Mechanistic origin of high retained strength in refractory BCC high entropy alloys up to 1900K

The body centered cubic (BCC) high entropy alloys MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. Unlike pure BCC metals and dilute alloys that are controlled by screw dislocation motion at low temperatures, the strength of BCC HEAs can be controlled by edge dislocations, and especially at high temperatures, due to the barriers created for edge glide through the random field of solutes. A parameter-free theory for edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for the MoNbTaW and MoNbTaVW alloys. A reduced analytic version of the theory then enables screening over >600,000 compositions in the Mo-Nb-Ta-V-W family, identifying promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of stronger high entropy alloys.Comment: This version corrects the theory and provides more extensive explanation

Strengthening of high entropy alloys by dilute solute additions: CoCrFeNiAlx and CoCrFeNiMnAlx alloys

A theory to predict the initial flow stress of an arbitrary N-component fcc random alloy is extended to predict the additional strengthening when a dilute concentration of a substitutional element is introduced. Assuming properties for the N-component alloy to be established, the theory requires only information on the elastic and lattice constants of the new N + 1-alloy, and makes a parameter-free prediction for the strength increment due to the added N + 1st element. The theory is applied to the CoCrFeNiAlx and CoCrFeNiMnAlx systems, achieving good agreement with experiments. The theory thus serves as a valuable tool for guiding design of new fcc random alloys. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

First-principles-based prediction of yield strength in the RhIrPdPtNiCu high-entropy alloy

High-entropy alloys are random alloys with five or more components, often near equi-composition, that often exhibit excellent mechanical properties. Guiding the design of new materials across the wide composition space requires an ability to compute necessary underlying material parameters via ab initio methods. Here, density functional theory is used to compute the elemental misfit volumes, alloy lattice constant, elastic constants, and stable stacking fault energy in the fcc noble metal RhIrPdPtNiCu. These properties are then used in a recent theory for the temperature and strain-rate dependent yield strength. The parameter-free prediction of 583 MPa is in excellent agreement with the measured value of 527 MPa. This quantitative connection between alloy composition and yield strength, without any experimental input, motivates this general density functional theory-based methodological path for exploring new potential high-strength high-entropy alloys, in this and other alloy classes, with the chemical accuracy of first-principles methods

Predicting yield strengths of noble metal high entropy alloys

Recent data on the Noble metal (Pd-Pt-Rh-Ir-Au-Ag-Cu-Ni) high entropy alloys (HEAs) shows some of these materials to have impressive mechanical properties. Here, a mechanistic theory for the temperature-, composition-, and strain-rate-dependence of the initial yield strength of fcc HEA5 is applied to this alloy class, with inputs obtained through "rule-of-mixtures" models for both alloy lattice and elastic constants. Predictions for PdPtRhIrCuNi are in good agreement with available experiment and the model provides useful insights into this system. The model is then used to explore other alloy compositions within this broad class to guide design of new stronger Noble metal HEA5. (C) 2017 Acta Materialia Inc. Published by. Elsevier Ltd. All rights reserved

A New Mechanism for Twin Growth in Mg Alloys

Twinning is an important deformation mode in lightweight Mg alloys, but the mechanisms of nucleation and growth of twins and their interactions with solutes remain largely unresolved. Here, a new model for thermally-activated, stress-driven growth of twin boundaries is presented and the role of random and segregated solutes in controlling this growth is studied analytically and using direct molecular dynamics simulations. Twin growth occurs by the thermally-activated nucleation and expansion of twin dislocation loops on a pre-existing twin boundary. Fluctuations in the local concentration of random solutes lowers the energy barrier for this process and thus facilitates twin growth. Segregation of solutes to the twin boundary strongly increases the energy barrier and suppresses twin growth. In random solid solutions at experimental strain rates, the thermally-activated nucleation process is sufficiently fast at low applied stresses so that growth of the twin is controlled by solute strengthening of the nucleated twin loop as it expands across the twin boundary. Annealing of a deformed sample leads to solute segregation and very strong pinning so that further twinning can only be accomplished by nucleation of new twins, consistent with experimental observations. The new mechanism also (i) operates for detwinning and rationalizes complex twin shapes observed in experiments and (ii) suggests a process for dynamic strain aging commonly observed in Mg alloys at elevated temperatures

