分子動力學模擬與微結構演進分析: 以鎳鈦形狀記憶合金與奈米雙晶銅為例

Microstructural Evolution Analysis in NiTi Shape Memory Alloys and Nano-twinned Cu by Molecular Dynamics

楊仲齊(Chung-Chi Yang)

指導教授 ： 鄒年棣

國立交通大學/工學院/材料科學與工程系所/碩士(2020年)

摘要

形狀記憶效應與超彈性是鎳鈦形狀記憶合金中非常重要的特性，主要應用於致動器與阻尼器等綠能元件中。眾多的研究顯示，麻田散體相變化是材料具有形狀記憶效應與超彈性的主要原因。因此掌握麻田散體相變化的機制成為近年來眾多研究努力的方向。溫度誘發以及應力誘發麻田散體相變化是形狀記憶效應與超彈性中重要的環節，透過原子體積-溫度曲線、負載-壓印深度曲線和能量變化曲線可以知道相轉變發生的時機點以及機械性質的變化。故本研究致力於了解在溫度誘發後壓印試驗中，負載-壓印深度曲線與能量變化曲線對應之微結構特徵，藉此了解鎳鈦形狀記憶合金的微結構演變路徑、型態和壓痕殘留的改變。本研究利用分子動力學來模擬變形環境後壓印和溫度對微結構的影響，並透過自行開發的麻田散體兄弟晶辨認方法（Martensite variants identification method, MVIM）分析微結構的演進。變形環境分別為等向性變形、異相性變形和雙軸耦合變形；壓印溫度包含100K、325K與400K。最後，本研究致力於在不同壓印條件中，掌握鎳鈦形狀記憶合金微結構變化趨勢，進而達到設計材料的可能性。
柱狀奈米雙晶銅之低電阻率與高延展性，使不斷微縮的積體電路封裝製程突破傳統技術的限制，而特定方向的負載有助於提升材料之機械性質。文獻指出，差排與晶粒微結構為奈米雙晶銅特性變化的主要原因，故了解差排與晶粒的演進成為近年來研究的重點。透過負載試驗中的應力-應變曲線對應之差排分布與晶界特徵，進而分析差排之滑行路徑、差排與晶界的交互作用以及晶粒融合的過程。本研究利用分子動力學模擬兩組座標系統、三種模型結構與不同方向的負載對機械性質與差排運動的影響，接著透過晶體結構後處理軟體分析微結構的演進。其中，座標系統分別為（001）與（111）座標系統；模型結構分別為非雙晶結構、雙晶結構與奈米多晶基底結構；負載為拉伸與壓縮。最終，本研究致力於優化材料並整理出微結構演變趨勢。

關鍵字

鎳鈦形狀記憶合金 ； 溫度誘發麻田散體相變化 ； 麻田散體兄弟晶 ； 奈米壓印 ； 奈米雙晶銅 ； 奈米多晶基底 ； 負載變形

並列摘要

Shape memory effect (SME) and superelasticity (SE) are very important material properties in NiTi shape memory alloys. Renewable energy devices like actuator and damper are the main applications. Many studies have shown that the martensitic transformation is the main reason for the SME and SE of the material. Therefore, understanding the mechanism of the martensitic transformation has become a popular research topic in recent years. Thermal-induced and stress-induced martensitic transformation is an important part of the SME and SE, and the atomic volume-temperature curve as well as the load-indentation depth curve can help us understand the onset of the phase transformation and also the change of the mechanical properties. Since our study is devoted to understand the microstructural evolution and dent residue in post temperature-induced indentation. Molecular dynamics is used to simulate the effects of the post boundary deformation conditions with different indentation temperatures. Moreover, the microstructural evolution is identified by Martensite variants identification method (MVIM). The boundary conditions include isotropic deformation, anisotropic deformation and coupled deformation environments. Additionally, 100K, 325K and 400K are set to be the indentation temperature. Lastly, the study aims to understand the trends of the microstructural evolution in NiTi shape memory alloys in order to reach the possibilities of materials design.
Columnar-grained nano-twinned Cu possesses low electrical resistivity and high ductility, and it helps break though the assembly process constrains of traditional technologies while scaling down the IC. And also the material’s mechanical properties are improved with specified loading directions. Studies indicate that the property change is owing to the microstructure of dislocation and grains orientation which tends to be a hot topic in recent years. By the distribution of dislocations and the characterization of grains during loading, we analyze the sliding paths of dislocation, interaction between grain boundary and dislocation and grain merging process. The study simulates two coordinate systems, three modeling structure and two loadings by molecular dynamics, in oreder to identify the effects that brings to mechanical properties and the motions of dislocation. Additionally, we analyze the microstructural evolution by crystal structure post process software. The coordinate system includes (001) and (111) model; modeling structure contains twin-free, twin and nano-crystalline substrate structures; loading includes compressive and tensile loading. Last but not least, our study contributes to optimizing the materials and summarizing the trends of microstructural evolution.

並列關鍵字

NiTi shape memory alloys ； Temperature-induced martensitic phase transformation ； Martensitic variants ； Nano-indentation ； Nano-twinned Cu ； Nano-crystalline substrate ； Loading deformation

參考文獻

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