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1. Multiplicity of dislocation pathways in a refractory multiprincipal element alloy

   https://doi.org/10.1126/science.aba3722  

   Wang, Fulin; Balbus, Glenn H.; Xu, Shuozhi; Su, Yanqing; Shin, Jungho; Rottmann, Paul F.; Knipling, Keith E.; Stinville, Jean-Charles; Mills, Leah H.; Senkov, Oleg N.; et al (October 2020, Science)

   Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum. 

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2. Polarization near dislocation cores in SrTiO3 single crystals: The role of flexoelectricity

   https://doi.org/10.1063/5.0202453  

   Cheng, Xiaoxing; Wang, Bo; Chen, Long-Qing (March 2024, Applied Physics Letters)

   Spontaneous polarization as large as ∼28 μC/cm2 was recently observed around the dislocation cores in non-polar SrTiO3 bulk crystals, and its origin was attributed to the flexoelectric effect, i.e., polarization induced by strain gradients. However, the roles of flexoelectricity, relative to other electromechanical contributions, and the nature of dislocations, i.e., edge vs screw dislocations in the induced polarization, are not well understood. In this work, we study the role of flexoelectricity in inducing polarization around three types of dislocation cores in SrTiO3: b=a(100) edge dislocation, b=a(110) edge dislocation, and b=a(010) screw dislocation, where b is the Burgers vector. For the edge dislocations, polarization can be induced by electrostriction alone, while flexoelectricity is essential for stabilizing the symmetric polarization pattern. The shear component of the flexoelectric tensor has a dominant effect on the magnitude and spatial distribution of the flexoelectric polarization. In contrast, no polarization is induced around the b=a(010) screw dislocation through either electrostriction or flexoelectricity. Our findings provide an in-depth understanding of the role of flexoelectricity in inducing polarization around dislocation cores and offer insights into the defect engineering of dielectric/ferroelectric materials. 

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3. Cross-kinks control screw dislocation strength in equiatomic bcc refractory alloys

   Zhou, X.; He, S.; Marian, J. (April 2021, Acta materialia)

   null (Ed.)

   Refractory multi-element alloys (RMEA) with body-centered cubic (bcc) structure have been the object of much research over the last decade due to their high potential as candidate materials for high- temperature applications. Most of these alloys display a remarkable strength at high temperatures, which cannot be explained by the standard model of bcc plasticity based on thermally-activated screw disloca- tion motion. Several works have pointed to chemical energy fluctuations as an essential aspect of RMEA strength that is not captured by standard models. In this work, we quantify the contribution of screw dis- locations to the strength of equiatomic Nb-Ta-V alloys using a kinetic Monte Carlo model fitted to solu- tion energetics obtained from atomistic calculations. In agreement with molecular dynamics simulations, we find that chemical energy fluctuations along the dislocation line lead to measurable concentrations of kinks in equilibrium in a wide temperature range. A fraction of these form cross-kink configurations, which are ultimately found to control screw dislocation motion and material strength. Our simulations (i) confirm that the evolution of cross kinks and self-pinning are strong contributors to the so-called ‘cocktail’ effect in this alloy at low temperature, and (ii) substantiate the notion that screw dislocation plasticity alone cannot explain the high temperature strength of bcc RMEA. 

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4. On the computational solution of vector-density based continuum dislocation dynamics models: a comparison of two plastic distortion and stress update algorithms

   https://doi.org/https://doi.org/10.1016/j.ijplas.2021.102943  

   P. Lin, V. Vivekanandan. (January 2021, International journal of plasticity)

   null (Ed.)

   Continuum dislocation dynamics models of mesoscale plasticity consist of dislocation transport-reaction equations coupled with crystal mechanics equations. The coupling between these two sets of equations is such that dislocation transport gives rise to the evolution of plastic distortion (strain), while the evolution of the latter fixes the stress from which the dislocation velocity field is found via a mobility law. Earlier solutions of these equations employed a staggered solution scheme for the two sets of equations in which the plastic distortion was updated via time integration of its rate, as found from Orowan’s law. In this work, we show that such a direct time integration scheme can suffer from accumulation of numerical errors. We introduce an alternative scheme based on field dislocation mechanics that ensures consistency between the plastic distortion and the dislocation content in the crystal. The new scheme is based on calculating the compatible and incompatible parts of the plastic distortion separately, and the incompatible part is calculated from the current dislocation density field. Stress field and dislocation transport calculations were implemented within a finite element based discretization of the governing equations, with the crystal mechanics part solved by a conventional Galerkin method and the dislocation transport equations by the least squares method. A simple test is first performed to show the accuracy of the two schemes for updating the plastic distortion, which shows that the solution method based on field dislocation mechanics is more accurate. This method then was used to simulate an austenitic steel crystal under uniaxial loading and multiple slip conditions. By considering dislocation interactions caused by junctions, a hardening rate similar to discrete dislocation dynamics simulation results was obtained. The simulations show that dislocations exhibit some self-organized structures as the strain is increased. 

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5. Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy

   https://doi.org/10.1126/science.adn2428  

   Cook, David H.; Kumar, Punit; Payne, Madelyn I.; Belcher, Calvin H.; Borges, Pedro; Wang, Wenqing; Walsh, Flynn; Li, Zehao; Devaraj, Arun; Zhang, Mingwei; et al (April 2024, Science)

   Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy’s behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m^1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip. 

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