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Damage during cycling loading of polycrystalline metallic alloys involves localized plastic straining at the scale of individual grains. To better understand damage accumulation ...processes and to build models for material behavior there is a need for quantitative assessment of the heterogeneous strain fields at the grain and even more microscopic scales. In the present study, a digital image correlation (DIC) approach has been developed to measure the strains at the grain level and at finer scales where plastic strain localization is manifested as physical slip bands. Strain fields have been measured in situ and ex situ on a René 88DT polycrystalline nickel-based superalloy to assess the grain-scale deformation processes during monotonic straining in tension and compression. DIC analysis and transmission electron microscopy demonstrate that slip occurs in a highly localized manner. The highest localized strains developed in slip bands that formed on {111} planes parallel to, and slightly offset from, annealing twins. Enhanced local straining below yield was observed during compression loading. The degree of strain concentration caused by slip bands impinging on grain boundaries was also analyzed. The results are compared to predictions of plasticity models.
ABSTRACT
Blazars research is one of the hot topics of contemporary extragalactic astrophysics. That is because these sources are the most abundant type of extragalactic γ-ray sources and are ...suspected to play a central role in multimessenger astrophysics. We have used Swift$\_$xrtproc, a tool to carry out an accurate spectral and photometric analysis of the Swift-XRT data of all blazars observed by Swift at least 50 times between December 2004 and the end of 2020. We present a database of X-ray spectra, best-fit parameter values, count rates and flux estimations in several energy bands of over 31 000 X-ray observations and single snapshots of 65 blazars. The results of the X-ray analysis have been combined with other multifrequency archival data to assemble the broad-band Spectral Energy Distributions (SEDs) and the long-term light curves of all sources in the sample. Our study shows that large X-ray luminosity variability on different time-scales is present in all objects. Spectral changes are also frequently observed with a ‘harder-when-brighter’ or ‘softer-when-brighter’ behaviour depending on the SED type of the blazars. The peak energy of the synchrotron component (νpeak) in the SED of HBL blazars, estimated from the log-parabolic shape of their X-ray spectra, also exhibits very large changes in the same source, spanning a range of over two orders of magnitude in Mrk421 and Mrk501, the objects with the best data sets in our sample.
Tuberculosis (TB) remains a public health threat in low TB incidence countries, through a combination of reactivated disease and onward transmission. Using surveillance data from the United Kingdom ...(UK) and the Netherlands (NL), we demonstrate a simple and predictable relationship between the probability of observing a cluster and its size (the number of cases with a single genotype). We demonstrate that the full range of observed cluster sizes can be described using a modified branching process model with the individual reproduction number following a Poisson lognormal distribution. We estimate that, on average, between 2010 and 2015, a TB case generated 0.41 (95% CrI 0.30,0.60) secondary cases in the UK, and 0.24 (0.14,0.48) secondary cases in the NL. A majority of cases did not generate any secondary cases. Recent transmission accounted for 39% (26%,60%) of UK cases and 23%(13%,37%) of NL cases. We predict that reducing UK transmission rates to those observed in the NL would result in 538(266,818) fewer cases annually in the UK. In conclusion, while TB in low incidence countries is strongly associated with reactivated infections, we demonstrate that recent transmission remains sufficient to warrant policies aimed at limiting local TB spread.
Damage initiation during cycling loading of polycrystalline metallic alloys involves localized damage at the scale of individual grains. To better understand damage processes and to build models for ...material behavior, there is a need for quantitative assessment of the microstructural configurations that favor fatigue crack initiation. In materials that form annealing twins during processing, these special interfaces are often locations of particular interest for their role in strain and damage accumulation. In the present study, fatigue experiments in the very high and low cycle fatigue regime on a René 88DT polycrystalline nickel-base superalloy were performed to statistically evaluate grain-scale features that favor crack initiation. Combined elastic and plastic criteria at the grain scale have been developed. A crack distribution function is defined to compare and assess the effect of the microstructural parameters for the two fatigue regimes.
