Ti50Ni25Cu25形狀記憶合金箔帶麻田散體變態及超彈性能之研究

Abstract

本研究利用快速凝固製程(RSP)製備Ti50Ni25Cu25合金箔帶(ribbon)，並針對其退火後之麻田散體變態、顯微結構、超彈性等進行探討。本研究所使用之4000rpm Ti50Ni25Cu25合金箔帶幾乎為完全非晶質的，其wavenumber (Qp)、結晶起始溫度(Tx)以及結晶活化能(Ea)皆較Ti-Ni二元合金箔帶、薄膜及Ti50Ni40Cu10合金箔帶低，可知非晶質合金箔帶的熱穩定性隨箔帶中Cu含量增加而降低。非晶質之Ti50Ni25Cu25箔帶分別經過高於結晶溫度的500℃以及低於結晶溫度的435℃退火後，皆產生B2↔B19一階相變態，惟在435℃退火之試片需要更長之退火時間方能結晶。透過XRD檢視退火結晶化後箔帶之相組成，可發現箔帶接觸面以及自由面具有不同的織構，織構形成的原因為RSP過程中，熔融金屬接觸銅輪焠冷時之熱流方向影響所致；並可透過SEM於箔帶與銅輪接觸面發現一層具優選方位之晶粒。EBSD結果顯示，退火時間對晶粒大小之影響相當小，不論是在結晶溫度以上或以下退火之箔帶皆具有細小、等軸且均勻的組織。由奈米壓痕硬度試驗以及超彈性之研究可歸納出兩點影響超彈性行為之主要因素：一為變態潛熱，即ΔH值；另一為析出物之量以及析出形貌。變態潛熱隨退火時間增加，即結晶化程度增加而上升，使得硬度下降、可回復之變形量增加；但亦會因析出物析出，即可變態之材料體積減少而下降，使得硬度上升、可回復之變形量減少。而材料之析出行為則隨退火溫度之不同而有相當大之差異，在高於結晶溫度退火之箔帶，析出物成長較快，整合性很快喪失，因此使箔帶脆化，可回復之變形量減少；低於結晶溫度退火之箔帶，析出物則可保有其整合性較長時間，其特殊之胞格狀結構，雖能增加箔帶硬度但也使箔帶嚴重脆化。本研究之箔帶試片，其最佳之超彈性可回復應變量可大於80%，顯示經過適當之退火處理之Ti50Ni25Cu25形狀記憶合金箔帶具有相當良好之超彈性。

Ti50Ni25Cu25 ribbons prepared by rapid solidification process (RSP) at 4000 rpm are studied to investigate the transformation behavior, microstructure and mechanical properties. As-spun Ti50Ni25Cu25 ribbons are nearly fully amorphous with a lower wavenumber Qp than the amorphous Ti-Ni alloys and the partially amorphous Ti50Ni40Cu10 ribbons owing to its high Cu content. Both crystallization activation energy Ea and onset temperature Tx for Ti50Ni25Cu25 ribbons are lower than those for Ti50Ni50 and Ti50Ni40Cu10 ribbons/thin films, which also indicate a worse thermal stability exhibited in amorphous Ti50Ni50-xCux alloys with high Cu content. Texture of {100}B2 orientation is discovered on the contact side of the ribbons, due to that the heat flow associated with ribbon processing is normal to the spin wheel during RSP. Nanoindentation tests show that proper annealing process is required to obtain well crystallized ribbons with promising pseudoelasticity (PE). The PE property of these annealed ribbons is found to be related to two factors: the transformation latent heat and the precipitation behavior. The PE recovery ratio of nanoindentation tests can reach > 80% in these ribbons, indicating quite good PE property exhibited in annealed Ti50Ni25Cu25 ribbons.