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201.
We have developed a unique processing technique to fabricate Zinc (Zn) nanocomposites with controlled microstructures. In this method, pulsed laser deposition of Zn in conjunction with a few monolayers of tungsten (W) is used to control the grain size of nanocrystalline composites. The grain size of Zn was controlled by the amount of Zn and the substrate temperature. The Zn islands of lower surface energy nucleate on W-layer of high surface energy, which is also insoluble in Zn resulting in lower interfacial energy. Using this approach, we have fabricated nanocomposites of grain sizes ranging from 30nm down to 6nm. The hardness of these nanocrystalline films increases with the decrease in grain size, following approximately Hall–Petch relationship. The most interesting observation is the decrease in hardness below a critical size, which is explained on the basis of grain boundary deformation/sliding. The role of W in grain boundary deformation is of particular interest in strengthening and stabilizing the nanocrystalline composites. The potential of this technique to attain even lower grain sizes is discussed. 相似文献
202.
结构静力分析中常因材料应变软化使得相应定解问题失去适定性,从而导致有限元分析不收敛。为解决此问题,在已有的相关研究基础上,采用动力松弛法(DRM)求解结构非线性有限元静力分析的增量步,将其应用于损伤型本构所描述的结构软化问题。本文方法依据两个原理,其一是苏联《数学百科全书》论述的原理——定义于时间域的任何定解问题适定可解,其二是DRM所用的原理——质量系统静力解为相应动力解的稳态部分。且DRM无需进行隐式静力分析时的总体刚度矩阵组装和求逆计算。本文用加荷载增量求解静力平衡路径硬化段,用加位移增量求解极值点和软化段。数值试验表明,本文方法能完成应变软化类结构的静力平衡路径求解。 相似文献
203.
A magnon‐phonon interaction model is set up in a two‐dimensional ferromagnetic compound square‐lattice system. Using the Matsubara Green function theory we calculated the magnon dispersion curves on the main symmetric line in Brillouin zone, compared the influences of the magnetic ion optical phonon with non‐magnetic ion optical phonon on the magnetic excitation of the system and discussed the influences of various parameters on the magnon softening. The lower Debye temperature of ferromagnetic materials is, the more likely the magnon softening occurs. It turned out that the optical phonon‐magnon coupling plays an important role on the magnon softening, the longitudinal optical phonon contributes the most to the magnon softening and magnon damping. It is also found that the contribution of the non‐magnetic ion to the magnon softening and magnon damping is more significant than that of magnetic ion when the mass of the magnetic ion is less than that of the non‐magnetic ion, or the mass of magnetic ion and the non‐magnetic ion are equal. 相似文献
204.
Based on the assumption of laminated microstructure, a micromechanical model of stress induced martensite transformation for NiTi shape memory alloys single crystal is proposed. Elasticity anisotropy and different proper-ties for two phases are considered. Martensite volume fraction is chosen as the internal variable that controls the phase transformation quantitatively. An effective macroscopic elasticity matrix based on the different elasticity characteristics of each phase and the martensite volume fraction are obtained with the help of the perfect interfa-cial relationships. A phase transformation driving force is derived to construct the transformation criterion. The model corresponds to a non-convexity free energy function during phase transformation, so softening behavior can be well simulated by the model. A numerical simulation is implemented for the uniaxial loading of NiTi single crystal alloy according to the model, and simulation results are proved by experimental results of polycrystal with strong {111} texture. Superelasticity, Anisotropy, the evolution of microstructure and softening behavior can be well simulated. 相似文献
205.
通过Gleeble-1500D型热模拟机对REⅡ稀土重轨钢进行应变速率为5 s-1、变形量均为25%的双道次热压缩模拟试验,分别测定820,850,880,1000℃下的真应力-真应变曲线,采用后插法计算奥氏体等温变形后道次间隙时间1~1200 s内的软化率,研究REⅡ稀土重轨钢的静态再结晶规律。结果显示:当变形温度为>1000℃时,REⅡ重轨钢完成静态再结晶弛豫时间<90 s;当变形温度<820℃时,静态再结晶很难进行,即使弛豫时间延长至1000 s,再结晶百分数也只有38.8%;当变形温度为850和880℃时,再结晶过程会出现析出现象,对抑制静态再结晶的进行有影响,导致软化率曲线上出现了平台。 相似文献
206.
207.
微裂纹模型是研究砂土变形中应变局部化问题的微观力学模型。本文在前文的基础上建立了有限平面的徽裂纹模型的基本方程。数值解析表明,微裂纹模型可再现应变局部化及应变软化现象。同时,本文简要地讨论了边界约束、尺寸效应及侧压对应变局部化的影响。 相似文献
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210.
MA Dong-Ping ZHANG Ji-Ping 《理论物理通讯》2001,(10)
By taking into account all the irreducible representations and their components in the electron-phonon interaction (EPI) as well as all the levels and the admixtures of basic wavefunctions within d3 electronic configuration,the values of the parameters in the expressions of thermal shift (TS) from EPI for the ground level, R level and R line of SrTiO3:Mn4 have been evaluated; the R-line TS and various contributions to it have been calculated in the low-temperature region. It is found that all the three terms of R-line TS from EPI relevant to the lattice vibration are red shifts. The Raman term is the largest, the neighbor-level term is the second, and the optical-branch term is very small over the range of T ≤ 80 K. The contribution to R-line TS from thermal expansion has been approximately neglected in this work. The very strong EPI relevant to its lattice vibration for SrTiO3:Mn4 causes its R-line TS to be an unusually large red-shift. Only by taking into account the strong softening of the low-frequency acoustic modes of the lattice vibration at low temperatures, can we successfully explain the variation of R-line TS of SrTiO3:Mn4 with temperature.`` 相似文献