時效處理對富鎳鈦鎳形狀記憶合金多階相變態行為之研究

Abstract

本研究對Ti49.2Ni50.8、Ti48.7Ni51.3SMAs熔煉均質化後時效之，因未熱/冷軋延，故得平均50μm以上之大晶粒。時效後二合金都產生Ti3Ni4析出物，其分布會直接影響到合金之多階相變態。Ti49.2Ni50.8合金在350℃及400℃時效者主要為三階相變態，而450℃時效者因Ti3Ni4成核快，且因晶粒大使晶粒中心的Ni原子來不及擴散至晶界，故於晶粒內出現了三個區域Ⅰ(近晶界處)、Ⅱ(Ⅰ、Ⅲ中間)及Ⅲ(近晶粒中心)，因此形成四階相變態，由高溫至低溫分別為 B2Ⅰ,Ⅱ,Ⅲ→RⅠ,Ⅱ,Ⅲ, RⅠ,Ⅲ→B19’Ⅰ,Ⅲ, B2→B19’ (no ppt)及RⅡ→B19’Ⅱ。時效溫度的高低決定原子之擴散速率及Ti3Ni4?的成長速率，因而與Ti3Ni4之析出狀況極為相關，由實驗結果發現Ti3Ni4在450℃析出最快，350℃及650℃者均不易析出。由SEM可觀察到長時間時效後，析出物會均勻分布在晶粒內使多階相變態消失。Ti48.7Ni51.3合金因Ni的過飽和度高，Ti3Ni4成核之能量驅動力大，使晶粒內部及晶界處的析出速度相當，故Ti3Ni4分布均勻使之呈B2→R→B19’二階相變態。本研究也同時研究熱軋延後經固溶處理再經500 ℃時效的Ti49Ni41Cu10合金之多階相變態行為。

Ti49.2Ni50.8 and Ti48.7Ni51.3 shape memory alloys (SMAs) melt, homogenized and aged without any hot/cold rolling have grain size >50μm. The distribution of Ti3Ni4 precipitates directly affects the transformation behavior of these alloys. After 350℃ and 400℃ aging, Ti49.2Ni50.8 shows a normal three-step transformation. At 450℃×6hr, the nucleation rate of Ti3Ni4 is so high that the supersaturated Ni atoms in the grain center can not diffuse such long distance to grain boundaries and thus, there are three distinct regions resulted in the grain, i.e., regions I, II and III. Four-step transformation of B2Ⅰ,Ⅱ,Ⅲ→RⅠ,Ⅱ,Ⅲ, RⅠ,Ⅲ→B19’Ⅰ,Ⅲ, B2→B19’(no ppt) and RⅡ→B19’Ⅱ is induced from these regions. Experiment results show that the aging temperature decides the diffusion rate of Ni atoms, the growth rate and the distribution of Ti3Ni4 precipitates in the grain. The Ti3Ni4 precipitation rate has its maximum at around 450℃ aging, but can hardly be observed at 350℃ and 650℃ aging. According to SEM observations, after longer aging time, the precipitates distribute more homogeneously in the grain and the multi-step transformation no longer exists. For Ti48.7Ni51.3 alloys, its higher Ni supersaturation causes the Ti3Ni4 nucleation having higher driving force and causes the precipitation rates being almost the same at grain boundaries and grain interior. This feature results in a simple B2→R→B19’ two-step transformation in aged Ti48.7Ni51.3 SMA. The multi-step transformation behavior of Ti49Ni41Cu10 SMA is also investigated in this study.