納米晶晶界演化行為的晶體相場法研究
發(fā)布時間：2018-08-05 13:22
【摘要】：納米晶材料作為新興的材料門類,因其具有獨特的機械、電磁及化學性質,在凝聚態(tài)物理、應用化學、材料科學等研究領域受到了廣泛的重視。存在大量的晶界及三晶交線是納米晶材料的重要組織特征,晶界在曲率及外力驅動條件下的演化行為,在很大程度上決定了納米晶材料的性能。納米晶晶界演化是一個涉及原子重排和界面擴散的多尺度復雜物理過程。為從本質上認識晶界在不同條件下是如何運動進而影響納米晶粒演化的,必須深入理解涉及空間和時間多尺度的晶界及三晶交線演化問題。晶體相場模型是近年來興起的在原子尺度上描述材料微觀組織演化的模擬方法,這種基于密度泛函理論的原子尺度模擬方法最大優(yōu)勢在于能夠在反映材料原子尺度信息的基礎上描述其在擴散時間尺度上的演化過程。本文采用標準和最小體耗散的晶體相場模型,以晶界原子尺度特征的準確描述為基礎,對BCC結構納米晶中[001]傾斜晶界在曲率驅動及外加剪切應變條件下的演化行為以及三晶交線對晶界演化的影響等進行了深入研究,這可為實現(xiàn)納米晶材料的組織調控提供理論基礎。論文獲得了如下主要結論:(1)基于所編制的晶體相場模型并行計算求解程序,成功實現(xiàn)了曲率驅動及外加剪切應變驅動下納米晶晶界演化過程的數(shù)值模擬。成功獲得了BCC晶體中[001]傾斜晶界的平衡結構、晶界能及其附近彈性應變場等原子尺度特征的準確表征,發(fā)現(xiàn)模擬所得晶界結構符合位錯理論描述,均由兩類刃位錯(a100和a110/2)混合而成,單個位錯附近的應變場亦與各向異性的位錯彈性理論定量相符。(2)獲得了純曲率驅動、純耦合效應及混合方式等晶界運動機制在BCC雙晶內(nèi)嵌系統(tǒng)中不同取向差下的適用性范圍。取向差在27.5度以上的晶界按照經(jīng)典純曲率驅動方式做遷移運動,內(nèi)嵌晶粒取向保持恒定,晶粒面積隨時間呈線性快速變化;取向差在20度以下的晶界按照純耦合方式運動,內(nèi)嵌晶粒取向逐漸升高,而晶粒面積仍保持線性變化但演化速率較慢;而取向差在20-27.5度之間的晶界則按照部分曲率驅動部分耦合效應的混合方式運動,中心晶粒取向變化小,而晶粒面積呈非線性緩慢變化。(3)闡明了極小取向差的彎曲晶界運動誘發(fā)晶粒轉動現(xiàn)象的發(fā)生機制。當晶界取向差較小時,相鄰晶界位錯間的共格區(qū)域內(nèi)存在明顯彈性畸變,從而導致晶界位錯向晶粒中心徑向運動,同時對內(nèi)部晶粒產(chǎn)生轉動力矩,造成晶粒轉動;當取向差較高時,相鄰晶界位錯的間距小,其間的彈性畸變也非常弱,位錯湮滅、分解等反應過程占主導,整個晶界在純曲率驅動下運動,不會引起晶粒轉動。從晶界原子結構上而言,不同取向差下晶界表現(xiàn)出不同的演化行為可歸因于晶界內(nèi)多種位錯的平衡間距隨取向差的變化不同步。(4)平直對稱傾斜晶界在外加剪切應變條件下按照剪切耦合的方式遷移運動,其微觀本質是連錯的形核及擴展。連錯對的存在對晶界遷移過程的影響顯著,連錯的形核勢壘決定了晶界遷移的PN勢,其同時具有的不全位錯和臺階特征決定了晶界遷移的耦合因子。(5)晶界偏轉角對非對稱傾斜晶界剪切耦合運動的影響作用取決于兩類晶界位錯的占比及其對剪切應變的響應。晶界偏轉角較小時,一類位錯承擔了變形主體,而另一類位錯則隨之發(fā)生運動,此時晶界的PN勢和耦合因子與對稱傾斜晶界相比變化不大;而當偏轉角轉至兩類位錯占比相近的角度時,兩類位錯共同作為變形主體,晶界開啟運動和遷移都很困難,造成晶界PN勢和耦合因子的顯著提升。(6)獲得了納米晶中三晶交線對晶界演化的影響規(guī)律及機制,建立了三晶交線不連續(xù)遷移運動模型。三晶交線的拖拽作用改變了晶界位錯的運動方式從而減緩了晶粒的演化動力學,三晶交線附近的位錯釋放是由晶界的運動不協(xié)調造成;三晶系統(tǒng)在變形條件和曲率驅動條件下表現(xiàn)出截然不同的穩(wěn)定性。
[Abstract]:As a new type of material, nanocrystalline materials have been widely paid attention to in the fields of condensed matter physics, applied chemistry and material science because of their unique mechanical, electromagnetic and chemical properties. The existence of a large number of grain boundaries and intersecting lines is an important fabric of nanocrystalline materials, and the grain boundary is performed under the conditions of curvature and external force. The properties of nanocrystalline materials are largely determined by chemical behavior. The evolution of nanocrystalline boundary is a multi-scale and complex physical process involving atomic rearrangement and interface diffusion. It is necessary to understand how the grain boundary is moving in different conditions and influence the evolution of nanocrystalline grain in essence. It is necessary to understand the multiscale of space and time. The crystal phase field model is a new method to describe the evolution of microstructures on the atomic scale in recent years. The greatest advantage of the atomic scale simulation method based on density functional theory is that it can describe its diffusion time scale on the basis of reflecting the information of the atomic scale. Based on the accurate description of the atomic scale characteristics of the grain boundary, the evolution process of the crystal phase field of the standard and the minimum body dissipation is studied in this paper. The evolution behavior of the inclined grain boundary in the BCC structure nanocrystals under the conditions of curvature driven and external shear strain and the influence of the intersection of three crystals on the evolution of the grain boundary are studied. It can provide the theoretical basis for the organization and control of nanocrystalline materials. The main conclusions are obtained as follows: (1) a numerical simulation of the evolution of nanocrystalline boundaries driven by curvature driven and external shear strain was successfully realized on the basis of the parallel computation program of the crystal phase field model. The [001] tilt in BCC crystal has been successfully obtained. The equilibrium structure of grain boundary, the energy of the grain boundary and the elastic strain field near the grain boundary are accurately characterized. It is found that the grain boundary structure of the simulated grain conforms to the