ELASTIC WAVE LOCALIZATION IN TWO-DIMENSIONAL PHONONIC CRYSTALS WITH ONE-DIMENSIONAL QUASI-PERIODICITY AND RANDOM DISORDER

The band structures of both in-plane and anti-plane elastic waves propagating in two-dimensional ordered and disordered （in one direction） phononic crystals are studied in this paper. The localization of wave propagation due to random disorder is discussed by introducing the concept of the localization factor that is calculated by the plane-wave-based transfer-matrix method. By treating the quasi-periodicity as the deviation from the periodicity in a special way, two kinds of quasi phononic crystal that has quasi-periodicity （Fibonacci sequence） in one direction and translational symmetry in the other direction are considered and the band structures are characterized by using localization factors. The results show that the localization factor is an effective parameter in characterizing the band gaps of two-dimensional perfect, randomly disordered and quasi-periodic phononic crystals. Band structures of the phononic crystals can be tuned by different random disorder or changing quasi-periodic parameters. The quasi phononic crystals exhibit more band gaps with narrower width than the ordered and randomly disordered systems.

Genetic control of compound leaf development in the mungbean(Vigna radiata L.)

Many studies suggest that there are distinct regulatory processes controlling compound leaf development in different clades of legumes.Loss of function of the LEAFY(LFY)orthologs results in a reduction of leaf complexity to different degrees in inverted repeat-lacking clade(IRLC)and non-IRLC species.To further understand the role of LFY orthologs and the molecular mechanism in compound leaf development in non-IRLC plants,we studied leaf development in unifoliate leaf(un)mutant,a classical mutant of mungbean(Vigna radiata L.),which showed a complete conversion of compound leaves into simple leaves.Our analysis revealed that UN encoded the mungbean LFY ortholog(VrLFY)and played a significant role in leaf development.In situ RNA hybridization results showed that STM-like KNOXI genes were expressed in compound leaf primordia in mungbean.Furthermore,increased leaflet number in heptafoliate leaflets1(hel1)mutants was demonstrated to depend on the function of VrLFY and KNOXI genes in mungbean.Our results suggested that HEL1 is a key factor coordinating distinct processes in the control of compound leaf development in mungbean and its related non-IRLC legumes.

Molecular dynamics simulations of mechanical properties of epoxy-amine:Cross-linker type and degree of conversion effects

Molecular dynamics(MD)simulations are conducted to study the thermo-mechanical properties of a family of thermosetting epoxy-amines.The crosslinked epoxy resin EPON862 with a series of cross-linkers is built and simulated under the polymer consistent force field(PCFF).Three types of curing agents(rigidity1,3-phenylenediamine(1,3-P),4,4-diaminodiphenylmethane(DDM),and phenol-formaldehyde-ethylenediamine(PFE))with different numbers of active sites are selected in the simulations.We focus on the effects of the cross-linkers on thermo-mechanical properties such as density,glass transition temperature(T_(g)),elastic constants,and strength.Our simulations show a significant increase in the Tg,Young’s modulus and yield stress with the increase in the degree of conversion.The simulation results reveal that the mechanical properties of thermosetting polymers are strongly dependent on the molecular structures of the cross-linker and network topological properties,such as end-to-end distance,crosslinking density and degree of conversion.

密排六方金属中的孪生及孪晶位错机制

密排六方晶体结构金属中可同时启动的滑移系少,孪生成为密排六方金属中重要的塑性变形形式.由于密排六方金属复杂的晶体结构,均匀切变不能保证所有晶格点都能与基体形成对称的晶体结构,因此密排六方金属的孪生通常为滑移和原子重组(shuffle)机制相结合.论文以密排六方金属中常见的{1012}、{1011}、{1122}及{1121}孪生为例,阐述不同类型孪生过程中的孪晶位错机制.分析表明,由于原子重组机制的参与,密排六方金属的孪生可以通过不同形式的孪晶位错实现.以上四种密排六方金属孪晶中,只有{1122}孪生中的一层孪晶位错是纯剪切机制,其余的孪生机制都需要原子重组的参与.孪生机制可以大致分为滑移主导、原子重组主导以及滑移-重组相结合的机制.当孪生类型确定时,即第一不畸变面(孪晶面)k1(和孪晶剪切方向η1)确定时,不同孪晶位错机制对应的孪晶剪切大小和方向均不同,第二不畸变面k2和共轭剪切方向η2也不相同,所导致孪晶的拉压性质也不同.不同剪切方向和大小的孪晶位错机制有可能在不同应力和温度条件下被激活,从而作为密排六方金属塑性的重要来源.

Compression Deformation Mechanisms at the Nanoscale in Magnesium Single Crystal

The dominant deformation mode at low temperatures for magnesium and its alloys is generally regarded to be twinning because of the hcp crystal structure. More recently, the phenomenon of a ＂loss＂ of the twins has been reported in microcompression experiments of the magnesium single crystals. Molecular dynamics simulation of compression deformation shows that the pyramidal 〈α ＋ c〉 slip dominates compression behavior at the nanoscale. No compression twins are observed at different temperatures at different loadings and boundary conditions. This is explained by the analyses, that is, the {10^_12} and {10^11} twins can be activated under c-axis tension, while compression twins will not occur when the c/α ratio of the hcp metal is below √3. Our theoretical and simulation results are consistent with recent microcompression experiments of the magnesium （0001） single crystals.

Atomistic simulation of tension deformation behavior in magnesium single crystal

The deformation behavior in magnesium single crystal under c-axis tension is investigated in a temperature range between 250 K and 570 K by molecular dynamics simulations. At a low temperature, twinning and shear bands are found to be the main deformation mechanisms. In particular, the {102} tension twins with the reorientation angle of about 90 °are observed in the simulations. The mechanisms of {102} twinning are illustrated by the simulated motion of atoms. Moreover, grain nucleation and growth are found to be accompanied with the {102} twinning. At temperatures above 450 K, the twin frequency decreases with increasing temperature. The {102} extension twin almost disappears at the temperature of 570 K. The non-basal slip plays an important role on the tensile deformation in magnesium single crystal at high temperatures.