2NN 及 FS 勢能模擬鎳鈦形狀記憶合金之微結構演變

2NN and FS Potential Simulation of Microstructural Evolution in NiTi Shape Memory Alloys

李紀廷(Ji-Ting Li)

指導教授 ： 鄒年棣

國立交通大學/工學院/材料科學與工程系所/碩士(2018年)

摘要

超彈性及形狀記憶效應是形狀記憶合金中固態相變化所造成之兩種極為重要的特性。而相變化過程中的微結構演進主宰了這些行為。近年來已有許多研究利用了分子動力學模擬形狀記憶合金，對相變化過程中的微結構行為進行分析。目前大多數的分析方法都是以CNA方法或晶格的角度來判定微結構。但這些分析方法並不完善，無法將微結構中各種麻田散體的兄弟晶一一辨認出，甚至存在誤判的可能性。因此本研究提出了一套通過計算原子模型中每簡單四方結構之轉變矩陣的判定方法。藉由此方法，即可以精確的辨認出模擬結果中的沃斯田體，以及各種麻田散體的兄弟晶。將模擬的結果進行分析後，可以清楚的觀察到合金發生相變化時的微結構演進，成功的繪製出體積分率-模擬時間的分布圖，並發現了鎳鈦合金在受到不同方向的應力時，誘發出不同相的兄弟晶的機制。此外，本研究藉由此方法比較了FS勢能與2NN勢能在鎳鈦合金的溫度、應力誘發相變以及奈米壓印的模擬中的表現。透過了微結構的觀察後，發現了2NN勢能在各方面的表現均優於FS勢能。最後經由相容性方程式的驗證以及DXA差排分析方法對比後，在上述之每個模擬中皆在雙晶介面處觀察到了非常密集的差排。此現象已在實驗中所觀察到，因此證明出本研究發展的分析方法具有足夠的可靠性。

關鍵字

形狀記憶合金 ； 分子動力學 ； 微結構 ； 溫度誘發麻田散體相變化 ； 應力誘發麻田散體相變化 ； 奈米壓印

並列摘要

Superelasticity and shape memory effect are the most important properties caused by solid state phase transformation of shape memory alloys(SMAs). These behaviors are dominated by microstructure evolutions. In the recent years, many studies adopted molecular dynamics (MD) to simulate SMAs and the evolution of microstructures. Most of these studies use the CNA method or compare angles of the unit cell to identify the martensite phases. However, these methods may lead ambiguity and can not identify each variant of martensite. This study develops a method, which calculate the transformation matrices of each tetragonal unit cell in the model and, compare the matrices with the theoretical ones. In this way, austenite and variants of martensite can be identified accurately. Thus, microstructure evolution can be illustrated; the volume fraction of variants vs. time can be analyzed; the mechanism of stress induced martensite variants in SMAs is revealed.
Also, this study compares the performance of 2NN potential to FS potential by temperature-induced, stress-induced matetnsitic transformation and indentation. The results show that 2NN potential is better than FS potential. Finally, the resulting microstructures mentioned above are examined by compatibility equations and DXA. There are high density dislocations between the twin boundaries which has good agreement in the literature.

並列關鍵字

Shape memory alloys ； Molecular dynamics ； Variant identification ； Temperature-induced martensitic transformation ； Stress-induced martensitic transformation ； nano-indentaiton

參考文獻

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