以分子動力模擬探討奈米壓印之變形行為與差排機制

Abstract

本論文針對鋁金屬材料在奈米壓印下的變形行為和其背後所對應的差排機制進行研究。在本論文中，我們分別使用圓形壓印子、方形壓印子和Berkovich壓印子來進行探討。此外，我們還對此鋁金屬材料的二個不同晶面在奈米壓印時所反應出來的彈性異向性行為進行探討。 本論文中，我們使用分子動力法作為探討奈米壓印實驗的模擬方法。此方法對觀察原子運動狀態的細部資訊提供了相當高的解析度，因此我們能夠利用它來對材料的變形行為進行相當精確的討論。在網路上，支援分子動力法模擬的軟體非常多，本研究選擇使用一套由康乃爾理論中心 (Cornell Theory Center) 所開發的分子動力法模擬軟體-分子動力套件(Molecular Dynamics package) 作為模擬工具，並在分子動力套件這套軟體的架構下，加入了一個根據Ercolessi and Adams (Ercolessi and Adams 1994) 所提出的鋁金屬勢能類別，以便我們對鋁金屬材料進行探討。我們在本論文中使用滑動向量分析 (Slip Vector Analysis, Zimmerman et al. 2001) 來將產生差排缺陷的原子找出，並用二個視算輔助軟體RasMol和PVWin來將這些缺陷原子畫出，以便我們分析材料的變形行為和其背後的差排機制。此外，我們也利用Hardy (1982) 所提出的原子尺度下的應力定義來分析在奈米壓印後，壓印區正下方產生變形的區域之臨界平均剪切應力。 根據本研究結果，我們可提出下來八點結論: (1) 在load-separate displacement關係圖中所出現的突然不連續點或負載力突然下降的現象可歸因於差排結構的產生或差排結構的變化。(2) 從奈米壓印的彈性行為分析中，我們可得知Ercolessi-Adams鋁金屬勢能對鋁(001)面的模擬能力比對鋁(111)面來的好。(3) 壓印區正下方產生變形的區域之臨界平均剪切應力在使用圓形和方形壓印子時，量測值落在3.84GPa到3.33GPa之間，很接近由Ercolessi-Adams鋁金屬勢能計算出的理論值5.28GPa。然而在Berkovich 壓印子的例中，由於一個四面體缺陷的產生，使得量測值退化到1.83GPa到2.5GPa之間。(4) 在奈米壓印時，鋁(111)面的楊氏模數大約比鋁(001)面的楊氏模數大約高66%。(5) 奈米壓印中的塑性行為主要是由差排的活動來主導。一些主要的差排活動，如dislocation lock formation、dislocation cross slip和dislocation double cross slip等，都可在本論文的模擬裡觀察到。(6) 壓印子的形狀對差排的活動有很大的影響，我們觀察到在不同形狀的壓印子之下，差排活動的形式有很大的不同。(7) 在奈米壓印模擬中，我們發現使用不同形狀的壓印子，材料所展現出的變形行為相當的不同。 (8) 本研究成功地將分子動力套件用於模擬奈米壓印實驗，驗証此多用途材料模擬軟體可以容易地被特製化成特定的模擬應用。

In this study, we study the deformation responses and dislocation mechanisms during nanoindentation into aluminum metal with three geometrically different indenters. The spherical indenter, rectangular indenter, and Berkovich indenter are employed to use in the nanoindentation simulations. Besides the dislocation microstructure developments, the elastic anisotropy of the aluminum sample for two different crystallographic planes is also discussed. We use molecular dynamics as our modeling approach because its capability of elucidating the atomic information with high resolution can greatly help us to study the deformation process in the atomistic length scale. A general purpose materials simulation tool, the Molecular Dynamics package from Cornell Theory Center, is employed. The Ercolessi-Adams glue potential (Ercolessi and Adams 1994) is incorporated into this package to model the aluminum metal. The slip vector (Zimmerman et al. 2001) is used to detect the nucleation of dislocation defects. Two visualization tools, RasMol and PVWin, are used to visualize the deformed configuration and the dynamic deformation process. A stress definition proposed by Hardy (1982) is employed to analyze the critical mean resolved shear stress of the deformed region under the indentation site. Based on this study, the following conclusions can be drawn: (1) The discontinuity or load drop event presenting in the load-separate displacement curve can be viewed as the signal for dislocation nucleation or defect structure transition. (2) From the elastic analysis, we find that the Ercolessi-Adams glue potential models better than . (3) The critical mean resolved stress in the deformed region for the spherical and rectangular indenter cases are in a range between 3.84GPa and 3.33GPa, which is close to the calculated theoretic shear strength for the Ercolessi-Adams glue potential, 5.28GPa. However, that value reduces to a range between 1.83GPa to 2.5GPa for the Berkovich indenter case because of the presence of a tetrahedral defect region. (4) The effective Young’s modulus of is about 66% greater than that of when the aluminum sample is subjected to indentation loading condition. (5) The plasticity behavior in the nanoindentation experiment is dominated by dislocation activities. Prosperous dislocation activities, including dislocation lock formations, dislocation cross slip and double cross slip events, are observed during the nanoindentation simulation. (6) The dislocation activities under three geometrically different indenters are observed to be significantly different. Our observations indicate that the dislocation activities are substantially affected by the indenter geometrical shape. (7) In nanoindentation simulation, the deformation behavior is different for each indenter case. (8) We successfully extend the Molecular Dynamics package to simulate the nanoindentation experiment. This general purpose software for materials simulation is proven to be extensible and flexible for specific applications.