RESUMO
Electron correlation microscopy (ECM) is a way to measure structural relaxation times, τ, of liquids with nanometer-scale spatial resolution using coherent electron scattering equivalent of photon correlation spectroscopy. We have applied ECM with a 3.5nm diameter probe to Pt57.5Cu14.7Ni5.3P22.5 amorphous nanorods and Pd40Ni40P20 bulk metallic glass (BMG) heated inside the STEM into the supercooled liquid region. These data demonstrate that the ECM technique is limited by the characteristics of the time series, which must be at least 40τ to obtain a well-converged correlation function g2(t), and the time per frame, which must be less than 0.1τ to obtain sufficient sampling. A high-speed direct electron camera enables fast acquisition and affords reliable g2(t) data even with low signal per frame.
RESUMO
Multiprincipal element high entropy alloys stabilized as a single alloy phase represent a new material system with promising properties, such as high corrosion and creep resistance, sluggish diffusion, and high temperature tensile strength. However, the mechanism of stabilization to form single phase alloys is controversial. Early studies hypothesized that a large entropy of mixing was responsible for stabilizing the single phase; more recent work has proposed that the single-phase solid solution is the result of mutual solubility of the principal elements. Here, we demonstrate the first self-consistent study of the relative importance of these two proposed mechanisms. In situ high-throughput synchrotron diffraction studies were used to monitor the stability of the single phase alloy in thin-film (Al1-x-yCuxMoy)FeNiTiVZr composition spread samples. Our results indicate that a metastable solid solution can be captured via the rapid quenching typical of physical vapor deposition processes, but upon annealing the solid-solution phase stability is primarily governed by mutual miscibility.
Assuntos
Ligas/química , Técnicas de Química Combinatória , Corrosão , Entropia , Teste de Materiais , Estrutura Molecular , Solubilidade , Propriedades de Superfície , TemperaturaRESUMO
Electron correlation microscopy (ECM) is a new technique that utilizes time-resolved coherent electron nanodiffraction to study dynamic atomic rearrangements in materials. It is the electron scattering equivalent of photon correlation spectroscopy with the added advantage of nanometer-scale spatial resolution. We have applied ECM to a Pd40Ni40P20 metallic glass, heated inside a scanning transmission electron microscope into a supercooled liquid to measure the structural relaxation time τ between the glass transition temperature T g and the crystallization temperature, T x . τ determined from the mean diffraction intensity autocorrelation function g 2(t) decreases with temperature following an Arrhenius relationship between T g and T g +25 K, and then increases as temperature approaches T x . The distribution of τ determined from the g 2(t) of single speckles is broad and changes significantly with temperature.
RESUMO
High-temperature alloy coatings that can resist oxidation are urgently needed as nuclear cladding materials to mitigate the danger of hydrogen explosions during meltdown. Here we apply a combination of computationally guided materials synthesis, high-throughput structural characterization and data analysis tools to investigate the feasibility of coatings from the FeCrAl alloy system. Composition-spread samples were synthesized to cover the region of the phase diagram previous bulk studies have identified as forming protective oxides. The metallurgical and oxide phase evolution were studied via in situ synchrotron glancing incidence x-ray diffraction at temperatures up to 690 K. A composition region with an Al concentration greater than 3.08 at%, and between 20.0 at% and 32.9 at% Cr showed the least overall oxide growth. Subsequently, a series of samples were deposited on stubs and their oxidation behavior at 1373 K was observed. The continued presence of a passivating oxide was confirmed in this region over a period of 6 h.
