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MAX phases are layered carbides or nitrides with the general formula Mn+1AXn, which exhibit a unique combination of ceramic- and metal-like properties. The effect of stacking a number (determined by n) remains to be elucidated and a priori is not clear whether, for a given chemistry, n significantly changes the intrinsic deformation behavior of these systems. In this work, we have studied the intrinsic deformation behavior of Tin+1AlCn and Tan+1AlCn (n = 1 … 5) using DFT-based calculations. Surprisingly, the results suggest that the stacking number tends to have a minimal effect on the intrinsic mechanical behavior of the systems studied. [ABSTRACT FROM AUTHOR]

Recently, it has been predicted that Ta4AlC3 is likely to exhibit an (α-β) polymorphic transformation at temperatures above 1873 K. However, recent X-ray diffraction and transmission electron microscopy experiments suggest on the other hand that the α phase remains stable up to temperatures close to the limit of experimental capabilities and no transition has yet been observed. While the matter has already been settled experimentally, in this work, we re-investigate the phase stability problem in Ta4AlC3 by using first-principles methods. The study was carried out by considering both thermodynamic and mechanical stabilities of the Ta4AlC3 polymorphs. Particularly, finite-temperature Gibbs free energies and elastic properties of the polymorphs were calculated using density functional theory. Calculation results reveal that the α phase continue to be stable even at temperatures exceeding 1875 K, which is in agreement with experimental results reported in literature. [ABSTRACT FROM AUTHOR]

In this work, the electronic, structural, elastic, and thermodynamic properties of Ti2AX MAX phases (A = Al or Ga, X = C or N) were investigated using density functional theory (DFT). It is shown that the calculations of the electronic, structural, and elastic properties of these structures, using local density approximation (LDA) and generalized gradient approximation (GGA) coupled with projected augmented-wave (PAW) pseudopotentials, agree well with experiments. A thermodynamic model, which considers the vibrational and electronic contributions to the total free energy of the system, was used to investigate the finite-temperature thermodynamic properties of Ti2AX. The vibrational contribution was calculated using the supercell method, whereas the electronic contribution resulted from one-dimensional integration of electronic density of states (DOSs). To verify the model, the specific heats of pure elements were calculated and compared to experimental data. The DFT-D2 technique was used to calculate the heat capacity of graphite, taking into account the van der Waals (vdW) effect. Good agreement between the calculations and experiments for the specific heats of graphite and other pure elements lends validity to the approach used. The calculated results for the specific heats of Ti2AlC and Ti2AlN match well with experimental data. These strengthen the results of specific heats of Ti2GaC and Ti2GaN as well as other calculated thermodynamic properties, including the energies of formation and thermal expansion coefficient. [ABSTRACT FROM AUTHOR]

Abstract In a previous work by the present authors (Acta Mater. 148, 2018, 263–279), the thermodynamics of V–Nb–Ta–Mo–W highly concentrated alloy at equiatomic concentration was investigated using the cluster variation method in the irregular tetrahedron approximation. It was shown that the disordered solid solution is stabilized in this highly concentrated area of the system as prescribed by the high entropy alloy (HEA) paradigm. The configurational entropy, however, has only a minor (and sometimes contrary) influence in this stabilization. It was then postulated that configurational frustration, i.e. the competition between conflicting interactions, is the more relevant factor in the stabilization of the highly-concentrated alloys. The present work elaborates this idea by further considering the cases of V–Nb–Mo–Al and V–Nb–W–Al quaternary systemas and the corresponding quinary systems with Ta addition. The quaternary system are characterized by strong Al – M pair interactions that could enhance configurational frustration. The introduction of Ta, with weaker Ta – Al and practically non-existing Ta – Nb interactions, is meant to disrupt the frustration, hence leading to a decrease of the stability of the disordered alloys. It was shown that V–Nb–Mo–Al and its quinary do not exhibit configurational frustration as expected, since a stable compound with stoichiometry VMoNbAl is formed, and the stability of their disordered alloys (A2) is low. On the contrary, V–Nb–W–Al and its quinary demonstrated strong configurational frustration and correspondingly a stronger A2 stabilization. Deviations from equiatomic composition were also investigated and even stronger A2 stabilization was observed in this case. These results show that the stabilization of the disordered phase can only be attributed to configurational frustration, corroborating the previous work's conclusion that the effect of configurational entropy is only marginal in this stabilization. Graphical abstract Image 1 Highlights • Systems V – Nb – (Ta) – Mo – Al and V – Nb – (Ta) – W – Al behave differently. • Thermodynamics of disordered highly concentrated alloys is defined by the enthalpy. • Deviations of the ideal activity are large and even non-linear both in x and T. • Short-range order parameters are large and even non-linear in T. • Role of the entropy is secondary and thus no need to restrain to equiatomic alloys. [ABSTRACT FROM AUTHOR]

