中高熵Ti30(ZrHf)20Ni35Cu15形狀記憶合金中Zr與Hf元素換置對於相變態、顯微結構與形狀記憶效應之影響

Abstract

本研究針對Ti30Hf20-xZrxNi35Cu15 (x=0, 5, 10, 15, 20) 擬二元中、高熵形狀記憶合金進行Hf、Zr元素相互替換的影響，分別命名為Hf20、Hf15、Hf10Zr10、Zr15、Zr20。特別針對Hf20及Zr20兩種尚未被研究之合金的相變態溫度、顯微結構、晶體結構、形狀記憶效應進行研究。Hf20合金在經過400℃、500℃時效處理後，相變溫度下降，且循環穩定性增加；Zr合金在經過400℃、500℃時效處理後相變溫度消失，推測降至低於儀器量測範圍；Hf20與Zr20合金皆在600℃、700℃時效處理後相變溫度上升。在掃描電子顯微鏡及場發射電子微探儀成分分析實驗中，400℃與500℃時效處理後的五種合金中，唯有Zr20被觀察到明顯的H相析出物，在穿透式電子顯微鏡中也觀察到相關繞射圖；在600℃時效處理後，在Hf20及Hf15觀察到層狀結構，Zr元素含量越高之合金則有更密集的Zr7Cu10白色析出物，證明Hf元素因沒有Zr元素的ZrCu中間相，更容易產生Hf7Cu10與Ti2Cu組成的層狀析出物；700℃時效處理後，五種合金皆展現出對比略有差異之新生Ti2Ni以及白色(Hf,Zr)7Cu10析出物，與600℃時效後的顯微結構趨勢一致，Zr元素含量與析出物密度呈正相關。X光繞射儀實驗中，驗證顯微結構與成分分析觀察結果外，也觀察到此兩種合金皆在完全相變（或逆相變）後仍有殘留的B2 相（B19’相），顯示有部分材料體積沒有參與相變過程。Hf20及Zr20合金固溶後皆擁有優秀的形狀記憶效應，Hf20合金在不同溫度的時效後皆有短暫的析出強化效果；而Zr20則是在時效處理後可恢復應變減少。總結所有實驗，相比於Hf元素，Zr元素使合金在熱處理後更容易產生析出物，因此相變溫度的變化更為劇烈。

This study investigates the effect of mutual substitution of Hf and Zr elements on the shape memory effect of Ti30Hf20-xZrxNi35Cu15 (x=0, 5, 10, 15, 20) pseudo-binary medium and high entropy shape memory alloys. The alloys were named Hf20, Hf15, Hf10Zr10, Zr15, and Zr20, respectively. The research particularly focuses on the transformation temperatures, microstructure, crystal structure, and shape memory effect of Hf20 and Zr20 alloys, which have not been studied before. The results showed that the transformation temperatures of Hf20 alloy decreased after aging at 400°C and 500°C, but its cyclic stability increased. The transformation temperatures of Zr20 alloy disappeared after aging at 400°C and 500°C, and it is speculated that they dropped below the measurement range of the instrument. The transformation temperatures of both Hf20 and Zr20 alloys increased after aging at 600°C and 700°C. In Scanning Electron Microscope microstructure and Electron Probe Microanalyzer composition analysis experiments, Zr20 among the five alloys aged at 400°C and 500°C was observed to have obvious H-phase precipitates, and the corresponding diffraction patterns were also observed in Transmission electron microscope. After aging at 600°C, layered structures were observed in Hf20 and Hf15, and alloys with higher Zr content had denser Zr7Cu10 white precipitates. This proves that Hf elements, which do not have the ZrCu intermediate phase of Zr elements, are more likely to produce layered precipitates composed of Hf7Cu10 and Ti2Cu. After aging at 700°C, all five alloys exhibited slightly different new Ti2Ni and white (Hf,Zr)7Cu10 precipitates, consistent with the trend of the microstructure after aging at 600°C. The atomic percentage of Zr elements fraction and precipitates is positively correlated. In X-ray diffractometer experiments, in addition to verifying the results of microstructure and composition analysis, it was also observed that both alloys still had residual B2 phases (B19' phases) after complete martensite transformation (or reverse transformation), indicating that part of the material volume did not participate in the transformation process. Both Hf20 and Zr20 alloys have excellent shape memory effect after solid solution. Hf20 alloy has a short-term precipitation strengthening effect after aging at different temperatures; while Zr20 alloy can restore strain reduction after aging. In summary, compared to Hf elements, Zr elements make alloys more prone to precipitates after heat treatment, and therefore the changes in transformation temperature are more drastic.