Investigation of the size of plastic zones in nano indentation and nano scratching

Friction and the associated wear are important but still poorly understood phenomena with strong impacts on our every day lives. Several mechanisms, such as plasticity, lattice vibration, and third-body interactions contribute to the dissipation of energy in friction phenomena. This physical complexity is further increased by the inherently multiscale nature of contact. Indeed, it is well known that roughness exists over multiple length scales, which imposes a multiscale numerical treatment. Our objective in this study is to analyse the development of plastic events at contacting asperities in fcc metals. Dislocation nucleation can happen at the contact surface or – in special cases – as bulk nucleation [1] underneath the surface. We capture these dislocations by molecular dynamics (MD) mod- elling of the contact zones. As dislocation activity extends far away from the contact, it is not feasible to tackle this problem via MD alone. Therefore, to reduce computational cost, we resort to coupling MD to a discrete dislocation dynamics (DD) domain [2], into which MD dislocations may enter. The coupling method used is the recently proposed coupled atomistics and discrete dislocations (CADD) method [3, 4, 5]. It has so far been implemented only in 2D and therefore effectively models asperities with an infinite third dimension (cylindrical asperities). In the first part of our presentation, we evaluate the systematic differences between 2D and 3D contact in pure MD calculations. We use this comparison to motivate 2D simulations. Finally, we present simulation results obtained at different scratching speeds for several normal forces and indenter sizes and shapes. We monitor the friction coefficient and scratching forces and relate them to the energy dissipated in the form of discrete plasticity events

Average-atom interatomic potential for random alloys

An average-atom (A-atom) embedded-atom-method potential for random multicomponent alloys at any composition is derived analytically and validated by comparing A-atom and true random alloys bulk and defect properties, in model Fe-Ni-Cr systems. The A-atom can be mixed with the individual alloying-element potentials, thus enabling computation of defect/defect interactions. Its use provides quantitative insight into the physical role of the fluctuations, and has many applications, such as in atomistic/continuum modeling of random alloys and the development of new potentials with controlled properties

Sedimentary DNA and molecular evidence for early human occupation of the Faroe Islands

The Faroe Islands, a North Atlantic archipelago between Norway and Iceland, were settled by Viking explorers in the mid-9th century CE. However, several indirect lines of evidence suggest earlier occupation of the Faroes by people from the British Isles. Here, we present sedimentary ancient DNA and molecular fecal biomarker evidence from a lake sediment core proximal to a prominent archaeological site in the Faroe Islands to establish the earliest date for the arrival of people in the watershed. Our results reveal an increase in fecal biomarker concentrations and the first appearance of sheep DNA at 500 CE (95% confidence interval 370-610 CE), pre-dating Norse settlements by 300 years. Sedimentary plant DNA indicates an increase in grasses and the disappearance of woody plants, likely due to livestock grazing. This provides unequivocal evidence for human arrival and livestock disturbance in the Faroe Islands centuries before Viking settlement in the 9th century

Sedimentary DNA and Molecular Evidence for Early Human Occupation of the Faroe Islands

The Faroe Islands, a North Atlantic archipelago between Norway and Iceland, were settled by Viking explorers in the mid-9th century CE. However, several indirect lines of evidence suggest earlier occupation of the Faroes by people from the British Isles. Here, we present sedimentary ancient DNA and molecular fecal biomarker evidence from a lake sediment core proximal to a prominent archaeological site in the Faroe Islands to establish the earliest date for the arrival of people in the watershed. Our results reveal an increase in fecal biomarker concentrations and the first appearance of sheep DNA at 500 CE (95% confidence interval 370-610 CE), pre-dating Norse settlements by 300 years. Sedimentary plant DNA indicates an increase in grasses and the disappearance of woody plants, likely due to livestock grazing. This provides unequivocal evidence for human arrival and livestock disturbance in the Faroe Islands centuries before Viking settlement in the 9th century