富鈦高溫形狀記憶合金Ti52-xNi48Hfx (x=5、10、15)箔帶之時效熱處理對相變態溫度與機械性能研究

Abstract

本研究使用快速冷凝製程(熔旋噴鑄)製備一系列之富Ti之TiNiHf形狀記憶合金，成分為Ti_(52-x) Ni_48 Hf_x （x=5、10、15）形狀記憶合金，探討成分組成與時效處理對材料之相變態溫度、形狀記憶效應、超彈性及微結構之影響。本研究結果顯示，加入越多第三元素(Hf)會使試片在快速凝固中不易結晶，在5000rpm條件下Hf10、Hf15 5000rpm as-spun大部分區域呈現非晶態，為結晶狀態的Hf5 5000 rpm與2000 rpm樣品，也因快速冷凝所導入之高差排密度與缺陷，而未觀察到明顯相變態行為，在500 °C熱處理後隨時間增加，相變態溫度皆往高溫移動，且在24小時候趨於飽和。箔帶樣品整體表現出優於塊材的熱循環穩定性，相變溫度最大變化在7°C以內，其中以15分鐘短時間熱處理之箔帶穩定性最佳。Hf5與Hf10試片在15分鐘熱處理後可回復應變分別達5.89%與6.03%，而Hf15則需1小時熱處理後方可達4.82%，在相同應力條件下，Hf含量越高，其可回復應變越小，主要因高Hf含量造成內應力上升與compound twin密度增加。超彈性行為方面則觀察到，隨Hf含量增加，其臨界應力由520 MPa上升至610 MPa，此現象與Hf元素造成晶格扭曲（lattice distortion）有關，進一步提高了應力誘發麻田散體所需之應力。微結構觀察結果指出，Hf5 2000 rpm樣品經500 °C熱處理24小時後產生(Ti,Hf)2Ni析出物，尺寸約為50–100 nm，並與基地具整合性（coherent），對材料仍具有良好的基地強化效果。然而，過長時效處理導致析出相成長，使得可回復應變較短時間(15min)處理樣品略低，顯示短時間熱處理能兼顧析出強化與形狀記憶性能，為未來應用開發之重要依據。

This study utilized rapid solidification processing (melt spinning) to fabricate a series of Ti-rich TiNiHf shape memory alloys with compositions of Ti52−xNi48Hfx (x = 5, 10, 15), aiming to investigate the effects of composition and aging treatment on phase transformation temperature, shape memory effect, superelasticity, and microstructure.The results show that increasing the amount of the third element (Hf) hinders crystallization during rapid solidification. Under the 5000 rpm condition, the as-spun Hf10 and Hf15 ribbons exhibited predominantly amorphous structures. Even the crystalline Hf5 5000 rpm and all 2000 rpm ribbons did not exhibit phase transformation behavior due to the high density of dislocations and defects introduced by rapid solidification. After aging at 500 °C, the phase transformation temperatures increased with aging time and tended to saturate after 24 hours.The ribbon samples showed better thermal cycling stability compared to bulk alloys, with a maximum transformation temperature shift within 7 °C. Among all conditions, ribbons subjected to short-time aging (15 minutes) exhibited the best stability. The recoverable strain reached 5.89% and 6.03% for Hf5 and Hf10 samples aged for 15 minutes, respectively, while Hf15 required 1 hour of aging to reach 4.82%. At the same stress level, higher Hf content led to lower recoverable strain, primarily due to increased internal stress and higher compound twin density.Regarding superelastic behavior, the critical stress increased from 520 MPa to 610 MPa with increasing Hf content. This phenomenon is attributed to the lattice distortion induced by Hf addition, which raises the stress required to trigger the martensitic transformation.Microstructural analysis revealed that in the Hf5 2000 rpm sample aged at 500 °C for 24 hours, (Ti,Hf)2Ni precipitates with sizes ranging from 50 to 100 nm were formed and exhibited coherent interfaces with the matrix, contributing to matrix strengthening. However, prolonged aging led to precipitate coarsening, resulting in lower recoverable strain compared to the 15-minute aged samples. This indicates that short-time aging effectively balances precipitation strengthening and shape memory performance, serving as an important reference for future material development.