RESUMO
The microstructure of minerals and rocks can significantly alter reaction rates. This study focuses on identifying transport paths in low porosity rocks based on the hypothesis that grain boundary widening accelerates reactions in which one mineral is replaced by another (replacement reaction). We conducted a time series of replacement experiments of three limestones (CaCO3) of different microstructures and solid impurity contents using FeCl2. Reacted solids were analyzed using chemical imaging, small angle X-ray and neutron scattering and Raman spectroscopy. In high porosity limestones replacement is reaction controlled and complete replacement was observed within 2 days. In low porosity limestones that contain 1-2% dolomite impurities and are dominated by grain boundaries, a reaction rim was observed whose width did not change with reaction time. Siderite (FeCO3) nucleation was observed in all parts of the rock cores indicating the percolation of the solution throughout the complete core. Dolomite impurities were identified to act as nucleation sites leading to growth of crystals that exert force on the CaCO3 grains. Widening of grain boundaries beyond what is expected based on dissolution and thermal grain expansion was observed in the low porosity marble containing dolomite impurities. This leads to a self-perpetuating cycle of grain boundary widening and reaction acceleration instead of reaction front propagation.
RESUMO
Single-phase high- and medium-entropy alloys with face-centred cubic (fcc) structure can exhibit high tensile ductility1,2 and excellent toughness2,3, but their room-temperature strengths are low1-3. Dislocation obstacles such as grain boundaries4, twin boundaries5, solute atoms6 and precipitates7-9 can increase strength. However, with few exceptions8-11, such obstacles tend to decrease ductility. Interestingly, precipitates can also hinder phase transformations12,13. Here, using a model, precipitate-strengthened, Fe-Ni-Al-Ti medium-entropy alloy, we demonstrate a strategy that combines these dual functions in a single alloy. The nanoprecipitates in our alloy, in addition to providing conventional strengthening of the matrix, also modulate its transformation from fcc-austenite to body-centred cubic (bcc) martensite, constraining it to remain as metastable fcc after quenching through the transformation temperature. During subsequent tensile testing, the matrix progressively transforms to bcc-martensite, enabling substantial increases in strength, work hardening and ductility. This use of nanoprecipitates exploits synergies between precipitation strengthening and transformation-induced plasticity, resulting in simultaneous enhancement of tensile strength and uniform elongation. Our findings demonstrate how synergistic deformation mechanisms can be deliberately activated, exactly when needed, by altering precipitate characteristics (such as size, spacing, and so on), along with the chemical driving force for phase transformation, to optimize strength and ductility.
RESUMO
The Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes (TALSPEAK) process is a solvent extraction based method for separating trivalent lanthanides (Ln(3+)) from trivalent actinide cations in used nuclear fuel reprocessing. In conventional TALSPEAK, the extractant solution is di(2-ethylhexyl)phosphoric acid (HDEHP) in 1,4-diisopropylbenzene (DIPB). The aqueous medium is diethylenetriamine-N,N,N',Nâ³,Nâ³-pentaacetic acid (DTPA) in a concentrated lactic acid (HL) buffer. Lanthanides are extracted by HDEHP/DIPB, while the actinides remain in the aqueous phase as DTPA complexes. Lactic acid is extracted both independently of the lanthanides and as Ln/HL/HDEHP mixed complex(es). Previous results indicate that lanthanides are extracted both as the mixed complex and as a binary Ln(DEHP·HDEHP)(3) species. Small-angle neutron scattering (SANS) has been applied to study the self-organization properties of solute molecules in xylene solutions containing HDEHP, HL, selected lanthanide ions, and water. The scattering results demonstrate that the dominant HDEHP species is the hydrogen bonded dimer, (HDEHP)(2). Absent lanthanides, lactic acid is extracted as the 1:3 complex (HL·(HDEHP)(3)). Scattering in samples containing up to 0.005 M lanthanides (prepared by extracting lanthanides from aqueous media containing 1.0 M buffered lactic acid) indicates that the dominant metal complex is Ln(DEHP·HDEHP)(3). At 0.013 M extracted lanthanide, the scattering results indicate lower Ln:DEHP stoichiometry and larger scattering particles. At higher metal concentrations, the SANS results indicate large aggregates, the largest aggregates achieving a size equivalent to 20 HDEHP monomers as the primary scattering entity. Analysis of particle shapes indicates best fits with a uniform oblate spheroid particle. These results are discussed in connection with the results of a number of complementary observations that have been made on this system.