Ti50Ni50及Ti49.3Ni50.7鈦鎳形狀記憶合金變態及機械性能之研究

Abstract

Ti49.3Ni50.7與Ti50Ni50 SMAs之PE、SME與超彈性應力應變循環等性能之提升作一系列的探討。Ti49.3Ni50.7合金經由300℃與400℃時效處理後，300℃ 者雖需較長時間才能達到最大硬度，但其硬度相較於400℃者大上許多，也因此300℃時效者具有較優異的PE、SME與超彈性應力應變循環性能。Ti50Ni50合金若未經任何熱機處理強化，其PE、SME與超彈性應力應變循環等性質都是表現最差的；但若經一定程度之冷軋延與低溫退火後其拉伸性質會大幅提升，甚至較Ti49.3Ni50.7 經400℃最大時效硬化者來的優異。冷加工量對TiNi合金的超彈性應力應變循環性能也有很大的影響，將Ti50Ni50合金施予0%、10%、20%、30%及40%之冷軋延後，發現冷軋延量小於20%者隨著循環次數的增加而逐漸轉變成線性超彈性，經過拉伸循環訓練後其儲能效率高但可儲存之能量小；冷軋延量大於20%者則不隨著循環次數的增加而有太大的改變，儲能效率稍差，但可儲存較大之能量；而冷軋延後之退火時間越長，材料的拉伸強度及應力應變循環表現越差，最好的條件為1 min退火；而退火時間太長者經應力應變循環時產生的差排累積，阻礙了相變態之發生，特別是R相變態的壓抑最為明顯。拉伸時之最大應變越大，材料的殘留應變越多，SIM逆變態越為困難，故超彈性在工程應用上其應變量不宜超過7%。在應變速率2.5×10-4s-1~1.0×10-2s-1的範圍內，於越快的應變速率下作超彈性之應力應變循環，循環對SIM之順變態的助益越明顯，但對SIM逆變態的影響則越有限；反之，若應變速率越慢，對SIM逆變態之助益則大於順變態者。

In this study, the property improvement of shape memory effect (SME), pseudoelasticity (PE) and stress-strain (σ-ε) cycling of Ti49.3Ni50.7 and Ti50Ni50 shape memory alloys (SMAs) is investigated. Ti49.3Ni50.7 SMA aged at 300℃×100h and 400℃×8h can reach the maximal precipitation-hardening with the hardness of the former being higher than that of the latter. Tensile test indicates that the specimen aged at 300℃×100h has better SME/PE and σ-ε cycling properties than that aged at 400℃×8h. Cold-rolling effect on the property improvement is studied on Ti50Ni50 SMA. Experimental results show that the degree of cold-rolling lower than 20% is insufficient to strengthen the SMAs to improve their properties, such as the σ-ε cycling stability and the recoverable storage energy in σ-ε curve. If the annealing of cold-rolled specimen is over, the SMAs’ properties can also be deteriorated. At the same time, the σ-ε cycling test indicates that, after 20th cycles, both R-phase and B19’ martensitic transformations are depressed due to the dislocations pile-up during the cycling, and the B2→R transformation is more depressed than R→B19’ one. In this study, the maximal PE strain induced by stress-induced martensite (SIM) is found to be lower than ~7% and the plasticity deformation occurs if the strain is higher than 7% which will deteriorate the SMAs’ PE property. For the strain rate (ε ̇) effect on the property improvement of Ti50Ni50 SMA, in the ε ̇ range of 2.5×10-4s-1~1.0×10-2s-1, the σ-ε cycling with higher ε ̇ will be more beneficial to the forward SIM transformation, instead of the reverse SIM transformation during the cycling.