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The effect of temperature over the range of ambient to 704 °C and strain rate from 10−4 to 10−2 s−1 on the tensile properties and fracture behavior of ATI 718Plus was investigated. The results showed that with increase in temperature at a strain rate 10−4 s−1, there is a small reduction in the yield strength, but a large drop in ductility at 704 °C. This reduction was accompanied by a change in fracture mode from ductile transgranular to brittle intergranular cracking. Detailed analysis of the microstructure and microchemistry of the areas around the crack using electron microscopy showed that the driving mechanism behind the failure at elevated temperatures and slow strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism. In addition, the results suggest that the δ precipitates on the grain boundaries tend to oxidize and may facilitate the oxygen-induced intergranular cracking. Finally, an increase in strain rate at 704 °C caused a small increase in the yield strength and a huge increase in ductility. This increase in ductility was accompanied by a change in fracture mode from brittle-to-ductile failure. Possible mechanisms for the deformation, failure mechanisms, and strain rate dependence are discussed.

A deformation mechanism, rarely reported in Ti alloys, has been observed involving α/β interfacial and colony boundary sliding. This paper reports the initial characterization of this mechanism, involving a comparative study of the deformation behaviors of the new alloy (Ti–0.85Al–4.0V–0.25Fe–0.25Si–0.15O, in wt%) and an established α/β alloy. Both alloys exhibited slip steps on their surfaces. However, the new alloy may also deform by a mechanism which manifests itself as sliding at α/β interfaces and colony boundaries. This mechanism is consistent with the provision of significant ductility during very high strain rate testing, a property exhibited by the new alloy.

The US plan for high-level nuclear waste includes the immobilization of long-lived radionuclides in glass or ceramic waste forms in stainless-steel canisters for disposal in deep geological repositories. Here we report that, under simulated repository conditions, corrosion could be significantly accelerated at the interfaces of different barrier materials, which has not been considered in the current safety and performance assessment models. Severe localized corrosion was found at the interfaces between stainless steel and a model nuclear waste glass and between stainless steel and a ceramic waste form. The accelerated corrosion can be attributed to changes of solution chemistry and local acidity/alkalinity within a confined space, which significantly alter the corrosion of both the waste-form materials and the metallic canisters. The corrosion that is accelerated by the interface interaction between dissimilar materials could profoundly impact the service life of the nuclear waste packages, which, therefore, should be carefully considered when evaluating the performance of waste forms and their packages. Moreover, compatible barriers should be selected to further optimize the performance of the geological repository system. Immobilization of radionuclides in glass or ceramic forms in stainless steel in deep geological repositories is planned in the US for disposal of nuclear waste. Under simulated repository conditions, corrosion could be significantly accelerated at the interfaces of different barrier materials.

Using advanced aberration-corrected scanning transmission electron microscopy coupled with high-resolution energy-dispersive spectroscopy techniques, transitions to microstructure and composition occurring at a grain boundary (GB) crack-tip in Waspaloy after dwell-fatigue at 704 °C are characterized at the nano to atomic scale. The simple microstructure of this γ/γ′ superalloy, which enables elucidation of oxygen effect ahead of the oxide intrusion—dissolution of the precipitates fostering selective segregation of Al to the GB to form oxides—has been experimentally observed.

The strength of a commercial β‑titanium alloy was tuned by systematically controlling the size-scale of α precipitation via two-step heat treatments. While the first step annealing at 350 °C forms the precursor metastable ω phase, the subsequent annealing at 600 °C/1H resulted in fine-scale α precipitation. We find that annealing duration at 350 °C greatly impacts the α size-scale and, this consequently can be used to tune the ultimate tensile strength (UTS) from ~1 to 2 GPa. A combination of high volume-fraction of non-shearable super-refined α precipitates, coupled with their pyramidal arrangement, and a constrained β matrix leads to very high strength.

Hydrogen embrittlement behavior of four heats of alloy 725 (H1, H2, H3, and H4) were characterized by measuring the crack growth rate response over a range of cathodic potentials. Significant grain boundary precipitation in two heats (H1 and H3) may have contributed to higher susceptibility, as evidenced by higher values of crack growth rate. The crack growth rate increased with increasing twin fraction. The crack morphology exhibited evidence of intergranular as well as slip band cracking. The crack growth rate response of the two heats that were less susceptible (H2 and H4) were evaluated in more detail over a range of applied potentials and stress intensity factors. Evidence of nano void formation at the intersection of dislocation slip bands, and grain boundaries suggests that hydrogen stabilized vacancies may play a role in the crack growth mechanism. Crack growth rate exhibited a strong response with applied potential varying by as much as 100 times in the potential range of −1050mV SCE to −850mV SCE. Over the range of K values evaluated, the crack growth rate increased by a factor of 3to 30 with applied K depending on the applied potential. The crack growth rate was related to the water adsorption on fresh metal surfaces of alloy 725 in the anticipated crack tip conditions. A crack tip strain rate based approach was applied to model the measured crack growth response.

