Molecular dynamics simulations of structure behavior of Au poly-crystalline and single crystal

• Main Authors: Hsing-Wei Tsai, 蔡信緯
• Other Authors: Cheng-Da Wu
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/94bcwp

碩士 === 中原大學 === 機械工程研究所 === 106 === 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.