Showing papers by "Mark Asta published in 2022"

Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin

Abstract: CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters½ (MPa·m½) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness. Description Too cold to fracture Finding structural materials that have good fracture properties at very low temperatures is challenging but is important for fields such as space exploration. Liu et al. discovered a high-entropy chromium-cobalt-nickel alloy that has an incredibly high fracture toughness at 20 kelvin (see the Perspective by Zhang and Zhang). This behavior is caused by an unexpected phase transformation that, when combined with other microstructures, prevents crack formation and propagation. The fracture toughness of this alloy makes it potentially useful for a range of cryogenic applications. —BG CrCoNi-based alloys have very high fracture toughness at 20 kelvin.

Molecular dynamics studies of 〈a〉 -type screw dislocation core structure polymorphism in titanium

Abstract: The atomic scale computation of dislocation core structures has become an essential tool in the development of models for the plasticity of metals. Competing dislocation core structures are often analyzed at $T=0$ K (with $T$ the temperature), and the dislocation core structure with the lowest energy is assumed to be the structure dictating the dynamics of the individual dislocation at finite temperatures. It is shown here that, for some hexagonal-close-packed (HCP) metals, this approach may be too simplistic. As a prototypical example, $\ensuremath{\langle}a\ensuremath{\rangle}$-type screw dislocations within HCP Ti modeled using an empirical interatomic potential are considered. It is shown using molecular dynamics simulations that, at room temperature and above, the core structure of the dislocation is remarkably complex and variable. The implications of this complexity for the dynamics of the dislocations are discussed.

Modeling antiphase boundary energies of Ni3Al-based alloys using automated density functional theory and machine learning

Abstract: Abstract Antiphase boundaries (APBs) are planar defects that play a critical role in strengthening Ni-based superalloys, and their sensitivity to alloy composition offers a flexible tuning parameter for alloy design. Here, we report a computational workflow to enable the development of sufficient data to train machine-learning (ML) models to automate the study of the effect of composition on the (111) APB energy in Ni 3 Al-based alloys. We employ ML to leverage this wealth of data and identify several physical properties that are used to build predictive models for the APB energy that achieve a cross-validation error of 0.033 J m −2 . We demonstrate the transferability of these models by predicting APB energies in commercial superalloys. Moreover, our use of physically motivated features such as the ordering energy and stoichiometry-based features opens the way to using existing materials properties databases to guide superalloy design strategies to maximize the APB energy.

One dimensional wormhole corrosion in metals

Abstract: Corrosion is a ubiquitous failure mode of materials. Often, the progression of localized corrosion is accompanied by the evolution of porosity in materials previously reported to be either three-dimensional or two-dimensional. However, using new tools and analysis techniques, we have realized that a more localized form of corrosion, which we call 1D wormhole corrosion, has previously been miscategorized in some situations. Using electron tomography, we show multiple examples of this 1D and percolating morphology. To understand the origin of this mechanism in a Ni-Cr alloy corroded by molten salt, we combined energy-filtered four-dimensional scanning transmission electron microscopy and ab initio density functional theory calculations to develop a vacancy mapping method with nanometer-resolution, identifying a remarkably high vacancy concentration in the diffusion-induced grain boundary migration zone, up to 100 times the equilibrium value at the melting point. Deciphering the origins of 1D corrosion is an important step towards designing structural materials with enhanced corrosion resistance.

The effect of Cr alloying on defect migration at Ni grain boundaries

Abstract: Abstract Mass transport along grain boundaries in alloys depends not only on the atomic structure of the boundary, but also its chemical make-up. In this work, we use molecular dynamics to examine the effect of Cr alloying on interstitial and vacancy-mediated transport at a variety of grain boundaries in Ni. We find that, in general, Cr tends to reduce the rate of mass transport, an effect which is greatest for interstitials at pure tilt boundaries. However, there are special scenarios in which it can greatly enhance atomic mobility. Cr tends to migrate faster than Ni, though again this depends on the structure of the grain boundary. Further, grain boundary mobility, which is sometimes pronounced for pure Ni grain boundaries, is eliminated on the time scales of our simulations when Cr is present. We conclude that the enhanced transport and grain boundary mobility often seen in this system in experimental studies is the result of non-equilibrium effects and is not intrinsic to the alloyed grain boundary. These results provide new insight into the role of grain boundary alloying on transport that can help in the interpretation of experimental results and the development of predictive models of materials evolution.

