【作者】 徐娜；

【导师】 宗亚平； 张芳；

【作者基本信息】 东北大学 ， 材料学， 2008， 博士

【摘要】 随着颗粒增强金属基复合材料在工程上的广泛应用,要求人们对其力学性能、变形与强化机制等问题进行深入的研究,因此,需要建立科学、合理的力学模型为颗粒增强材料的微观结构设计、力学性能分析提供必要的参考和重要的理论依据,通过模拟探讨新的材料设计和性能改进的途径。本文采用基体平均扰动应变的方法,确定了扰动应变与多种粒子本征应变值之间的归一化关系,建立了一个Eshelby颗粒复合体力学分析模型,推导并修订了颗粒受力的正确表达式,使用了全张量表达,得出了多重粒子强化复合材料顺度张量的完整表达式。在模拟复合材料非线性阶段的应力-应变曲线时,提出引入基体、颗粒复合体割线模量、切线模量的方法和等效泊松比的计算,借用颗粒复合体弹性顺度张量表达式,实现了Eshelby分析模型关于颗粒复合体屈服强度的计算和屈服附近应力-应变的关系的预测。并以此模型来研究基体经不同处理的两种SiCp/Al-2618复合材料的应力-应变曲线、材料中各组分的受力情况以及粒子的几何因素对材料力学性能的影响。并将模拟结果与实验值进行了比较,来探讨颗粒增强金属基复合材料的强化机理,研究颗粒复合体的力学行为。建立了有限元轴对称单胞模型和平面应变多颗粒模型,利用ANSYS软件,设定颗粒和基体之间为理想连结状态并限定合理边界条件,采用位移加载求解复合材料平均应力的方法,实现了应力-应变曲线模拟并与实验值进行了比较。同时,研究了颗粒的几何因素对金属基复合材料力学性能的影响,仅计算平面应变模型的横向应力-应变曲线用来研究颗粒复合体的力学行为,以便于对粒子形状、分布的影响进行研究,为复合材料的显微组织优化设计提供一定的理论依据。设计了两种工艺处理雾化沉积15v%SiCp/Al-2618复合材料作为模拟对比的软基体和硬基体模型材料,系统测定了其力学性能、应力-应变曲线和显微组织分析作为本模拟工作的验证和修订参考依据。研究表明,对于经不同处理的复合材料应采用不同的模型进行模拟研究。时效硬化基体的复合材料应采用切线模量Eshelby颗粒复合体力学模型方法来研究其力学性能,而单相塑性基体的复合材料则应采用割线模量Eshelby颗粒复合体力学模型法；有限元单胞模型对于硬基体复合材料的应力-应变曲线的模拟是可行的,但不适于模拟软基体复合材料。透射电镜对15v%SiCp/Al-2618复合材料位错密度的观察表明.,位错密度的微观增强机制对此复合材料是微不足道的；弥散作用、细化晶粒对复合材料强度的贡献分别仅为0.38MPa和7MPa,说明基体的微观强化效果不明显。通过对发生颗粒断裂的复合材料显微组织的观察,表明颗粒在复合材料应变过程中承担了很大的载荷；同时用两种不同的模型模拟并与实验曲线进行比较表明,颗粒的存在并没有提高基体的强度,而是颗粒在复合材料应变过程中承担了大部分的载荷，得出了载荷传递机制是该颗粒增强复合材料的主要强化机制。并且软基体复合材料是以应变失配机制来进行载荷传递的；而对于硬基体复合材料，载荷传递机制主要依赖于增强粒子对基体应变的阻碍作用来实现。采用有限元轴对称模型和平面应变模型的模拟研究得出,颗粒形状对复合材料的屈服强度有重要影响。在所研究的基本形状粒子中,柱状颗粒具有最好的强化效果；三角形、四边形等尖锐颗粒便于载荷传递,表现出比球形更好的强化效果。在所模拟的多边形粒子形状中,当多变形颗粒的边数大于5时,材料的屈服强度受边数的增加影响不大但略有降低。随着粒子长径比的逐渐增大,粒子中的受力逐渐增大,复合材料的弹性模量和屈服强度也逐渐增大。与Eshelby模型相比,FEM模型对于颗粒长径比的变化更明显,表明颗粒的形状对应变约束效应的贡献要大于对错配应变的贡献。通过用两种模型对具有不同弹性模量增强粒子的模拟可以看出,弹性模量高的增强颗粒在复合材料应变过程中所承担的载荷也较大,更利于载荷传递,因此其强化效果也更好；随着颗粒体积分数的逐渐增大,增强粒子弹性模量的作用更加显著。随着颗粒体积分数的增加,复合材料的屈服强度也逐渐增大。与FEM模型相比,Eshelby模型受颗粒体积分数变化的影响更明显,表明颗粒的体积分数对错配应变的贡献要大于对应变约束效应的贡献。在模拟增强粒子的受力时,用FEM模型预测出的粒子受力随着颗粒体积分数的增加而增大,而用Eshelby模型预测的结果随着颗粒体积分数的增加而减小,这是由于模型建立的原理不同所造成的,经分析,Eshelby模型的预测更为合理。模拟研究得出,颗粒增强复合材料中强化粒子的几何因素对材料性能有重要影响,影响规律非常敏感地依赖于基体的应力-应变曲线形状及粒子与基体弹塑性性能的匹配,影响关系没有简单解析表达式,只能通过模拟数据进行具体归结。因此,通过模拟对颗粒增强复合材料进行显微组织设计是必要和可行的。更多还原

