Near-threshold fatigue crack growth in bulk metallic glass composites

A major drawback in using bulk metallic glasses (BMGs) as structural materials is their extremely poor fatigue performance. One way to alleviate this problem is through the composite route, in which second phases are introduced into the glass to arrest crack growth. In this paper, the fatigue crack growth behavior of in situ reinforced BMGs with crystalline dendrites, which are tailored to impart significant ductility and toughness to the BMG, was investigated. Three composites, all with equal volume fraction of dendrite phases, were examined to assess the influence of chemical composition on the near-threshold fatigue crack growth characteristics. While the ductility is enhanced at the cost of yield strength vis-à-vis that of the fully amorphous BMG, the threshold stress intensity factor range for fatigue crack initiation in composites was found to be enhanced by more than 100%. Crack blunting and trapping by the dendritic phases and constraining of the shear bands within the interdendritic regions are the micromechanisms responsible for this enhanced fatigue crack growth resistance

Effect of hydrogen on the nanomechanical behavior of dual-phase nanocrystalline high-entropy alloy

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Spherical indentation study on incipient plasticity of medium-/high-entropy alloys

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Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass

Instrumented indentation experiments on a Zr-based bulk metallic glass (BMG) in as-cast, shot-peened and structurally relaxed conditions were conducted to examine the dependence of plastic deformation on its structural state. Results show significant differences in hardness, H, with structural relaxation increasing it and shot peening markedly reducing it, and slightly changed morphology of shear bands around the indents. This effect is in contrast to uniaxial compressive yield strength, σ y, which remains invariant with the change in the structural state of the alloys investigated. The plastic constraint factor, C = H/σ y, of the relaxed BMG increases compared with that of the as-cast glass, indicating enhanced pressure sensitivity upon annealing. In contrast, C of the shot-peened layer was found to be similar to that observed in crystalline metals, indicating that severe plastic deformation could eliminate pressure sensitivity. Microscopic origins for this result, in terms of shear transformation zones and free volume, are discusse

Structure-property correlations in piracetam polytypes

Polymorphs II and III of piracetam exhibit a polytypic relationship comprising identical layers of molecules with different relative arrangements. Polymorph II has an interlayer structure in which the piracetam molecules adopt face-to-face and edge-to-edge alignments, while polymorph III adopts a herringbone type arrangement in the interlayer region. The structures are analysed using energy-vector models derived from PIXEL pairwise intermolecular interaction energies. Thermal expansion measurements show that the principal expansion axes are approximately aligned with the unit-cell axes in polymorph III, corresponding to directions within the polytypic layers and perpendicular to them. Expansion perpendicular to the layers is almost twice as large as that along any direction within the layers. Polymorph II shows greater volumetric expansion than polymorph III, and its principal expansion axes are aligned parallel and perpendicular to the planes of the piracetam molecules, rather than along the unit-cell axes. Nanoindentation experiments performed on single crystals along the direction perpendicular to the polytypic layers show that the polymorphs have similar hardness (H) values, but polymorph III has a significantly larger elastic modulus (E). Along the direction nearly parallel to the polytypic layers, polymorph II shows a very similar E value to that perpendicular to the layers, but a significantly smaller H value, implying easier slip between the polytypic layers. The tableting behaviour of bulk polymorph II is superior to that of polymorph III, suggesting greater plasticity for polymorph II, which is likely due to a greater degree of slip.We are grateful to the Danish Council for Independent Research | Natural Sciences (DFF-1323-00122), and to the Department of Pharmacy, University of Copenhagen, for funding this wor

Chemically stabilized epitaxial wurtzite-BN thin film

We report on the chemically stabilized epitaxial w-BN thin film grown on c-plane sapphire by pulsed laser deposition under slow kinetic condition. Traces of no other allotropes such as cubic (c) or hexagonal (h) BN phases are present. Sapphire substrate plays a significant role in stabilizing the metastable w-BN from h-BN target under unusual PLD growth condition involving low temperature and pressure and is explained based on density functional theory calculation. The hardness and the elastic modulus of the w-BN film are 37 & 339 GPa, respectively measured by indentation along direction. The results are extremely promising in advancing the microelectronic and mechanical tooling industry

対流圏に見られる鉛直微細構造

We report the design, synthesis, detailed characterization, and analysis of a new multifunctional pi-conjugated bola-amphiphilic chromophore: oligo-(p-phenyleneethynylene)-dicarboxylic acid with dialkoxyoctadecyl side chains (OPE-C-18-1). OPE-C-18-1 shows two polymorphs at 123 K (OPE-C-18-1') and 373 K (OPE-C-18-1 `'), whose crystal structures were characterized via single crystal X-ray diffraction. OPE-C-18-1 also exhibits thermotropic liquid crystalline property revealing a columnar phase. The inherent pi-conjugation of OPE-C-18-1 imparts luminescence to the system. Photoluminescence measurements on the mesophase also reveal similar luminescence as in the crystalline state. Additionally, OPE-C-18-1 shows mechano-hypsochromic luminescence behavior. Density functional theory (DFT)-based calculations unravel the origins behind the simultaneous existence of all these properties. Nanoindentation experiments on the single crystal reveal its mechanical strength and accurately correlate the molecular arrangement with the liquid crystalline and mechanochromic luminescence behavior

Epimers with distinct mechanical behaviours

This study highlights the impact of relative stereochemistry in epimer compounds on their mechanical properties; the crystals of one series of esters are ductile and deform plastically upon bending, whereas the other series are all brittle. Nanoindentation studies show that the hardness, H, and elastic moduli, E, of the brittle crystals are substantially larger than those of the ductile ones. For the brittle crystals, the H values range from 153(10) to 293(37) MPa and E from 2.85(0.33) to 9.10(0.51) GPa, whereas for the ductile crystals, the H values range from 76(2) to 125(11) MPa and E from 1.40(0.36) and 2.75(0.06) GPa. These are rationalized by recourse to the distinct crystal structural features, especially in terms of interdigitation in the molecular planes in the brittle crystals and slip planes in the ductile crystals. The indentation fracture toughness, Kc, values of the (2′S) crystals are higher than those typically reported for molecular crystals, due to the corrugated nature of their crystal packing which enhances the crack tortuosity. The Kc values are in the range 0.215 (0.08) to 0.278 (0.06) MPa m½ and the brittleness index values are in the range 711(19) to 1053(50) m−½