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An evolving material structure is in a non-equilibrium state, with free energy expressed by the generalized coordinates. A global approach leads to robust computations for the generalized thermodynamic forces. Those forces drive various kinetic processes, causing dissipation at spots, along curves, surfaces and interfaces, and within volumetric regions. The actual evolution path, and therefore the final equilibrium state, is determined by the energetics and kinetics. A virtual work principle links the free energy landscape and the kinetic processes, and assigns a viscous environment to every point on the landscape. The approach leads to a dynamical system that governs the evolution of generalized coordinates. The microstructural evolution is globally characterized by a basin map in the coordinate space; and by a diversity map and a variety map in the parameter space. The control of basin boundaries raises the issue of energetic and kinetic bifurcations. The variation of basin boundaries under different sets of controlling parameters provides an analytical way to plot the diversity maps of structural evolution. The project supported by the National Science Foundation (USA) through grant MSS-9258115, and by the National Natural Science Foundation of China  相似文献
通过计算机模拟的方法对不同温度下Cu//Cu/FeS摩擦副的摩擦性能进行计算研究,结果发现:当环境的温度在500 K时,摩擦副的摩擦系数出现第一个谷值,而此时金属铜发生再结晶,黏着磨损更为明显;700 K时,黏着层数减小而摩擦系数却有所增大,主要是由于复合材料内部的FeS结构发生转变造成.计算结果还证明:在高温情况下,增强相FeS存在先分解再重新结合的2个过程,并指出Cu/FeS复合材料在900 K时,仍然具有很好的减摩耐磨作用.还对不同温度摩擦时复合材料内部的组织演变过程进行系统分析,分析结果表明,随着温度的升高,FeS的结构发生演变,从六方NiAs型超结构相转变为简单NiAs型结构相,并且随着温度的升高,FeS结构的 c/a 也在不断减小,这些都影响着复合材料的摩擦性能.  相似文献
This work presents the application of a recently proposed second-order homogenization method (Ponte Castañeda, 2002) to generate estimates for effective behavior and loss of ellipticity in hyperelastic porous materials with random microstructures that are subjected to finite deformations. The main concept behind the method is the introduction of an optimally selected linear thermoelastic comparison composite, which can then be used to convert available linear homogenization estimates into new estimates for the nonlinear hyperelastic composite. In this paper, explicit results are provided for the case where the matrix is taken to be isotropic and strongly elliptic. In spite of the strong ellipticity of the matrix phase, the homogenized second-order estimates for the overall behavior are found to lose ellipticity at sufficiently large compressive deformations corresponding to the possible development of shear band-type instabilities (Abeyaratne and Triantafyllidis, 1984). The reasons for this result have been linked to the evolution of the microstructure, which, under appropriate loading conditions, can induce geometric softening leading to overall loss of ellipticity. Furthermore, the second-order homogenization method has the merit that it recovers the exact evolution of the porosity under a finite-deformation history in the limit of incompressible behavior for the matrix. Mathematics Subject Classifications (2000) 49S05, 74B20, 74Q15, 74Q05.  相似文献
