【摘要】：晶體失穩(wěn)作為材料學研究的基礎性課題之一,在模擬和實驗上已經進行了廣泛的研究。但由于受到時間和空間尺度的限制,材料模擬和實驗之間的差距較大。材料多尺度模擬的快速發(fā)展,為縮小模擬與實驗之間的差距創(chuàng)造了條件。建立在Cauchy-Born法則、超彈性本構理論和原子勢基礎上的相互原子勢有限元模型(IPFEM),實現(xiàn)了原子模擬與有限元模擬間的無縫連接。本文對IPFEM模型進行改進,將其應用范圍從簡單晶體擴展到復式晶體。此外,將軟聲子分析引入到有限元模擬中,建立晶格動力學有限元模型(LDFEM),進一步增強預測晶體失穩(wěn)的準確性。運用IPFEM、LDFEM和分子動力學(MD)法研究HCP Co、L10型γ-TiAl、L12型FeNi_3和Ni_3Al等金屬晶體的納米力學行為。采用改進的IPFEM模型研究L12有序結構FeNi_3合金的晶體失穩(wěn)。在圓柱型納米壓痕中,從IPFEM模擬得到的載荷-位移曲線、壓痕應力場和激活的滑移系與MD模擬結果相符。在球型納米壓痕中,晶體失穩(wěn)位置和壓痕應力場與晶體取向有關。采用插值法將晶體失穩(wěn)時的臨界載荷、臨界平均接觸壓強、壓痕模量和臨界分切應力與晶體取向的關系表示在反極圖中。此外,MD模擬結果表明,FeNi_3納米壓痕中位錯形核所需的臨界載荷以及形核過程與晶體取向有關。納米壓痕中出現(xiàn)的pop-in行為與晶體內位錯的形成和反應有關。首個pop-in事件與1/61 1 2型不全位錯均勻形核有關。激活的不全位錯發(fā)生復雜的位錯反應形成位錯鎖。運用IPFEM模型研究了晶體取向對L10有序結構γ-TiAl晶體失穩(wěn)的影響。研究結果表明,納米壓痕載荷-位移曲線、臨界壓入深度和臨界載荷以及壓痕模量與晶面取向有關。詳細分析了(0 0 1)、(1 0 0)、(1 0 1)、(1 1 0)和(1 1 1)五個典型晶面納米壓痕過程中的晶體失穩(wěn)位置和激活的滑移。IPFEM模擬過程中預測到的晶體失穩(wěn)位置和激活的滑移系與MD模擬過程中觀察到的位錯形核位置和滑移方式一致。將LDFEM模型用于研究γ-TiAl晶體的晶體失穩(wěn)。模擬結果表明,γ-TiAl的晶體失穩(wěn)受加載模式和晶體取向的影響。在單向加載過程中,γ-TiAl晶體失穩(wěn)表現(xiàn)出明顯的拉伸-壓縮不對稱性。LDFEM模擬得到的應力-應變曲線以及激活的滑移系與MD模擬結果相吻合。在納米壓痕中,接觸面法向對應力分布、晶體失穩(wěn)和位錯形核均有顯著的影響。LDFEM模型能準確預測晶體的失穩(wěn)位置和激活的滑移系。LDFEM模型還被用于研究Co晶體在單向拉伸和納米壓痕過程中的聲子失穩(wěn)。模擬結果表明,Co晶體的晶體失穩(wěn)受加載模式和晶體取向的影響。LDFEM模型能夠準確描述Co晶體的拉伸力學行為。在圓柱型納米壓痕過程中,從LDFEM和MD模擬得到的載荷-位移曲線在晶體失穩(wěn)前保持一致。Co晶體失穩(wěn)后主要發(fā)生基面位錯形核。在(0 0 0 1)面球型納米壓痕過程中,出現(xiàn)六個對稱的失穩(wěn)位置。此外,對Ni_3Al晶體的納米壓痕過程進行MD模擬,以研究Ni_3Al晶體的初始塑性。模擬結果表明,Ni_3Al晶體的初始塑性與Shockley不全位錯的均勻形核有關。晶體取向、原子勢、模型尺寸和溫度對晶體發(fā)生初始塑性時的臨界載荷、臨界接觸壓強、位錯形核位置和激活的滑移系均有顯著影響。隨著壓頭半徑的增加,壓痕模量和位錯形核深度增加,但最大切應力隨壓頭半徑的增加而減小。最大切應力和壓痕模量隨溫度的升高線性下降,位錯形核本質上屬于應力輔助熱激活過程。
[Abstract]:Crystal instability has been extensively studied in both simulation and experiment as one of the fundamental subjects in materials science. However, due to the limitation of time and space scales, the gap between material simulation and experiment is large. The rapid development of multi-scale simulation of materials has created conditions for narrowing the gap between simulation and experiment. Based on Cauchy-Born rule, superelastic constitutive theory and atomic potential, a finite element model of mutual atomic potential (IPFEM) is proposed to realize the seamless connection between atomic simulation and finite element simulation. In this paper, the lattice dynamics finite element model (LDFEM) is established to further enhance the accuracy of predicting crystal instability. The nanomechanical behaviors of HCP Co, L10 type gamma-TiAl, L12 type FeNi_3 and Ni_3Al are studied by using IPFEM, LDFEM and molecular dynamics (MD) methods. The crystal instability of L12 ordered structure FeNi_3 alloy is studied by using the improved IPFEM model. In cylindrical nanoindentation, the load-displacement curves obtained from IPFEM simulation, the indentation stress field and the activated slip system are in agreement with the MD simulation results. In spherical nanoindentation, the crystal instability position and the