An in-situ approach for preparing atom probe tomography specimens by xenon plasma-focused ion beam

A method for the rapid preparation of atom probe tomography (APT) needles using a xenon plasma-focused ion beam (FIB) instrument is presented and demonstrated on a test sample of Ti-6Al-4V alloy. The method requires significantly less operator input than the standard lift-out protocol, is site-specific and produces needles with minimal ion-beam damage; electron microscopy indicated the needle's surface amorphised/oxidised region to be less than 2 nm thick. The resulting needles were routinely analysable by APT, confirming the expected microstructure and showing negligible Xe contamination

On the effect of Ti on Oxidation Behaviour of a Polycrystalline Nickel-based Superalloy

Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion

Nanomagnetic properties of the meteorite cloudy zone.

Meteorites contain a record of their thermal and magnetic history, written in the intergrowths of iron-rich and nickel-rich phases that formed during slow cooling. Of intense interest from a magnetic perspective is the "cloudy zone," a nanoscale intergrowth containing tetrataenite-a naturally occurring hard ferromagnetic mineral that has potential applications as a sustainable alternative to rare-earth permanent magnets. Here we use a combination of high-resolution electron diffraction, electron tomography, atom probe tomography (APT), and micromagnetic simulations to reveal the 3D architecture of the cloudy zone with subnanometer spatial resolution and model the mechanism of remanence acquisition during slow cooling on the meteorite parent body. Isolated islands of tetrataenite are embedded in a matrix of an ordered superstructure. The islands are arranged in clusters of three crystallographic variants, which control how magnetic information is encoded into the nanostructure. The cloudy zone acquires paleomagnetic remanence via a sequence of magnetic domain state transformations (vortex to two domain to single domain), driven by Fe-Ni ordering at 320 °C. Rather than remanence being recorded at different times at different positions throughout the cloudy zone, each subregion of the cloudy zone records a coherent snapshot of the magnetic field that was present at 320 °C. Only the coarse and intermediate regions of the cloudy zone are found to be suitable for paleomagnetic applications. The fine regions, on the other hand, have properties similar to those of rare-earth permanent magnets, providing potential routes to synthetic tetrataenite-based magnetic materials.European Research Counci

Increased Expression of PLS3 Correlates with Better Outcome in Sézary Syndrome.

Dermatology-oncolog

A SANS and APT study of precipitate evolution and strengthening in a maraging steel

In this work a combination of the characterisation techniques small angle neutron scattering (SANS) and atom probe tomography (APT) are used to study the precipitation in a maraging steel. Three similar maraging steel alloys were aged at different temperatures and ageing times, and then characterised using SANS, APT and microhardness. The alloys consist of two types of precipitates, namely Laves phase and β-NiAl, the precipitates have different composition and hence precipitate ageing, which makes it complicated to model. The SANS experimental set-up was relatively simple and allowed the precipitate size and fraction of a large number of samples to be measured in a single experiment. The APT results were used for constraining the SANS modelling, particularly the composition, shape and distribution of phases. The characterisation led to the following description of precipitation: NiAl phase reaches coarsening at early stages of ageing and shifts its strength mechanisms from shearing to Orowan looping, which cause the characteristic peak strength; the Laves phase is in growth throughout and its strength contribution increases with ageing time. These observations were shown to be consistent with precipitate evolution and strengthening models, and the work of others. Although, there are some issues with the combination of SANS and APT approach, which are discussed, the methodology provides a valuable tool to understand complex precipitation behaviours

Characterization of Ordering in A-Site Deficient Perovskite Ca1-xLa2x/3TiO3 Using STEM/EELS

The vacancy ordering behavior of an A-site deficient perovskite system, Ca1-xLa2x/3TiO3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems

Effect of alloying on the microstructure, phase stability, hardness and partitioning behavior of a new dual-superlattice nickel-based superalloy

A novel y-y'-y" dual-superlattice superalloy, with promising mechanical properties up to elevated temperatures was recently reported. The present work employs state of the art chemical and spatial characterization techniques to study the effect systematic additions of Mo, W and Fe and variations in Nb and Al contents have on the phase fraction, thermal stability, elemental partitioning and mechanical properties. Alloys were produced through arc melting followed by heat treatment. Multi-scale characterization techniques and hardness testing were employed to characterize their microstructure, thermal stability and mechanical properties. Alterations in such properties or in elemental partitioning behaviour were then explained through thermodynamic modelling. A modest addition of 1.8 at.% Mo had a strong effect on the microstructure and thermal stability: it minimized microstructural coarsening during heat treatments while not significantly decreasing the y' solvus temperature. A reduction of Nb by 0.6 at.%, strongly reduced the y" volume fraction, without affecting the y' volume fraction. The reduced precipitate fraction led to a significant reduction in alloy hardness. Fe, added to achieve better processability and reduced material cost, decreased the y' solvus temperature and caused rapid microstructural coarsening during heat treatments, without affecting alloy hardness. A reduction of Al by 0.4 at.%, reduced the y' volume fraction and the y' solvus temperature, also without affecting alloy hardness. The addition of 0.9 at.% W decreased the y' solvus temperature but increased both precipitate volume fractions. These data will be invaluable to optimize current alloy design and to inform future alloy design efforts