無間隙型原子鋼中巨大肥粒鐵和Ti-6Al-4V中巨大相變化之研究

Abstract

低碳鋼及鈦合金因其優秀的強度，至今已被廣泛運用在各個產業，如建築、汽車、航太等等。在西元1970年，日本冶金學家發現去除間隙型原子能大幅提升低碳鋼的成型性能，此鋼種即為現今廣泛運用於汽車外殼的無間隙型原子鋼。早期的研究中已發現極低碳鋼及鈦合金在快速冷卻下會引入巨大相變化，然而，關於此相變化的研究仍然有限。本研究的目的旨在釐清巨大相變化在無間隙原子鋼及鈦合金中的影響。透過光學顯微鏡、掃描式電子顯微鏡及穿透式電子顯微鏡來觀察顯微結構並搭配機械性能的測試來了解巨大肥粒鐵對無間隙型原子鋼的成型性能與機械性能所造成的改變，並透過背向散射電子繞射技術搭配母相重構演算法來檢查巨大相變化在Ti-6Al-4V中的晶體學特徵及其生長機制。 第四章深入探討巨大肥粒鐵及等軸肥粒鐵的機械性能及其變形組織演化過程，並發現此二肥粒鐵變形組織的序列皆是由差排胞、密集差排牆 (DDWs) 和變形微帶 (MBs) 所組成。透過固定應變量的間斷拉伸測試，我們發現DDWs和MBs可在較低應變量下於巨大肥粒鐵晶粒內大量發展，此差異是由於巨大肥粒鐵初始的差排密度較高。較高的差排密度促進了差排的增生和堆積，因而在巨大肥粒鐵中產生了更明顯的應變硬化效應。此外，巨大相變化所產生的不規則晶界及晶粒內部的次晶界使得強度提升的同時，延展性得以維持。 第五章則是檢視冷軋和退火處理對巨大肥粒鐵及等軸肥粒鐵的微觀結構及織構演變的影響。我們發現巨大肥粒鐵組織在經過相同的冷軋和退火製程後能產生更細小的晶粒及更強的γ-纖維，而γ-纖維已被證實是影響成型性能的關鍵因子。我們亦發現在冷軋時，巨大肥粒鐵的主要織構會由{111}〈112〉轉向{111}〈110〉，這個現象是由巨大肥粒鐵中的差排及次晶界所形成的預應變效應造成的。相反地，在退火階段，我們在二肥粒鐵中均無觀察到在高應變量下常見的{111}〈112〉再結晶織構，這是因為巨大肥粒鐵內的預應變效應仍不夠顯著，使得主要的織構仍為{111}〈110〉。此外，巨大肥粒鐵所儲存的應變能明顯高於等軸肥粒鐵，前者在600°C退火15分鐘後再結晶分率即來到88.4%，而後者在相同條件下只有48.2%，此結果顯示巨大肥粒鐵能有效減少再結晶完成所需的時間並節省成本。 第六章旨在鑑定Ti-6Al-4V合金中的巨大相變化。我們在Ti-6Al-4V中觀察到兩種類型的巨大相變化晶粒形貌：晶界αm和塊狀αm。此二組織的化學成分與麻田散體相同，並具有不規則且無法分辨的晶界。本研究發現，塊狀αm是由晶界αm跨越先前β晶界生長而形成的。前β相的重構分析表明，Ti-6Al-4V中的αm在先前的β晶界處以Burgers方位關係成核。然而，αm的生長並不一定需要維持Burgers方位關係。此外，結合β重構和KAM分析的結果顯示，在塊狀αm的生長過程中，先前的β晶界會在塊狀 αm 跨過時轉變為小角度晶界。最後，本研究發現跨晶界生長之 αm兩側的前β晶粒有共用{110}晶面的現象。

Low carbon steels and titanium alloys, known for their excellent strength, have been widely utilized in various industries, such as construction, automotive, aerospace, and more. In 1970, Japanese metallurgists discovered that removing interstitial atoms significantly enhances the formability of low carbon steel, leading to the development of Interstitial-Free (IF) steels, now extensively used in automotive bodies. Early research found that ultra-low carbon steels and titanium alloys undergo massive transformation during rapid cooling, yet studies on this transformation remains limited. This research aims to clarify the impact of massive transformation in IF steels and titanium alloys, employing optical microscopy, scanning electron microscopy, and transmission electron microscopy to observe microstructures and coordinate with mechanical performance tests to understand the changes in formability and mechanical properties caused by massive phase. Moreover, the parent grain reconstruction algorithm was utilized in the present study to explore the crystallographic nature of the massive phase in Ti-6Al-4V alloy. Chapter 4 delves into the deformation structures of massive and allotriomorphic ferrite, revealing that both consist of dislocation cells, Dense Dislocation Walls (DDWs), and microbands (MBs). Through interrupted tensile tests at fixed strain levels, it was found that, under the same strain, DDWs and MBs developed earlier in the massive ferrite compared to allotriomorphic ferrite, due to higher dislocation density of the original massive ferrite grain-matrix. This higher dislocation density enhances dislocation multiplication and accumulation, leading to a more pronounced strain hardening effect in massive ferrite. Furthermore, the sub-boundaries and irregular grain boundaries produced by the massive transformation contribute to increased strength while maintaining ductility. Chapter 5 examines the impact of cold-rolling and annealing treatments on the microstructures and textures of massive and allotriomorphic ferrite grains. It was found that massive ferrite structures developed finer grains and stronger γ-fiber textures after the same cold-rolling and annealing processes. γ-fiber has been proven to be a key factor affecting formability. It was also observed that during cold rolling, the primary texture component of massive ferrite shifted from {111}〈112〉 to {111}〈110〉, believed to be caused by the pre-strain effect induced by dislocations and sub-boundaries in massive ferrite. In contrast, during the annealing phase, the common {111}〈112〉 recrystallization texture observed in cold-rolled steels subjected to high strains was not seen in either type of ferrite, as the pre-strain effect in massive ferrite was not significant enough, keeping the primary texture as {111}〈110〉. Additionally, the strain energy stored in massive ferrite was notably higher than in allotriomorphic ferrite, with the former reaching a recrystallization fraction of 88.4% after annealing at 600°C for 15 minutes, while the latter only reached 48.2% under the same conditions. This result indicates that massive ferrite can effectively reduce the time required for complete recrystallization, thus saving costs. Chapter 6 identifies the massive transformation in Ti-6Al-4V alloys. Two types of massive phase: GB-αm and blocky αm were observed in the Ti-6Al-4V. These phases are characterized by their identical chemical composition to martensite and their patchy morphologies. The blocky αm was found to result from the growth of GB- αm across the prior β grain boundaries. Prior β reconstruction demonstrated that αm in Ti-6Al-4V nucleates at prior β grain boundaries with Burgers orientation relationship. However, the growth of αm does not necessarily requires the Burgers OR. Additionally, the incorporation of prior β reconstruction and KAM analyses revealed that the prior β grain boundaries transformed into low-angle boundaries during the growth of blocky αm. Finally, it was found that the neighboring prior β grains of the transgranular blocky-αm shares the same {110} poles.