Realistic magnetic thermodynamics by local quantization of a semiclassical Heisenberg model

Abstract: Abstract Classical Monte Carlo simulation of the Heisenberg model poorly describes many thermodynamic phenomena due to its neglect of the quantum nature of spins. Alternatively, we discuss how to semiclassically approach the quantum problem and demonstrate a simple method for introducing a locally approximate form of spin quantization. While the procedure underestimates magnetic short-range order, our results suggest a simple correction for recovering realistic spin–spin correlations above the critical temperature. Moreover, ensemble fluctuations are found to provide reasonably accurate thermodynamics, largely reproducing quantum mechanically calculated heat capacities and experimental magnetometry for ferromagnetic Fe and antiferromagnetic RbMnF 3 . Extensions of the method are proposed to address remaining inaccuracies.

Constant-potential molecular dynamics simulations of molten salt double layers for FLiBe and FLiNaK.

Abstract: We report the results of constant-potential molecular dynamics simulations of the double layer interface between molten 2LiF-BeF2 (FLiBe) and 23LiF-6NaF-21KF (FLiNaK) fluoride mixtures and idealized solid electrodes. Employing methods similar to those used in studies of chloride double layers, we compute the structure and differential capacitance of molten fluoride electric double layers as a function of applied voltage. The role of molten salt structure is probed through comparisons between FLiBe and FLiNaK, which serve as models for strong and weak associate-forming salts, respectively. In FLiBe, screening involves changes in Be-F-Be angles and alignment of the oligomers parallel to the electrode, while in FLiNaK, the electric field is screened mainly by rearrangement of individual ions, predominantly the polarizable potassium cation.

Theoretical antiferromagnetism of ordered face-centered cubic Cr-Ni alloys

Abstract: Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semi-local density-functional theory. The optimization of local magnetic moments significantly increases the driving force for the formation of CrNi 2 , the only experimentally observed intermetallic phase. This structure’s ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin fluctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, specifically a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. Implications for related commercial and high-entropy alloys are discussed for each case.

Using Jupyter Tools to Design an Interactive Textbook to Guide Undergraduate Research in Materials Informatics

Abstract: With the growing desire to incorporate data science and informatics into STEM curricula, there is an opportunity to integrate research-based software and tools (e.g., Python) within existing pedagogical methods to craft new, accessible learning experiences. We show how the open-source Jupyter Book software can achieve this goal by creating a digital, interactive textbook compiled from Jupyter notebooks, which are already commonplace in research. Using Jupyter Book, we design an open-source, introductory materials informatics research curriculum where the Python programming exercises are supplemented with prose, graphics, slides, and discussion questions, all of which are embedded into a uniform web interface for streamlined access. Interactive programming capabilities, enabled through the JupyterHub cloud infrastructure, provide opportunities in these digital spaces for students to interrogate the code, test their own hypotheses, and deepen their comprehension. These authentic learning experiences demonstrate the broad utility of the Jupyter ecosystem in sustaining the growth of materials informatics education.

Extra electron reflections in concentrated alloys may originate from planar defects, not short-range order

Abstract: In many concentrated alloys of current interest, the observation of diffuse superlattice intensities by transmission electron microscopy has been attributed to the presence of chemical short-range order. This interpretation is questioned on the basis of crystallographic considerations and theoretical predictions of ordering. The work of Xiao and Daykin [ Ultramicroscopy 53 (1994)], which shows how planar defects can produce the exact set of observed peaks, is highlighted as an alternative explanation that would impact the conclusions of a number of recent studies.

Resolving the Chemical Formula of Nesquehonite via NMR Crystallography, DFT Computation, and Complementary Neutron Diffraction.

Abstract: Nesquehonite is a magnesium carbonate mineral relevant to carbon sequestration envisioned for carbon capture and storage of CO2. Its chemical formula remains controversial today, assigned as either a hydrated magnesium carbonate [MgCO3•3H2O], or a hydroxy - hydrated - magnesium bicarbonate [Mg(HCO3)OH•2H2O]. The resolution of this controversy is central to understanding this material's thermodynamic, phase, and chemical behavior. We present an NMR crystallography study, using rotational-echo double-resonance 13C{1H} (REDOR) to determine 13C - 1H distances with precision, and the combination of 13C static NMR lineshapes and density - functional theory (DFT) calculations to model different H atomic coordinates. We find [MgCO3•3H2O] to be accurate, and evidence from neutron powder diffraction bolstered these assignments. Refined H positions can help us understand how H-bonding stabilizes this structure against dehydration to MgCO3. More broadly, our results illustrate the power of NMR crystallography as a technique for resolving questions where X-ray diffraction is inconclusive.

