本研究旨在探討在奈米尺度下，不同晶粒尺寸製備出的銅銀合金模型受拉伸負載後的機械性質，以及當銅銀合金模型為梯度奈米晶粒結構時，其破壞行為與應變傳播模式的差異。本研究使用分子動力學方法搭配Finnis-Sinclair勢能函數進行數值模擬，均勻與梯度晶粒結構的模型晶粒尺寸為16.97nm、12nm及2.68nm。同時，使用徑向分布函數法(RDF)與共同近鄰原子分析法(CNA)進行模型結構分析，接著對模型進行單軸拉伸試驗，探討材料的機械性質，並觀察材料的變形與破壞機制以及原子結構的相變情形。本研究亦將預裂紋分別置入均勻與梯度晶粒結構的模型中，探討裂紋傳播的差異性。此外，對具有相同條件的純銅金屬與純銀金屬亦進行相同參數設定的拉伸試驗，探討銅銀合金、銅金屬與銀金屬間材料性質的差異。模擬結果顯示，均勻晶粒尺寸的銅銀合金模型中，晶粒尺寸12nm為銅銀合金的H-P關係(Hall-Petch Relationship)轉折點，具有最大的強度，其抗拉極限應力高達1.84GPa。而隨著晶粒尺寸的減小，極限抗拉強度減小，延展性提升，塑性變形越均勻。此外，具有梯度奈米晶粒結構的銅銀合金，受拉伸負載後，原子結構會產生應力引發的相變，從原本的晶態結構轉變成高比例的非晶態結構，導致原始晶粒間存在的晶界不復存在，使得此梯度結構造成的影響較不顯著。另外，觀察裂紋所造成的影響，結果顯示裂紋尖端受拉伸後因塑性變形而發生鈍化，與脆性材料相比擁有較好的抵抗裂紋傳播的能力。綜合以上實驗結果做一總結，不論銅銀合金晶粒結構為何，破壞模式皆為塑性斷裂，延展性極佳，伸長量皆可高達80%以上，且擁有阻擋裂紋傳播的能力，另外，12nm晶粒尺寸為銅銀合金具有最佳極限抗拉強度的晶粒結構尺寸，亦能提高梯度結構的強度。

In this work, the mechanical properties and fracture behaviors of the structures with different grain sizes and Gradient-Nano-Grain (GNG) of Cu50Ag50 alloy were individually investigated under the uniaxial tensile stress. Molecular dynamics (MD) simulation with Finnis-Sinclair (FS) potential was adopted in numerical simulation. Grain sizes of 16.97 nm, 12 nm and 2.68 nm were selected in the samples with the mean grain sizes and GNG structure. Common neighbor analysis (CNA) and radial distribution function (RDF) method were employed for structure identification. Pre-cracked Cu50Ag50 alloy models under uniaxial tensile were also considered to investigate the crack growth and propagation. Simulation results indicate that the maximum strength and the inverse Hall-Petch (H-P) relationship can be observed at the grain size of 12 nm, and the ultimate tensile strength (UTS) is as high as 1.84 GPa. The UTS decreases with the decrease of the mean grain sizes, while the ductility increases and the plastic deformation become more uniform. Moreover, for GNG structure, stress induced phase transformation can be found during uniaxial tensile test. The initial crystalline structure transforms into a high proportion of amorphous structure, and the grain boundaries between the initial grains disappear. The effect of GNG structure on mechanical properties is not significant. Moreover, it was found that the crack tip exhibits blunting due to plastic deformation after the action of uniaxial tensile stress. Therefore, the Cu50Ag50 alloy shows good resistance of crack propagation than other brittle materials.

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