A dissimilar weld between a low alloy steel (LAS) butter weld joined to a F65 steel pipe using a narrow groove hot wire gas tungsten arc welding (HW-GTAW) procedure with Alloy 625 filler metal was investigated. The weld interpass microstructure is comprised of large swirls formed by a macrosegregation mechanism involving partial, non-uniform mixing of liquid base metal with the lower melting temperature weld pool, followed by fast solidification. This mechanism produces steep gradients in composition and solidification behavior. The resulting swirls are composed of alternating iron-rich peninsulas and partially mixed zones (PMXZ) that are surrounded by planar and cellular zones exhibiting multiple solidification directions. Large austenitic grains, encompassing planar, cellular, and dendritic morphologies, nucleate off peninsulas in direct contact with the weld pool. The highest hardness was found in nickel and chromium rich PMXZs that exhibited a lath martensite microstructure. In the event of exposure to hydrogen containing environments, the PMXZs could serve as nucleation sites for hydrogen assisted cracking.

The compositional and order-disorder transition widths of γ′/γ interfaces in a Co–Al–W alloy have been established for the first time by coupling of aberration corrected high resolution scanning transmission electron microscopy and orientation-specific atom probe tomography. While the compositional width is approximately 2.5 nm, the order-disorder transition width is very sharp measuring 0.5 nm. A comparative study revealed that the γ′/γ interfaces, in a model Ni–Al–Cr alloy, are both compositionally and structurally diffuse with nearly the same width (~ 2.5–3 nm). These differences between the γ′/γ interfaces in Co and Ni-base alloys can impact the coarsening behavior of the ordered precipitates.

Electron microscopy, X-ray energy dispersive spectroscopy, and tomographic reconstructions were used to characterize a high entropy alloy predominantly composed of refractory elements. The intragranular microstructure was found to consist of a periodic two phase mixture, where a disordered bcc phase is aligned orthogonally in an ordered B2 phase. The phases were found to exhibit continuous lattice registry and an orientation relationship given by bcc// B2, {001}bcc//{001}B2. X-ray energy dispersive spectroscopy was used to determine the compositions of the phases, and spectral images were used in the tomographic reconstruction of the alloy to reveal the morphology and the elemental partitioning between phases.

The microstructure of a high entropy alloy with composition of Mo0.5Al1Nb1Ta0.5Ti1Zr1 (the digits refer to molar volumes) has been characterised directly in three dimensions using TEM dark field (DF) imaging and by recording tilt pair micrographs using STEM high angle annular DF (HAADF) imaging. The microstructure contains disordered bcc precipitates that appeared as orthogonal stacks of plate-like features. A tapered needle sample was prepared in a focused ion beam/SEM and was used to acquire a 180° tomographic dataset of STEM/HAADF images in 2° increments. The tilt series images were registered, and the algebraic reconstruction technique was used to reconstruct the three-dimensional microstructure. The bcc precipitates were segmented using a combinative approach involving two threshold techniques. The precipitates were then visualised using commercial software, which revealed surprisingly the existence of both cuboidal and plate-like morphologies. Colouring each precipitate according to its morp...

The crystallization of molecular sieve ZSM-12 has been investigated by employing benzyltrimethylammonium chloride (BTMACl), tetraethylammonium bromide (TEABr) and methyltriethylammonium bromide (MTEABr) as templates. The crystallization of pure ZSM-12 framework was accomplished at Si/Al ratio of 90. The pure ZSM-12 samples so obtained were judged with respect to the differences in X-ray diffraction patterns, morphology, crystal size and total acidity to understand the role of template during crystallization. Interestingly, crystallization of ZSM-12 phase with rice, cubic and elongated cubic shaped morphology was favored in the presence of BTMACl, TEABr, and MTEABr, respectively. Finally, the effect of zeolite crystal size and morphology on hydroisomerization reaction was investigated by employing ZSM-12 (Si/Al = 90) based hydroisomerization catalysts. The performance of the prepared catalysts was judged in terms of their activity, selectivity and product distribution for the hydroisomerization reaction using n-hexadecane as a model feed. Based on the obtained results, the possible advantages of using a ZSM-12 sample of specific morphological features and crystal size for hydroisomerization of long chain n-paraffins are discussed. � 2013 Elsevier Inc. All rights reserved.

