介原子分子動(dòng)力學(xué)方法的發(fā)展與應(yīng)用
發(fā)布時(shí)間：2018-08-18 12:14
【摘要】：合金廣泛應(yīng)用于工業(yè)生產(chǎn),由此也得到學(xué)術(shù)界的廣泛關(guān)注與研究。但在經(jīng)典的嵌入式原子勢(shì)體系下,科學(xué)家們很難優(yōu)化得到可靠的合金勢(shì)函數(shù),這使得合金分子動(dòng)力學(xué)研究嚴(yán)重滯后于實(shí)驗(yàn)與理論的研究進(jìn)展。為能夠利用分子動(dòng)力學(xué)從原子層面研究合金的變形機(jī)制,我們基于嵌入式原子勢(shì)發(fā)展出用于合金分子動(dòng)力學(xué)模擬的介原子分子動(dòng)力學(xué)方法。所取得的研究進(jìn)展如下：本文基于兩個(gè)基本假設(shè)“相似性假設(shè)”和“平均化假設(shè)”提出了介原子分子動(dòng)力學(xué)方法。在這一方法中,所有的原子都不再代表合金中的不同元素的原子而是一個(gè)虛擬的原子—“介原子”。其勢(shì)函數(shù)的擬合也不再考慮不同元素之間的相互作用,轉(zhuǎn)而直接使用合金的實(shí)測(cè)材料屬性來優(yōu)化擬合�；谶@一方法,本文自主開發(fā)了一套計(jì)算機(jī)程序,通過加權(quán)最小二乘法優(yōu)化模擬值與目標(biāo)值的殘差,最后輸出滿足嵌入式原子勢(shì)格式的勢(shì)函數(shù)文件。為了降低單次優(yōu)化的時(shí)間并提升計(jì)算效率,本文提出了“幾何因子”和“白噪聲”等加速算法,這有效提升了程序優(yōu)化得到勢(shì)函數(shù)的求解能力。為了驗(yàn)證本文所提出方法的可靠性,本文利用介原子分子動(dòng)力學(xué)方法制作得到了一系列的銅鋁合金勢(shì)函數(shù)和虛擬可發(fā)生脆斷的面心立方勢(shì)函數(shù),并利用這些勢(shì)函數(shù)模擬研究層錯(cuò)能對(duì)合金變形機(jī)制的影響和金屬的韌脆轉(zhuǎn)變效應(yīng)。本文的研究結(jié)果發(fā)現(xiàn)層錯(cuò)能對(duì)于合金的變形機(jī)制有著重要影響：隨著層錯(cuò)能的降低,金屬的變形機(jī)制中孿晶所占比例迅速提升。同時(shí),自由表面能和不穩(wěn)定層錯(cuò)能的比值直接決定了金屬的韌性或脆性斷裂屬性。這組勢(shì)函數(shù)的成功應(yīng)用證明了本方法的可靠性與普適性。為研究固溶體合金中的固溶強(qiáng)化效應(yīng),本文又對(duì)介原子分子動(dòng)力學(xué)方法做了改進(jìn),提出了放縮因子介原子分子動(dòng)力學(xué)方法。放縮因子的加入可以有效地在金屬中引入彌散分布的晶格畸變。并且,隨著晶格畸變的加大,當(dāng)超過晶格自身的穩(wěn)定性時(shí),晶格就會(huì)垮塌成為非晶結(jié)構(gòu)。放縮因子的加入并不會(huì)對(duì)材料屬性有很大的影響,但又會(huì)使位錯(cuò)滑動(dòng)的臨界應(yīng)力Peierls應(yīng)力大幅度上升,明顯提升位錯(cuò)滑動(dòng)所面對(duì)的晶格阻力,進(jìn)而影響材料變形機(jī)制。為了更好地推廣本文提出的介原子勢(shì)函數(shù),本文選擇了學(xué)術(shù)界近期的研究熱點(diǎn)“孿晶誘導(dǎo)塑形變形鋼”(TWIP鋼),并利用介原子分子動(dòng)力學(xué)方法發(fā)展得到層錯(cuò)能為19 mJ/m2的TWIP鋼介原子勢(shì)函數(shù),隨后利用這一勢(shì)函數(shù)系統(tǒng)研究低層錯(cuò)能金屬形變孿晶的形核、生長(zhǎng)以及位錯(cuò)與孿晶的相互作用。研究發(fā)現(xiàn),形變孿晶與擴(kuò)展位錯(cuò)均在TWIP鋼的塑形變形過程中發(fā)揮著重要作用。而形變孿晶的孿晶界可以有效地細(xì)化晶粒。在阻礙位錯(cuò)運(yùn)動(dòng)的同時(shí),共格的孿晶界又提升了晶粒的容納位錯(cuò)能力。在經(jīng)歷一定量塑性變形后,晶粒內(nèi)會(huì)出現(xiàn)大量點(diǎn)空穴或空穴管道。這些空穴主要來自于位錯(cuò)割階的非保守運(yùn)動(dòng)或特定伯格斯矢量層錯(cuò)的交叉。相對(duì)于傳統(tǒng)的嵌入式原子勢(shì)方法,本文所提出的介原子分子動(dòng)力學(xué)方法可以有效降低合金勢(shì)函數(shù)的發(fā)展難度。我們相信,這一方法的提出和應(yīng)用必將對(duì)合金的分子動(dòng)力學(xué)研究產(chǎn)生重要貢獻(xiàn)。
[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.
【學(xué)位授予單位】：浙江大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：O56
本文編號(hào)：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.
【學(xué)位授予單位】：浙江大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：O56
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