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1. 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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2. Models of dislocation glide and strengthening mechanisms in bcc complex concentrated alloys

   https://doi.org/10.1557/s43577-023-00571-y  

   Zhou, Xinran ; Wang, Xinyi ; Fey, Lauren ; He, Sicong ; Beyerlein, Irene ; Cao, Penghui ; Marian, Jaime ( August 2023 , MRS Bulletin)

   The mechanical response of complex concentrated alloys (CCAs) deviates from that of their pure and dilute counterparts due to the introduction of a combinatorially sized chemical concentration dimension. Compositional fluctuations constantly alter the energy landscape over which dislocations move, leading to line roughness and the appearance of defects such as kinks and jogs under stress and temperature conditions where they would ordinarily not exist in pure metals and dilute alloys. The presence of suchchemicaldefects gives rise to atomic-level mechanisms that fundamentally change how CCAs deform plastically at meso- and macroscales. In this article, we provide a review of recent advances in modeling dislocation glide processes in CCAs, including atomistic simulations of dislocation glide using molecular dynamics, kinetic Monte Carlo simulations of edge and screw dislocation motion in refractory CCAs, and phase-field models of dislocation evolution over complex energy landscapes. We also discuss pathways to develop comprehensive simulation methodologies that connect an atomic-level description of the compositional complexity of CCAs with their mesoscopic dislocation-mediated plastic response with an eye toward improved design of CCA with superior mechanical response.

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3. 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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4. Additive manufacturing of an ultrastrong, deformable Al alloy with nanoscale intermetallics

   https://doi.org/10.1038/s41467-024-48693-4  

   Shang, Anyu ; Stegman, Benjamin ; Choy, Kenyi ; Niu, Tongjun ; Shen, Chao ; Shang, Zhongxia ; Sheng, Xuanyu ; Lopez, Jack ; Hoppenrath, Luke ; Zhang, Bohua_Peter ; et al ( June 2024 , Nature Communications)

   Light-weight, high-strength, aluminum (Al) alloys have widespread industrial applications. However, most commercially available high-strength Al alloys, like AA 7075, are not suitable for additive manufacturing due to their high susceptibility to solidification cracking. In this work, a custom Al alloy Al₉₂Ti₂Fe₂Co₂Ni₂is fabricated by selective laser melting. Heterogeneous nanoscale medium-entropy intermetallic lamella form in the as-printed Al alloy. Macroscale compression tests reveal a combination of high strength, over 700 MPa, and prominent plastic deformability. Micropillar compression tests display significant back stress in all regions, and certain regions have flow stresses exceeding 900 MPa. Post-deformation analyses reveal that, in addition to abundant dislocation activities in Al matrix, complex dislocation structures and stacking faults form in monoclinic Al₉Co₂type brittle intermetallics. This study shows that proper introduction of heterogeneous microstructures and nanoscale medium entropy intermetallics offer an alternative solution to the design of ultrastrong, deformable Al alloys via additive manufacturing.

    

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5. Hydrogen embrittlement in metallic nanowires

   https://doi.org/10.1038/s41467-019-10035-0  

   Yin, Sheng ; Cheng, Guangming ; Chang, Tzu-Hsuan ; Richter, Gunther ; Zhu, Yong ; Gao, Huajian ( May 2019 , Nature Communications)

   Although hydrogen embrittlement has been observed and extensively studied in a wide variety of metals and alloys, there still exist controversies over the underlying mechanisms and a fundamental understanding of hydrogen embrittlement in nanostructures is almost non-existent. Here we use metallic nanowires (NWs) as a platform to study hydrogen embrittlement in nanostructures where deformation and failure are dominated by dislocation nucleation. Based on quantitative in-situ transmission electron microscopy nanomechanical testing and molecular dynamics simulations, we report enhanced yield strength and a transition in failure mechanism from distributed plasticity to localized necking in penta-twinned Ag NWs due to the presence of surface-adsorbed hydrogen. In-situ stress relaxation experiments and simulations reveal that the observed embrittlement in metallic nanowires is governed by the hydrogen-induced suppression of dislocation nucleation at the free surface of NWs.

    

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