Ni3Al alloys exhibit remarkable deformation characteristics at small scales that are tied to their unique dislocation core structures. The present work examines microcrystal strengthening and flow for a binary Ni3Al alloy and two alloys containing −0.25% Hf and −1.0% Ta, and evaluates their response relative to known dislocation mechanisms for this material and sample size effects in other materials. The work includes analysis of the flow-stress anomaly mechanisms, dislocation velocity, the single-arm source model, exhaustion strengthening and dislocation multiplication for describing the response of Ni3Al alloy microcrystals. Primary remaining needs are to understand the nature of double cross slip and dislocation sources that trigger the onset of flow and limit size-dependent strengthening.

Samples of Hot Isostatically Pressed (HIPped) powder of TIMETAL 6-4 (Ti-6Al-4V, compositions in wt pct unless indicated), which was HIPped at 1203 K (930 °C), and of forged bar stock, which was slowly cooled from above the beta transus, were both subsequently held at 773 K (500 °C) for times up to 5 weeks and analyzed using scanning and transmission electron microscopy and atom probe analysis. It has been shown that in the samples aged for 5 weeks at 773 K (500 °C), there is a high density of alpha2 (α2, an ordered phase based on the composition Ti3Al) precipitates, which are typically 5 nm in size, and a significantly smaller density was present in the slowly cooled samples. The fatigue and tensile properties of samples aged for 5 weeks at 773 K (500 °C) have been compared with those of the HIPped powder and of the forged samples which were slowly cooled from just above the transus, and although no significant difference was found between the fatigue properties, the tensile strength of the aged samples was 5 pct higher than that of the as-HIPped and slowly cooled forged samples. The ductility of the forged samples did not decrease after aging at 773 K (500 °C) despite the strength increase. Transmission electron microscopy has been used to assess the nature of dislocations generated during tensile and fatigue deformation and it has been found that not just is planar slip observed, but dislocation pairs are not uncommon in samples aged at 773 K (500 °C) and some are seen in slowly cooled Ti6Al4V.

Microcompression testing was conducted on the cast single crystal nickel-base superalloy Rene N5. Microcrystals were selectively fabricated from either dendrite core or interdendritic regions. The compression axis was oriented for single-slip deformation and microcrystal diameters ranged from 2.5 to 80 μm. All microcrystals displayed several hallmarks of size-affected plastic flow, including a size-affected and stochastic flow-stress and initial strain hardening rate, as well as an intermittent flow response. The magnitude of size-affected flow-stress scaling behavior was dependent upon the plastic strain level of the flow-stress measurement, with increasing size-dependence for increasing strain levels. TEM analysis demonstrated the activation of multiple slip-systems, despite the microcrystals being oriented for single-slip deformation. Zig-zag slip was also observed in microcrystals that achieved flow stresses of ∼1300 MPa or higher. For microcrystals fabricated within interdendritic regions the flow-stress values are, on average, lower compared to dendrite core microcrystals. This difference in flow-stress is especially pronounced for microcrystals which are 5 μm in diameter. The microcrystal diameter for which bulk-like properties are estimated to be observed is approximately 350 μm, which is approaching the measured primary dendrite arm spacing for this crystal (430 μm).

Optical constants of cadmium sulfide (CdS) thin films were determined in the spectral range of 400–1200 nm from optical absorption and transmittance measurements for different bath temperatures. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) were the techniques used to determine the crystallite structure and morphology of the films. EDX images showed that a sample had a stoichiometric composition. The crystallite size and microstrain were calculated using the Williamson–Hall method. The optical band gap values of the films varied from 2.35 eV to 2.5 eV, depending on the bath temperature. Optical study was performed to calculate the refractive index (n), extinction coefficient (k), optical conductivity (σ), dielectric constant (real and imaginary), and optical band gap using transmission spectra. It has been observed that the conductivity of the synthesized films has a close relationship with the size of the crystallites. The optical conductivity and the crystallite size increase at a temperature of up to 70°C and then start decreasing when the temperature is still increased due to the change in phase from hexagonal to cubic in nature. Moreover, we observed that beyond this temperature the XRD peak shifts toward the nanoregion. The values of microstructure parameters change drastically, whereas the trend of optical constants remains the same.