High Entropy Alloys (HEAs) designate a class of multicomponent metallic alloys in nearly equiatomic compositions. According to the constituting postulate, a larger number of constituents in a solid solution increases its configurational entropy, which has a maximal value when constituents exist in equiatomic concentrations and this is sufficient to overcome enthalpic contributions between the alloy components which would otherwise favor compound formation or phase separation. This entropy effect would, thus, stabilize disordered, crystallographically simple, solid solutions. Since then, numerous HEA candidate systems have been experimentally studied. It is unclear, however, if, and to which extent, the configurational entropy can be accessed by the system in the way it is suggested. The present work deals with this question using theoretical/computational methods. First, a series of model Body Centered Cubic (BCC) systems involving strong symmetric interactions between unlike atom pairs (referred to as “equinteracting” systems), containing up to five components, is investigated using the Cluster Variation Method in the irregular tetrahedron approximation. The symmetry of interactions, though artificial, allows for the straightforward presentation of phase diagram sections even for quaternary and quinary systems. Next, a “real” HEA candidate system, VNbTaMoW, which presents the BCC structure, is investigated by ab initio calculations, allowing to extend the conclusions to a realistic case with asymmetric interactions. The results show that configurational entropy has a small, even marginal, effect on phase transitions and the competition between conflicting interactions in the solid solution ( i.e. frustration ) seems to be the relevant factor behind the observed stabilization in the disordered states in HEA systems. [ABSTRACT FROM AUTHOR]

In this work, thermodynamic and kinetic diffusivities of uranium–niobium (U–Nb) are re-assessed by means of the CALPHAD (CALculation of PHAse Diagram) methodology. In order to improve the consistency and reliability of the assessments, first-principles calculations are coupled with CALPHAD. In particular, heats of formation of γ -U–Nb are estimated and verified using various density-functional theory (DFT) approaches. These thermochemistry data are then used as constraints to guide the thermodynamic optimization process in such a way that the mutual-consistency between first-principles calculations and CALPHAD assessment is satisfactory. In addition, long-term aging experiments are conducted in order to generate new phase equilibria data at the γ 2 / α + γ 2 boundary. These data are meant to verify the thermodynamic model. Assessment results are generally in good agreement with experiments and previous calculations, without showing the artifacts that were observed in previous modeling. The mutual-consistent thermodynamic description is then used to evaluate atomic mobility and diffusivity of γ -U–Nb. Finally, Bayesian analysis is conducted to evaluate the uncertainty of the thermodynamic model and its impact on the system's phase stability. [ABSTRACT FROM AUTHOR]

In this work, we calculate the structural, elastic, thermodynamic properties of Ti 3 SiC 2 and Ti 3 AlC 2 using a density functional theory (DFT) framework. The vibrational, electronic and quasi-harmonic contributions as well as an anharmonic correction to the total free energy of the system are computed and extrapolated to determine the finite-temperatures properties of the systems. Charge densities, electron localization functions (ELF), the electronic density of states (EDOS) and the vibrational density of states (VDOS) are analyzed in order to deepen our understanding of the interactions giving rise to the calculated properties. Calculated values of Young’s modulus, Heat Capacity and Coefficient of Thermal Expansion (CTE) show good agreement with experimental values. [ABSTRACT FROM AUTHOR]

