Multi-scale investigation of short-range order and dislocation glide in MoNbTi and TaNbTi multi-principal element alloys

Refractory multi-principal element alloys(RMPEAs)are promising materials for high-temperature structural applications.Here,we investigate the role of short-range ordering(SRO)on dislocation glide in the MoNbTi and TaNbTi RMPEAs using a multi-scale modeling approach.Monte carlo/molecular dynamics simulations with a moment tensor potential show that MoNbTi exhibits a much greater degree of SRO than TaNbTi and the local composition has a direct effect on the unstable stacking fault energies(USFEs).From mesoscale phase-field dislocation dynamics simulations,we find that increasing SRO leads to higher mean USFEs and stress required for dislocation glide.The gliding dislocations experience significant hardening due to pinning and depinning caused by random compositional fluctuations,with higher SRO decreasing the degree of USFE dispersion and hence,amount of hardening.Finally,we show how the morphology of an expanding dislocation loop is affected by the applied stress.

An electrostatic spectral neighbor analysis potential for lithium nitride

Machine-learned interatomic potentials based on local environment descriptors represent a transformative leap over traditional potentials based on rigid functional forms in terms of prediction accuracy.However,a challenge in their application to ionic systems is the treatment of long-ranged electrostatics.Here,we present a highly accurate electrostatic Spectral Neighbor Analysis Potential(eSNAP)for ionicα-Li3N,a prototypical lithium superionic conductor of interest as a solid electrolyte or coating for rechargeable lithium-ion batteries.We show that the optimized eSNAP model substantially outperforms traditional Coulomb–Buckingham potential in the prediction of energies and forces,as well as various properties,such as lattice constants,elastic constants,and phonon dispersion curves.We also demonstrate the application of eSNAP in long-time,large-scale Li diffusion studies in Li3N,providing atomistic insights into measures of concerted ionic motion(e.g.,the Haven ratio)and grain boundary diffusion.This work aims at providing an approach to developing quantum-accurate force fields for multi-component ionic systems under the SNAP formalism,enabling large-scale atomistic simulations for such systems.

Complex strengthening mechanisms in the NbMoTaW multiprincipal element alloy

Refractory multi-principal element alloys(MPEAs)have exceptional mechanical properties,including high strength-to-weight ratio and fracture toughness,at high temperatures.Here we elucidate the complex interplay between segregation,short-range order,and strengthening in the NbMoTaW MPEA through atomistic simulations with a highly accurate machine learning interatomic potential.In the single crystal MPEA,we find greatly reduced anisotropy in the critically resolved shear stress between screw and edge dislocations compared to the elemental metals.In the polycrystalline MPEA,we demonstrate that thermodynamically driven Nb segregation to the grain boundaries(GBs)and W enrichment within the grains intensifies the observed short-range order(SRO).The increased GB stability due to Nb enrichment reduces the von Mises strain,resulting in higher strength than a random solid solution MPEA.These results highlight the need to simultaneously tune GB composition and bulk SRO to tailor the mechanical properties of MPEAs.