高熵輕量鋼之顯微結構與機械性質研究

Study on Microstructure and Mechanical Behaviors of a High-Entropy Lightweight Steel

賴人豪(Zen-Hao Lai)

指導教授 ： 顏鴻威

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000297

摘要

近年來因為節能減碳的目標，汽車產業對於輕量鋼的需求日益上升，因此發展高比強度的鋼成為近年的主流。提高比強度可以藉由提高強度或是降低密度的方式達成，本研究係著重於後者。在此研究中，我們設計了一個新的高熵輕量鋼，它結合了低密度、高熵以及適當的疊差能以導入雙晶誘發塑性三個特色。它的化學成分為Fe-25Mn-5.5Al-3Si-1C-0.2Mo-0.1V (以重量百分比表示)。因為它同時添加了鋁和矽，其密度為6.98 g/cm3，比起純鐵的7.87 g/cm3少了約10%。高熵鋼的觀念被使用，也就是採用複雜的合金設計，來幫助我們決定合金成分。其中疊差能可以藉由用矽取代鋁的方式大幅降低。此高熵輕量鋼成功達到所有預設的目標，而且其機械性質與其他的輕量鋼相近或甚至更好。
我們研究了此高熵輕量鋼的相變態以及其顯微結構與機械性質之關聯，顯微結構特別著重於退火以及時效處理。在900°C退火後，許多二次相形成在晶界上且沿著軋延方向，因為二次相都生成在晶界上，它們可以提供二次相強化但會造成大幅度的延性下降。在1030°C退火後，其結構為完全再結晶之單相FCC結構。我們研究此條件階段變形之顯微結構，發現其在約20%的變形量時開始形成變形雙晶，它把原先的晶粒切割成更小的晶粒，於是產生動態晶粒細化的效果進而貢獻加工硬化，所以此條件可以達到穩定的加工硬化(Yield ratio~0.5)良好的均勻延伸率(＞70%)及高的抗拉強度(~900MPa)。經過時效後，許多種類的析出物生成，有些在原本晶粒的晶界上，有些在晶粒中。晶粒中的kappa碳化物大幅提高降伏強度，卻會降低加工硬化的能力及伸長量，過量的晶粒內DO3介金屬化合物也會傷害延性，但層狀或蜂窩狀的晶界析出物造成更嚴重的延性劣化，它會導致嚴重脆性並使得材料在降伏前就破裂。
此論文研究了此一新設計之高熵輕量鋼之相變態及其顯微結構與機械性質之關聯，它說明了藉由導入高熵鋼的概念可以創造出具有良好性質的先進輕量鋼。

關鍵字

高熵鋼 ； 輕量化 ； 雙晶誘發塑性 ； 析出行為 ； 變形顯微結構 ； 電子顯微鏡 ； 機械性質

並列摘要

The demands for weight-lightening in steel industry are increasing in the recent years for energy-saving purpose, resulting in the trend of developing high specific strength steels. Specific strength can be enhanced by increasing strength or reducing density. The current study is focused on the latter one. In this work, a high-entropy lightweight steel (HELWS) which combines the characteristics of low-density, high-entropy, and suitable stacking fault energy for twinning-induced plasticity is designed. Its chemical composition is Fe-25Mn-5.5Al-3Si-1C-0.2Mo-0.1V (in wt%). Its density is 6.98 g/cm3, which is about 10% lower than that of pure iron (7.87 g/cm3) due to the synergetic additions of Al and Si. The concept of high-entropy steel is used, i.e. applying complex alloy design, to determine the alloy composition. Stacking fault energy is noticeably decreased by replacing Al with Si. Desired properties are all successfully achieved in HELWS, and the mechanical properties are comparable with or even better than current lightweight steel.
The phase transformation of HELWS and the relationship between microstructure and mechanical behaviors are studied. Microstructural characterization is especially focused on annealing and aging. After annealing below 900°C, plenty of 2nd phases form along cold-rolling direction and on grain boundary. Because these 2nd phases form on grain boundary, they can provide 2nd phase strengthening while significantly harm the ductility. It becomes fully recrystallized single FCC structure at 1030°C. At single-FCC state, the deformed microstructure at various strain level is investigated, and the onset strain of deformation twins is around 20% of engineering strain. Deformation twins subdivide original grain into smaller grains, resulting in dynamic Hall-Petch effect. Therefore, steady work hardening (Yield ratio~0.5), outstanding uniform elongation (EL>70%), and high ultimate tensile strength (UTS~900MPa) are achieved. After aging, several kinds of precipitates forms with some on the original FCC grain boundary and some inside grain. Precipitation of kappa carbide inside grain significantly raises the yield strength, while decreasing the work hardening capability and elongation. Excessive DO3 intermetallic compound within grain harms the ductility severely as well. However, the most severe ductility loss comes from the lamellar or cellular precipitation on grain boundary, as it results in brittleness and causes the steel to fracture before yielding.
This thesis investigates the phase transformation and the relationship between microstructure and mechanical behaviors of the newly designed HELWS. It demonstrates the strategy of high-entropy steel creates advanced low-density steels with excellent properties.

並列關鍵字

high-entropy steel ； lightweight ； twinning-induced plasticity ； precipitation ； deformed microstructure ； electron microscopy ； mechanical property

參考文獻

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