The use of different electron loss edges in energy-filtered transmission electron microscopy (EFTEM) has allowed researchers to capture images of the morphology and size of precipitates in nickel-based superalloys. In this work, the authors discuss a computational methodology for automated detection of secondary and tertiary γ′ precipitates in EFTEM images. The optimum parameters for the automated region growing technique were identified using a combination of visual inspection and intensity information from the EFTEM images. The microstructural statistics obtained from the segmented γ′ precipitates agreed with those of the manually segmented precipitates. Then, automated segmented precipitates are used to extract microstructural information about the distributions of equivalent diameters of 656 tertiary precipitates along with the distances to the nearest secondary precipitates. The significance of this technique is its ability to automate segmentation of precipitates in a reproducible manner for...

Synchrotron X-ray diffraction and neutron diffraction techniques are employed to characterise the lattice parameters and volume fraction of γ and γ′ phases in a high strength nickel base superalloy. Samples of Rene88DT were solutionised at 1150°C and cooled under three different rates to provide fine and coarse γ′ size distributions. Samples were aged at 760°C to precipitate tertiary γ′ and coarsen secondary precipitates. Lattice parameter misfit and coefficient of thermal expansion parameters were also determined. Results indicate significant microstructure changes within the samples during the initial 25 h of aging.

The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.

Constant-load creep experiments at a temperature of 750 °C and a nominal stress of 300 MPa were conducted on a fine-grained Ti–45Al–5Nb–0.2B–0.2C (in at.%) alloy with a duplex microstructure. Microstructures before and after creep (accumulated strain: 9.6%) were analyzed using scanning and transmission electron microscopy (SEM and TEM). TEM analysis after creep indicates that the individual microstructural constituents of the fine duplex microstructure, namely, the equiaxed γ and the lamellar α2/γ colonies, undergo varying degrees of deformation and develop various substructures. Lamellar grains deform by dislocation creep. They show clear evidence for dislocation and twin activity. In contrast, only few dislocations are found in the equiaxed grains. We show that the regions with small equiaxed γ grains, representing 65–75 vol.% of the microstructure, deform by grain-boundary sliding.

Single crystal sheets of niobium with initial orientations of (0 0 1) [ 1 1 ¯ 0 ] , (1 1 0) [ 1 1 ¯ 0 ] and (1 1 1) [ 1 1 ¯ 0 ] were rolled at room temperature in the strain range of 25–50%. The deformed specimens were vacuum annealed at temperatures of 800 °C, 1000 °C, and 1200 °C for 3 h. TEM, SEM-OIM and optical microscopy revealed orientation stability in (0 0 1) and (1 1 0) rolled samples with no recrystallization observed after annealing. Samples rolled along (1 1 1) partially recrystallized after annealing at 1000 °C and 1200 °C. A relatively small increase was observed in hardness of (0 0 1) rolled crystals between 25% and 50% strain, implying low work hardening rates. (1 1 1) rolled samples showed higher hardening rates, and enhanced recovery in hardness values after annealing, due to partial recrystallization. Conditions have been identified for the deformation and annealing of niobium single crystals, enabling the preservation of single crystal structure and near-complete recovery of mechanical properties. A simple crystallographic model is proposed, giving an explanation for the observed orientation stability in (0 0 1) and (1 1 0) rolled samples, and the tendency towards instability and recrystallization in (1 1 1) rolled samples.

The chemical partitioning of alloying elements between the disordered γ and ordered γ′ phases in the nickel-base superalloy Rene 88 DT, has been characterized in detail using three-dimensional atom probe tomography (3DAP) coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After a homogenization treatment followed by a water quench, it is observed that the morphology of the γ′ precipitates remains near spherical even after long aging times. Compositional variation, observed between small (

A creep deformation mechanism in a polycrystalline nickel-base superalloy (Rene 88 DT) has been examined using crystallographic analysis and comparison between simulated and experimental high-resolution transmission electron microscopy (HRTEM) images. Confirmation of the magnitude of the Burgers vector of the partial dislocations involved in the creep deformation was critical in conclusively determining its nature. A new approach to determining a complex stacking fault was developed using superlattice image contrast from an experimental HRTEM image.

It has been recently shown that microtwinning is the principal deformation mechanism under certain creep conditions in several polycrystalline nickel-base superalloys. A creep model based on this mechanism is developed in this paper. Model parameters, supplied by microscopy and by atomistic calculations, are used to compare the model against experiment.

