Ni50Ti45Hf5與Ni49Ti51-xHfx (x=5,10,15)系列形狀記憶合金箔帶的相變態行為與機械性能之研究

Abstract

本研究主要是針對Ni50Ti45Hf5與Ni49Ti51-xHfx (x=5,10,15)系列形狀記憶合金箔帶的相變態行為、微結構與形狀記憶效應等等特性進行探討。箔帶按製程時的銅輪轉速，可以分為冷卻速率較慢的2000 RPM箔帶，以及冷卻速率較快的4000 RPM箔帶兩種。藉由光學顯微鏡與SEM檢視後，可以確認Hf徹底融入箔帶當中。其中Hf的含量與冷卻速率的快慢，會影響箔帶在As-spun狀態下的微結構與相變態行為。在2000 RPM箔帶系列中，Ni50Ti45Hf5和Ni49Ti46Hf5合金箔帶，缺陷含量最少，可以利用DSC量測到相變態發生。而在經過XRD與頻率依賴性實驗後，發現Ni49Ti41Hf10合金2000 RPM箔帶內的缺陷濃度超過臨界值，結構是由應變玻璃所組成的。Ni49Ti36Hf15合金2000 RPM箔帶則在經過XRD實驗後，發現內部結構是處於半晶質狀態。而4000 RPM箔帶系列在經過TEM實驗後，發現有大量的奈米級細晶組織存在其中，這些結構會抑制麻田散體相變態的發生。對於這些在As-spun狀態下無法量測到麻田散體相變態行為的材料，皆須要經過至少大於500°C的時效熱處理後，才能發揮出其形狀記憶合金相關的特性。相對高溫的600°C時效條件，擁有較強的消除缺陷能力，在短時間時效下可令箔帶擁有最高的相變態溫度。其同時有相對最好的析出物生成能力，在經過較長時間的時效會令相變態溫度顯著下降，並可在SEM觀察結果中清楚觀察到明顯的析出物生成。與此相對的，在經過短時間較低溫500°C時效熱處理後的箔帶，雖然會有相對較低的相變態溫度，但可以更有效利用固溶強化的效果，可在熱循環穩定性與形狀記憶效應上可以得到比600°C時效條件下更優良的結果。研究結果顯示，藉由改變箔帶的成分、製程的冷卻速率、時效熱處理的溫度大小和時間長短，能夠有效調整箔帶的微結構組成，控制其相變態溫度和行為。

This study investigates the phase transformation behavior, microstructure, and shape memory effect of Ni50Ti45Hf5 and Ni49Ti51−xHfx (x = 5, 10, 15) shape memory alloy (SMA) ribbons. The ribbons were categorized based on the rotational speed of copper wheel rotation during fabrication: 2000 RPM ribbons (slower cooling rate) and 4000 RPM ribbons (faster cooling rate). The complete dissolution of Hf into the ribbon matrix can be verify through optical microscopy and scanning electron microscopy (SEM) observations. The Hf content in composition and cooling rate during fabrication both significantly influence the microstructure and phase transformation behavior of as-spun ribbons. Among the 2000 RPM ribbons, the Ni50Ti45Hf5 and Ni49Ti46Hf5 alloy ribbons exhibited the lowest defect concentrations, allowing phase transformations detection via differential scanning calorimetry (DSC). X-ray diffraction (XRD) analysis and frequency-dependent experiments revealed that Ni49Ti41Hf10 alloy ribbons (2000 RPM) contained defect concentration exceeding a critical concentration, resulting in a strain glass structure. Through XRD analysis, it is determined that Ni49Ti36Hf15 alloy ribbons (2000 RPM) exhibited a semi-crystalline structure. Transmission electron microscopy (TEM) analysis of the 4000 RPM ribbons revealed that these ribbons consist predominantly of nanocrystalline structures, which inhibited martensitic phase transformations. For materials lacking detectable martensitic transformation behavior in its as-spun state, it is understood that aging heat treatment at temperatures above 500°C is necessary to manifest their shape memory alloys properties. Among the aging heat treatments, the 600°C aging condition is particularly effective at eliminating defects, achieving the highest phase transformation temperatures after short-term aging. Furthermore, this condition exhibits the most effective precipitate formation ability, which became evident through clearly observable precipitate formation in SEM after prolonged aging, which is accompanied by a marked decrease in the transformation temperature. In contrast, ribbons subjected to short-term aging at 500°C, despite exhibiting relatively lower phase transformation temperatures, are able to more effectively utilize the benefits of solid solution strengthening. These conditions provided superior thermal cycling stability and shape memory effects when compared to its higher temperature counterpart. Overall, this research demonstrates that by adjusting the ribbon composition, cooling rates during fabrication, aging temperature and duration of heat treatments, the microstructure of the ribbons can be effectively tailored, thereby enabling control over their phase transformation temperatures and behaviors.