時效對富鎳Ti48Ni52及Ti48.5Ni41.5Cu10形狀記憶合金相變態影響之研究

Abstract

本文探討Ti48Ni52及Ti48.5Ni41.5Cu10 SMAs在固溶處理後時效於250~600°C之性質。在Ti48Ni52 SMA之研究中，我們提出了一個時效下經歷三個階段(phase I~III)之顯微結構及變態順序之示意圖，來解釋所觀察到的相變態行為。在phase I時，Ti3Ni4均勻的析出，因此發生一階B2↔R、二階B2↔R↔B19’1或一階B2↔B19’1相變態，其中R相是否產生取決於Ti3Ni4界面的整合性應力場；在phase II時，Ti3Ni4與Ti2Ni3共存， Ti2Ni3板片周圍會形成一個Ti3Ni4空乏區，此Ti3Ni4空乏區會發生B2↔B19’2相變態；而持續時效到達phase III時，只有Ti2Ni3板片及B2↔B19’2被觀察到。由本研究可知，Ti3Ni4界面整合性應力場，除了與時效溫度有關外，亦與時效時間有關聯。Ti48Ni52在250~350°C時效者的硬度vs.時間曲線皆有一停頓點及一極大點；在400~500°C時效者只有一極大點；而時效於550°C及600°C者，硬度在時效後均一路下滑。在Ti48.5Ni41.5Cu10 SMA中，as-quenched者的DSC曲線中有B2↔B191及B19↔B19’的相變態，其變態溫度都較Ti50Ni40Cu10者為低。在250~400°C時效不同時間的DSC結果幾乎與as-quenched者相同；但時效於500°C及600°C時，基地會析出片狀Ti(Ni,Cu)2，其周圍的應力場可為B19麻田散體的成核點，也誘發新的B2↔B192變態；隨著時效時間及溫度的增加，片狀Ti(Ni,Cu)2佔據了整個基地，使原先基地的B2↔B191變態無法產生。Ti48.5Ni41.5Cu10 SMA在時效後硬度僅增加30HV，硬化效果並不顯著。在硬度最高者的DMA曲線中，B2→B19變態的tanδ值可達到0.15，其storage modulus有相當明顯的變態軟化現象。

In this study, transformation characteristics of aged Ni-rich Ti48Ni52 and Ti48.5Ni51.5Cu10 SMAs are investigated. For Ti48Ni52 aged at 250~600°C, schematic diagrams for phases I~III are proposed to interpret the observed transformation behavior. In phase I, Ti3Ni4 ppts distribute homogeneously and show B2↔R, B2↔R↔B19’1 or B2↔B19’1 transformations with/without R phase depending on the existence of coherent interface around Ti3Ni4. In phase II, Ti3Ni4 and Ti2Ni3 coexist, and Ti3Ni4 depletion zones occur around Ti2Ni3 plates to induce B2↔B19’2. In phase III, only Ti2Ni3 ppts and B2↔B19’2 are observed. According to this study, the occurrence of coherent stress field around Ti3Ni4 depends both on aging temperature and time. The hardness vs. aging time curves of Ti48Ni52 aged at 250~350°C show a step before reaching maximum; while those aged at 400~500°C have one maximum; and those aged at 550~600°C never show a hardness increase. For Ti48.5Ni41.5Cu10, the as-quenched specimen exhibits B2↔B191 and B19↔B19’, and both transformation temperatures are lower than those of Ti50Ni40Cu10. The DSC curves of specimens aged at 250~400°C for different times are almost the same as the as-quenched ones; while those aged at 500~600°C precipitate plate-like Ti(Ni,Cu)2 to induce a B2↔B192 transformation simultaneously. As the aging time increases, plate-like Ti(Ni,Cu)2 ppts fill up the matrix, and no B2↔B191 occurs. The aging hardening effect is not significant. Tanδ value of the maximum hardness specimen reaches 0.15 with an obvious softening storage modulus during B2→B19 transformation.