The development of a set of computational tools that permit microstructurally based predictions for the tensile properties of commercially important titanium alloys, such as Ti-6Al-4V, is a valuable step toward the accelerated maturation of materials. This paper will discuss the development of neural network models based on a Bayesian framework to predict the yield and ultimate tensile strengths of Ti-6Al-4V at room temperature. The development of such rules-based model requires the population of extensive databases, which in the present case are microstructurally based. The steps involved in database development include producing controlled variations of the microstructure using novel approaches to heat treatments, the use of standardized stereology protocols to characterize and quantify microstructural features rapidly, and mechanical testing of the heat-treated specimens. These databases have been used to train and test neural network models for prediction of tensile properties. In addition, these models have been used to identify the influence of individual microstructural features on the tensile properties, consequently guiding the efforts toward development of more robust mechanistically based models. Based on the neural network model, it is possible to investigate the influence of individual microstructural features on the tensile properties, and in certain cases these dependencies can point toward unrecognized phenomena. For example, the apparently unexpected trend of increase in tensile strength with increasing prior β-grain size has led to the determination of the pronounced role of the basketweave microstructure in strengthening these alloys, especially in case of larger prior β grains.

This paper reports a pioneering investigation of the orientation change of the α phase after recrystallization and the mode of transformation from the α to the β phase on heating above the β transus in the α/β alloy Ti–6Al–4V by in situ texture measurements done at room temperature, 800 °C, 1020 °C and again at room temperature using the HIPPO neutron time-of-flight diffractometer at the Los Alamos Neutron Science Center.

Titanium alloys are widely used in various industrial, domestic, and medical applications such as turbine blades, bicycle frames, knee implants, etc. The two-phase titanium alloy Ti-6Al-4V (wt. percent) is considered to be a workhorse alloy for many applications in these diverse fields. Despite the large body of work on this alloy, the question of the transformation mechanism from the hcp a to the bcc b phase, occurring on heating to temperatures above the a/b transus at ~980°C, is still unresolved. Due to experimental difficulties, it has not yet been clearly determined whether the increase in b volume fraction occurs by fresh nucleation of b crystals within a phase grains or the growth of preexisting b grains. Since the Burgers orientation relationship holds only if the b grains are nucleated within the a grains, the outcome of this question greatly affects texture-modeling efforts for this system. The Burgers orientation relationship predicts that the {0001} crystal direction in a grain of the a phase becomes a {110} crystal direction in a grain of the b phase after the transformation. In this work we present experimental results from in-situ texture measurements performed on the HIPPO neutron diffractometer at LANSCE. Using the combination of time-offlight neutrons and full-pattern Rietveld analysis allowed us to determine the orientation distribution functions of both phases at room temperature, 800°C, 1020°C and again at room temperature. We found strong indications that the b phase indeed grows from grains preexisting at room temperature. Upon re-transformation from b to a we found that the Burgers relationship is followed.

Creep deformation substructures in superalloy Rene 88 DT have been investigated at two applied stress levels after small-strain (0.5%) creep at 650 °C using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and substructures have been observed as a function of applied stress. It has been established that at intermediate temperatures microtwinning caused by the passage of Shockley partial dislocations on successive {1 1 1} planes is the dominant deformation process at low applied stress. At higher applied stress the mechanism changes to planar shearing of the matrix by 1/2〈1 1 0〉 unit dislocations and Orowan looping of the precipitates. Detailed experimental evidences for these operating processes are shown and possible explanation is provided.

Crept microstructures in g-TiAl based alloys reveal a preponderance of 1/2[110]-type jogged-screw dislocations, suggesting that the rate of creep deformation is controlled by the glide of such dislocations. A creep model based on these microstructural observations has been recently developed. This leads to an excellent prediction of creep rates and stress exponents. In this paper, the framework of this model including the verification and validation of the functional dependencies of various microstructural model parameters is reviewed. It has also been observed that creep phenomenology is extremely sensitive to microstructure – fully lamellar g-based alloys exhibit lower creep rates and higher stress exponents even though the deformation microstructure is similar to that in equiaxed alloys. The modifications made to the model that account for the constrained nature of deformation in lamellar alloys are discussed. The applicability of the model is explored in materials systems, including a-Ti and a+b Ti alloys where similar creep exponents and deformation structures have been observed. Finally, the relevance, applicability and shortcomings of the model are critically analyzed.

