocation interactions are studied by tracking individual threading dislocations with high spatial and temporal resolution. The ECCI technique has an inherently high resolution and was conducted in a convenient scanning electron microscope environment. The results and methods described in this Rapid Communication provide insight into dislocation-filtering processes and failure mechanisms in technologically important III-V devices on silicon, as well as other mismatched heteroepitaxial systems.

Room-temperature non-radiative recombination is studied at single dislocations in Ga0.5In0.5P quantum wells grown on metamorphic templates using cathodoluminescence and electron channeling contrast imaging. An analysis of the light emission intensity profiles around single dislocations reveals that the average recombination strength of a dislocation decreases by a factor of four and seven as a result of decreasing growth temperature of the GaInP quantum well from 725 to 675 and 625 °C, respectively. This reduction occurs despite little change in the diffusion length, precluding the prospect of inducing carrier localization by ordering and phase separation in GaInP at lower growth temperatures. These observations are rationalized by the premise that point defects or impurities are largely responsible for the recombination activity of dislocations, and the extent of decoration of the dislocation core decreases with temperature. Preliminary evidence for the impact of the Burgers vector is also presented. The lowest growth temperature, however, negatively impacts light emission away from dislocations. Carrier recombination in the bulk and at dislocations needs to be considered together for metamorphic devices, and this work can lead to new techniques to limit non-radiative recombination.

International audience; In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.

Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSD [1] and ECP [2] to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries.

We demonstrate that the surface topography of a sample can be reconstructed from electron backscatter diffraction (EBSD) patterns collected with a commercial EBSD system. This technique combines the location of the maximum background intensity with a correction from Monte Carlo simulations to determine the local surface normals at each point in an EBSD scan. A surface height map is then reconstructed from the local surface normals. In this study, a Ni sample was machined with a femtosecond laser, which causes the formation of a laser-induced periodic surface structure (LIPSS). The topography of the LIPSS was analyzed using atomic force microscopy (AFM) and reconstructions from EBSD patterns collected at 5 and 20 kV. The LIPSS consisted of a combination of low frequency waviness due to curtaining and high frequency ridges. The morphology of the reconstructed low frequency waviness and high frequency ridges matched the AFM data. The reconstruction technique does not require any modification to existing EBSD systems and so can be particularly useful for measuring topography and its evolution duringin situexperiments.

The new capabilities of a FEG scanning electron microscope (SEM) equipped with a scanning transmission electron microscopy (STEM) detector for defect characterization have been studied in parallel with transmission electron microscopy (TEM) imaging. Stacking faults and dislocations have been characterized in strontium titanate, a polycrystalline nickel-base superalloy and a single crystal cobalt-base material. Imaging modes that are similar to conventional TEM (CTEM) bright field (BF) and dark field (DF) and STEM are explored, and some of the differences due to the different accelerating voltages highlighted. Defect images have been simulated for the transmission scanning electron microscopy (TSEM) configuration using a scattering matrix formulation, and diffraction contrast in the SEM is discussed in comparison to TEM. Interference effects associated with conventional TEM, such as thickness fringes and bending contours are significantly reduced in TSEM by using a convergent probe, similar to a STEM imaging modality, enabling individual defects to be imaged clearly even in high dislocation density regions. Beyond this, TSEM provides significant advantages for high throughput and dynamic in-situ characterization.

High performance III-V lasers at datacom and telecom wavelengths on on-axis (001) Si are needed for scalable datacenter interconnect technologies. We demonstrate electrically injected quantum dot lasers grown on on-axis (001) Si patterned with {111} v-grooves lying in the [110] direction. No additional Ge buffers or substrate miscut was used. The active region consists of five InAs/InGaAs dot-in-a-well layers. We achieve continuous wave lasing with thresholds as low as 36 mA and operation up to 80°C.

International audience; A methodology is presented for the use of the oxide scale that develops in polycrystalline Ni-base super-alloys at service temperature, as a speckle pattern for μm-scale resolution strain measurements. Quantitative assessment of the heterogeneous strain field at the grain scale is performed by high-resolution SEM digital image correlation under monotonic and cyclic loading in polycrystalline Ni-base superalloys up to 650 • C. In the René 88DT superalloy, strain localization is observed near twin boundaries during low cycle fatigue (LCF) at intermediate temperatures, correlating with activation of {111}}110 and {111}}112 slip systems. A strong correlation between the microstruc-tural configuration that promotes strain localization during monotonic loading and crack initiation at 650 • C in low cycle fatigue was observed.

The growth and the density dependence of the low temperature mobility of a series of two-dimensional electron systems confined to un-intentionally doped, low extended defect density InAs quantum wells with Al$_{1-x}$Ga$_{x}$Sb barriers are reported. The electron mobility limiting scattering mechanisms were determined by utilizing dual-gated devices to study the dependence of mobility on carrier density and electric field independently. Analysis of the possible scattering mechanisms indicate the mobility was limited primarily by rough interfaces in narrow quantum wells and a combination of alloy disorder and interface roughness in wide wells at high carrier density within the first occupied electronic sub-band. At low carrier density the functional dependence of the mobility on carrier density provided evidence of coulombic scattering from charged defects. A gate-tuned electron mobility exceeding 750,000 cm$^{2}$/Vs was achieved at a sample temperature of 2 K., 23 pages, 7 figures, 1 table

A high energy ball milling process was used to produce dysprosium nitride and cerium nitride powders at room temperature. Dysprosium and cerium metal flakes were milled in a 275 kPa nitrogen atmosphere for 24 h at ambient temperatures. X-ray diffraction confirmed the formation of phase pure dysprosium nitride and cerium nitride powders. The median particle size of the resultant dysprosium nitride was measured as 4 μm using a laser scattering technique. The particle size of the cerium nitride was not measured due to its reactive nature.

We report a systematic study of high quality GaAs growths on on-axis (001) GaP/Si substrates using molecular beam epitaxy. Various types of dislocation filter layers and growth temperatures of initial GaAs layer were investigated to reduce the threading dislocation densities in GaAs on GaP/Si. Electron channeling contrast imaging techniques revealed that an optimized GaAs buffer layer with thermal cycle annealing and InGaAs/GaAs dislocation filter layers has a threading dislocation density of 7.2 × 106 cm−2, which is a factor of 40 lower than an unoptimized GaAs buffer. The root-mean-square surface roughness was greatly decreased from 7.8 nm to 2.9 nm after the optimization process. A strong enhancement in photoluminescence intensity indicates that the optimized GaAs template grown on on-axis (001) GaP/Si substrates is a promising virtual substrate for Si-based optoelectronic devices.

The feasibility of a novel transient liquid phase (TLP) joining method has been demonstrated in joining La{sub 0.9}Ca{sub 0.1}FeO{sub 3} materials. Metal oxide powders were processed to form the TLP compositions which were used in the joining process. The method has been successful in producing joint interfaces that effectively disappear, as they are the same material and have the same properties as the joined parts. The feasibility of the method has been demonstrated for a single system, but many systems where the method can potentially be applied have been identified.