Highlights: [•] Finite-temperature elastic properties of Ti2AlC and Ti2AlN were calculated. [•] Good agreement between calculation and experiment for Ti2AlC verified the approach. [•] Prediction of finite-temperature elastic properties for Ti2AlN was presented. [•] The approach’s applicability is limited to high temperatures, 1600K for this work. [ABSTRACT FROM AUTHOR]

The supercell approach to first-principles calculations of an alloy's full phonon spectrum is computationally expensive. It is therefore rarely used for calculating temperature dependent free energies of alloy phases or for calculating alloy phase diagrams. Amid the wider acceptance of uncertainty evaluation and the use of more efficient methods for materials design, the supercell approach is revisited and a simpler and more computationally efficient approximation is proposed. The new method is then used to calculate the Cr-Ni temperature dependent thermodynamic properties and phase diagram, resulting in predictions that are in good agreement with previous thermodynamic assessment. The new approach can facilitate accurate and rapid CALPHAD thermodynamic assessments of multi-component systems. [ABSTRACT FROM AUTHOR]

U-Nb's discontinuous precipitation, γ matrix bcc → α cellular orth + γ cellular ′ bcc , is intriguing in the sense that it allows formation and growth of the metastable γ ′ phase during the course of its occurrence. Previous attempts to explain the thermodynamic origin of U-Nb's discontinuous precipitation hypothesized that the energy of α forms an intermediate common tangent with the first potential of the double-well energy of γ at the γ ′ composition. In this work, we examine different possible mechanisms by which the discontinuous precipitation product in the U-Nb system can be stabilized. We put forward a mechanism by which the bulk free energy of the γ can develop a non-monotonic dependency with composition. Additionally we posit that local contributions due to lattice mismatch between the α and γ phases may be responsible for the generation of metastable states that may stabilized by thermodyna mics as well as by kinetics. Our work suggests that local misfit strain tends to play a crucial role in the growth of the discontinuous precipitation product. Depending on the magnitude of strain developed at the α / γ ′ interfaces, either an increasing γ ′ composition or a random distribution of γ ′ compositions around the equiatomic value with respect to increasing temperature could be expected. Moreover, we show how it is possible to stabilize the discontinuous precipitation front through highly anisotropic and fast interface diffusion. [ABSTRACT FROM AUTHOR]

ICME approaches provide decision support for materials design by establishing quantitative process-structure-property relations. Confidence in the decision support, however, must be achieved by establishing uncertainty bounds in ICME model chains. The quantification and propagation of uncertainty in computational materials science, however, remains a rather unexplored aspect of computational materials science approaches. Moreover, traditional uncertainty propagation frameworks tend to be limited in cases with computationally expensive simulations. A rather common and important model chain is that of CALPHAD-based thermodynamic models of phase stability coupled to phase-field models for microstructure evolution. Propagation of uncertainty in these cases is challenging not only due to the sheer computational cost of the simulations but also because of the high dimensionality of the input space. In this work, we present a framework for the quantification and propagation of uncertainty in a CALPHAD-based elastochemical phase-field model. We motivate our work by investigating the microstructure evolution in Mg 2 Si x Sn 1 − x thermoelectric materials. We first carry out a Markov Chain Monte Carlo-based inference of the CALPHAD model parameters for this pseudobinary system and then use advanced sampling schemes to propagate uncertainties across a high-dimensional simulation input space. Through high-throughput phase-field simulations we generate 200,000 time series of synthetic microstructures and use machine learning approaches to understand the effects of propagated uncertainties on the microstructure landscape of the system under study. The microstructure dataset has been curated in the Open Phase-field Microstructure Database (OPMD), available at http://microstructures.net. [ABSTRACT FROM AUTHOR]