Transmission Electron microscopy studies on crept samples of an equiaxed Ti–48Al alloy deformed to strains near the minimum strain rate show a microstructure dominated by unit 1/2[110] type dislocations. These dislocations are pinned by jogs of varying heights. The jogged-screw model is adopted, where the rate controlling step is assumed to be the non-conservative dragging of the jogs along the length of the screw dislocations. The presence of tall jogs and the existence of a stress-dependent upper bound to the height of tall jogs which can be dragged have been incorporated into the model. These modifications lead to excellent agreement with experimental data. The evolution of the creep curve is also qualitatively predicted. In contrast, lamellar structures show a highly inhomogeneous deformation behavior with dislocation activity in lamellae above a critical thickness and negligible activity below that limit. This limit is related to the minimum stress required to cause channeling of dislocations. The observation of jogged segments in the thicker lamellae suggests that a modification of the jogged-screw velocity law could be used by incorporating an effective stress approach. Qualitative analysis of a probable method of evaluating the creep rate is discussed. Finally, microstructural changes during the aging process in K5 and K5SC alloys have also been studied. Aging causes the dissolution of metastable α2. The effect of Si and C on the aging behavior and precipitation of the silicide and carbide particles during aging and following creep is discussed. These results suggest that microstructural stability is critically important in order to achieve the highest possible creep strengths.

Stress exponents for creep, in the range of 5, are typically associated with dislocation creep processes, normally associated with a strong tendency for subgrain formation. In this article, we will demonstrate that there are several important alloy systems that have similar stress dependence and, yet, lack this tendency for subgrain formation. Specifically, dislocations in the intermetallic compound γ-TiAl and the hexagonal close-packed (hcp) α phase of the commercial Ti alloy, Ti-6242, tend to be homogeneously distributed with a tendency for alignment along screw orientation. In both alloy systems, the screw dislocations exhibit a large density of pinning points, which detailed transmission electron microscopy (TEM) investigation indicate are locations of tall jogs. These observations suggest that the jogged-screw model for creep should be appropriate after suitable modification for the presence of these tall jogs. This modified jogged-screw (MJS) model is presented, together with a discussion of the assumptions made, and the results of this model are shown to compare favorably with experiment for both alloy systems. The possible criteria for the formation of tall jogs are also described, and the potential application of this modified model to other alloy systems is discussed.

Using state-of-the-art energy dispersive spectroscopy, it has been established for the first time that there exists significant compositional variation (enrichment of Co and Cr and deficiency of Ni and Al) associated with superlattice intrinsic stacking faults created in the ordered γ′ precipitates following intermediate temperature deformation of two commercial superalloys. The results indicate that long range diffusion of these elements is intimately involved in the precipitate shearing process and is therefore closely linked to the time-dependent deformation of the alloys.

The objective of this paper is to revisit the present status of the high temperature creep of alloys with particular reference to models based on the motion of jogged-screw dislocations. Models based on this concept have been previously attempted to describe the creep in pure metals and alloys (Barrett and Nix, Acta Metall., 13 (1965) 1247). Now, in light of emerging new experimental evidence, a modified version of this model appear to be appropriate for the case of creep in both disordered α-Ti alloys and ordered Ti–Al based intermetallic alloys based alloy. TEM investigations have shown that in both cases dislocations of screw character dominate the deformation microstructures in these alloys and that jogs much taller than atomic dimensions form along the dislocations. The predictions of the modified jogged-screw model are compared with creep data for Ti-6242 and γ-TiAl alloys.

Minimum creep rates as a function of stress (100–300 MPa) and T=815 °C have been obtained for a Ti–48Al–2Cr–2Nb alloy with a fully lamellar microstructure. Transmission electron microscopy (TEM) investigation reveals that the deformation structures are dominated by jogged screw 1/2[110] dislocations in γ-laths. It is suggested that the creep in this alloy is controlled by the non-conservative motion of 1/2[110] unit dislocations, which are normally seen in varying proportions depending on the width of the γ-laths. The creep results are then compared with a binary Ti–48Al alloy. The difference in creep rates and stress exponents between these alloys (although differing in composition) have been explained in terms of the active volume fraction of γ-laths that are participating in deformation for a given applied stress. In this context, the effect of lamellar spacing on the creep rates are also been discussed.

