介原子分子动力学方法的发展与应用
发布时间：2018-08-18 12:14
【摘要】：合金广泛应用于工业生产,由此也得到学术界的广泛关注与研究。但在经典的嵌入式原子势体系下,科学家们很难优化得到可靠的合金势函数,这使得合金分子动力学研究严重滞后于实验与理论的研究进展。为能够利用分子动力学从原子层面研究合金的变形机制,我们基于嵌入式原子势发展出用于合金分子动力学模拟的介原子分子动力学方法。所取得的研究进展如下：本文基于两个基本假设“相似性假设”和“平均化假设”提出了介原子分子动力学方法。在这一方法中,所有的原子都不再代表合金中的不同元素的原子而是一个虚拟的原子—“介原子”。其势函数的拟合也不再考虑不同元素之间的相互作用,转而直接使用合金的实测材料属性来优化拟合。基于这一方法,本文自主开发了一套计算机程序,通过加权最小二乘法优化模拟值与目标值的残差,最后输出满足嵌入式原子势格式的势函数文件。为了降低单次优化的时间并提升计算效率,本文提出了“几何因子”和“白噪声”等加速算法,这有效提升了程序优化得到势函数的求解能力。为了验证本文所提出方法的可靠性,本文利用介原子分子动力学方法制作得到了一系列的铜铝合金势函数和虚拟可发生脆断的面心立方势函数,并利用这些势函数模拟研究层错能对合金变形机制的影响和金属的韧脆转变效应。本文的研究结果发现层错能对于合金的变形机制有着重要影响：随着层错能的降低,金属的变形机制中孪晶所占比例迅速提升。同时,自由表面能和不稳定层错能的比值直接决定了金属的韧性或脆性断裂属性。这组势函数的成功应用证明了本方法的可靠性与普适性。为研究固溶体合金中的固溶强化效应,本文又对介原子分子动力学方法做了改进,提出了放缩因子介原子分子动力学方法。放缩因子的加入可以有效地在金属中引入弥散分布的晶格畸变。并且,随着晶格畸变的加大,当超过晶格自身的稳定性时,晶格就会垮塌成为非晶结构。放缩因子的加入并不会对材料属性有很大的影响,但又会使位错滑动的临界应力Peierls应力大幅度上升,明显提升位错滑动所面对的晶格阻力,进而影响材料变形机制。为了更好地推广本文提出的介原子势函数,本文选择了学术界近期的研究热点“孪晶诱导塑形变形钢”(TWIP钢),并利用介原子分子动力学方法发展得到层错能为19 mJ/m2的TWIP钢介原子势函数,随后利用这一势函数系统研究低层错能金属形变孪晶的形核、生长以及位错与孪晶的相互作用。研究发现,形变孪晶与扩展位错均在TWIP钢的塑形变形过程中发挥着重要作用。而形变孪晶的孪晶界可以有效地细化晶粒。在阻碍位错运动的同时,共格的孪晶界又提升了晶粒的容纳位错能力。在经历一定量塑性变形后,晶粒内会出现大量点空穴或空穴管道。这些空穴主要来自于位错割阶的非保守运动或特定伯格斯矢量层错的交叉。相对于传统的嵌入式原子势方法,本文所提出的介原子分子动力学方法可以有效降低合金势函数的发展难度。我们相信,这一方法的提出和应用必将对合金的分子动力学研究产生重要贡献。
[Abstract]:However, under the classical embedded atomic potential system, it is difficult for scientists to optimize the reliable alloy potential function, which makes the research of alloy molecular dynamics seriously lag behind the experimental and theoretical progress. Based on the embedded atomic potential, we developed a mesoatomic molecular dynamics method for alloy molecular dynamics simulation. The research progress is as follows: Based on two basic hypotheses, similarity hypothesis and averaging hypothesis, a mesoatomic molecular dynamics method is proposed. In one method, all the atoms no longer represent the atoms of different elements in the alloy, but are a virtual atom called "mesoatom". The fitting of the potential function does not consider the interaction between different elements, but directly uses the measured material properties of the alloy to optimize the fitting. In order to reduce the time of single optimization and improve the computational efficiency, some acceleration algorithms such as "geometric factor" and "white noise" are proposed, which effectively improve the program optimization. In order to verify the reliability of the proposed method, a series of potential functions for Cu-Al alloys and virtual face-centered cubic potential functions for brittle fracture have been fabricated by using the mesoatomic molecular dynamics method. These potential functions are used to simulate the effect of stacking fault energy on the deformation mechanism of alloys and metals. The results show that stacking fault energy has an important effect on the deformation mechanism of alloys: with the decrease of stacking fault energy, the proportion of twins in the deformation mechanism of metals increases rapidly. Meanwhile, the ratio of free surface energy to unstable stacking fault energy directly determines the ductile or brittle fracture properties of metals. In order to study the solid solution strengthening effect in solid solution alloys, the mesoatomic molecular dynamics method has been improved and a scaling factor mesoatomic molecular dynamics method has been proposed. Moreover, with the increase of lattice distortion, the lattice collapses into amorphous structure when the lattice is more stable than the lattice itself. The addition of the scaling factor does not have a great influence on the material properties, but also makes the critical stress Peierls stress of the dislocation slip increase greatly, which obviously increases the lattice resistance faced by the dislocation slip, and then shadow. In order to generalize the mesoatomic potential function proposed in this paper, the twin-induced plastic deformation steel (TWIP steel) has been selected as a research hotspot in recent years, and the mesoatomic potential function of TWIP steel with stacking fault energy of 19 mJ/m2 has been developed by using the mesoatomic molecular dynamics method. The nucleation and growth of deformation twins in low stacking fault energy metals and the interaction between dislocations and twins are studied.It is found that deformation twins and extended dislocations play an important role in the deformation process of TWIP steel.The twin boundaries of deformation twins can effectively refine grains.The coherent twin boundaries are raised while the dislocation movement is hindered. After a certain amount of plastic deformation, there will be a large number of point holes or holes in the grains. These holes mainly come from the non-conservative motion of the dislocation cut order or the crossover of specific Burgers vector stacking faults. Compared with the traditional embedded atomic potential method, the mesoatomic molecular dynamics proposed in this paper is more effective. It is believed that the proposed method and its application will contribute greatly to the study of molecular dynamics of alloys.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O56