Grain Boundary Effects in Dealloying Metals: A Multi-Phase Field Study

Abstract: A multi-phase field model is employed to predict the microstructural evolution of a bicrystal metal precursor made of model elements ’A’ and ’B,’ in contact with a liquid dealloying agent of species ’C.’ The model assumes that ’A’ is immiscible with ’C,’ which leads to the selective dissolution of ’B’ from the precursor. Spinodal driving forces and surface diffusion within the solid-liquid interface cause ’A’ to segregate, initiating the growth of ’A’-rich solid ligaments and promoting the bicontinuous penetration of the dealloying agent ’C’ into the precursor. We demonstrate that this dealloying process leads to coupled grain-boundary migration to maintain equilibrium contact angles with this topologically-complex solid-liquid interface. This leads to dealloying being locally accelerated by dissolving ’B’ from (and rejecting ’A’ into) the grain boundary. The migrating grain boundary promotes formation of a locally deeper corrosion channel that asymmetrically disrupts the ligament connectivity of the final dealloyed structure, in qualitative agreement with published experimental observations. The influence of varying grain boundary properties is explored within the model, showing that, in a polycrystalline sample, corrosion will be significantly deeper at the grain boundaries with large mobilities and fast solute diffusivities.

Tacking Directional Movement of Nanomotors with Liquid Cell Electron Microscopy

Abstract: Liquid cell transmission electron microscopy (TEM) has become a powerful tool for the study of nanoparticle movement and self-assembly [1-2]. By tracking the individual nanoparticle motion mechanisms of nanoparticle movement can be achieved. Extensive studies have demonstrated that various interaction forces contribute to the nanoparticle motion, which lead to the self-assembly of nanoparticles into one-dimensional chains [3-4], two-dimensional patterns [4-5], and three-dimensional architectures [6]. For example, the magnetic dipole interaction and Van der Waals force were considered playing a major role in the self-assembly of PtFe nanoparticles [4]. For the self-assembly of CoO nanoparticles around a liquid droplet, the curved surface of a liquid droplet may create a force gradient driving the CoO nanoparticles to the droplet perimeter [6].

Structure and glide of Lomer and Lomer-Cottrell dislocations: Atomistic simulations for model concentrated alloy solid solutions

Abstract: Lomer (L) and Lomer-Cottrell (LC) dislocations have long been considered to be central to work hardening in face-centered cubic (FCC) metals and alloys. These dislocations act as barriers of motion for other dislocations, and can serve as sites for twin nucleation. Recent focus on multicomponent concentrated FCC solid solution alloys has resulted in many reported observations of LC dislocations. While these and L dislocations are expected to have a role in the mechanical behavior of these alloys, little is understood about how variations in composition and associated fault energies change the response of these dislocations under stress. We present atomistic simulations of L and LC dislocations in a model Cu-Ni system and find that changes in composition and applied stress conditions result in a wide variety of responses, including changes in core configuration and (100) glide. The results are compared to and extend previous literature related to the nature of L/LC core structures and how they vary with respect to intrinsic materials properties and stress states. This study also provides insights into mechanisms such as twin nucleation that could have important implications for work hardening in FCC solid-solution alloys.

Exploring the Morphotropic Phase Boundary in Epitaxial PbHf 1 − x Ti x O 3 Thin Films

Abstract: : Epitaxial PbHf 1 − x Ti x O 3 /SrTiO 3 (001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr 1 − x Ti x O 3 system. End members, PbHfO 3 and PbTiO 3 , were found to possess orthorhombic ( Pbam ) and tetragonal ( P 4 mm ) crystal structures and antiferroelectric and ferroelectric ( ∼ 87 μ C/cm 2 ) behavior, respectively. PbHf 0.75 Ti 0.25 O 3 and PbHf 0.25 Ti 0.75 O 3 solid solutions were both found to be ferroelectric with rhombohedral ( R 3 c , ∼ 22 μ C/cm 2 ) and tetragonal ( P 4 mm , ∼ 46 μ C/ cm 2 ) structures, respectively. For intermediate PbHf 1 − x Ti x O 3 compositions (e.g., x = 0.4, 0.45, 0.5, and 0.55), a structural transition was observed from rhombohedral (hafnium-rich) to tetragonal (titanium-rich) phases. These intermediate compositions also exhibited mixed-phase structures including R 3 c , monoclinic ( Cm ), and P 4 mm symmetries and, in all cases, were ferroelectric with remanent (5 − 22 μ C/cm 2 ) and saturation (18.5 − 36 μ C/cm 2 ) polarization and coercive field (24 − 34.5 kV/cm) values increasing with x . While the dielectric constant was the largest for PbHf 0.6 Ti 0.4 O 3 , the MPB is thought to be near x = 0.5 after separation of the intrinsic and extrinsic contributions to the dielectric response. Furthermore, piezoelectric displacement − voltage hysteresis loops were obtained for all chemistries revealing displacement values as good as PbZr 0.52 Ti 0.48 O 3 films in the same geometry. Thereby, the PbHf 1 − x Ti x O 3 system is a viable alternative to the PbZr 1 − x Ti x O 3 system offering comparable performance.