The interfaces between the phase separated regions in the dendritic grains of laser-deposited samples of the high entropy alloy CoCrCuFeNiAl have been studied using aberration-corrected analytical (scanning) transmission electron microscopy ((S)TEM). The compositional variations have been determined using energy dispersive x-ray spectroscopy (EDS) in (S)TEM. It was found that between B2, consisting mainly of Al, Ni, Co, and Fe, and disordered bcc phase, consisting mainly of Cr and Fe, there is a transition region, approximately 1.5 nm in width, over which the chemical composition changes from the B2 to that of the bcc phase. The crystal structure of this interfacial region is also B2, but with very different sublattice occupancy than that of the adjacent B2 compound. The structural aspects of the interface between the ordered B2 phase and the disordered bcc phase have been characterized using high angle annular dark-field (HAADF) imaging in STEM. It has been determined that the interfaces are essentially coherent, with the lattice parameters of the two B2 regions and the disordered bcc phase being more or less the same, the uncertainty arising from possible relaxations from the proximity of the surfaces of the thin foils used in imaging of the microstructures. Direct observations show that there is a planar continuity between all three constituent phases.

Ni3Al alloys exhibit unique deformation characteristics at small scales that are tied to their unique dislocation core structures. The present work examines microcrystal strengthening and flow for a binary Ni3Al alloy and two alloys containing -0.25 % Hf and -1.0 % Ta, and evaluates their response relative to known dislocation mechanisms for this material and sample size effects in other materials. The work includes analysis of the flow-stress anomaly mechanisms, dislocation velocity, the single-arm source model, exhaustion strengthening and dislocation multiplication for describing the response of Ni3Al alloy microcrystals. While aspects of the flow-stress anomaly mechanisms and a form of the single-arm source model work remarkably well to describe the response, other limitations exist for understanding Ni3Al behavior from current data and models. A primary need is to understand the nature of double cross slip and dislocation sources that trigger the onset of flow and limit size-dependent strengthening.

A study of helium bubble growth by positron lifetime measurements made on helium-implanted palladium (100at.p.p.m. He) is reported. The temperature of formation of stable bubbles which retain helium in post-implantation annealing is identified to be 750±20K. The average bubble radius, bubble concentration and helium pressure in the bubble are obtained from an analysis of experimental data. The variation in the deduced bubble parameters in the annealing temperature range 973–1473 K is compared with that calculated on the basis of surface diffusion controlled migration and coalescence model, allowing for pressure effects as suggested by Mikhlin in 1979. The comparison is found to be satisfactory. The observed results are explained as due to suppression of bubble mobility at high internal pressures and subsequent enhancement of the mobility on pressure relaxation.

Two distinct ferromagnetic phases of LaMn$_{0.5}$Co$_{0.5}$O$_{3}$ having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature $\textit{T}_{c}$ is found to be different for both the phases. The origin of such contrasting characteristics is assigned to the changes in the distance(s) and angle(s) between Mn - O - Co resulting from distortions observed from neutron diffraction studies. Investigations on the temperature dependent Raman spectroscopy provide evidence for such structural characteristics, which affects the exchange interaction. The difference in B-site ordering which is evident from the neutron diffraction is also responsible for the difference in $\textit{T}_{c}$. Raman scattering suggests the presence of spin-phonon coupling for both the phases around the $\textit{T}_{c}$. Electrical transport properties of both the phases have been investigated based on the lattice distortion., Comment: 9 figures

Angular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions of GaAs are measured using the long-slit geometry. The shapes of the profiles indicate in addition to the ideal sp3 orbitals contribution, the contribution of σ bonding along [111] bonding direction and an admixture of σ and π* bondings for the [100] and [110] directions. The ACPAR anisotropies in this case are somewhat less compared to that of Ge which is caused because of the presence of the ionicity in compound semiconductors. Although the zero points of the autocorrelation functions for ACPAR data are shifted from the actual lattice positions, their periodicity remains same as the actual lattice spacing. Es wird die Winkelkorrelation der Positronenannihilationsstrahlung (ACPAR) in verschiedenen kristallographischen Richtungen von GaAs in Langschlitzgeometrie gemessen. Die Formen der Profile zeigen zusatzlich zum sp3-Orbitalbeitrag den Beitrag einer σ-Bindung in der [111]-Bindungsrichtung und eine Mischung von σ- und π*-Bindungen fur die [100]- und [110]-Richtungen. Die ACPAR-Anisotropien sind in diesem Fall etwas geringer, verglichen mit dem von Ge, was durch die vorhandene Ionizitat in Verbindungshalbleitern verursacht wird. Obwohl die Kreuzungsnullpunkte der Autokorrelationsfunktionen fur die ACPAR-Daten von den tatsachlichen Gitterpositionen verschoben sind, bleibt ihre Periodizitat dieselbe wie beim tatsachlichen Gitterabstand.

