Evaluation of microstructure and mechanical property variations in AlxCoCrFeNi high entropy alloys produced by a high-throughput laser deposition method

Twenty-one distinct AlxCoCrFeNi alloys were rapidly prepared by laser alloying an equiatomic CoCrFeNi substrate with Al powder to create an alloy library ranging x=0.15-1.32. Variations in crystal structure, microstructure and mechanical properties were investigated using X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy and nanoindentation. With increasing Al content, the crystal structure transitioned from a disordered FCC to a mixture of disordered BCC and ordered B2 structures. While the onset of BCC/B2 formation was consistent with previously reported cast alloys, the FCC structure was observed at larger Al contents in the laser processed materials, resulting in a wider two phase regime. The FCC phase was primarily confined to the BCC/B2 grain boundaries at these high Al contents. The nanoindentation modulus and hardness of the FCC phase increased with Al content, while the properties of the BCC/B2 structure were insensitive to composition. The structure and mechanical properties of the laser-processed alloys were surprisingly consistent with reported results for cast alloys, demonstrating the feasibility of applying this high-throughput methodology to multicomponent alloy design.Comment: 20 pages, 8 figures and 1 tabl

First principles calculation of polarization induced interfacial charges in GaN/AlN heterostructures

We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 +/- 0.4) x 10^13 cm-2 to be in excellent agreement with experiments and the value of 6.58 x 10^13 cm-2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.Comment: 3 pages, 2 figures; submitted to Applied Physics Letter

Nuclear Terrorism: Statutory Shortcomings and Prosecutorial Opportunities

In 2016, President Barack Obama warned that “[t]he danger of a terrorist group obtaining and using a nuclear weapon is one of the greatest threats to global security.” Thus far, however, U.S. and international efforts to address nuclear terrorism have faced a fundamental dilemma: While the importance of preventing this threat is unquestioned, there has been limited opportunity or need to conduct prosecutions that hinge on nuclear terrorism charges. This dilemma reflects the current piecemeal approach to nuclear terrorism, which prioritizes policies that address the “back-end” risk of nuclear terrorism (i.e., the detonation of nuclear weapons or attack of nuclear facilities) over efforts that address the front-end, preparatory steps that a would-be terrorist would take in advance of an attack. Given the importance of developing a more holistic approach to nuclear terrorism, this article proposes that the U.S. government promulgates a clearer framework for federal nuclear terrorism prosecutions. Part I first sets the legal landscape, introducing the key statutory provisions and prosecutions that comprise the existing federal jurisprudence on nuclear terrorism. Part II then evaluates the principal statutory shortcomings in the U.S. criminal system’s current approach to nuclear terrorism. Finally, Part III argues that in order to address these shortcomings, the U.S. Department of Justice (DOJ) should craft a nuclear terrorism prosecution framework (NTPF). This proposed framework consists of two guidelines premised on the principles of consistency and coordination and several recommended courses of action

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a ( LaFe O 3 ) 8 / ( SrFe O 3 ) 1 superlattice film capped with four layers of LaFe O 3 . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from F e 3 + in the bulk to F e 2 + at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a ( LaFe O 3 ) 8 / ( SrFe O 3 ) 1 superlattice film capped with four layers of LaFe O 3 . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from F e 3 + in the bulk to F e 2 + at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen

First-principles study of point defects in β-Ga2O3

Gallium oxide (Ga2O3) has been proposed as a promising candidate for power devices. Under the high electric field and high operating temperatures in such power devices, point defects are expected to form in Ga2O3 that can limit the device performance. We have calculated the thermodynamic stability of intrinsic point defects in stable monoclinic β-Ga2O3, such as VO, VGa, Oi, Gai, OGa, and GaO, under various chemical and electron potential using first-principles density functional theory calculations. We find that Vo, Gai, and GaO exhibit deep donor levels in gallium-rich conditions and do not contribute to n-type doping. GaO and Gai have high formation energy at low fermi levels in oxygen-rich conditions and can act as electron acceptors

Machine learning formation enthalpies of intermetallics

Developing fast and accurate methods to discover intermetallic compounds is relevant for alloy design. While density-functional-theory (DFT)-based methods have accelerated design of binary and ternary alloys by providing rapid access to the energy and properties of the stable intermetallics, they are not amenable for rapidly screening the vast combinatorial space of multi-principal element alloys (MPEAs). Here, a machine-learning model is presented for predicting the formation enthalpy of binary intermetallics and used to identify new ones. The model uses easily accessible elemental properties as descriptors and has a mean absolute error (MAE) of 0.025 eV/atom in predicting the formation enthalpy of stable binary intermetallics reported in the Materials Project database. The model further predicts stable intermetallics to form in 112 binary alloy systems that do not have any stable intermetallics reported in the Materials Project database. DFT calculations confirm one such stable intermetallic identified by the model, NbV2 to be on the convex hull. The model trained with binary intermetallics can also predict ternary intermetallics with similar accuracy as DFT, which suggests that it could be extended to identify compositionally complex intermetallics that may form in MPEAs