透過添加Co及時效熱處理控制 Ti20Zr15Hf15Ni50-xCox(x=15、20)合金 之相變態行為與機械性質

Abstract

本研究針對Ti20Zr15Hf15Ni35Co15高熵形狀記憶合金及Ti20Zr15Hf15Ni30Co20高熵應變玻璃合金，探討添加不同Co含量及400°C ~ 700°C時效熱處理對合金的麻田散體相變態行為、顯微結構、形狀記憶效應之研究。實驗結果顯示，添加Co會穩定B2相並抑制析出物生成，時效72小時後XRD均未觀察到析出物峰值，顯示析出效果不明顯。當添加Co超過臨界濃度時，麻田散體相變態被抑制，合金由Co15形狀記憶合金轉變為Co20應變玻璃合金。Co15經過低溫時效(400°C、500°C)，生成奈米級析出物，形成應力場抑制麻田散體相變態，對基底造成析出強化效果，進而改善熱循環穩定性；600°C時效後生成(Zr,Hf)7Ni10相，由於基底與析出物為非整合介面(incoherent)，未產生析出強化效果；700°C時效後生成沿晶界細小析出物，導致機械性質下降。Co20經過時效處理後，SEM結果皆顯示基底及富Zr、Hf的第二相為主要觀察特徵，由於添加更多的Co，析出的效果較Co15不明顯，且麻田散體相變態被嚴重抑制，推測在應力及降溫作用下僅能誘發R相變態，Co20 ST楊氏模數幾乎不隨溫度改變，展現艾林瓦效應，透過700°C時效熱處理可以提高相變溫度並使相變行為更明顯。形狀記憶實驗顯示，Co15與Co20低溫時效(400℃、500℃)生成之奈米級析出物有助於延遲材料斷裂並增加材料強度，此現象在Co20合金更為明顯；600℃時效熱處理生成的(Zr,Hf)7Ni10以及700℃時效熱處理生成細小沿晶界析出物使基底可相變區域變少，進而導致可回復應變量下降，不過強度仍與固溶狀態相同。Co15合金透過不同條件之時效熱處理，生成析出物不會犧牲太多可回復應變量，且低溫時效有助於增加材料強度；Co20合金析出效果則更不明顯，但是添加過量的Co導致麻田散體相變被嚴重抑制，奈米域會阻礙基底相變，同時透過應力及降溫只能誘發R相變態，導致Co20形狀記憶恢復能力不如Co15，但是時效後材料仍維持高強度(900MPa 以上)，顯示Co20需要較大的應力及過冷度誘發相變。本研究結果顯示，透過添加Co及時效熱處理可以控制麻田散體相變態行為與機械性質，有助於調整合金的相變態溫度、相變機制、微觀結構、機械性質等，提供未來設計合金之策略參考。

This study investigates the effects of varying Co content and aging heat treatments at 400°C to 700°C on the martensitic transformation behavior, microstructure, and shape memory effect of Ti20Zr15Hf15Ni35Co15 high-entropy shape memory alloy and Ti20Zr15Hf15Ni30Co20 high-entropy strain glass alloy. The experimental results indicate that the addition of Co stabilizes the B2 phase and suppresses the formation of precipitates, with no precipitate peaks observed in XRD after 72 hours of aging, indicating negligible precipitation effects. When the Co content exceeds a critical concentration, martensitic transformation is inhibited, and the alloy transitions from a Co15 shape memory alloy to a Co20 strain glass alloy. For Co15, low-temperature aging (400°C, 500°C) generates nanoscale precipitates that form stress fields, suppressed martensitic transformation, thus strengthening the matrix via precipitation hardening which enhancing its thermal cycling stability. The (Zr, Hf)7Ni10 phase forms after aging at 600°C, since the matrix and precipitates are incoherent, no precipitation hardening effect is observed. Aging at 700°C produces fine precipitates along grain boundaries, resulting in reduced mechanical properties. For Co20, post-aging treatment, SEM observations revealed that its primary features to be its matrix and a Zr, Hf-rich second phase. Due to the higher Co content, precipitation is less pronounced compared to Co15, and martensitic transformation is significantly suppressed. It is presumed that under stress and cooling, only the R-phase transformation is induced. The Young’s modulus of solution-treated Co20 remains almost constant with temperature, demonstrating an Elinvar effect. Aging at 700°C raises the transformation temperature and enhances transformation behavior. Shape memory effect experiments show that nanoscale precipitates formed during low-temperature aging (400°C, 500°C) for both Co15 and Co20 help delay material fracture and increase strength, with this effect more pronounced in Co20. However, aging at 600°C, which forms (Zr, Hf)7Ni10, and 700°C, which produces fine grain boundary precipitates, reduces the transformable region in the matrix, leading to decreased recoverable strain, although the strength remains comparable to the solution-treated state. For Co15, aging under different conditions produces precipitates without sacrificing recoverable strain significantly, while low-temperature aging increases material strength. In contrast, the precipitation effect in Co20 is less pronounced; the excessive Co addition severely suppresses martensitic transformation, and the nanoscale domains further hinder the matrix transformation. Under stress and cooling, only R-phase transformation is induced, resulting in Co20 exhibiting less shape memory recovery than Co15. However, the alloy retains high strength (above 900 MPa) after aging, indicating that Co20 requires greater stress and undercooling to induce phase transformation. This study demonstrates that Co addition and aging heat treatments can effectively control martensitic transformation behavior and mechanical properties, offering insights for adjusting transformation temperatures, mechanisms, microstructures, and mechanical properties, thereby providing strategic references for future alloy design.