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11.
Previous atomistic simulations and experiments have shown an increased Young's modulus and yield strength of fivefold twinned (FT) face-centered cubic metal nanowires (NWs) when compared to single crystalline (SC) NWs of the same orientation. Here we report the results of atomistic simulations of SC and FT Ag, Al, Au, Cu and Ni NWs with diameters between 2 and 50 nm under tension and compression. The simulations show that the differences in Young's modulus between SC and FT NWs are correlated with the elastic anisotropy of the metal, with Al showing a decreased Young's modulus. We develop a simple analytical model based on disclination theory and constraint anisotropic elasticity to explain the trend in the difference of Young's modulus between SC and FT NWs. Taking into account the role of surface stresses and the elastic properties of twin boundaries allows to account for the observed size effect in Young's modulus. The model furthermore explains the different relative yield strengths in tension and compression as well as the material and loading dependent failure mechanisms in FTNWs. 相似文献
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Nanoparticles usually exhibit pronounced anisotropic properties, and a close insight into the atomic-scale deformation mechanisms is of great interest. In present study, atomic simulations are conducted to analyse the compression of bcc nanoparticles, and orientation-dependent features are addressed. It is revealed that surface morphology under indenter predominantly governs the initial elastic response. The loading curve follows the flat punch contact model in [1 1 0] compression, while it obeys the Hertzian contact model in [1 1 1] and [0 0 1] compressions. In plastic deformation regime, full dislocation gliding is dominated in [1 1 0] compression, while deformation twinning is prominent in [1 1 1] compression, and these two mechanisms coexist in [0 0 1] compression. Such deformation mechanisms are distinct from those in bulk crystals under nanoindentation and nanopillars under compression, and the major differences are also illuminated. Our results provide an atomic perspective on the mechanical behaviours of bcc nanoparticles and are helpful for the design of nanoparticle-based components and systems. 相似文献
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The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels. 相似文献
16.
Phase‐pure ammonium catena‐polyphosphate IV [NH4PO3]x was synthesized by heating NH4H2PO4 in a tube furnace under an ammonia gas flow. The product contained single crystals of [NH4PO3]x IV appropriate for an X‐ray structure determination enabling structure refinement of this compound. The pseudo‐merohedrally twinned crystals of [NH4PO3]x crystallize in the monoclinic crystal system (P21/c, no. 14, a = 2270.3(5), b = 458.14(9), c = 1445.1(3) pm, β = 108.56(3)°, Z = 4, 2264 data, R1 = 0.076). In the unit cell the catena‐polyphosphate chain anions run parallel [010] with a chain‐periodicity P = 2 and a stretching factor fs = 0.94. The chain anions are interconnected through extensive hydrogen bonding towards the ammonium ions. Due to ‘chemical twinning’ a novel catena‐polyphosphate structure type is realized in [NH4PO3]x IV. The vibrational spectra of [NH4PO3]x IV are reported as well. 相似文献
17.
Ng Seik Weng 《结构化学》2009,28(12):1657-1660
The procedure for collecting diffraction data at –173 °C on a twinned specimen of methyl 2-aminopyrazine-3-carboxylate by using the APEX-II software followed by de-twinning the non-merohedrally-twinned reflection data with PLATON is described. De-twinning significantly lowers the R index from 0.141 to 0.038 owing to 49% twinning. Crystal data: C6H7N3O2,monoclinic,P21/c (a = 6.3149(1),b = 16.5274(2),c = 6.4544(1) A,β = 95.759(1)°,V = 670.24(2) A^3). 相似文献
18.
Inversion twinning in a second polymorph of the hydrochloride salt of the recreational drug ethylone
T. Stanley Cameron J. Stuart Grossert Chad R. Maheux Idralyn Q. Alarcon Catherine R. Copeland Anthony Linden 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(4):266-270
A second polymorph of the hydrochloride salt of the recreational drug ethylone, C12H16NO3+·Cl−, is reported [systematic name: (±)‐2‐ethylammonio‐1‐(3,4‐methylenedioxyphenyl)propane‐1‐one chloride]. This polymorph, denoted form (A), appears in crystallizations performed above 308 K. The originally reported form (B) [Wood et al. (2015). Acta Cryst. C 71 , 32–38] crystallizes preferentially at room temperature. The conformations of the cations in the two forms differ by a 180° rotation about the C—C bond linking the side chain to the aromatic ring. Hydrogen bonding links the cations and anions in both forms into similar extended chains in which any one chain contains only a single enantiomer of the chiral cation, but the packing of the ions is different. In form (A), the aromatic rings of adjacent chains interleave, but pack equally well if neighbouring chains contain the same or opposite enantiomorph of the cation. The consequence of this is then near perfect inversion twinning in the structure. In form (B), neighbouring chains are always inverted, leading to a centrosymmetric space group. The question as to why the polymorphs crystallize at slightly different temperatures has been examined by density functional theory (DFT) and lattice energy calculations and a consideration of packing compactness. The free energy (ΔG) of the crystal lattice for polymorph (A) lies some 52 kJ mol−1 above that of polymorph (B). 相似文献
19.
Y16I19C8B4 – a Yttrium Boride Carbide Halide Containing B2C4 Units The new compound Y16I19C8B4 was prepared from Y, YI3, C and B at 1050–1150 °C. The structure of a twinned crystal was determined by means of X-ray diffraction (space group P 1¯, a = 12.311(2) Å, b = 13.996(3) Å, c = 19.695(3) Å, α = 74.96(2)°, β = 89.51(2)°, γ = 67.03(2)°, Z = 2). Y16I19C8B4 is a semiconductor and contains nearly planar B2C4 units which are located in cages built up by 12 yttrium atoms. Assuming (B2C4)12–, these units can be regarded as isoelectronic with B2F4. The yttrium cages are connected via faces to form rods, which are surrounded by iodine atoms. Bridging iodine atoms connect the rods so that layers are formed. The characteristic twinning observed can be understood from the geometry of the crystal structure. 相似文献
20.
The evolution of texture during the tensile deformation of Mg–3.1%Al–1.05%Zn was studied by microtexture and macrotexture techniques. The deformation texture is shown to be the direct consequence of the variant selection that occurs during primary (contraction) and secondary (extension) twinning. During subsequent annealing, the secondary twins have a higher rate of nucleation than the primary twins and expanded within the latter rather than in the matrix grains. As a result, the primary twins did not contribute significantly to the annealing texture. 相似文献