Understanding changes in chemistry, microstructure, and physical properties during synthesis, processing, testing, and even service is vital for materials design and performance. Compared to ...traditional postmortem material characterization tools, in situ crystallographic characterization can provide considerable data and information on evolution of chemistry, dislocations, twinning, texture, and strains when a material is under external stimuli. Neutrons especially are able to probe material bulk properties and behaviors in extreme environments, thanks to their deep penetrating power and unique sensitivity to differentiate elements from lightweight to transition-metal atoms. In this article, we introduce and describe a diffractometer named VULCAN, which is located at Oak Ridge National Laboratory. This represents a powerful tool to understand materials properties and behaviors under complex environments, in particular, at high temperatures.
The atomic structure of metallic glasses has been a long-standing scientific problem. Unlike crystalline metals, where long-range ordering is established by periodic stacking of fundamental building ...blocks known as unit cells, a metallic glass has no long-range translational or orientational order, although some degrees of short- and medium-range order do exist. Previous studies have identified solute- (minority atom)-centred clusters as the fundamental building blocks or short-range order in metallic glasses. Idealized cluster packing schemes, such as efficient cluster packing on a cubic lattice and icosahedral packing as in a quasicrystal, have been proposed and provided first insights on the medium-range order in metallic glasses. However, these packing schemes break down beyond a length scale of a few clusters. Here, on the basis of neutron and X-ray diffraction experiments, we propose a new packing scheme-self-similar packing of atomic clusters. We show that the medium-range order has the characteristics of a fractal network with a dimension of 2.31, and is described by a power-law correlation function over the medium-range length scale. Our finding provides a new perspective of order in disordered materials and has broad implications for understanding their structure-property relationship, particularly those involving a change in length scales.
In this work, diffraction and single-crystal elastic constants of Inconel 625 have been determined by means of in situ loading at room and elevated temperatures using time-of-flight neutron ...diffraction. Theoretical models proposed by Voigt, Reuss, and Kroner were used to determine single-crystal elastic constants from measured diffraction elastic constants, with the Kroner model having the best ability to capture experimental data. The magnitude of single-crystal elastic moduli, computed from single-crystal elastic constants, decreases and the single crystal anisotropy increases as temperature increases, indicating the importance of texture in affecting macroscopic stress at elevated temperatures. The experimental data reported here are of great importance in understanding additive manufacturing of metallic components as: diffraction elastic constants are required for computing residual stresses from residual lattice strains measured using neutron diffraction, which can be used to validate thermomechanical models of additive manufacturing, while single-crystal elastic constants can be used in crystal plasticity modeling, for example, to understand mechanical deformation behavior of additively manufactured components.
Dynamic strain aging (DSA), observed macroscopically as serrated plastic flow, has long been seen in nickel-base superalloys when plastically deformed at elevated temperatures. Here we report the ...absence of DSA in Inconel 625 made by additive manufacturing (AM) at temperatures and strain rates where DSA is present in its conventionally processed counterpart. This absence is attributed to the unique AM microstructure of finely dispersed secondary phases (carbides, N-rich phases, and Laves phase) and textured grains. Based on experimental observations, we propose a dislocation-arrest model to elucidate the criterion for DSA to occur or to be absent as a competition between dislocation pipe diffusion and carbide-carbon reactions. With in situ neutron diffraction studies of lattice strain evolution, our findings provide a new perspective for mesoscale understanding of dislocation-solute interactions and their impact on work-hardening behaviors in high-temperature alloys, and have important implications for tailoring thermomechanical properties by microstructure control via AM.
The complex thermal histories present during additive manufacturing (AM) of metals result in the generation of residual stresses, which may result in distortion and early failure of the fabricated ...component. The amount of residual stress built up or relieved during deposition depends on the stress relaxation behavior of the deposited material as well as the substrate onto which the component is built, over the typical timescale for depositing a few layers in AM (seconds to minutes), which corresponds to the timescale over which the material is subjected to both stress and elevated temperature. This work presents a method for investigating stress relaxation behavior and mechanisms in conventionally processed and additively manufactured Ti-6Al-4V (CP Ti-6Al-4V and AM Ti-6Al-4V) through compression tests at 600°C and 700°C with in situ neutron diffraction. The results show that with an applied plastic deformation, 60–80% of the initial stress in Ti-6Al-4V was relieved in ten minutes and the stress stabilized at a negligibly low level. With the same applied strain, the stress relaxation rate at 700°C was 2–4 times higher than that at 600°C, and the peak stress at 600°C was twice as high as that at 700°C. It was determined that neither stress partitioning nor phase transformation were active in Ti-6Al-4V at the temperatures studied. Thus, it was hypothesized that the stress relaxation was primarily due to dislocation glide and climb. The presently reported relaxation behavior can be used in the development and validation of thermomechanical models used to predict and mitigate residual stresses and distortion in AM, or to predict distortion in Ti-6Al-4V used in structural applications at elevated temperatures.