Positron lifetime measurements in He-implanted Ni and Ni-Ti alloys containing dilute concentrations of Ti, during isochronal annealing, are reported. In the initial annealing stage of Ni-Ti alloys, only a single lifetime ranging from 160 to 180ps is observed, in contrast with the two lifetimes seen in pure Ni. This indicates saturation positron trapping at helium-bound Ti-vacancy complexes, formed in high concentrations. Lattice statics calculations of the He binding energy at various defect complexes in Ni-containing Ti give credence to the above interpretation. Above 800 K, two lifetimes are resolved in Ni-Ti alloys, where the longer lifetime τ2 increases with a sharp reduction in its intensity. This is indicative of He bubble growth. The bubble radius r B and bubble concentration C B are obtained from an analysis of positron lifetime parameters. These results indicate that, for a given annealing temperature, r B is smaller by a factor of two and C B higher by nearly an order of magnitude in Ni...

Results of a Mossbauer study of Al 13 Fe 4 alloy, obtained by rapid solidification of the melt and subsequent annealing for crystallization are presented. The Mossbauer spectrum has been analysed on the basis of the structure of Al 13 Fe 4 , as determined from an earlier reported X-ray diffraction study. Accordingly, the spectrum has been fitted to two doublets and a Lorentzian representing grossly two types of Fe sites with different local atomic environments in Al 13 Fe 4 and Fe solid solution in Al respectively. The area functions of the two doublets are in agreement with the relative abundances of the two types of sites in Al 13 Fe 4 . These results should be helpful in understanding local atomic environments in a related quasicrystalline phase of Al-Fe alloy.

Positron lifetime and Doppler broadening measurements have been performed in homogeneously helium implanted (715 appm) molybdenum. A new stage beyond the temperature range of stability of voids has been observed. The shorter lifetime componet τ1 = 158 ± 2 ps of 60% intensity is assigned to the positron state in helium-decorated dislocation loops while the longer lifetime component τ2 = 408 ± 5 ps of 40% intensity is explained in terms of positron trapping at voids with a small percentage of helium associated with them. Dissociation of helium from loops is observed in the range 300–800 K followed by loop annealing above 900 K. A sharp reduction of τ2 around 1300 K is assigned to multiple helium occupancy of voids, resulting in their transformation into stable bubbles. Helium retention in bubbles is found stable up to the highest annealing temperature of 1700 K. The bubble parameters deduced from the positron lifetime results reveal that the pressure in the bubbles is maintained at the near-equilib...

This paper reviews the presently proposed mechanisms for creep of {\gamma^{\prime}} strengthened Ni-base superalloys that are typically used for disk applications. Distinct creep strength controlling modes, such as dislocation-coupled antiphase-boundary shearing, shearing configurations involving superlattice stacking faults, Orowan looping, climb by-pass, and microtwinning have been observed. These are strongly influenced by the scale of the {\gamma^{\prime}} precipitating phase and are operative within specific ranges of temperature and stress. Insight from more recent experimental findings concerning microtwinning and extending stacking fault mechanisms suggest important similarities between these deformation modes. It is suggested that local atomic reordering in the wake of Shockley partials is responsible for the temperature dependence exhibited in this regime.

Deformation mechanisms, operative during intermediate temperature creep of Ni-based polycrystalline superalloys, are poorly understood. The creep deformation substructure has been characterized in Rene 88DT following rapid cooling from the super-solvus temperature, yielding a fine $\gamma^{\prime}$-precipitate microstructure. After creep to modest strain levels (up to 0.5% strain) at 650°C and an applied tensile stress of 838 MPa, microtwinning is found to be the predominant deformation mode. This surprising result has been confirmed using diffraction contrast and high-resolution transmission electron microscopy. Microtwinning occurs via the sequential movement of identical 1/6[11-2] Shockley partials on successive (111) planes. This mechanism necessitates reordering within the $\gamma^{\prime}$ precipitates in the wake of the twinning partials, so that the $L1_2$ structure can be restored. A quantitative model for creep rate has been derived on the basis that the reordering process is rate-limiting. The model is in reasonable agreement with experimental data. The results are also discussed in relation to previous studies under similar deformation conditions.

The fine structure of dislocations in intermetallic compounds can have a profound influence on their macroscopic mechanical properties. The development of appropriate models of deformation requires consideration of dislocation core structure, possible dissociation or decomposition reactions, overall dislocation morphology and relevant dislocation interactions based on detailed transmission electron microscopy study. This empirical information may be rationalized based on both atomistic and continuum-level dislocation modeling. The specific cases of jogged 1/2 superdislocations in NiAl are discussed.