以分子動力學模擬形狀記憶合金並分辨其麻田散變異體之研究

Abstract

形狀記憶合金具有別於一般合金的重要力學特性，而這樣的特性是固態之間的相變化所造成的。從更微觀的角度來看，可以發現是微結構的演進過程主宰了這樣的相變行為。近年來，形狀記憶合金中微結構行為的分析已趨近完備，並且已經有許多分子動力學的模型在進行相關的電腦模擬。然而無論是在電腦模擬或者實驗中，由於原子的數量眾多，要區分麻田散體相裡頭的變異體仍然是一項困難的挑戰。本研究提出一得已區分出分子動力學模型中的沃斯田體以及各個麻田散變異體的方法。藉由計算模型中每個晶格的轉變矩陣，就可以找到其相對應的晶體變異體。此方法已經運用在鎳鈦合金中熱致相變的模擬，也已成功得出低溫相中各變異體的體積分率與模擬時間的分布圖；微結構的空間分布情形亦有所記錄。而透過將數個分子動力學模型中的雙晶交界面與兼容條件方程式產生的理論解的比較，可以驗證此方法據有足夠的可靠性。

Shape memory alloys (SMAs) behave interesting and important mechanical properties due to the solid-solid phase transformation. These phenomena are dominated by the evolution of microstructures. In recent years, the microstructures in SMAs have been studied extensively and modeled using molecular dynamics (MD) simulations. However, it remains challenging to identify the crystal variants in the simulation results which consist of large amount of atoms. In the present work, a method is developed to identify the austenite parent phase and the monoclinic martensite crystal variants from MD results. The transformation matrix of the lattices is calculated to determine the corresponding crystal variants. Evolution of the volume fraction of the crystal variants and the microstructure in the Ni-Ti SMAs under thermal cycling induced phase transition are examined. The method is validated by comparing several interface normal observed from MD results with the theoretical solutions via the compatibility equation.