【Abstract】 With the increasing applications of particulate reinforced metal matrix composites in engineering, it is necessary to conduct intensive investigation on their mechanical behavior and strengthening mechanism. Therefore, analytical and numerical modeling are interested to provide necessary theoretical fundament for the design of micro-structure and properties of the composites, and to explore possible new ways to develop new materials and to improve performance.Based on the approach of mean disturbance strain of matrix, the normalized relationship between the disturbance strain and eigen strain of multiple kinds of reinforcing particles was studied. An Eshelby particle-compound mechanical model was established, and the valid expression of stress in particles was deduced in detail. A full expression for compliance tensor of multi-particle reinforcing compound was formulated eventually. The secant modulus and equivalent Poisson’s ratio of matrix and the compound were introduced to simulate nonlinear stage of the stress-strain curve. Employing the elastic compliance tensor of the compound, the calculation of yield strength and the prediction of stress-strain relation near the yielding point of the compound were obtained by the model. The Eshelby model was also used to study the stress-strain curve, the stress in each component and the effect of geometrical factors of the particles on mechanical behavior of SiCp/Al-2618 composite. The simulated results were compared with experimental data to investigate validity of the model and strengthening mechanisms.An axisymmetric unit-cell model and multi-particle plane strain model by finite element method were also built up based on the software ANSYS. Simulation of stress-strain curve was carried out by adopting the approach of solving average stress of composite through application of displacement load. Additionally, the influence of geometrical factors of particles on mechanical properties of the composite was examined by the FE analysis.Two types of 15v%SiCp/Al-2618 composites, which were produced by a spray-forming-deposition process, were used as experimental comparison with soft matrix and hard matrix respectively by different aging treatment. The mechanical properties, microstructure and stress-strain curve of the composites were measured to provide referencebasis for the verification and revision of the modeling work.The Eshelby particle-compounded mechanical model combing tangent modulus method should be adopted to model hard matrix composite; while for composite with soft matrix the secant modulus method should be adopted. As for the FEM unit-cell model, it is feasible to be used for predicting stress-strain curve of the composite with hard matrix but unsuitable to predict that of the composite with soft matrix.The observation of dislocation density in SiCp/Al-2618 composite by TEM indicates that micro-strengthening mechanism due to dislocation density has a little effect on-the composite. The contribution of dispersion strengthening and grain refining to the strength of the composite are only 0.38 MPa and 7 MPa respectively. Therefore, the effect of micro-strengthening mechanisms on the matrix is not obvious. The microstructure observation of the composite in which many SiC particles have been broken indicates that the particles, may take much load during straining. Meanwhile, the modeling proves that loadin the particles is much higher than in matrix during straining. Consequently, load transfer mechanism is suggested the main strengthening mechanism of these particle-reinforced composites.Through the modeling work of the composite by FEM axisymmetry model and plane strain model, it is shown that particle shape has significant influence on yield strength of the composite. Among the particles of basic shapes, cylindrical particle possesses the best strengthening effect; particles of sharp shape like triangle and rectangle have better strengthening effect than spherical particles. Among the polygonal particles, the sides of the: polygon have little effect on yield stress of the composite when they are greater than 5. Additionally, the stress in particles, young’s modulus and yield stress of the composite gradually increase with the. increase of particle aspect ratio. Compared with the Eshelby model, the FEM model is more sensitive to the variation of particle aspect ratio, which indicates.that particle shape contributes more to the strength by means, of strain constraint effect than of the mismatch strain effect.The simulation results show that the reinforcing particle with higher young’s modulus has better reinforcing effect. With the increase of volume fraction of particles, the effect becomes more, obvious. The yield stress, of the composite gradually increases with the increase of volume fraction of particle. Compared with the FEM model, the Eshelby model is more sensitive to the variation of volume fraction, which indicates that volume fraction contributes more to the effect of mismatch strain than to the strain constraint. With the increase of volume fraction, the stress in the particles gradually increases predicted by FEM model but decreases by Eshelby model due to the different principles of the two models and it seems that the Eshelby model is more reasonableIt can be concluded that geometrical factors of reinforcing particles have significant effect on mechanical properties of the particle-reinforced composites, and the effect is sensitively relied on the stress-strain curve of the matrix alloy and the matching of elastoplastic properties between reinforcing particles and matrix. There is no simple analytic expression of the effect, and it can only be represented by specific modeling data. Therefore, it is necessary and feasible to carry out microstructure design of the particle-reinforced composites by modeling.更多还原