No abstract

ratio, high stiffness, high damping capacity, good elastic modulus, good castability and unique biodegradability in the physiological environment [1][2][3][4]. However, their applications are limited by the poor high-temperature strength and creep resistance.To improve the strength of magnesium alloy, it is a reliable way to refine the grain size using effective nucleants during the casting process. ZrB 2 nanoparticle are such effective nucleants and they are capable of inducing finer long-period stacking ordered phase (nano-LPSO-layer) formation due to the nano-surface effect and finally resulting in the formation of nanograins in magnesium alloys [5]. Also, it is a suitable way to improve the magnesium alloys' stiffness, elastic modulus and wear resistances by preparing particle reinforced magnesium matrix composites since ceramic particles have high strength, hardness and high-wearing features [6,7]. Compared with SiC and TiC particles, ZrB 2 particle reinforced magnesium-based composites fabricated by a direct melt-mixing method [8] have the best strength [9]. The stress transfers from Mg matrix to ZrB 2 reinforcement through the ZrB 2 /Mg interface, and the micro-hardness, fatigue resistance and friction factor of the composites are directly affected by interface bonding [10,11].Up to now, it is known that for effective heterogeneous nucleants in magnesium melt or effective reinforcements in particle reinforced magnesium matrix composites, the lattice mismatch and the chemical interaction play important roles in the overall interfacial energy [12]. And, the ZrB 2 /Mg interface will seriously influence the nucleating in Mg melt [5] or the strength of the composites [13]. Hence, the main purpose of this work was to investigate the mechanism of interface bonding of Mg/ZrB 2 in atomic level and help to improve the mechanical properties of the material.First-principles calculation is a powerful method to provide fundamental information at atomic or electronic level. Generally, (001) plane is a stable low index plane for

Abstract. The effective model assumption based on the average field theory has been modified and extended to investigate the effects of interfacial parameters including the bonding strength and interfacial thickness on the yield strength of particle reinforced composites. The formulation is applied to a model case of SiC particle reinforced Al matrix composite. The theoretical results agree well with the experimental ones of the SiC/Al composite produced by a PM route. The modified theoretical model can effectively predict the interfacial behavior and provide the preparation of SiC/Al composites with scientific foundation to control the interfaces.

Abstract. The low-frequency internal friction behavior of SiC particle reinforced Al matrix composites was studied over a temperature range from 23 to 550 at frequencies of 0.1, 1 and 5 Hz. The experimental data were analyzed in terms of the K-G-L dislocation theory and the anelastic relaxation of grain boundary sliding. Two internal friction peaks of the composites were respectively observed over the temperature ranges of 100~250 and 200~500 . The dislocation motion is the important damping mechanism of the composites at low temperatures, while the grain boundary relaxation plays a dominant role at high temperatures. The activation energies for dislocation relaxation and grain boundary sliding are 1.2 eV and 1.57 eV, respectively.

Lattice parameters, electronic structures and elastic constants of lithium niobate and lithium tantalate were calculated with the plane wave pseudopotential method based on the first-principles density functional theory. The results show that calculated lattice parameters and elastic constants are in consistent with the corresponding experimental values. It was found that the bottom of the valence band and the top of the conductive band are mainly determined by electron orbits of O-2p and Nb-4d (Ta-5d). The chemical bonds theory indicate that Li, Nb (Ta) and O atoms have two types of bonds, and the Mulliken population analysis exhibits that there are two corresponding bond populations. The Nb-O (Ta-O) covalence is stronger than that of Li-O, and band length shorter than that of Li-O. Moreover, the planar acoustic velocities, studied by Christoffel equation, shows that the three-dimensional images of the planar acoustic wave consisting of a longitudinal wave and two transverse waves, indicating the anisotropic feature. The velocity of the longitudinal wave is larger than those of the two transverse waves. In xz and yz planes, not only the plane projections of the planar acoustic waves show the stronger anisotropy than those in xy plane which have a six-fold symmetry, but also the velocities of the two transverse waves are equal in   001 and [00 1] directions. The 无 机 材 料 学 报 第 31 卷 calculated static dielectric constants and optical permittivity indicate the refractive index of LiNbO 3 is stronger than that of LiTaO 3 .