以分子動力學模擬鎳鈦形狀記憶合金之微結構演變

Abstract

形狀記憶合金具有固態相變所造成的重要力學特性，例如超彈性與形狀記憶效應。而相變化造成的微結構演進主宰了這些行為。近年來進行許多分子動力學的電腦模擬形狀記憶合金中微結構行為的分析。但目前尚無完善且有效率的方法來分析模擬結果，使得吾人對形狀記憶元件之巨觀表現與內部微結構之間的關係認知仍不甚清楚。本研究提出能分辨出分子動力學模型中的沃斯田體以及各個麻田散體兄弟晶的方法。藉由計算原子模型中每個晶格的轉變矩陣，就可以找到其相對應之晶系的兄弟晶。此方法成功運用在鎳鈦合金中應力誘發相變的模擬，其可以清楚觀測出應力-應變曲線所對應之微結構，也已成功得出在高溫相中因應變造成相變之各兄弟晶的體積分率與模擬時間的分布圖。藉此更加了解鎳鈦合金透過相變時麻田散體結構交換下，超彈性行為應力釋放的機制。此方法藉著不同剪應變下產生數個分子動力學模型中的雙晶界面，與相容性方程式所產生的理論解比對後，證明出本研究發展的分析方法具有足夠的可靠性。

The mechanical properties, deformation and shape recovery mechanism of NiTi shape memory alloys are dominated by the microstructures. Many features of the microstructure are studied by molecular dynamics(MD) simulation, such as minimum energy state, the evolution of interfaces and martensitic transformations. However the relationship between the macroscopic behavior and microstructural evolution has not yet been fully understood due to the high nonlinearity. In this study, MD method is used to simulate the stress-induced phase transformation in NiTi shape memory alloys. Several different direction loading cases are applied by pure shear. A post-processing method which can identify variants and crystal systems based on the lattice transformation is developed to examine the resulting microstructure. The evolution of the volume fraction of the crystal variants and twin structures due to the various loading cases are predicted. The twinning modes and interface orientation are verified by the compatibility theory. The resulting microstructures and its corresponding behavior under loads have good agreement with those reported in the literature. The mechanism of martensitic transformation is revealed and discussed in detail.