時效處理對Ti20Zr15Hf15Ni35-xCoxCu15 (x= 0, 5, 10) 高熵形狀記憶合金之顯微結構與相變態的影響

Abstract

本研究針對經過時效處理之Ti20Zr15Hf15Ni35-xCoxCu15 (x= 0, 5, 10) 高熵形狀記憶合金之相變態溫度、顯微結構、晶體結構進行研究，使用400-700°C的時效溫度進行比較和探討。三種合金經過400、500°C時效，相變態溫度因為析出奈米級H相，使麻田散體相變態受到抑制，造成相變態溫度下降，而添加Co可以使相變態溫度下降趨緩。三種合金經過600、700°C時效，因為共析反應使基地成分改變，(Ti+Zr+Hf)之比例上升，造成相變態溫度上升。並發現添加Co可以使相變態溫度上升趨緩。因此在相變態溫度之結果可以顯示，添加Co減少了高溫和低溫時效的析出反應，所以相變態溫度變化較小。在SEM中，觀察到三種合金經過400、500°C時效之試片皆只有基地和擬二元Ti2Ni析出物所組成，因為H相析出物太細小難以觀察，也有進一步在TEM中觀察到Co5之H 相。三種合金經過600°C時效之試片除了基地和擬二元Ti2Ni，還有觀察到Zr7Cu10和Ti2Cu之共析結構，而添加Co會使共析受到抑制，因此添加Co後共析結構較少。三種合金經過700°C時效之試片除了基地和擬二元Ti2Ni，還有觀察到Zr7Cu10和新形成之擬二元Ti2Ni析出物，而添加Co會使析出受到抑制，因此添加Co後析出物較少。另外，也在TEM中觀察到Co5經過600°C時效168小時析出元素Ti。在XRD分析中，觀察到三種合金經過400、500°C時效之試片皆只含B2沃斯田體和B19’麻田散體；在600、700°C時效之試片則是產生了多又豐富的析出物，因此XRD結果皆和DSC、SEM十分吻合。因此，可以發現使用Co取代Ni會減少TiZrHfNiCoCu高熵形狀記憶合金在高溫、低溫時效的析出現象，因此使時效後相變態溫度變化較小。綜合上述這些結果顯示可透過不同成分和不同時效溫度析出不同的微結構以設計高熵形狀記憶合金不同的相變態溫度和應用。

This study focuses on the phase transformation temperature and microstructure of Ti20Zr15Hf15Ni35-xCoxCu15 (x= 0, 5, 10) high-entropy shape memory alloys after aging treatment. Aging temperatures of 400-700°C were applied to compare the effects of aging treatments. After aging at 400 and 500°C, the martensitic transformation was suppressed due to the precipitation of nanoscale H-phase, resulting in a decrease in phase transformation temperature. The addition of Co decreased the decline of phase transformation temperature after aging at these low temperatures. On the other hand, after aging at 600 and 700°C, the composition of the matrix changed due to the eutectoid reaction, which caused the ratio of (Ti+Zr+Hf) in the matrix to increase and thus increased the phase transformation temperature. It was confirmed that adding Co reduced the magnitude of the rise in phase transition temperature after aging treatments at high temperatures. The results of the phase transformation temperature showed that the addition of Co reduces the precipitation reaction of the alloy, so the change in the phase transformation temperature becomes less evident with increasing Co addition. SEM observations showed that the three alloys aged at 400 and 500°C were composed only of the matrix and pseudo-binary Ti2Ni precipitate. The H-phase precipitates were too small to be observed by SEM and were confirmed to form in the Co5 alloy aged at 500°C by TEM. On the other hand, for the aging treatment of 600°C, except the matrix and pseudo-binary Ti2Ni precipitate, the eutectoid structures of Zr7Cu10 and Ti2Cu were observed. It was noted that the addition of Co inhibited the eutectoid reaction, so the amount of eutectoid structure becomes less with increasing Co content. Similarly, for the aging treatment of 700°C, except the matrix and originally formed pseudo-binary Ti2Ni, Zr7Cu10 and newly formed pseudo-binary Ti2Ni precipitates were observed. It was found that the addition of Co also inhibited the precipitation phenomenon, so fewer precipitates were observed in the alloy with high Co content. In addition, it was also observed in TEM that Co5 precipitated elemental Ti after aging at 600°C for 168 hours. The XRD analysis demonstrated that all three alloys aged at 400 and 500°C contained only B2 austenite and B19' martensite. Nevertheless, precipitates were identified in all three alloys after aging at 600 and 700°C, which were consistent with DSC and SEM results. Therefore, it can be confirmed that replacing Ni with Co can reduce the precipitation reaction in TiZrHfNiCoCu high-entropy shape memory alloys during aging treatment so that the change in phase transformation temperature after aging became smaller with increasing Co addition. These experimental results showed that the microstructure and transformation temperatures of TiZrHfNiCoCu high-entropy shape memory alloys could be adjusted and modified by careful compositional design and aging treatments, which can be applied to design TiZrHfNiCoCu high-entropy shape memory alloys for various applications and circumstances.