Investigation of boron-enriched Cottrell atmospheres in FeAl on an atomic scale by three-dimensional atom-probe field-ion microscopy

In 1949, on the basis of theoretical considerations, Cottrell proposed the concept of 'atmospheres' (called later by his name) to explain some specific behaviour of materials during plastic deformation, such as sharp yield-point formation or the Portevin-LeChatelier effect. In this letter, atomic-scale observations and three-dimensional analyses of a Cottrell atmosphere are reported. They have been performed by three-dimensional atom-probe field-ion microscopy techniques. The ability of this new experimental method to provide atomic-resolution images, both structural and chemical, was confirmed; the basic stacking structure of (001) planes in FeAl could be visualized with success. Moreover the presence of a <001> edge dislocation was also detected in the analysed zone. Further, B enrichment was measured in the vicinity of this defect; the B-rich region appeared as a pipe 5 nm in diameter, parallel to the dislocation line. The concentration of B in the core reached 3 at.%; this local enrichment in boron was accompanied by an Al depletion of more than 10 at.%. Boron in FeAl has a well known tendency to segregate to internal interfaces. In this letter, we show experimental evidence of the solute segregation to dislocation lines. The observed effects of this segregation on mechanical properties of FeAl, both at room temperature and high temperatures, are also discussed.     

Effect of V addition on the structure of ZrCr2 Laves phase: a high-resolution transmission electron microscopy study

The structures of the binary ZrCr₂ and ZrCr₂ Laves phase with addition of V have been investigated by high-resolution transmission electron microscopy (HRTEM). The results show that annealed ZrCr₂ is the cubic C15 phase with micro-twins. In contrast, annealed Zr(CrV)₂ has a lamellar microstructure with a character of long-period stacking sequences. Further studies by HRTEM indicate that annealed Zr(CrV)₂ is the di-hexagonal C36 with a coalescence of 5-, 6- and 8-layer long-period stacking structures. The effect of V addition on the structural stability of ZrCr₂ is discussed.     

High-angle annular dark field scanning transmission electron microscopy of the antiphase boundary in a rapidly solidified B2 type TiPd compound

High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast is applied to characterize the antiphase boundary (APB) of the B2 structure in a rapidly solidified TiPd melt-spun compound. The atomic shift associated with the R ?=?(1/2) a ₀ ?111? type displacement vector is directly observed at the boundary. A microstructure modification of the melt-spun compound with the cooling rate during solidification is also described.     

Effect of deep cryogenic treatment on carbon segregation in Cr8Mo2SiV tool steel during tempering

Carbon segregation in Cr8Mo2SiV tool steel after common heat treatment (CHT) and deep cryogenic treatment (DCT) has been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) extraction replica and three-dimensional atom probe (3DAP) tests. The XRD results show that most of the retained austenite transformed to martensite after DCT. The TEM results confirm the transformation of blocky retained austenite to twin martensite during DCT. The 3DAP results reveal the segregation of carbon atoms and the maximum carbon fraction in carbon-rich regions increases after DCT. The peak carbon concentration in carbon-rich regions of SDC99 steel after CHT and DCT are 8 at. % and 15 at. %, respectively. The segregated carbon atoms grew into nuclei for nanoscale carbides precipitation on subsequent tempering.     

Epifluorescence optical microscopy: a sensitive tool for determining the spatial distribution of europium-dihalide precipitates in KCl : KBr : Eu2+ crystals

The spatial distribution of europium-dihalide-type precipitates in KCl?:?KBr?:Eu²⁺ single crystals annealed at 200°C for long periods of time was determined by epifluorescence optical microscopy. Laue-type patterns and optical spectroscopy were used to test the degree of long-range ordering of the specimens and to monitor the precipitation during annealing, respectively. Precipitates smaller than about 0.08?µm were found (2.84?×?10¹² precipitates cm^?3) all across the host, whereas larger precipitates of about 0.17?µm were found (2.9?×?10⁴ precipitates cm^?1) along certain linear structural singularities of the matrix. These singularities, identified as crystal dislocations, were observed to terminate always either at a triple node of singularities or at the crystal surface, forming a three-dimensional network of crystal singularities (2.0?×?10⁶ singularities cm^?2), identified as the Frank network. The presence of europium exhaustive matrix zones accompanying the observed europium-decorated paths indicates that impurity segregation processes, occurring during annealing, favour the precipitation phenomena.