Abstract B2–L2 1 configurational order resulting from thermal processing of Ni–Co–Mn–In Heusler alloys is one of the main factors defining the characteristics of the martensitic transformation experienced by these alloys. While the effects of the quenched-in disorder on the martensitic transformation have been extensively investigated, the kinetics of the ordering process is not well known and mostly inferred from the indirect results such as the dependence of the martensitic transformation or magnetic transition temperatures to thermal processing. However, recent results suggested that the kinetics of B2– L 2 1 ordering could have a more significant role than expected in the final state of the material and its functional properties. In the current work, an attempt has been made to investigate the kinetics of B2–L2 1 ordering in order to evaluate the dependence of the quenched-in disorder on the different variables of the thermal procedures. Results showed that ordering rate has a strong dependence on annealing temperature. Below 600 K, slow kinetics is forcing the crystal to always contain a certain amount of quenched-in disorder at room temperature. Between 600 K and 750 K, kinetics is fast enough to reach thermal equilibrium state within tens of minutes during the annealing process. Above 750 K, kinetics is very fast and thermal equilibrium can be reached during heating, so no isothermal holding is required. These predictions have been confirmed using differential scanning calorimetry and in-situ high-energy X-Ray diffraction measurements for non-direct estimations of the degree of order. Graphical abstract Image 1 Highlights • Ordering rate has a strong dependence on annealing temperature. • Below 600 K, the alloys to contain some quenched-in disorder at room temperature. • From 600 K to 750 K, ordering reaches equilibrium in tens of minutes when annealing. • Above 750 K, ordering can reach equilibrium during heating process. [ABSTRACT FROM AUTHOR]

Abstract In this work, we integrate different computational tools based on multi-phase-field simulations to account for the evolution of morphologies and crystallographic defects of Cu/Sn/Cu sandwich interconnect structures that are widely used in three dimensional integrated circuits (3DICs). Specifically, this work accounts for diffusion-driven formation and disappearance of multiple intermetallic phases during accelerated electromigration and takes into account the non-equilibrium formation of vacancies due to electromigration. The work compares nucleation, growth, and coalescence of intermetallic layers during transient liquid phase bonding and virtual joint structure evolution subjected to accelerated electromigration conditions at different temperatures. The changes in the rate of dissolution of Cu from intermetallics and the differences in the evolution of intermetallic layers depending on whether they act as cathodes or anodes are accounted for and are compared favorably with experiments. The model considers non-equilibrium evolution of vacancies that form due to differences in couplings between diffusing atoms and electron flows. The significance of this work lies in understanding the vacancy transport due to the difference in intrinsic diffusion of elements in different features of the microstructure, and the severe unidirectional convective flux of atoms due to the enforced electron wind that ultimately paves the road to study nucleation of microvoids. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Insufficient availability of molten salt corrosion‐resistant alloys severely limits the fruition of a variety of promising molten salt technologies that could otherwise have significant societal impacts. To accelerate alloy development for molten salt applications and develop fundamental understanding of corrosion in these environments, here an integrated approach is presented using a set of high‐throughput (HTP) alloy synthesis, corrosion testing, and modeling coupled with automated characterization and machine learning. By using this approach, a broad range of CrFeMnNi alloys are evaluated for their corrosion resistances in molten salt simultaneously demonstrating that corrosion‐resistant alloy development can be accelerated by 2 to 3 orders of magnitude. Based on the obtained results, a sacrificial protection mechanism is unveiled in the corrosion of CrFeMnNi alloys in molten salts which can be applied to protect the less unstable elements in the alloy from being depleted, and provided new insights on the design of high‐temperature molten salt corrosion‐resistant alloys. [ABSTRACT FROM AUTHOR]

Introduction: Nosocomial pneumonia is a common infection associated with high mortality in hospitalized patients. Nosocomial pneumonia, caused by gram-negative bacteria, often occurs in the elderly and patients with co-morbid diseases. Methods: Original research using a prospective cross-sectional design was conducted on 281 patients in an intensive care unit setting with nosocomial pneumonia between July 2015 and July 2019. For each nosocomial pneumonia case, data regarding comorbidities, risk factors, patient characteristics, Charlson comorbidity index (CCI), Systemic Inflammatory Response Syndrome (SIRS), and quick Sepsis-Related Organ Failure Assessment (qSOFA) points and treatment outcomes were collected. Data were analyzed by SPSS 22.0. Results: Nosocomial pneumonia due to gram-negative bacteria occurred in patients with neurological disorders (34.87%), heart diseases (16.37%), chronic renal failure (7.12%), and post-surgery (10.68%). Worse outcomes attributed to nosocomial pneumonia were high at 75.8%. Mechanical ventilation, change of antibiotics, and CCI ≥ 3 and qSOFA ≥ 2 were significantly negative prognostic factors (p < 0.05) on outcomes of nosocomial pneumonia. There was no difference in treatment effects between gender, age, time of onset pneumonia, SIRS score (p > 0.05). The pathogens were significant factors that influence treatment effects, but they weren't independent risk factors for poor outcomes (p = 0.823). Conclusions: Patients with nosocomial pneumonia hospitalized in intensive care units are usually associated with many underlying diseases, including neurological diseases. Mechanical ventilation, a change in antibiotics, CCI ≥ 3, and qSOFA ≥ 2 are also associated with a worse prognosis of nosocomial pneumonia. CCI and qSOFA might be used in predicting the outcome of nosocomial pneumonia. [ABSTRACT FROM AUTHOR]