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Metallic materials experience irreversible deformation with increasing applied stress, manifested in localized slip events that result in fatigue failure upon repeated cycling. We discerned the ...physical origins of fatigue strength in a large set of face-centered cubic, hexagonal close-packed, and body-centered cubic metallic materials by considering cyclic deformation processes at nanometer resolution over large volumes of individual materials at the earliest stages of cycling. We identified quantitative relations between the yield strength and the ultimate tensile strength, fatigue strength, and physical characteristics of early slip localization events. The fatigue strength of metallic alloys that deform by slip could be predicted by the amplitude of slip localization during the first cycle of loading. Our observations provide a physical basis for well-known empirical fatigue laws and enable a rapid method of predicting fatigue strength as reflected by measurement of slip localization amplitude.
Slipping into fatigue
Materials that are cyclically deformed become easier to break due to fatigue. However, tying fatigue strength to microstructure has been challenging. Stinville
et al
. used nanometer-resolution digital image correlation to observe the slip localization on the surface of a wide range of alloys (see the Perspective by Omar and El-Awady). They found that after one deformation cycle, the amplitude of the early slip localization events determines fatigue strength. This observation helps to provide a physical basis for well-known fatigue laws and paves the way to easily predicting fatigue strength. —BG
Observations of slip amplitude on the surface of a metal after deformation predict fatigue strength.
The structure and formation mechanism of extended planar defects in the γ/γ′ microstructure of creep deformed CoNi-base single crystal superalloys have been studied by conventional and advanced ...transmission electron microscopy (TEM). Planar defects in numerous isolated as well as contiguous γ′ precipitates on {111} planes reveal a characteristic configuration whereby superlattice intrinsic stacking faults (SISF) are fully embedded within antiphase boundaries (APB). Detailed analysis revealed that a leading 1/31¯1¯2 superpartial dislocation first creates an SISF. The SISF is then transformed into an APB by a trailing 1/61¯1¯2 partial dislocation. The partial is left inside the precipitate and remains as a dislocation loop. Thus, the entire shearing process constitutes a crystallographic slip of type 1/21¯1¯2. A force balance analysis indicates that the initial APB energy exceeds the SISF energy. However, energy-dispersive X-ray spectroscopy (EDXS) indicates pronounced local reordering and diffusion processes near both types of planar defects. The APB qualitatively adopts the composition of the γ phase whereas the SISF locally changes its composition towards that of the Co3W–D019 phase. We propose that these atomic diffusion processes determine the formation and shrinkage of the loops. A post mortem in situ TEM heating experiment shows that with increasing temperature the APBs exhibit complete faceting into {100} planes followed by coarsening, eventually leading to disintegration of the γ′ precipitate. This indicates a detrimental impact of APBs as potential nuclei for fragmentation of the γ/γ′ microstructure in CoNi-base superalloys.
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Abstract
The long-period, highly eccentric Wolf-Rayet star binary system WR 140 has exceptionally well-determined orbital and stellar parameters. Bright, variable X-ray emission is generated in ...shocks produced by the collision of the winds of the WC7pd+O5.5fc component stars. We discuss the variations in the context of the colliding-wind model using broadband spectrometry from the RXTE, Swift, and NICER observatories obtained over 20 yr and nearly 1000 observations through three consecutive 7.94 yr orbits, including three periastron passages. The X-ray luminosity varies as expected with the inverse of the stellar separation over most of the orbit; departures near periastron are produced when cooling shifts to excess optical emission in C
iii
λ
5696 in particular. We use X-ray absorption to estimate mass-loss rates for both stars and to constrain the system morphology. The absorption maximum coincides closely with the inferior conjunction of the WC star and provides evidence of the ion-reflection mechanism that underlies the formation of collisionless shocks governed by magnetic fields probably generated by the Weibel instability. Comparisons with
K
-band emission and He
i
λ
10830 absorption show that both are correlated after periastron with the asymmetric X-ray absorption. Dust appears within a few days of periastron, suggesting formation within shocked gas near the stagnation point. The X-ray flares seen in
η
Car have not occurred in WR 140, suggesting the absence of large-scale wind inhomogeneities. Relatively constant soft emission revealed during the X-ray minimum is probably not from recombining plasma entrained in outflowing shocked gas.
