冷軋延對鈦鎳合金相變態之研究

Abstract

本實驗使用VAR熔配Ti-50.3at. %Ni與Ti-50.7at.%Ni兩種不同成分之形狀記憶合金，鑄錠試片經均質化以及熱滾軋後，在1123K進行固溶處理，之後淬至冰水中，再進行不同縮減量的冷滾軋，利用DSC和XRD量測以及穿透式電子顯微鏡觀察其不同縮減量之顯微組織的變化。Ti-50.3at. %Ni成分在DSC量測裡發現多階段的相轉變行為，因而更進一步利用partial cycle來鑑定每個吸放熱峰的相轉變。而Ti-50.7at.%Ni成分之試片再進行in-situ TEM觀察相轉變的變化，比較塊材與薄片TEM試片之間相轉變行為的差異。 實驗結果得知TiNi 形狀記憶合金之變形調適機制會隨著合金成分之差異而有顯著的不同。以Ti-50.3at. %Ni 合金觀察其冷加工之調適機制主要是麻田散體兄弟晶以雙晶型式作變形調適。隨著不同的冷軋延量，其雙晶調適機制也隨之改變。在較低的冷軋延量（5％）時，主要以 <011> typeⅡ twinning 做調適。 當冷軋延量增加時（15％），麻田散體次組織帶內之調適雙晶型式逐漸改變，不再以 <011> typeⅡ twinning 為主，出現一些交插之針狀雙晶平板，以及大量的差排和缺陷。當冷軋延量達30％時，局部區域出現非晶質化，且可能有應力誘發母相SIP(stress-induced parent phase)的機制發生。在DSC 的觀察裡發現四階段的相轉變，而在降溫過程四階段相轉變發生的位置首先是在晶界上的B2→B19’，第二個相轉變行為發生在第一個相轉變麻田散體板條之間，也是B2→B19’。第三個相轉變是發生晶粒內部，在缺陷聚集位置上先產生B2→B19’相變態，生成的麻田散體板條以夾特定的角度排列。最後一個相轉變是發生在其他未轉變的母相基地裡，為B2→B19’連續性的相變態。 以Ti-50.7at.%Ni 7 合金觀察其冷加工之調適機制主要是應力誘發缺陷及麻田散體為主要的變形調適。應力誘發麻田散體的數量並非與冷軋延量呈線性成長。在10％之冷軋延量下，麻田散體與缺陷在母相基地內部做應變調適。當冷軋延量達15％時，差排沿著麻田散體界面排列，此排列有利於內部應力之調適，應力誘發麻田散體被大量缺陷抑制住。隨著冷軋延量達22％時，因缺陷在基地內達到最大值，所以在缺陷聚集的界面中，以微雙晶的區域(domain)做更 進一步的調適。當冷軋延量達到最大值35％時，基地內部達到一個奈米晶(nanocrystalline)與非晶質(amorphization)混合組織，仍具有織構(texture)，且似乎有應力誘發母相的行為發生。在in-situ TEM 之觀察與DSC 量測發現，相變化的順序會因為試片之厚薄而有些許的差異。當冷軋延量低於10％，在加熱與冷卻過程均發生相同的相轉變B19’→B2 和B2→R→B19’。當冷軋延量達15~22 ％時，在冷卻過程發現不同的相轉變，TEM 觀察裡相轉變只發生B2→R，而R→B19’的相轉變被完全抑制。冷軋延量達最大值35％時，原本微雙晶區域變成具有織構的奈米晶粒，且相轉變行為完全被抑制了。

Ti-50.3at. %Ni and Ti-50.7at.%Ni shape memory alloy were prepared by vacuum arc remelter (VAR) in this study. Bulk specimens were first homogenized in vacuum (24 h, 1323 K), quenched in ice water (276 K) and then hot rolled at 1123 K. After annealing at 1123 K for 1 h, some specimens were cold rolled by 5 ~ 35%. The microstructure and crystal structure of cold-rolled specimens were analyzed by Differential Scanning Calorimeter (DSC), X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The main object of this study is to perform a four-stage transformation of Ti-50.3at.%Ni caused only by cold-rolling without other thermomechanical treatments and provide a small-scale heterogeneity of local stress field explanation. And to observe the microstructure evolution of Ti-50.7at.%Ni alloy with various degrees of thickness reduction of particular interest is the accommodation mechanism by generation of lattice defects. Subsequently, the cold-rolling NiTi sample is further studied by in situ TEM observation to investigate the phase transformation during the heating and cooling processes and comparison with the results of DSC measurements of bulk materials. The results show that the deformation mechanism of Ti49.7Ni50.3 SMA is dominant by twinning accommodation in the martensite variants. The twinning mode is changed with degree of cold-working. At slight cold reduction (5％) , the <011> typeⅡ twinning mode is dominant .However at heavy cold reduction (15％), the twinning modes inside the martensite substructural bands are different. Some cross-hatched twinning plates, heavy dislocation and defects are observed. When the cold-reduction finally reaches 30％ , mechanical amorphisation happens in local regions. At the same time, the stress-induced parent phase also happens. The cold rolled deformation of Ti-50.3at. %Ni causes defect generation, resulted in a four-stage transformation was discovered to occur in TiNi alloy deformed 5% at RT. The transformation sequence on cooling was identified to be all the same B-M transformation. The first two transformations on cooling were suggested to occur in the regions close to grain boundary. The third transformation was suggested to occur in the grain interior. And the last one was suggested that it is retain B2 transformed B19’in the whole matrix. The heterogeneous defects cause the formation of small-scale heterogeneities which induced localized stress fields and affect the phase transformation sequence. The effects of cold rolling on deformation mechanism, martensite transformation and grain refinement were studied on Ti-50.7at.%Ni SMA. The cold rolled deformation of TiNi causes SIM, defect generation, micro twinning domains and finally amorphisation. Increasing strain results in a decrease of the size of micro twinning domains inside fractured plates. These nano-crystallites embedded in the amorphous matrix after in situ cycle possessed the preferred orientation. In the lower deformation, all of the SIM to B2, B2 to R and R to B19’ can occur during heating and cooling. After severe plastic deformation, the formation of nano-crystallite will suppress all phase transformations.