本文编号：2189439
[Abstract]:However, under the classical embedded atomic potential system, it is difficult for scientists to optimize the reliable alloy potential function, which makes the research of alloy molecular dynamics seriously lag behind the experimental and theoretical progress. Based on the embedded atomic potential, we developed a mesoatomic molecular dynamics method for alloy molecular dynamics simulation. The research progress is as follows: Based on two basic hypotheses, similarity hypothesis and averaging hypothesis, a mesoatomic molecular dynamics method is proposed. In one method, all the atoms no longer represent the atoms of different elements in the alloy, but are a virtual atom called "mesoatom". The fitting of the potential function does not consider the interaction between different elements, but directly uses the measured material properties of the alloy to optimize the fitting. In order to reduce the time of single optimization and improve the computational efficiency, some acceleration algorithms such as "geometric factor" and "white noise" are proposed, which effectively improve the program optimization. In order to verify the reliability of the proposed method, a series of potential functions for Cu-Al alloys and virtual face-centered cubic potential functions for brittle fracture have been fabricated by using the mesoatomic molecular dynamics method. These potential functions are used to simulate the effect of stacking fault energy on the deformation mechanism of alloys and metals. The results show that stacking fault energy has an important effect on the deformation mechanism of alloys: with the decrease of stacking fault energy, the proportion of twins in the deformation mechanism of metals increases rapidly. Meanwhile, the ratio of free surface energy to unstable stacking fault energy directly determines the ductile or brittle fracture properties of metals. In order to study the solid solution strengthening effect in solid solution alloys, the mesoatomic molecular dynamics method has been improved and a scaling factor mesoatomic molecular dynamics method has been proposed. Moreover, with the increase of lattice distortion, the lattice collapses into amorphous structure when the lattice is more stable than the lattice itself. The addition of the scaling factor does not have a great influence on the material properties, but also makes the critical stress Peierls stress of the dislocation slip increase greatly, which obviously increases the lattice resistance faced by the dislocation slip, and then shadow. In order to generalize the mesoatomic potential function proposed in this paper, the twin-induced plastic deformation steel (TWIP steel) has been selected as a research hotspot in recent years, and the mesoatomic potential function of TWIP steel with stacking fault energy of 19 mJ/m2 has been developed by using the mesoatomic molecular dynamics method. The nucleation and growth of deformation twins in low stacking fault energy metals and the interaction between dislocations and twins are studied.It is found that deformation twins and extended dislocations play an important role in the deformation process of TWIP steel.The twin boundaries of deformation twins can effectively refine grains.The coherent twin boundaries are raised while the dislocation movement is hindered. After a certain amount of plastic deformation, there will be a large number of point holes or holes in the grains. These holes mainly come from the non-conservative motion of the dislocation cut order or the crossover of specific Burgers vector stacking faults. Compared with the traditional embedded atomic potential method, the mesoatomic molecular dynamics proposed in this paper is more effective. It is believed that the proposed method and its application will contribute greatly to the study of molecular dynamics of alloys.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O56
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