dislocation theory description, which is composed of two kinds of edge dislocations (A100 and a110/2), and the strain field near the single dislocation is also consistent with the anisotropic dislocation elasticity theory. (2) the pure curve is obtained. The grain boundary movement mechanism, such as rate driven, pure coupling effect and mixing mode, is applicable to the different orientation difference in the BCC embedded system. The grain boundary in the grain boundary is more than 27.5 degrees in the classical pure curvature driving mode. The grain orientation keeps constant, the grain surface product changes linearly with time, and the orientation difference is 20 degrees. The grain boundary in the following grain boundaries is moving in the pure coupling mode, and the grain orientation increases gradually, while the grain size remains linear but the evolution rate is slow, while the grain boundary between the orientation difference and the 20-27.5 degree is moving with the partial curvature which drives the partial coupling effect, and the orientation of the central grain is small and the grain area is nonlinear slowly. Slow change. (3) the mechanism of grain rotation induced by the curved grain boundary movement of the small orientation difference is clarified. When the grain boundary orientation difference is small, the common area between the adjacent grain boundary dislocations is obviously elastic, which leads to the grain boundary dislocation moving to the grain center, and the rotation moment of the grain at the same time, resulting in the grain rotation. When the orientation difference is high, the distance between the adjacent grain boundary dislocations is small, the elastic distortion is also very weak, the dislocation annihilation, the decomposition and other reaction processes dominate, the grain boundary is driven by pure curvature and does not cause the grain rotation. From the grain boundary structure, the different evolution behavior of the grain boundary under the different orientation difference can be attributed to the different evolution behavior. The equilibrium spacing of a variety of dislocation in grain boundary is not synchronized with the variation of orientation difference. (4) the vertical symmetry inclined grain boundary is migrated under the shear coupling mode under the external shear strain condition, and its microscopic nature is the nucleation and expansion of the dislocation, and the existence of the fault pair has a significant influence on the grain boundary migration process, and the grain boundary determines the grain boundary with the wrong nucleation barrier. The PN potential of the migration determines the coupling factor of grain boundary migration. (5) the influence of the grain boundary deflection angle on the shear coupling motion of the asymmetric slant grain boundary depends on the proportion of the two kinds of grain boundary dislocation and the response to the shear strain. The dislocation of grain boundary is small, and a class of dislocation bears the deformation body. The PN potential and coupling factor of the grain boundary have little change compared with the symmetrical slant grain boundary. When the deflection angle turns to the two kind of dislocation, the two kind of dislocation is the main body of deformation, and the movement and migration of grain boundary opening are very difficult, resulting in the remarkable enhancement of the grain boundary PN potential and the coupling factor. 6) the influence law and mechanism of the intersection of three crystals in nanocrystalline on the evolution of grain boundary was obtained. The movement model of discontinuous migration of intersecting lines was established. The drag and drop of the intersection of three crystals changed the movement mode of grain boundary dislocation and slowed the dynamics of grain evolution. The dislocation release near the intersection of three crystals was caused by the incoordination of grain boundary movement. The three crystal system shows a completely different stability under deformation conditions and curvature driving conditions.
