Co含量對時效處理後的Ti20Zr15Hf15(NiCoCu)50高熵形狀記憶合金之顯微結構和功能性能的影響

Abstract

本研究以 Co 取代 Ni 或 Cu 的四種成分之 Ti20Zr15Hf15(NiCuCo)50 高熵形狀記憶合金 ， 分別為 Ni30Cu15Co5、 Ni25Cu15Co10 、 Ni35Cu5Co10 和 Ni35Cu10Co5 ，進行 400-700°C 的時效處理後研究合金的麻田散體相變態行為 、顯微結構、 熱循環穩定性和形狀記憶效應。所有合金在 400°C 和 500°C 下進行時效處理後會析出分布密集之H 相，此 H 相周圍會產生應力場抑制麻田散體相變態且對基地造成析出強化的效果，導致合金的相變溫度變低而熱循環穩定性則有所提升。在 SEM 中，所有合金在 400°C 和 500°C 下進行時效處理後之試片皆只有觀察到基地和 Ti2Ni 二次相，在 TEM 中進一步觀察到 H 相，並發現添加 Co 可以抑制此 H 相的析出反應以減緩相變溫度變低的幅度。另一方面，Ni30Cu15Co5 、Ni25Cu15Co10 和 Ni35Cu5Co10 合金在在 600°C 和 700°C 下進行時效處理後，因為析出富(Ni, Cu)的 Zr7Cu10 析出物使基地中的(Ti+Hf+Zr)之含量增加，導致相變溫度變高，而此 Zr7Cu10 析出物因尺寸太大無法有效地對基地造成析出強化使熱循環穩定性沒有變佳的趨勢，且此Zr7Cu10 析出物會使合金之形狀記憶效能較差，最大可恢復應變下降。在 SEM 觀察中，發現添加 Co 也可以抑制 Zr7Cu10 的析出，因此使相變溫度變高和最大可恢復應變下降的幅度皆趨緩 ，而以 Co 取代 Cu 更加顯著地抑制 Zr7Cu10 析出物的反應 ，其中 Ni35Cu5Co10 因沒有 Zr7Cu10 析出物而使相變態溫度和最大可恢復應變幾乎維持恆定。最後在 XRD 分析中，得到 Ni35Cu5Co10 和 Ni35Cu10Co5 合金的晶格常數並計算 B19’麻田散體相和 B2 沃斯田體相之間的幾何晶格相容性，發現 400°C 時效處理後的試片之晶格相容性無明顯的改變，因此合金的熱循環穩定性提升仍主要歸因於 H 相的析出強化現象。本研究結果發現以 Co 取代 Ni 或 Cu 皆會抑制TiZrHfNiCuCo 高熵形狀記憶合金在低溫和高溫時效的析出反應，進而影響合金的性能，因此可藉由調整時效處理條件和合金成分來設計應用於不同領域的高熵形狀記憶合金。

This study researches following four compositions of Ti20Zr15Hf15(NiCuCo)50 high-entropy shape memory alloys (HESMAs) with Co substituting for Ni or Cu, Ni30Cu15Co5, Ni25Cu15Co10, Ni35Cu5Co10, and Ni35Cu10Co5, to investigate their martensitic phase transformation behaviors, microstructures, thermal cyclic stability, and shape memory effect aged at 400-700°C. After 400°C and 500°C aging treatments, widely distributed nano-scale H-phases precipitates were observed, inhibiting the martensitic phase transformation and inducing precipitation hardening in the matrix, which lowered the phase transformation temperature and enhanced the thermal cycling stability. SEM observations exhibited only the matrix and Ti2Ni precipitates, while TEM observations further showed the presence of H-phases. Adding Co suppressed the precipitation of the H-phases, mitigating the decrease in the phase transformation temperature. On the contrary, aging at 600°C and 700°C led to the precipitation of (Ni, Cu)-rich Zr7Cu10, increasing the (Ti+Hf+Zr) ratio of the matrix, which increased the phase transformation temperature. However, these Zr7Cu10 precipitates were too large to cause precipitation hardening in the matrix; thus, the thermal cycling stability did not improve and caused deterioration of the shape memory effect with reduced maximum reversible strain. SEM observations indicated that Co addition also suppressed the Zr7Cu10 precipitation, mitigating the increase in the phase transformation temperature and the decrease in the maximum reversible strain. Co substitution for Cu showed more significant suppression of the Zr7Cu10 precipitation, with the Ni35Cu5Co10 alloy exhibiting no Zr7Cu10 precipitates and thus maintaining nearly constant phase transformation temperature and maximum reversible strain after aging. Lastly, XRD analysis determined the lattice constants of Ni35Cu5Co10 and Ni35Cu10Co5 alloys and calculated the geometric lattice compatibility between B19’ martensite and B2 austenite. No significant change in the lattice compatibility was observed after the 400°C aging treatment, suggesting that the enhancement in the thermal cyclic stability was mainly attributed to the precipitation hardening of the H-phases precipitates. It is concluded that the substitution of Co for Ni or Cu reduced the precipitation after low- and high-temperature aging, affecting the properties of the TiZrHfNiCuCo high-entropy shape memory alloys (HESMAs), which introduced a new approach for developing HESMAs for different applications by adjusting their compositions and aging treatments.