時效對鈦鎳合金多階段相變態之研究

Abstract

摘 要 本實驗以VAR熔配五種成分之TiNi形狀記憶合金，並進行400℃恆溫時效處理，來探討析出物對多階段相變態及顯微組織的影響。實驗結果顯示在Ti49.3Ni50.7合金的DSC曲線中發現了四階段相變態行為，此時效造成的四階段相變態可歸因於在400℃時效處理下的材料顯微組織演變--亦即大、小尺度上Ni4Ti3析出物不均勻析出的情形，並且此時效導致的四階段相變態是一種在特定的合金成分範圍下才能發生的現象。DSC曲線圖中的前兩個相變態反應峰可視為是R相的形成以及緊接在後的B19’麻田散體相變態反應，且是源自於在晶界區域聚集的Ni4Ti3析出物所導致的大尺度上應變場差異。而晶粒內部發生的小尺度上Ni4Ti3析出物不均勻分佈則是DSC曲線圖上後兩個相變態峰的主要因素。 在in-situ TEM的降溫觀察中，晶粒內部歷經三種變態的順序：分別為R相變態、第一階段B19’麻田散體相變態以及第二階段B19’麻田散體相變。其中R相變態的特徵為『連續』及『平順』的孕核成長模式，以小圓點形貌孕核並連續成長不中斷。相反地，B19’麻田散體相變態則瞬間『爆炸式』孕核，以『不連續』及『迅速』的方式成長，並長到一固定大小尺寸而不再改變，此R相變態為額外的相變態反應，在DSC曲線圖上並未被發現，其形成原因應該是試片的薄膜效應所致。在晶界部分也觀察到由B2母相變態成R相的過程，證明了晶界的B2-R-B19’變態的順序。 此外，本研究並以DSC及in-sutu TEM觀察熱循環對四階段相變態特性的影響，結果顯示經熱循環處理後，在材料的內部導入了差排缺陷，形成了不可逆的改變。這些差排的應變場造成了B19’麻田散體被抑制而殘存在晶粒內部。雖然晶粒內部的相變態反應在熱循環過程中被明顯抑制，然而晶界的相變態卻保持原來的變態特性。根據in-situ TEM的觀察，析出物在晶界及晶粒內部分佈不均的情況是導致熱循環過程中上述不同相變態演變結果的原因。晶界密集析出形成強大應變場，抑止差排產生，使晶界變態不受差排影響而保有原來特性，相反地，晶粒內部則因較稀疏析出物無法阻止差排產生，因而有相變態受差排阻礙的現象。

Abstract In this study, five kinds of binary NiTi alloys with different chemical compositions were prepared by VAR. The Ni-rich NiTi SMAs were subjected to aging treatment at 400℃ in order to study the effects of aging on the multi-stage transformation behavior and on the microstructural evolution. The experimental results show that a four-stage transformation has been observed in an aged Ti49.3Ni50.7 shape memory alloy by DSC. Such phenomenon is attributed to the complex microstructural evolution, i.e., formation of large-scale and small-scale heterogeneities in the Ni4Ti3 precipitation during the aging treatment. The age-induced four-stage transformation is able to occur in a narrow range of Ni content by aging at 400℃. The first two peaks on the DSC cooling curve are considered to be the formation of R phase and the subsequent transformation of B19’ martensite in the regions of grain boundary, attributed to the effects of large-scale heterogeneity in stress field induced by Ni4Ti3 precipitates congregating along grain boundary. A heterogeneous distribution of Ni4Ti3 precipitates in small-scale in grain interior is considered to account for the last twos. The in-situ TEM observation during cooling shows that three phase transformations occur sequentially in grain interior: the R phase transformation, the first-step B19’ martensitic transformation, and the second-step martensitic transformation. The R phase is found to transform in a “continuous” and ”smooth” manner, nucleating as a small round spot and growing smoothly without interruptions. On the contrary, the B19’ martensitic transformation nucleates in sudden burst, growing discontinuously and rapidly to an apparent size then stopping. However, the R phase transformation is unexpected owing to its absence in DSC curves and considered to result from the thin film effect. In addition, the B2-R transformation is observed in regions along grain boundary, thus the transformation sequence of B2-R-M is therefore convinced. Furthermore, the effects of thermal cycling on multiple-stage transformation in aged NiTi alloy were directly studied by DSC and in-situ TEM observations. The experimental results suggest that the irreversible changes such as dislocations were introduced during thermal cycling and constrained the B19’ martensite in grain interior. The transformations in grain interior were suppressed during thermal cycling whereas the others along grain boundary remained their characteristics. According to in-situ observations, the heterogeneous precipitations in grain interior and grain boundary were considered to account for the different evolutions of transformations during thermal cycling.