Precipitate shapes in nickel-base superalloys vary substantially with alloy composition, partitioning of elements to the disordered gamma matrix and the ordered gamma prime precipitates and the ...degree of coarsening during elevated temperature exposures. These shapes, which vary from spherical to cuboidal to rod-like, have typically not been quantified in Ni-base alloys containing relatively high γ′ volume fractions, in spite of their importance to mechanical properties. Precipitate shapes in a series of new platinum group metal (PGM)-containing Ni-base alloys have been quantified by their two-dimensional moment invariants. Precipitate morphologies were characterized in a total of 17 PGM-containing alloys in the solution treated and aged condition. The average γ′ volume fraction was measured as 0.60, typical of highly creep resistant Ni-base alloys. PGM additions resulted in an unusually large range of precipitate shapes. Precipitate morphologies were quantitatively analyzed using a shape parameter derived from the absolute moment invariant. For the compositions examined, the shape parameter reaches a maximum value at a precipitate–matrix misfit magnitude of 0.4%. A large set of commercial single crystals exhibits the same range in shape parameter values around this misfit magnitude, suggesting that favorable high temperature properties are correlated with an optimum precipitate shape.
The influence of high thermal gradient processing on the creep and low cycle fatigue properties of the AM1 Ni-based single crystal superalloy has been studied. Isothermal creep (from 750°C up to ...1200°C) and low cycle fatigue (750°C and 950°C) experiments were performed for AM1 alloy solidified with a conventional radiation cooled (Bridgman) and higher thermal gradient liquid-metal cooled (LMC) casting process to produce coarse and finer-scaled dendritic structures, respectively. There was no significant effect of the casting technique on creep properties, due to the very similar microstructures (γ′-size and γ-channel width) established after full heat treatment of both Bridgman and LMC samples. For low cycle fatigue properties, the benefit of the higher gradient LMC process was dependent on the testing temperature. At 750°C, cracks primarily initiated at pores created by solidification shrinkage in both Bridgman and LMC samples. Samples produced by the LMC technique demonstrated fatigue lives up to 4 times longer, compared to the Bridgman samples, due to refined porosity. At 950°C the low cycle fatigue properties of the LMC and conventionally solidified material were not distinguishable due to a shift of crack initiation sites from internal pores to oxidized surface layers or near-surface pores. The benefit of the LMC approach was, however, apparent in fatigue at 950°C when testing in a vacuum environment. Based on these results, a crack initiation model based on the local slip activity close to casting defect is proposed.
Abstract
Identifying trends between observational data and the range of physical parameters of massive stars is a critical step to the still-elusive full understanding of the source, structure, and ...evolution of X-ray emission from the stellar winds, requiring a substantial sample size and systematic analysis methods. As of 2022, the Chandra data archive contains 37 high-resolution spectra of O, B, and WR stars, observed with the Chandra/HETGS, and of sufficient quality to fit the continua and emission-line profiles. Using a systematic approach to the data analysis, we explore morphological trends in the line profiles (i.e., O, Ne, Mg, and Si) and find that the centroid offsets of resolved lines versus wavelength can be separated in three empirically defined groups based on the amount of line broadening and centroid offset. Using Fe
xvii
(15.01, 17.05 Å) and Ne
x
α
(12.13 Å) lines, which are prevalent among the sample stars, we find a well-correlated linear trend of increasing Full Width Half Maximum with faster wind terminal velocity. The H-like/He-like total line flux ratio for strong lines displays different trends with spectral class depending on ion species. Some of the sources in our sample have peculiar properties (e.g., magnetic and
γ
Cas-analog stars) and we find that these sources stand out as outliers from more regular trends. Finally, our spectral analysis is presented summarily in terms of X-ray spectral energy distributions in specific luminosity for each source, including tables of line identifications and fluxes.
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