Materials can stress relax for a variety of reasons, for example stress relaxation in classical viscoelastic bodies and that due to aging or damage. Here, within the context of a specific model we show that the stress relaxation that is characteristic of viscoelasticity is distinctly different from that of aging.  相似文献
为研究陶瓷和金属微波烧结时的微观演化机理,从而为优化不同材料的烧结过程提供依据,本文采用同步辐射技术对陶瓷(SiC)和金属(Al)的微波烧结微结构演化过程进行实时、无损的观测,并结合有限元模拟分析两者的微结构演化特征及微观机理。通过滤波反投影等数字图像处理技术得到烧结过程中样品内部的二维、三维重建图像,清晰地观察到SiC和Al在颗粒表面和界面演化上存在差异。定量地统计了陶瓷和金属烧结颈相对尺寸与时间的双对数关系,并与陶瓷和金属双球模型的微波烧结模拟结果进行了对比。运用模拟分别对实验中的烧结颈和微观形貌演化进行分析,得出结论:陶瓷和金属微波烧结时的加热机制不同,分别为整体介质损耗加热和表面涡流损耗加热。陶瓷的整体加热将会在材料内部特别是界面产生较高的温度,而金属的表面加热使颗粒表面温度高于界面。由相应的加热机制产生的温度分布差异,将会对材料的物质扩散过程产生不同程度的影响,进而产生不同的微结构。  相似文献
为实现对金属粉末压坯烧结过程的实时观测,从而验证现有的相关理论,本文利用同步辐射CT(SR-CT)技术,对铝压制陶瓷坯体的固相烧结过程进行实时投影成像;应用滤波反投影算法和数字图像处理技术,得到了陶瓷坯体在整个固相烧结过程中内部微结构演化的三维重建图像,实现了对铝压制陶瓷坯体整个固相烧结过程的无损原位观测,得到了样品由烧结中期进入烧结后期的完整的演化过程图像。通过重建图像,清晰观测了样品的固相烧结过程:在烧结前、中期(烧结时间t<180min、温度T<600℃),烧结颈形成并随烧结时间生长;由烧结中期开始进入后期时(烧结时间t≥180min、温度T≥600℃),样品内部结构演化加剧,气孔由相互连通演化为相互孤立并球化。进一步在实验图像的基础上分析了二面角等烧结特征的变化情况,得到烧结颈尺寸与时间对数有较好的线性关系,并可根据曲线分辨中期、后期。统计了样品在不同烧结时间的孔隙率,得到了孔隙率随烧结时间和烧结时间对数的变化曲线;分析了样品在不同烧结阶段的致密化特点,得到了烧结中期孔隙率和时间对数的线性关系。实验结果验证了现有的烧结理论,并为进一步完善烧结理论以及建立扩散和本构模型提供了高质量的实验数据。  相似文献
应用有限元及连续介质力学计算了平面应交条件下体心立方铁中不同取向裂纹的裂尖应力场和弹性应变能密度分布,从宏微观相结合角度分析了裂尖场分布与裂尖微结构演化的相互关联.指出裂尖塑性变形的具体形式与裂尖滑移面上分切应力的大小密切相关,并从能量角度解释了裂尖相变产生的原因,最后结合相关分子动力学研究成果探讨了裂尖奇异性区域.  相似文献
Analytical solutions for diffuse interface propagation are found for two recently developed Landau potentials that account for the phenomenology of stress-induced martensitic phase transformations. The solutions include the interface profile and velocity as a function of temperature and stress tensor. An instability in the interface propagation near lattice instability conditions is studied numerically. The effect of material inertia is approximately included. Two methods for introducing an athermal interface friction in phase field models are discussed. In the first method an analytic expression defines the location of the diffuse interface, and the rate of change of the order parameters is required to vanish if the driving force is below a threshold. As an alternative and more physical approach, we demonstrate that the introduction of spatially oscillatory stress fields due to crystal defects and the Peierls barrier, or to a jump in chemical energy, reproduces the effect of an athermal threshold. Finite element simulations of microstructure evolution with and without an athermal threshold are performed. In the presence of spatially oscillatory fields the evolution self-arrests in realistic stationary microstructures, thus the system does not converge to an unphysical single-phase final state, and rate-independent temperature- and stress-induced phase transformation hysteresis are exhibited.  相似文献
基于CuAlNi形状记忆合金单晶的一维准静态拉伸实验,着重研究相变过程中条带状马氏体的微观结构.通过金相显微镜对整个试样表面进行拍照,并利用图像处理技术和均匀化的方法,提取到了不同载荷下马氏体的含量(相分量)和条带的数量(界面数)在试样各处的分布与变化情况,找到了加载和卸载过程中相变微结构的演化规律,从而实现了对条带状马氏体相变微结构的量化处理.  相似文献
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