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1. A Multiscale Simulation Approach for the Mechanical Response of Copper/Nickel Nanofoams With Experimental Validation

   https://doi.org/10.1115/1.4051806  

   Ke, Hang ; Loaiza, Alexandra ; Jimenez, Andres G. ; Bahr, David F. ; Mastorakos, Ioannis ( January 2022 , Journal of Engineering Materials and Technology)

   null (Ed.)

   Abstract Metallic nanofoams, cellular structures consisting of interlinked thin nanowires and empty pores, create low density, high surface area materials. These structures can suffer from macroscopically brittle behavior. In this work, we present a multiscale approach to study and explain the mechanical behavior of metallic nanofoams obtained by an electrospinning method. In this multiscale approach, atomistic simulations were first used to obtain the yield surfaces of different metallic nanofoam cell structures. Then, a continuum plasticity model using finite elements was used to predict the alloy nanofoam's overall strength in compression. The manufactured metallic nanofoams were produced by electrospinning a polymeric non-woven fabric containing metal precursors for alloys of copper–nickel and then thermally processing the fabric to create alloy metallic nanofoams. The nanofoams were tested with nanoindentation. The experimental results suggest that the addition of nickel increases the hardening of the nanofoams. The multiscale simulation modeling results agreed qualitatively with the experiments by suggesting that the addition of the alloying can be beneficial to the hardening behavior of the metallic nanofoams and helps to isolate the effects of alloying from morphological changes in the foam. This behavior was related to the addition of solute atoms that prevent the free dislocation movement and increase the strength of the structure. 

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2. Improving the flame retardancy of polypropylene foam with piperazine pyrophosphate via multilayering coextrusion of film/foam composites

   https://doi.org/10.1002/app.48552  

   Lee, Sangjin ; Morgan, Alexander B. ; Schiraldi, David A. ; Maia, João ( October 2019 , Journal of Applied Polymer Science)

   A series of 16‐layer polypropylene/flame retardant (PP/FR) film/foam composite structures were produced by microlayer coextrusion. A highly branched PP was used in the foam layers to increase strain hardening and cell stability, while the PP used in the film layers was a high shear viscosity grade to confine bubble growth. In addition to improved tensile properties, the PP/FR composite film/foams exhibited five times the compression modulus of PP/FR composite foams at each FR loading level. The thermal stabilities of the composites were investigated, exhibiting three step decompositions. The FR particles were effective in decreasing flammability by forming intumescent char. The PP/FR‐film/foam‐20 showed self‐extinguishing behavior in a modified vertical burn test, while the PP/FR‐foam‐20 sample continued to burn. Cone calorimetry demonstrated that PP/FR film/foams had lower heat release than PP/FR foams due to the unique alternating film/foam structure of PP/FR film/foams. Scanning electron microscopy imaging of the residual chars from fire testing that the PP/FR composite film/foams showed a more continuous protective char surface when compared with PP/FR composite foams at each FR concentration. The combined data indicate that the formation of a surface film on top of a foam ensures a robust intumescent fire protective barrier for partly foamed materials and shows a new way toward lightweight materials with improved fire safety performance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2020,137, 48552.

    

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3. The impact of alloying on defect-free nanoparticles exhibiting softer but tougher behavior

   https://doi.org/10.1038/s41467-021-22707-x  

   Bisht, Anuj ; Koju, Raj Kiran ; Qi, Yuanshen ; Hickman, James ; Mishin, Yuri ; Rabkin, Eugen ( May 2021 , Nature Communications)

   The classic paradigm of physical metallurgy is that the addition of alloying elements to metals increases their strength. It is less known if the solution-hardening can occur in nano-scale objects, and it is totally unknown how alloying can impact the strength of defect-free faceted nanoparticles. Purely metallic defect-free nanoparticles exhibit an ultra-high strength approaching the theoretical limit. Tested in compression, they deform elastically until the nucleation of the first dislocation, after which they collapse into a pancake shape. Here, we show by experiments and atomistic simulations that the alloying of Ni nanoparticles with Co reduces their ultimate strength. This counter-intuitive solution-softening effect is explained by solute-induced local spatial variations of the resolved shear stress, causing premature dislocation nucleation. The subsequent particle deformation requires more work, making it tougher. The emerging compromise between strength and toughness makes alloy nanoparticles promising candidates for applications.

    

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4. Evolution of microstructure and strength of a high entropy alloy undergoing the strain-induced martensitic transformation

   https://doi.org/10.1016/j.msea.2023.145754  

   Weiss, Jacob ; Savage, Daniel J. ; Vogel, Sven C. ; McWilliams, Brandon A. ; Mishra, Rajiv S. ; Knezevic, Marko ( November 2023 , Materials Science and Engineering: A)

   In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage. 

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5. Bandgap Engineering in 2D Lateral Heterostructures of Transition Metal Dichalcogenides via Controlled Alloying

   https://doi.org/10.1002/smll.202106600  

   Nugera, Florence A. ; Sahoo, Prasana K. ; Xin, Yan ; Ambardar, Sharad ; Voronine, Dmitri V. ; Kim, Un Jeong ; Han, Yoojoong ; Son, Hyungbin ; Gutiérrez, Humberto R. ( January 2022 , Small)

   2D heterostructures made of transition metal dichalcogenides (TMD) have emerged as potential building blocks for new‐generation 2D electronics due to their interesting physical properties at the interfaces. The bandgap, work function, and optical constants are composition dependent, and the spectrum of applications can be expanded by producing alloy‐based heterostructures. Herein, the successful synthesis of monolayer and bilayer lateral heterostructures, based on ternary alloys of MoS_2(1−x)Se_2x–WS_2(1−x)Se_2x, is reported by modifying the ratio of the source precursors; the bandgaps of both materials in the heterostructure are continuously tuned in the entire range of chalcogen compositions. Raman and photoluminescence (PL) spatial maps show good intradomain composition homogeneity. Kelvin probe measurements in different heterostructures reveal composition‐dependent band alignments, which can further be affected by unintentional electronic doping during the growth. The fabrication of sequential multijunction lateral heterostructures with three layers of thickness, composed of quaternary and ternary alloys, is also reported. These results greatly expand the available tools kit for optoelectronic applications in the 2D realm.

    

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