- 學位論文
分子動力學分析奈米多晶與單晶金之結構變化
Molecular dynamics simulations of structure behavior of Au poly-crystalline and single crystal
摘要
研究以分子動力學來模擬具有不同的缺口大小的 FCC 結構多晶金在奈米尺度下的拉伸性質，以及扭轉柱狀的多晶金，並與單晶態做比較。 此篇文分為三個部份來做討論，首先從單晶和多晶在一般拉伸的情況，以及不同缺陷大小下的拉伸結果，並與相關文獻做分析討論。再來則是棒狀的單晶和多晶金在施加扭轉之下，觀察剪應變、位移流場、晶體結構的變化，並佐以溫度效應來探討。這部份可以比較單晶和多晶的異同外，也觀察了單晶金的扭轉溫度效應以及多晶金的扭轉溫度效應。最後則是扭轉不同長度的柱狀多晶金，觀察在不同長度的柱狀多晶金結構變化、剪應變等的異同。 在具有缺陷的多晶及單晶金拉伸模擬結果，觀察到缺口越大的多晶金具有越低的機械強度，在晶界處以及缺口附近有較高的剪應變。隨著缺口越大，越往缺口處集中。單晶金則在較小缺口時剪應變呈現隨機分佈。多晶金的差排在產生時，較平均分佈於各晶界處；單晶則好發於缺口附近。 而在扭轉試驗的部分，單晶金在扭轉至70度時產生大量差排，剪應變和應力分佈的結果也顯示較集中在模型中的特定區域，並在該區域斷裂。多晶金的剪應變及應力分佈能較平均分佈於晶界處，使其在相同扭轉角度下能保有較好的機械強度。
並列摘要
It's well known that different notch size in material may cause different result in tensile test. We use molecular dynamics method to simulate the tensile test of FCC-structured nanocrystalline / single crystal gold with different notch size under room temperature. Then we analysis the shear strain and stacking fault. We discovered that the bigger notch size has lower mechanical strength. There's higher shear strain around the notch and grain boundaries. Single-crystal's shear strain distributed randomly with smaller notch. Crystalline’s stacking fault spread in grain boundaries; single-crystal spread nearby the notch. And then, torsion test simulation of cylinder nanocrystalline / single crystal gold under different temperature, we build cylinder-shaped specimens, balance them properly and then make them twist. We discovered that in different temperature causes different result. For example, higher temperature model tends to be softer than low temperature ones. It’s might be the energy of its causes this consequence. Higher temperature means higher energy state. It will move shakier than lower’s. Not only it had done this effect, but also the structure behavior. We discovered with higher temp it tends to have more others structure come up. It’s actually not just come up from nowhere, it transformed from FCC structure. With the rotation angle increase, the simulation object starts to deform and to be twisted. Poly-crystalline have a little amount of HCC structure in the beginning because of heat balancing. And lots of atoms in other structure to construct grain boundary. Grain boundary. Grain boundary is the interface between two grains. It’s a defect in crystal system. With rotation goes on, grain will be stressed and FCC structure deformed to HCP or Others. In single crystalline, HCP structure will keep growing until meet the other boundary. Boundary of single crystalline is outside of object. And in the meantime poly-crystalline’s boundary is just another grain and it will not break through. In this research, we simulated multiple condition to find what if we changed the temperature or length or notch size. It will be helpful for every industry and world will be better tomorrow.