【學位授予單位】：西北工業(yè)大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TB383.1
本文編號：2165921
[Abstract]:As a new type of material, nanocrystalline materials have been widely paid attention to in the fields of condensed matter physics, applied chemistry and material science because of their unique mechanical, electromagnetic and chemical properties. The existence of a large number of grain boundaries and intersecting lines is an important fabric of nanocrystalline materials, and the grain boundary is performed under the conditions of curvature and external force. The properties of nanocrystalline materials are largely determined by chemical behavior. The evolution of nanocrystalline boundary is a multi-scale and complex physical process involving atomic rearrangement and interface diffusion. It is necessary to understand how the grain boundary is moving in different conditions and influence the evolution of nanocrystalline grain in essence. It is necessary to understand the multiscale of space and time. The crystal phase field model is a new method to describe the evolution of microstructures on the atomic scale in recent years. The greatest advantage of the atomic scale simulation method based on density functional theory is that it can describe its diffusion time scale on the basis of reflecting the information of the atomic scale. Based on the accurate description of the atomic scale characteristics of the grain boundary, the evolution process of the crystal phase field of the standard and the minimum body dissipation is studied in this paper. The evolution behavior of the inclined grain boundary in the BCC structure nanocrystals under the conditions of curvature driven and external shear strain and the influence of the intersection of three crystals on the evolution of the grain boundary are studied. It can provide the theoretical basis for the organization and control of nanocrystalline materials. The main conclusions are obtained as follows: (1) a numerical simulation of the evolution of nanocrystalline boundaries driven by curvature driven and external shear strain was successfully realized on the basis of the parallel computation program of the crystal phase field model. The [001] tilt in BCC crystal has been successfully obtained. The equilibrium structure of grain boundary, the energy of the grain boundary and the elastic strain field near the grain boundary are accurately characterized. It is found that the grain boundary structure of the simulated grain conforms to the dislocation theory description, which is composed of two kinds of edge dislocations (A100 and a110/2), and the strain field near the single dislocation is also consistent with the anisotropic dislocation elasticity theory. (2) the pure curve is obtained. The grain boundary movement mechanism, such as rate driven, pure coupling effect and mixing mode, is applicable to the different orientation difference in the BCC embedded system. The grain boundary in the grain boundary is more than 27.5 degrees in the classical pure curvature driving mode. The grain orientation keeps constant, the grain surface product changes linearly with time, and the orientation difference is 20 degrees. The grain boundary in the following grain boundaries is moving in the pure coupling mode, and the grain orientation increases gradually, while the grain size remains linear but the evolution rate is slow, while the grain boundary between the orientation difference and the 20-27.5 degree is moving with the partial curvature which drives the partial coupling effect, and the orientation of the central grain is small and the grain area is nonlinear slowly. Slow change. (3) the mechanism of grain rotation induced by the curved grain boundary movement of the small orientation difference is clarified. When the grain boundary orientation difference is small, the common area between the adjacent grain boundary dislocations is obviously elastic, which leads to the grain boundary dislocation moving to the grain center, and the rotation moment of the grain at the same time, resulting in the grain rotation. When the orientation difference is high, the distance between the adjacent grain boundary dislocations is small, the elastic distortion is also very weak, the dislocation annihilation, the decomposition and other reaction processes dominate, the grain boundary is driven by pure curvature and does not cause the grain rotation. From the grain boundary structure, the different evolution behavior of the grain boundary under the different orientation difference can be attributed to the different evolution behavior. The equilibrium spacing of a variety of dislocation in grain boundary is not synchronized with the variation of orientation difference. (4) the vertical symmetry inclined grain boundary is migrated under the shear coupling mode under the external shear strain condition, and its microscopic nature is the nucleation and expansion of the dislocation, and the existence of the fault pair has a significant influence on the grain boundary migration process, and the grain boundary determines the grain boundary with the wrong nucleation barrier. The PN potential of the migration determines the coupling factor of grain boundary migration. (5) the influence of the grain boundary deflection angle on the shear coupling motion of the asymmetric slant grain boundary depends on the proportion of the two kinds of grain boundary dislocation and the response to the shear strain. The dislocation of grain boundary is small, and a class of dislocation bears the deformation body. The PN potential and coupling factor of the grain boundary have little change compared with the symmetrical slant grain boundary. When the deflection angle turns to the two kind of dislocation, the two kind of dislocation is the main body of deformation, and the movement and migration of grain boundary opening are very difficult, resulting in the remarkable enhancement of the grain boundary PN potential and the coupling factor. 6) the influence law and mechanism of the intersection of three crystals in nanocrystalline on the evolution of grain boundary was obtained. The movement model of discontinuous migration of intersecting lines was established. The drag and drop of the intersection of three crystals changed the movement mode of grain boundary dislocation and slowed the dynamics of grain evolution. The dislocation release near the intersection of three crystals was caused by the incoordination of grain boundary movement. The three crystal system shows a completely different stability under deformation conditions and curvature driving conditions.
【學位授予單位】：西北工業(yè)大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TB383.1
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