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CoCrNi -medium-entropy alloy (MEA) has been widely investigated due to its superior mechanical properties that overcome strength-ductility tradeoff. Here we show further property enhancement of CoCrNi MEA by introducing nano-scale features. Both CoCrNi and oxide dispersion strengthened (ODS) CoCrNi are fabricated by mechanical alloying and spark plasma sintering. Microstructural characterization and mechanical testing of these nanostructured MEAs revealed that the nano-scale features significantly improves the strength of the alloys. In ODS-CoCrNi MEA, Y₂Ti₂O₇ oxides with an average diameter of 7.3 ± 3.2 nm are incoherent with the matrix, and a specific orientation relationship exists between Y₂Ti₂O₇ and the matrix, whichmore » is [011]_Y2Ti2O7//[011]Matrix, (400)_Y2Ti2O7//(200)Matrix and ($$22\bar2$$)_Y2Ti2O7//($$11\bar1$$)Matrix. The recrystallization and grain growth processes are effectively suppressed by the introduction of Y₂Ti₂O₇ nanoparticles. Strengthening mechanism analyses indicate that the strength improvement of ODS-CoCrNi is mainly ascribed to the precipitation strengthening of Y₂Ti₂O₇.« less

Microstructure and mechanical properties of two kinds of oxide dispersion strengthened concentrated solid solution alloys (CSAs), FeCoNi-1.5Y₂O₃ (FCNY) and FeCoNi-1.2Ti-1.5Y₂O₃ (FCNTY), are studied through a comparing investigation with FeCoNi (FCN) CSAs. All these alloys are fabricated by mechanical alloying, spark plasma sintering, hot rolling and annealing treatment. For three kinds of alloys, both the as-milled powders and bulk materials are of single face-centered cubic structure. Electron backscattered diffraction results reveal that the texture transformation is suppressed during the hot rolling process because the movement of grain boundaries is hindered by the oxide particles. Compared with FCN CSAs, grains are refinedmore » by 43% and 47% for FCNY and FCNTY CSAs, respectively. Nano-sized Y₂O₃ (monoclinic structure) and Y₂Ti₂O₇ (pyrochlore structure) particles are uniformly distributed in FCNY and FCNTY CSAs, respectively. Both Y2O3 and Y2Ti2O7 particles show a semi-coherent relationship with the matrix. Yield strength of FCN, FCNY and FCNTY CSAs is 559, 981 and 1050 MPa, respectively. Theoretical calculations illustrate that high strength of FCNY and FCNTY CSAs comes from refined grains and high-density nano-sized oxide particles.« less

FeCoNi concentrated solid-solution alloys (CSAs) and its modified version strengthened by in-situ formed nano-sized particles are successfully manufactured through mechanical alloying and spark plasma sintering method. Both the as-milled powders and the bulk materials are of single face-centered cubic structure. After sintering and heat treatment, grains are refined by 68% with the addition of oxides. In oxide dispersion strengthened (ODS) FeCoNi CSAs, the majority of nano-sized particles with diameter ranging from a few to ~ 40 nm are uniformly distributed in the matrix, and most of them are identified as Y₂Ti₂O₇. Electron backscattered diffraction results reveal that the recrystallization processmore » is suppressed due to the existence of the nano-sized particles. The size and distribution of oxide particles in the matrix can be controlled by adjusting experiment parameters of the mechanical alloying process and annealing treatment. Finally, refined grains and high-density nano-sized particles result in enhanced compressive yield strength for ODS-FeCoNi CSAs at room temperature and 700 °C, which is 1295 MPa and 127 MPa, respectively.« less

This work details an integrated investigation of liquid crystal (LC) oligomers that combines experiments and molecular dynamics simulations to obtain a detailed understanding of the molecular structure of LC oligomers and the mechanism underlying their phase transition temperatures. We synthesized and characterized a series of LC oligomers prepared from different lengths of methylene spacers in the reactive LC monomers and n-alkylamine chain extenders via the aza-Michael addition reaction. In parallel, we performed isothermal–isobaric (NPT) ensemble coarse-grained molecular dynamics (CG-MD) simulation of analogue mesogens that are connected to flexible spacers and extenders at varying temperatures, spacer lengths, and extender lengths. Thismore » approach allowed the effect of length in the flexible spacer as well as in the chain extender on the nematic–isotropic transition temperature (T_ni) to be determined. The results showed that increasing the length of the extender decreases T_ni for LC oligomers and amplifies the decrease of T_ni in LC oligomers when the spacer length is short. We infer that the combination of spacer and extender changes the shape anisotropy of LC oligomers, changing the packing behavior of constituent mesogens, thus affecting their ability to transition from the isotropic to the nematic phase. Here, the detailed molecular structure–property relationships formulated enable prescribing design rules for LC oligomers geared toward molecularly engineered shape changing materials.« less