In this paper we study the relation between the nonuniform stability in mean square and admissibility of stochastic differential equation in Hilbert spaces. We consider an adapted norms and thus we ...obtain a variant for the stochastic case of nonuniform exponential stability in mean square due to in deterministic case. In the qualitative theory of evolution equations, nonuniform exponential stability is one the most important asymptotic properties and in last years it was treated from various perspectives The main objective is to give a more general concept of nonuniform exponential stability in mean square of stochastic differential equations in Hilbert spaces.
We show that a variety of bulk metallic glasses (BMGs) inherit their Young's modulus and shear modulus from the solvent components. This is attributed to preferential straining of locally ...solvent-rich configurations among tightly bonded atomic clusters, which constitute the weakest link in an amorphous structure. This aspect of inhomogeneous deformation, also revealed by our in situ neutron diffraction studies of an elastically deformed BMG, suggests a rubberlike viscoelastic behavior due to a hierarchy of atomic bonds in BMGs.
The rapid solidification and subsequent thermal cycles that material is subjected to during additive manufacturing (AM) of a component result in a buildup of residual stresses, which lead to part ...distortion, and negatively impact the component's mechanical properties. We present a method for using neutron diffraction to validate thermomechanical models developed to predict the residual stresses in Inconel 625 walls fabricated by laser-based directed energy deposition. Residual stress calculations from neutron diffraction measurements depend strongly on the determination of stress-free lattice spacings. After measurement of stressed lattice spacings in Inconel 625 walls, reference samples were obtained by extracting thin slices from the walls and cutting comb-type slits into these slices. Reference lattice spacings were measured in these slices, as well as equivalent slices that were also subjected to stress-relieving heat treatment. These heat treatments changed the reference lattice spacings, and therefore affected residual strain measurements. Further, this study shows the importance of using location-dependent reference lattice spacing, as during AM, the thermal history, and therefore elemental composition and stress-free lattice spacing, vary with position. Residual stresses measured by neutron diffraction along the build direction using comb-type reference samples without heat treatment were in good agreement with thermomechanical modeling predictions.
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•Reference lattice spacings in neutron diffraction measurements can be determined from mechanically stress-relieved samples.•Stress-relief heat treatments change reference lattice spacings by modifying the microstructure of reference samples.•In additive manufacturing (AM), thermal history and therefore elemental composition vary with position.•Location-dependent reference lattice spacings must be measured in AM due to chemical and microstructural heterogeneity.•Neutron diffraction and thermomechanical modeling are complementary techniques to determine residual stress.
First In Situ Lattice Strains Measurements Under Load at VULCAN An, Ke; Skorpenske, Harley D.; Stoica, Alexandru D. ...
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
01/2011, Letnik:
42, Številka:
1
Journal Article
Recenzirano
The engineering materials diffractometer, VULCAN, at the Spallation Neutron Source began commissioning on June 26, 2009. This instrument is designed for materials science and engineering studies.
In ...situ
lattice strain measurements of a model metallic material under monotonic tensile load have been performed on VULCAN. The tensile load was applied under two different strain rates, and neutron diffraction measurements were carried out in both high-intensity and high-resolution modes. These experiments demonstrated VULCAN’s
in situ
study capability of deformation behaviors even during the early phases of commissioning.
Polymer electrolytes, using a poly(epichlorydrin-allyl glycidyl ether) copolymer as matrix, were prepared and characterized. Anion conducting networks were obtained by the incorporation of two cyclic ...diamines named 1,4-diazabicyclo-2.2.2-octane (DABCO) and 1-azabicyclo-2.2.2-octane (Quinuclidine), neither sensitive to Hoffman elimination. In order to improve the mechanical properties, the membrane was reinforced using polyamide supports. The physicochemical and electrochemical characteristics, namely ionic exchange capacity, swelling ratio, glass transition temperature, thermal stability and ionic conductivity, were evaluated.
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