Ti49.2Ni49.3Fe1.5形狀記憶合金塊材之超彈性與 彈熱效應研究

Abstract

本研究針對Ti49.2Ni49.3Fe1.5形狀記憶合金塊材進行研究，探討時效處理及應力循環對於形狀記憶合金之變態溫度與彈熱效應的影響，以及訓練後試片於不同操作溫度下之彈熱效應。經時效50小時與150小時之試片皆產生Ti3Ni4析出物，前者析出物大小平均為75nm，而後者則為140nm。以400°C時效處理150小時，之後經過900MPa應力訓練之h150X900試片，擁有最良好的應力循環穩定性，其於訓練過程中應變衰退量為實驗試片中最小，且DSC曲線之峰值溫度隨應力循環的變化幅度也皆最小。從彈熱效應的結果中顯示，h150X900試片的升溫衰減率為試片中最低，因為於時效處理時產生之Ti3Ni4析出物具有析出強化效果，能有效阻擋應力循環時所產生之差排，且以較小的應力進行訓練，材料內部導入的差排較少，性質劣化較不嚴重，因此能提高循環穩定性。反之，時效50小時試片之析出強化效果不如時效150小時者，若以較大應力進行訓練，則容易產生塑性變形，因此h50X1200試片於訓練後性質劣化最為嚴重。於-40°C~80°C的變溫彈熱效應測試中，試片的升/降溫變化量及應變皆有隨測試溫度下降而上升的趨勢，而h150X1200試片於各溫度下擁有最大溫度變化量及應變量，顯示時效150小時具更佳析出強化效果，而以較大的應力進行測試，能誘發更多麻田散體相變態，彈熱效應所產生的溫度變化量也較多。若同樣以900MPa測試h150X1200及h150X900試片，則h150X1200溫度變化量及應變皆不及h150X900試片，顯示在訓練過程中，以較大應力進行訓練容易於材料內部產生差排，導致塑性變形，進而影響到性質穩定後的彈熱效應表現；而同樣以1200MPa測試h150X1200及h150X900試片，則h150X900會因訓練時應變硬化程度不足，故承受較大之1200MPa應力時無法於高溫抵抗塑性變形，造成h150X900試片於高溫時展現的彈熱效應不及h150X1200試片。

In this study, the effects of heat treatment and stress cycles on transformation temperatures and elastocaloric effect of Ti49.2Ni49.3Fe1.5 shape memory alloy were investigated. Ti3Ni4 precipitations could be formed after 50hr and 150hr heat treatments at 400 °C. The average size of the Ti3Ni4 precipitations was 75nm for the former and was 140nm for the later one. The specimen h150X900, which was heat-treated at 400°C for 150hr and then trained at 900MPa, had the best stability under stress cycles. It experienced the smallest decay of deformation strain during the 500 training cycles, so did its transformation peak temperatures measured by DSC. This feature originated from the strengthening effect of Ti3Ni4 precipitations, which effectively hinder dislocations produced during stress cycles. On the other hand, the strengthening effect of the 50hr treated sample was less than that of the 150hr one. Besides, if a larger stress, 1200 MPa, was employed to train the 50hr sample, plastic deformation was introduced more easily, causing the properties of h50X1200 sample decayed rapidly. The elastocaloric effect of the trained samples was tested between -40 °C to 80 °C. Both the deformation strain and ΔT(heating/cooling) rose when the testing temperature decreased. The h150X1200 sample had the most substantial deformation strain and temperature change at this temperature range. Because of the 150hr treated sample had a better strengthening effect, testing at 1200MPa would cause less plastic deformation and induce more martensite transformation, which resulted in a larger temperature change during elastocaloric effect. If both the h150X1200 and h150X900 samples were tested under 900 MPa, the temperature change and deformation strain of the h150X1200 were less than those of the h150X900 one. This feature revealed that fewer martensitic transformation could be induced when a smaller triggering stress was used, owing to more dislocations were introduced during the training process with 1200 MPa. However, if both samples were tested under 1200MPa, the strain hardening effect of h150X900 was not enough, it would cause plastic deformation at the high-temperature end, exhibiting a worse elastocaloric performance than that of the h150X1200 one in high-temperature environments.