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1. Solid-state additive manufacturing of aluminum and copper using additive friction stir deposition: Process-microstructure linkages

   https://doi.org/https://doi.org/10.1016/j.mtla.2020.100967  

   Griffiths, R. Joey ; Garcia, David ; Song, Jie ; Vasudevan, Vijay K. ; Steiner, Matthew A. ; Cai, Wenjun ; Yu, Hang Z. ( March 2021 , Materialia)

   null (Ed.)

   Among metal additive manufacturing technologies, additive friction stir deposition stands out for its ability to create freeform and fully-dense structures without melting and solidification. Here, we employ a comparative approach to investigate the process-microstructure linkages in additive friction stir deposition, utilizing two materials with distinct thermomechanical behavior—an Al-Mg-Si alloy and Cu—both of which are challenging to print using beam-based additive processes. The deposited Al-Mg-Si is shown to exhibit a relatively homogeneous microstructure with extensive subgrain formation and a strong shear texture, whereas the deposited Cu is characterized by a wide distribution of grain sizes and a weaker shear texture. We show evidence that the microstructure in Al-Mg-Si primarily evolves by continuous dynamic recrystallization, including geometric dynamic recrystallization and progressive lattice rotation, while the heterogeneous microstructure of Cu results from discontinuous recrystallization during both deposition and cooling. In Al-Mg-Si, the continuous recrystallization progresses with an increase of the applied strain, which correlates with the ratio between the tool rotation rate and travel velocity. Conversely, the microstructure evolution in Cu is found to be less dependent on , instead varying more with changes to . This difference originates from the absence of Cu rotation in the deposition zone, which reduces the influence of tool rotation on strain development. We attribute the distinct process-microstructure linkages and the underlying mechanisms between Al-Mg-Si and Cu to their differences in intrinsic thermomechanical properties and interactions with the tool head. 

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2. Density‐Dependent Impact Resilience and Auxeticity of Elastomeric Polyurea Foams

   https://doi.org/10.1002/adem.202200578  

   Youssef, George ; Kokash, Yazeed ; Uddin, Kazi Zahir ; Koohbor, Behrad ( September 2022 , Advanced Engineering Materials)

   This research investigates the dynamic response of a novel polyurea foam with different densities by separately submitting samples to single and multiple impacts at different energies ranging from 1.77 to 7.09 J. The impact and transmitted force‐time histories are acquired during the impact events. Deformation of the samples is also recorded using high‐speed photography and analyzed using digital image correlation (DIC) to characterize density‐dependent strain rate and Poisson's ratio. The analyses of the force‐time histories highlight the interrelationship between the incoming impact energy and force characteristics, including amplitude and durations. The experimental results reveal that polyurea foams can absorb nearly 50% of the incoming impact energy irrespective of their density. The dynamic impact efficacy of the foam persists even after sequential impact events are imparted on the same samples, with only a 20% drop in the load‐bearing capacity after seven consecutive impacts. Furthermore, as verified via electron microscopy observations, the higher‐density foam does not exhibit any permanent damage. This high‐density polyurea foam shows reversible auxetic transition at all impact energies considered herein. The outcomes of this research indicate the suitability of polyurea foams for cushioning and impact mitigation applications, especially in repeated biomechanical impact scenarios.

    

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3. First principles predicting enhanced ductility of boride carbide through magnesium microalloying

   https://doi.org/10.1111/jace.16383  

   Tang, Bin ; He, Yi ; Goddard, III, William A. ; An, Qi ( February 2019 , Journal of the American Ceramic Society)

   The low fracture toughness of strong covalent solids prevents them from wide engineering applications. Microalloying metal elements into covalent solids may lead to a significant improvement on mechanical properties and drastical changes on the chemical bonding. To illustrate these effects we employed density functional theory (DFT) to examine the bonding characteristic and mechanical failure of recently synthesized magnesium boride carbide (Mg₃B₅₀C₈) that is formed by adding Mg into boron carbide (B₄C). We found that Mg₃B₅₀C₈has more metallic bonding charterer than B₄C, but the atomic structure still satisfies Wade's rules. The metallic bonding significantly affects the failure mechanisms of Mg₃B₅₀C₈compared with B₄C. In Mg₃B₅₀C₈, the B₁₂icosahedral clusters are rotated in order to accommodate to the extensive shear strain without deconstruction. In addition, the critical failure strength of Mg₃B₅₀C₈is slightly higher than that of B₄C under indentation stress conditions. Our results suggested that the ductility of Mg₃B₅₀C₈is drastically enhanced compared with B₄C while the hardness is slightly higher than B₄C.

    

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4. Potential energy landscape activations governing plastic flows in glass rheology

   https://doi.org/10.1073/pnas.1907317116  

   Cao, Penghui ; Short, Michael P. ; Yip, Sidney ( September 2019 , Proceedings of the National Academy of Sciences)

   While glasses are ubiquitous in natural and manufactured materials, the atomic-level mechanisms governing their deformation and how these mechanisms relate to rheological behavior are still open questions for fundamental understanding. Using atomistic simulations spanning nearly 10 orders of magnitude in the applied strain rate we probe the atomic rearrangements associated with 3 characteristic regimes of homogeneous and heterogeneous shear flow. In the low and high strain-rate limits, simulation results together with theoretical models reveal distinct scaling behavior in flow stress variation with strain rate, signifying a nonlinear coupling between thermally activated diffusion and stress-driven motion. Moreover, we find the emergence of flow heterogeneity is closely correlated with extreme values of local strain bursts that are not readily accommodated by immediate surroundings, acting as origins of shear localization. The atomistic mechanisms underlying the flow regimes are interpreted by analyzing a distance matrix of nonaffine particle displacements, yielding evidence of various barrier-hopping processes on a fractal potential energy landscape (PEL) in which shear transformations and liquid-like regions are triggered by the interplay of thermal and stress activations. 

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5. The L–G phase transition in binary Cu–Zr metallic liquids

   https://doi.org/10.1039/D1CP04157F  

   An, Qi ; Johnson, William L. ; Samwer, Konrad ; Corona, Sydney L. ; Shen, Yidi ; Goddard, William A. ( December 2021 , Physical Chemistry Chemical Physics)

   The authors recently reported that undercooled liquid Ag and Ag–Cu alloys both exhibit a first order phase transition from the homogeneous liquid (L-phase) to a heterogeneous solid-like G-phase under isothermal evolution. Here, we report a similar L–G transition and heterogenous G-phase in simulations of liquid Cu–Zr bulk glass. The thermodynamic description and kinetic features (viscosity) of the L-G-phase transition in Cu–Zr simulations suggest it corresponds to experimentally reported liquid–liquid phase transitions in Vitreloy 1 (Vit1) and other Cu–Zr-bearing bulk glass forming alloys. The Cu–Zr G-phase has icosahedrally ordered cores versus fcc/hcp core structures in Ag and Ag–Cu with a notably smaller heterogeneity length scale Λ . We propose the L–G transition is a phenomenon in metallic liquids associated with the emergence of elastic rigidity. The heterogeneous core–shell nano-composite structure likely results from accommodating strain mismatch of stiff core regions by more compliant intervening liquid-like medium. 

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