indentation stress field are related to the crystal orientation. The relationship between the critical shear stress and the crystal orientation is expressed in the inverse pole diagram. In addition, MD simulation results show that the critical load required for dislocation nucleation in FeNi_3 nanoindentation and the nucleation process are related to the crystal orientation. The effect of crystal orientation on the instability of ordered L10-TiAl crystals was studied by using IPFEM model. The results show that nanoindentation load-displacement curves, critical indentation depth, critical load and indentation modulus are related to the crystal plane selection. The position of crystal instability and the slip of activation during the nanoindentation of five typical crystal planes are analyzed in detail. The predicted position of crystal instability and the slip system of activation during IPFEM simulation are consistent with the dislocation nucleation position and slip mode observed during MD simulation. The simulation results show that the instability of the crystal is affected by the loading mode and crystal orientation. During uniaxial loading, the instability of the crystal exhibits obvious tensile-compressive asymmetry. The stress-strain curves obtained by LDFEM simulation and the activated slip system kiss the MD simulation results. In nanoindentation, the normal orientation of the contact surface has a significant effect on the stress distribution, crystal instability and dislocation nucleation. The LDFEM model can accurately predict the location of instability and the active slip system. The LDFEM model is also used to study the phonon instability of Co crystals during uniaxial tension and nanoindentation. The bulk instability is affected by the loading mode and crystal orientation. The LDFEM model can accurately describe the tensile mechanical behavior of Co crystals. The load-displacement curves obtained from LDFEM and MD simulation are consistent before the crystal instability. The dislocation nucleation mainly occurs after the instability of Co crystals. In addition, the nanoindentation process of Ni_3Al crystals was simulated by MD to study the initial plasticity of Ni_3Al crystals. The results show that the initial plasticity of Ni_3Al crystals is related to the homogeneous nucleation of Shockley incomplete dislocations. With the increase of indentation radius, the indentation modulus and the nucleation depth of dislocation increase, but the maximum shear stress decreases with the increase of indentation radius. The maximum shear stress and indentation modulus decrease linearly with the increase of temperature, and the dislocation nucleation base. Qualitatively, it belongs to the stress assisted thermal activation process.
【學位授予單位】：上海交通大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG111.2
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本文編號：2197671