During laser powder bed fusion (L-PBF) parts undergo multiple rapid heating-cooling cycles, leading to complex microstructures with nonuniform properties. In the present work, a computational framework which weakly couples a finite element thermal model to a non-equilibrium PF model was developed to investigate the rapid solidification microstructure of a Ni–Nb alloy during L-PBF. The framework is utilized to predict the spatial variation of the morphology and size of cellular segregation structures as well as the differences in melt pool microstructures obtained under different process conditions. A solidification map demonstrating the variation of microstructural features as a function of the temperature gradient and growth rate is presented. A planar to cellular transition is predicted in the majority of keyhole mode melt pools, while a planar interface is predominant in conduction mode melt pools. The predicted morphology and size of the cellular segregation structure agree well with experimental measurements. [ABSTRACT FROM AUTHOR]

Despite the fact that there is evidence for the important role that vacancies play in the martensitic transformation (MT) behavior of metamagnetic shape memory alloys (MMSMAs), little theoretical – and even experimental – work on the thermodynamics and kinetics of point defects in these systems has been carried out. Since the MT behavior of MMSMAs has a great influence on their magneto‐caloric response, investigating the vacancy evolution in MMSMAs has potentially a significant technological impact. Scarcity of studies may be due to the limited characterization capability available for studying vacancy properties as well as their impacts on the materials performance. The current work seeks to introduce the application of the grand‐canonical dilute‐solution model to the investigation of equilibrium (thermal) populations of point defects in Ni2MnGa, used as a prototypical MMSMA. The thermodynamic model is coupled to first‐principles calculations of the energetics of defect‐containing supercell structures. Such characterization capability allows for more realistic investigations of MMSMAs with the vacancy degree of freedom taken into account and subsequently opens up many interesting research topics. Here we demonstrate the capability of the first principles based characterization method by investigating the role of vacancy concentration in the kinetics of order–disorder (ODO) process and the MT temperature of Ni2MnGa. [ABSTRACT FROM AUTHOR]

Martensitic transformations of rapidly quenched and less rapidly cooled Heusler alloys of type Ni–Mn–X with X = Ga, In, and Sn are investigated by ab initio calculatioms. For the rapidly cooled alloys, we obtain the magnetocaloric properties near the magnetocaloric transition. For the less rapidly quenched alloys these magnetocaloric properties start to change considerably, each alloy transforms during temper‐annealing into a dual‐phase composite alloy. The two phases are identified to be cubic Ni–Mn–X and tetragonal NiMn. [ABSTRACT FROM AUTHOR]

In this work, the effects of small ternary additions to B2 NiTi structures was investigated through DFT calculations. The analysis considered deviations from stoichiometry arising from either simple substitution of host atoms in a given sublattice or from the formation of anti-sites. The calculations enabled the determination of the site preference of X ternary additions. Moreover, the results suggest that ternary additions located in the central region of the transition metal group across all periods tend to occupy Ni sites due to favorable X–Ti nearest neighbor (NN) interactions. This occupancy is achieved through substitution or through the generation of anti-site defects. On the other hand, ternary additions at both ends of a given transition metal row tend to occupy Ti sites due to favorable X–Ni NN interactions. Once site preferences are determined, the effect of alloying on the thermodynamic and mechanical properties of B2 NiTi–X structures are presented and trends are discussed. [ABSTRACT FROM AUTHOR]