富鎳Ti49Ni51形狀記憶合金時效後之相變態與性能最佳化之研究

Abstract

本研究針對Ti49Ni51 形狀記憶合金(SMA)之相變態、形狀記憶效應(SME)、超彈性(PE)與超彈性應力應變循環等性質之提升作一系列的探討。900℃×1hr固溶處理後之硬度值約為303HV。研究結果發現，一階段250℃時效者在25小時可達374HV之最大硬度。一階段時效於250℃、300℃及350℃等溫度，主要為B2↔R變態；而時效於400℃、450℃及500℃時，R相及B19’變態都非常明顯，由DSC結果可知上述之B2→R變態的RS溫度與反應熱都以300℃時效為最大。在300℃時效50小時者(硬度360HV)在SME及PE試驗中，其應力大於固溶處理者，殘留應變也較少，這是因為析出物強化基地，使塑性變形較不易產生所造成的。在二階段時效者中，其最大硬度值可達348Hv，雖然不及一階段時效者，但其SME之性能達100%，而在制振能的研究方面，發現二階段時效者相變態時之tanδ值相當高，可達0.073，僅次於固溶處理的試片，表示其比一階段時效者擁有良好的制振能。本研究之Ti49Ni51 SMA與Ti49.3Ni50.7者相比較，不論做一階段或兩階段時效者，發現Ni含量的增加對其SME及PE性能之改善是有顯著的效益，此因鎳含量提高，固溶強化及析出硬化的效果也增強。

In this study, the property improvement of shape memory effect (SME) and pseudoelasticity (PE) exhibited in Ni-rich Ti49Ni51 shape memory alloy (SMA) solid-soluted(SS) and aged at 250℃~500℃ for various time is investigated. The hardness of specimen at 900℃x 1 hr SS and water quenched is 303HV. Among all aged specimens, the maximum hardness is 374 HV for specimen aged at 250℃x25 hrs. From DSC tests, B2↔R transformation mainly appears in specimens aged at 250℃~350℃, but both B2↔R and R↔B19’/B2↔B19’ transformations occur in specimens aged at 400℃~500℃. The Rs temperature and ∆HR value of B2↔R transformation reach their maxima for specimens aged at 300℃. In the early aging, the curve of the specimen hardness at room temperature v.s. aging time has the 1st maximum, the minimum and the 2nd maximum , in which the 2nd maximum is also the highest hardness possessed in aged alloy. From DSC results and TEM observations, the formation of R phase and nano-precipitates is closely related to the hardness maximum and minimum exhibited in specimens aged at 250℃~400℃. The specimens with two step aging have 100% SME and higher damping capacity even though their the maximum hardness is 348HV. The tensile tests for specimens aged at 300℃x50 hrs (hardness 360HV) and at 250℃x5 hrs+400℃x1 hr (hardness 341HV) indicate their SME and PE properties are better than those of Ti49.3Ni50.7 due to the former has higher SS strengthening and precipitation hardening.