The three-dimensional distribution of individual hydrogen atoms within a complex steel microstructure is characterized using isotopic doping and cryogenic-transfer atom probe tomography. AbstractThe design of atomic-scale microstructural traps to limit the diffusion of hydrogen is one key strategy in the development of hydrogen-

Abstract:Ultrathin passive films effectively prevent the chemical attack of stainless steel grades in corrosive environments; their stability critically depends on the interplay between structure and chemistry of the constituents Fe-Cr-Mo. In particular, nanoscale inhomogeneities along the surface can have a tremendous impact on material failure, but are yet barely understood. Addressing a stainless-type glass-forming Fe 50 Cr 15 Mo 14 C 15 B 6 alloy and utilizing a combination of complementary high-resolution analytical techniques, we relate near-atomistic insight into different gradual nanostructures with time-and element-resolved dissolution behavior. The progressive elemental segregation on the nanoscale is followed in its influence on the concomitant degree of passivity. A detrimental transition from Cr-controlled passivity to Mo-controlled breakdown is dissected atom-by-atom demonstrating the importance of nanoscale knowledge for understanding corrosion.

We demonstrate experimentally that a part-per-million addition of Sn solutes in Al-Mg-Si alloys can inhibit natural aging and enhance artificial aging. The mechanism controlling the aging is argued to be vacancy diffusion, with solutes trapping vacancies at low temperature and releasing them at elevated temperature, which is supported by a thermodynamic model and first-principles computations of Sn-vacancy binding. This "diffusion on demand" solves the long-standing problem of detrimental natural aging in Al-Mg-Si alloys, which is of great scientific and industrial importance. Moreover, the mechanism of controlled buffering and release of excess vacancies is generally applicable to modulate diffusion in other metallic systems.

SummaryStandard atom probe tomography spatial reconstruction techniques have been reasonably successful in reproducing single crystal datasets. However, artefacts persist in the reconstructions that can be attributed to the incorrect assumption of a spherical evaporation surface. Using simulated and experimental field evaporation, we examine the expected shape of the evaporating surface and propose the use of a variable point projection position to mitigate to some degree these reconstruction artefacts. We show initial results from an implementation of a variable projection position, illustrating the effect on simulated and experimental data, while still maintaining a spherical projection surface. Specimen shapes during evaporation of model structures with interfaces between regions of low-and high-evaporation-field material are presented. Use of two-and three-dimensional projection-point maps in the reconstruction of more complicated datasets is discussed.

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First-principles calculations based on a plane-wave pseudopotential method, as implemented in the VASP code, are presented for the formation energies of several transition-metal and non-transition-metal dopants in Ti-Al alloys. Substitution for either Ti or Al in ␥-TiAl, ␣ 2 -Ti 3 Al, Ti 2 AlC, and Ti 3 AlC are considered. Calculated ͑zero-temperature͒ defect formation energies exhibit clear trends as a function of the periodic-table column of transition metal solutes. Early transition metals in TiAl prefer the Ti sublattice, but this preference gradually shifts to the Al sublattice for late transition metals; the Ti sublattice is preferred by all transition metal solutes in Ti 3 Al. Partitioning of solutes to Ti 3 Al is predicted for mid-period transition elements, and to TiAl for early and late transition elements. A simple Ising model treatment demonstrates the plausibility of these trends, which are in excellent overall agreement with experiment. The influence of temperature on formation energies is examined with a cluster expansion for the binary TiAl alloys and a low temperature expansion for dilute ternary alloys. Results for Nb-doped alloys provide insight into the relative sensitivity of solute partitioning to individual contributions to the free energy. Whereas the calculated formation energy of Nb ͑substitution͒ at zero temperature favors partitioning to ␣ 2 -Ti 3 Al, temperature-dependent contributions to the formation free energy, evaluated at 1075 K, favor partitioning to ␥-TiAl, in agreement with experiment.

Environmental control during transfer between instruments is required for samples sensitive to air or thermal exposure to prevent morphological or chemical changes prior to analysis. Atom probe tomography is a rapidly expanding technique for three-dimensional structural and chemical analysis, but commercial instruments remain limited to loading specimens under ambient conditions. In this study, we describe a multifunctional environmental transfer hub allowing controlled cryogenic or room-temperature transfer of specimens under atmospheric or vacuum pressure conditions between an atom probe and other instruments or reaction chambers. The utility of the environmental transfer hub is demonstrated through the acquisition of previously unavailable mass spectral analysis of an intact organic molecule made possible via controlled cryogenic transfer into the atom probe using the hub. The ability to prepare and transfer specimens in precise environments promises a means to access new science across many disciplines from untainted samples and allow downstream time-resolved in situ atom probe studies.

Passivation of grain boundaries (GBs) and interfaces to suppress recombination and to improve minority carrier lifetime (MCLT) is essential for the functionality of devices based on polycrystalline materials. Improvement of MCLT is believed to be a very promising way to bring CdTe solar cells to the next efficiency level. However, which parameters significantly affect MCLT is not well understood. Here, high‐efficiency CdTe solar cells in an unconventional inverted structure are used to approach this issue. Advanced characterization tools such as secondary ion mass spectroscopy 3D chemical imaging, atom probe tomography, and X‐ray photoelectron spectroscopy are used to detect small amounts of impurities at GBs and are synergetically used together with time resolved photoluminescence measurements to correlate impurity distribution with electronic properties in CdTe solar cells. MCLT increases by an order of magnitude upon sulfur diffusion along GBs of the CdTe layer, which can occur by an elemental exchange with oxygen. Chlorine segregates at GBs and at the CdS/CdTe interface and bonding to cadmium and tellurium is indicated. CdTe solar cells in the inverted structure are presented with a certified efficiency of 13.5%. The results give guidance to further improve the performance of CdTe solar cells.