40 research outputs found
Measurement of competing pathways in a shock-induced phase transition in zirconium by femtosecond diffraction
The traditional picture of solid-solid phase transformations assumes an
ordered parent phase transforms into an ordered daughter phase via a single
unique pathway. Zirconium and its prototypical phase transition from hexagonal
close-packed (hcp) to simple hexagonal (hex-3) structure has generated
considerable controversy over several decades regarding which mechanism
mediates the transformation. However, a lack of in situ measurements over the
relevant atomistic timescales has hindered our ability to identify the true
pathway. In this study, we exploit femtosecond X-ray diffraction coupled with
nanosecond laser compression to give unprecedented insights into the
complexities of how materials transform at the lattice level. We observe
single-crystal zirconium changing from hcp to a hex-3 structure via not one but
three competing pathways simultaneously. Concurrently, we also observe a broad
diffuse background underlying the sharp Bragg diffraction during the
transition. We corroborate our observation of the diffuse signal with
multimillion-atom molecular dynamics simulations using a machine-learned
interatomic potential. Our study demonstrates that the traditional mechanistic
view of transitions may fail for even an elemental metal and that the
mechanisms by which materials transform are far more intricate than generally
thought
Adaptive Value of Phenological Traits in Stressful Environments: Predictions Based on Seed Production and Laboratory Natural Selection
Phenological traits often show variation within and among natural populations of annual plants. Nevertheless, the adaptive value of post-anthesis traits is seldom tested. In this study, we estimated the adaptive values of pre- and post-anthesis traits in two stressful environments (water stress and interspecific competition), using the selfing annual species Arabidopsis thaliana. By estimating seed production and by performing laboratory natural selection (LNS), we assessed the strength and nature (directional, disruptive and stabilizing) of selection acting on phenological traits in A. thaliana under the two tested stress conditions, each with four intensities. Both the type of stress and its intensity affected the strength and nature of selection, as did genetic constraints among phenological traits. Under water stress, both experimental approaches demonstrated directional selection for a shorter life cycle, although bolting time imposes a genetic constraint on the length of the interval between bolting and anthesis. Under interspecific competition, results from the two experimental approaches showed discrepancies. Estimation of seed production predicted directional selection toward early pre-anthesis traits and long post-anthesis periods. In contrast, the LNS approach suggested neutrality for all phenological traits. This study opens questions on adaptation in complex natural environment where many selective pressures act simultaneously
The James Webb Space Telescope Mission
Twenty-six years ago a small committee report, building on earlier studies,
expounded a compelling and poetic vision for the future of astronomy, calling
for an infrared-optimized space telescope with an aperture of at least .
With the support of their governments in the US, Europe, and Canada, 20,000
people realized that vision as the James Webb Space Telescope. A
generation of astronomers will celebrate their accomplishments for the life of
the mission, potentially as long as 20 years, and beyond. This report and the
scientific discoveries that follow are extended thank-you notes to the 20,000
team members. The telescope is working perfectly, with much better image
quality than expected. In this and accompanying papers, we give a brief
history, describe the observatory, outline its objectives and current observing
program, and discuss the inventions and people who made it possible. We cite
detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space
Telescope Overview, 29 pages, 4 figure
Large expert-curated database for benchmarking document similarity detection in biomedical literature search
Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe
A direct method for the determination of the mean orientation-dependent elastic
A salient manifestation of anisotropy in the mechanical response of polycrystal-
line materials is the inhomogeneous partitioning of elastic strains over the
aggregate. For bulk samples, the distributions of these intergranular strains are
expected to have a strong functional dependence on grain orientations. It is then
useful to formulate a mean lattice strain distribution function (LSDF) over the
orientation space, which serves to characterize the micromechanical state of the
aggregate. Orientation-dependent intergranular stresses may be recovered from
the LSDF via a constitutive assumption, such as anisotropic linear elasticity.
While the LSDF may be determined directly from simulation data, its
experimental determination relies on solving an inverse problem that is similar
in character to the fundamental problem of texture analysis. In this paper, a
versatile and robust direct method for determining an LSDF from strain pole
figures is presented. The effectiveness of this method is demonstrated using
synthetic strain pole figures from a model LSDF obtained from the simulated
uniaxial deformation of a 1000-crystal aggregate
The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium
Dwell fatigue, the reduction in fatigue life experienced by titanium alloys
due to holds at stresses as low as 60% of yield, has been implicated in several
uncontained jet engine failures. Dislocation slip has long been observed to be
an intermittent, scale-bridging phenomenon, similar to that seen in earthquakes
but at the nanoscale, leading to the speculation that large stress bursts might
promote the initial opening of a crack. Here we observe such stress bursts at
the scale of individual grains in situ, using high energy X-ray diffraction
microscopy in Ti-7Al-O alloys. This shows that the detrimental effect of
precipitation of ordered Ti_3Al is to increase the magnitude of rare pri and
bas slip bursts associated with slip localisation. In contrast, the addition
of trace O interstitials is beneficial, reducing the magnitude of bas slip
bursts and increasing the homogeneity between basal and prismatic slip.
This is further evidence that the formation of long paths for easy basal plane
slip localisation should be avoided when engineering titanium alloys against
dwell fatigue.Comment: Revised after revie