扩散与应力耦合行为及模型研究

发布时间：2018-01-23 07:34

本文关键词： 力学-化学模型高温氧化力学特性元素扩散细观力学　出处：《华东理工大学》2017年博士论文　论文类型：学位论文

【摘要】：钛、镍及其合金由于其出色的力学性能,在航空发动机和重型燃气轮机等诸多尖端技术领域,有着广泛的应用。然而,高温环境下的抗氧化性能,严重制约着金属及合金材料的工作条件和服役寿命。苛刻条件下的金属材料热氧化行为及力学特性的研究,对于实际工程中的材料性能分析、构件寿命预测和表面结构优化设计,均有着重要的指导意义。针对金属材料的氧化、扩散行为与力学特性之间的多重耦合作用关系,作者展开了以下方面的模型及实验研究工作:(1)基于Clarke生长应变理论,建立综合考虑生长应变、弹性应变、蠕变应变和外部载荷的力学—化学耦合作用模型,用以预测应力、应变及氧化膜厚度随时间的变化规律。由氧化所致的氧化膜内产生明显的生长应变和生长应力,对氧化膜/基体系统的力学平衡造成影响。建模过程中,作者对氧化膜厚度与生长应变之间的关系作了线性描述。同时,根据Helmholtz自由能公式,应力状态对元素的扩散行为造成影响,从而改变了金属材料的氧化动力学特性。(2)通过考虑应力、应变及氧元素浓度在氧化膜内的梯度分布,对上述力学—化学模型作了进一步拓展和修正,使模型得到更为广泛而精确的应用。采用离散化方法,宏观尺度上的氧化膜被分解为多层的微观膜,用以描述氧化膜内的梯度结构分布。基于线性代数方法,对应力、应变及氧元素浓度的分布作了耦合求解。建模过程中,采用修正后的线性-抛物线型公式描述材料的氧化动力学特性,并证明该公式可视作传统的Wagner方程的广义形式。同时,通过比较应变和应力载荷在内的多种机械载荷的作用,澄清了机械载荷对于氧化动力学和力学行为的影响机制。(3)采用X射线衍射、扫描电镜、能谱分析、光学显微镜及纳米压痕等有效实验手段,系统性地研究了 Ti-6A1-4V合金的高温氧化机制、氧化动力学特性及氧化过程中的应力变化情况。实验结果证实了弯曲载荷对于钛合金的氧化膜、渗氧层的不同作用,并与模型计算结果相比较,验证了模型的准确性和可靠性。(4)基于细观力学的理论基础,对力学均匀化理论做了有效的推导和拓展,使其能够广泛应用于元素扩散的研究领域。借助傅里叶变换,求解了关于扩散性能的Esheby张量的闭合解,并由此求得了非均匀混合物的有效扩散张量。(5)基于细观力学,推导得到了合金元素扩散过程中微观应变与宏观应变之间的力学关系模型。在此基础上,通过考虑有效弹性张量对元素浓度的依赖关系,建立力学平衡方程。同时,不同载荷对于应力分布和扩散行为的影响也做了进一步探讨。基于元素化学式、有效扩散张量、有效弹性性能及应力状态之间的相互作用,建立了扩散行为与力学特性耦合作用下的扩散方程,实现了在宏观和微观两个尺度下应力状态及元素浓度的耦合求解。该方程可视作对传统Fick扩散定律的有效修正,提高了应力状态与元素浓度预测过程中的准确性。
[Abstract]:Titanium, nickel and its alloy because of its excellent mechanical properties, in many cutting-edge technology field of the aero engine and gas turbine, has been widely used. However, the antioxidant properties in high temperature environment, seriously restricting the metal and alloy materials working conditions and service life of metal materials. Thermal oxidation behavior and mechanical properties under harsh conditions, analysis of the material properties in practical engineering, component life prediction and optimization design of surface structure, has the important guiding significance. For the oxidation of metal materials, multiple coupling relationship between diffusion characteristics and mechanical behavior, the author carried out the work model and experimental research of the following aspects: (1 based on the theory of the growth of Clarke strain), considering the growth strain, elastic strain, creep strain and external load mechanical chemical coupling model is used to predict the stress should be. Changes and film thickness with time. Obvious growth strain and growth stresses caused by oxidation caused by the oxide film, the oxide film / substrate system affect the mechanical equilibrium. In the process of modeling, the author makes a line description of the relationship between film thickness and growth strain. At the same time, according to the Helmholtz free energy formula that should be the diffusion behavior of stress of elements of the impact, thus changing the oxidation kinetics of metal materials. (2) by considering the stress gradient in the oxide film in the strain and oxygen concentration distribution on the mechanical and chemical model is further extended and modified, more widely and accurately make model. Using the discrete method on macro scale film is decomposed into multilayer micro film, used to describe the structure of oxide film. The gradient distribution method based on linear algebra, the stress should be. The distribution and concentration of oxygen for the coupling solution. In the process of modeling, using the modified linear parabolic equations describing the oxidation kinetics characteristics of the materials, and proves that the generalized form of the formula can be regarded as the traditional Wagner equation. At the same time, through a variety of mechanical loads between strain and stress load, role clarification the mechanism of the effect of mechanical load on the oxidation kinetics and mechanical behavior. (3) by using X ray diffraction, scanning electron microscopy, energy spectrum analysis method, optical microscope and nano indentation experiments effectively, the mechanism of oxidation system of Ti-6A1-4V alloy, stress change dynamics and oxidation in the process of the experiment. The results confirmed that the film bending load for the titanium alloy, the effect of oxygen permeability of different layers, and compare the calculated results with the model, to verify the accuracy of the model and the reliability based on (4). The theoretical basis of meso mechanics, mechanics of homogenization theory the derivation and development, so that it can be widely used in the element diffusion research field. With the help of Fu Liye transform, solving the closed Esheby on the performance of the solution diffusion tensor, and thus obtained the effective diffusion tensor of non uniform mixture (5.) micromechanics based on derived alloy element diffusion mechanics model between micro strain and macroscopic strain in the process. On this basis, by considering the dependence of the effective elastic tensor element concentration, a mechanical equilibrium equation. At the same time, the influence of different load on the stress distribution and the diffusion behavior also made further to investigate the chemical elements. Based on the effective diffusion tensor, the effective elastic properties and stress interaction between States, set up the diffusion equation of coupling diffusion behavior and mechanical properties of the implementation The coupling solution of stress state and element concentration under two scales of macroscopical and microcosmic is solved. This equation can be used to correct the traditional Fick diffusion law effectively, and improve the accuracy of prediction of stress state and element concentration.

【学位授予单位】：华东理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG111.6

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